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Catalytic oxidation of cyclohexanol to cyclohexanone using a combination of rhodium (111), iron (111).. 1978

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CATALYTIC OXIDATION OF CY CL OH EX AN OL TO CYCLOHEXANONE USING A COMBINATION OF RHODIUM{|II) IRON ( I I I ) AND MOLECULAR OXYGEN. by JOHN ABBOT B.Sc. I m p e r i a l C o l l e g e , London, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e F a c u l t y o f G r a d u a t e S t u d i e s i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the r e g u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA May 1978 (cT) J o h n A b b o t , 1978 In presenting th i s thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ib rary sha l l make i t f ree l y ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r l y purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or p u b l i c a t i o n of th is thes is fo r f i nanc ia l gain sha l l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date 23ap Mfty i ABSTRACT The c a t a l y t i c conversion of cyclohexanol to cyclohexanone using a combination of rhodium t r i c h l o r i d e trihydrate and f e r r i c chloride in the presence of molecular oxygen was investigated. No conversion to cyclohexanone occurred i n the absence of rhodium t r i c h l o r i d e trihydrate, but some degree of conversion was found i n the absence of f e r r i c chloride. The optimum conditions f o r c a t a l y t i c oxidation were produced by using a combination of rhodium t r i c h l o r i d e trihydrate and f e r r i c chloride, and under these conditions the rate of conversion to the ketone declined s t e a d i l y , u n t i l the mixture contained approximately U0% cyclohexanone. For a fixed amount of cyclohexanol and rhodium t r i c h l o r i d e trihydrate i t was found that there was an optimum amount of f e r r i c chloride necessary to produce the maximum y i e l d i n the shortest possible time. Addition of f e r r i c chloride i n excess of t h i s optimum amount tended to suppress the rate of conversion to the ketone. This can probably be explained by the additional production of water and cyclohexene (see below). Using a cyclohexanol/ferric chloride r a t i o i n the optimum range at a given temperature, increasing the rhodium t r i c h l o r i d e trihydrate concentration beyond a certain l e v e l did not s i g n i f i c a n t l y increase the f i n a l y i e l d , or the reaction rate. The oxidation reaction occurred under a c i d i c conditions, th i s a c i d i t y being the re s u l t of i n t e r a c t i o n between f e r r i c chloride and cyclohexanol (and cyclohexanone) , The a c i d i t y of a t y p i c a l system was found to decline rapidly as the reaction p r o g r e s s e d . C y c l o h e x e n e was p r o d u c e d i n a s i d e r e a c t i o n , t o g e t h e r w i t h w a t e r . T h i s i s p r e s u m a b l y t h e r e s u l t o f c y c l o h e x a n o l u n d e r g o i n g an e l i m i n a t i o n r e a c t i o n u n d e r a c i d i c c o n d i t i o n s . U s i n g t h e o p t i m u m c y c l o h e x a n o l / f e r r i c c h l o r i d e r a t i o a t 1 0 0 d e g , t h e t h e c y c l o h e x e n e c o n t e n t r e m a i n e d a t l e s s t h a n 1 0 $ , d u r i n g t h e c o u r s e o f t h e r e a c t i o n . I n t r o d u c t i o n o f c y c l o h e x e n e i n a m o u n t s i n e x c e s s o f 20 % g r e a t l y s u p p r e s s e d t h e c o n v e r s i o n t o c y c l o h e x a n o n e , p r e s u m a b l y due t o s t r o n g c o m p l e x a t i o n o f t h e o l e f i n w i t h a r h o d i u m s p e c i e s . w a t e r was p r o d u c e d d u r i n g t h e c a t a l y t i c o x i d a t i o n i n a m o u n t s g r e a t e r t h a n c o u l d b e a c c o u n t e d f o r by p r o d u c t i o n o f c y c l o h e x e n e . T h i s a d d i t i o n a l w a t e r c o n t e n t o f t h e r e a c t i o n m i x t u r e i n a c l o s e d s y s t e m was i n g o o d a g r e e m e n t w i t h t h a t p r e d i c t e d on t h e b a s i s o f t h e e q u a t i o n : T h e p r e s e n c e o f w a t e r i n t h e r e a c t i o n m i x t u r e t e n d e d t o s u p p r e s s t h e o x i d a t i o n o f c y c l o h e x a n o l t o c y c l o h e x a n o n e . U s i n g t h e o p t i m u m r a t i o o f c o m p o n e n t s , v e r y l i t t l e c o n v e r s i o n t o t h e k e t o n e o c c u r r e d i n t h e p r e s e n c e o f o x y g e n , a t t e m p e r a t u r e s b e l o w 5 0 d e g . I n c r e a s i n g t h e t e m p e r a t u r e f r o m l O O d e g t o 150deg i n c r e a s e d t h e r a t e o f o x i d a t i o n b u t h a d l i t t l e e f f e c t on t h e f i n a l y i e l d o f c y c l o h e x a n o n e . , O x y g e n was f o u n d t o be n e c e s s a r y f o r c a t a l y t i c o x i d a t i o n t o O O H o i i i occur. The measured oxygen absorption for a reaction mixture containing an optimum r a t i o of components, was found to be i n good agreement with that predicted by the above equation. Using an optimum r a t i o of components in the presence of oxygen, the conversion to cyclohe.xano.ne was limited to approximately 40%. This l i m i t was probably due to an interaction between cyclohexanone and some active rhodium species e s s e n t i a l for c a t a l y t i c a c t i v i t y . i v TABLE OF C08TENTS Page ABSTRACT. ................. ... . . . .... ( i ) TABLE OF CONTENTS.... (iv) LIST OF TABLES.................... ...... ...... ..... ..... <vii) LIST OF FIGURES AND GRAPHS . (ix) ABBREVIATIONS. . . ...... (xvi) ACNOWLEDGEMENTS.. ., ..... . .. . ( x v i i ) CHAPTER 1 INTRODUCTION...... ................ ............... 1 CHAPTER 2. OXYGEN FLOW SYSTEMS.... ............ ...........10 2.1. Experimental setup .................................. 10 2.2. The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - c o n c e n t r a t e d h y d r o c h l o r i c a c i d system............... 13 2.3. The c y c l o h e x a n o l - f e r r i c c h l o r i d e - c o n c e n t r a t e d h y d r o c h l o r i c a c i d system............................13 2.4. The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c c h l o r i d e - c o n c e n t r a t e d h y d r o c h l o r i c a c i d - s y s . . t e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.4a. Dependence on i n i t i a l f e r r i c c h l o r i d e - c o n c e n t r a t i o n . ..................... .14 2.4fe. Dependence on rhodium t r i c h l o r i d e t r i h y d r a t e c o n c e n t r a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5 2.4c. Dependence on a c i d i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2. 4d. Dependence on water content .44 2.4e. Dependence on cyclohexene.........................47 V 2. 4f. Temperature Dependence......... ... ................ 51 2.4g. E f f e c t of a d d i t i o n o f a phosphine.................55 2.4h. Attempt t o de t e c t hydrogen 55 2.4i. Attempts to i n c r e a s e the f i n a l y i e l d of cyclohexanone. 57 2.4j. The i n t e r a c t i o n between rhodium t r i c h l o r i d e t r i h y d r a t e and cyclohexanone..................... 59 2.4k. Dependence on oxygen f l o w . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.41. Attempts to determine the f e r r o u s / f e r r i c r a t i o . . . . 6 3 CHAPTER 3. CLOSED SYSTEMS............r 72 3.1. Experimental setup ............... ................... 72 3.2. The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - c o n c e n t r a t e d h y d r o c h l o r i c a c i d system...............73 3.2a. Reaction under oxygen.•............•..•........... 73 3.2b. Reaction under helium.............................74 3.3. The c y c l o h e x a n o l - f e r r i c c h l o r i d e system..............74 3 . 4 . The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c c h l o r i d e system.................• •........... 75 3.4a. Reaction under oxygen........^....................75 3.4b. Reaction under n i t r o g e n . . 82 3.4c. E f f e c t of cyclohexene............................. 82 CHAPTER 4. RELATED SYSTEMS............ ...................85 v i 4 . 1 . U s e o f o t h e r m e t a l s a l t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 4 . 2 . T h e c y c l o h e x a n o l - r h o d i u m t r i c h l o r i d e t r i h y d r a t e - p - t o l u e n e s u l p h o n i c a c i d m o n o h y d r a t e s y s t e m . . . . . . . . . 8 5 4 . 3 . A d d i t i o n o f a s o l v e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8 4 . 4 . O x i d a t i o n o f c y c l o p e n t a n o l . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 0 C H A P T E R 5 . S U M M A R Y O P R E S U L T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 C H A P T E R 6 . D I S C U S S I O N AND C O N C L U S I O N S . . . . . . . . . . . . . . . . . . . . . 96 v i i LIST OF TABLES Page Table (1). 26 Variation i n water and cyclohexene content with f e r r i c chloride concentration f o r the system; Cyclohexanol (I90mmol)# BhCl 3^0 (0. 302mmol) , HCl (0.10ml), at lOOdeg. Table (2)., 29 Acidity of cyclohexanol/concentrated HCl mixtures. Table (3). 32 A c i d i t y of cyclohexanol/FeCl^ mixtures. Table (4) . 33 E f f e c t of water on a c i d i t y of the system: Cyclohexanol (190mmol), Feci (9.25mmol) , 3 HCl (0.10ml), Table (5). 35 E f f e c t of water on a c i d i t y of the system: Cyclohexanone (190ramol), F e c i <9.25mmol), 3 HCl (0.10ml), Table (6). 37 Variation of a c i d i t y with time for the system: Cyclohexanol (190mmol), ShCl 3H 0 (0. 302mmol) , Feci. (9.25mmol) , HCl (0.10ml), at 100deg. v i i i P a g e T a b l e (7) . 40 V a r i a t i o n i n c y c l o h e x a n o n e c o n t e n t , c y c l o h e x e n e c o n t e n t , w a t e r c o n t e n t , a n d a c i d i t y f o r t h e s y s t e m : C y c l o h e x a n o l ( 1 9 0 m m o l ) , E h C l ^ H ^ O ( 0 . 302mmol) , F e c i ( 9 . 2 5 m m o l ) , c o n c e n t r a t e d H C l ( 2 . 0 m l ) , a t 1 0 0 d e g . T a b l e ( 8 ) . 66 A b s o r b a n c e o f s o l u t i o n s a f t e r a d d i t i o n o f KSCN t o c y c l o h e x a n o l / F e C l s y s t e m s . T a b l e (9) . 6 9 A b s o r b a n c e o f s o l u t i o n s a f t e r a d d i t i o n o f K ( C N ) . t o c y c l o h e x a n o l / ? e e l s y s t e m s . 3 * " T a b l e (10). 70 A b s o r b a n c e o f s o l u t i o n s a f t e r a d d i t i o n o f K (CN) c y c l o h e x a n o l / F e C l s y s t e m s . 3 6 3 T a b l e ( 1 1 ) . C o m p a r i s o n o f d a t a f o r c a t a l y t i c o x i d a t i o n o f a l c o h o l s . i x LIST OF FIGURES AND GRAPHS Page Figure (1). 7 Mechanism (1) Graph (1). 17 Rate of cyclohexanone formation f o r the system: Cyclohexanol (I90mmol), 8hCl3H 0 <0. 302mmol), FeCl (9.25mmol), HCl (0.10ml), at 100aeg. Graph (1a)., 18 Rate of cyclohexanone formation for the system: Cyclohexanol (190mmol), RhCl,3HO (0.302mmol), F e c i , (9.25mmol) , HCl (0.10ml), at 100deg. Reproducibility of r e s u l t s . Graph (2). 19 Rate of cyclohexanone formation for the system: Cyclohexanol (190mttol), ShCl^H^O (0.302mmol), ? e C l 3 C2.3mmol) HCl (0.10ml), at 100deg. Graph (3) . , 20 Rate of cyclohexanone formation for the system: Cyclohexanol (190mmol), RhCl^H^O (0. 302mmol), FeCl (18.5mmol) HCl (0.10ml), at 100deg. Rate of cyclohexanone formation for the system: Cyclohexanol (190mmol}» RhCl 3H 0 (0.302mmol), PeCl (24.7mmol) HCl (0.10ml), at 100deg. Rate of cyclohexanone formation for the system: Cyclohexanol <190mmol), RhCl^H^O (0.302mmol), ^eCl^ (6.17mmcl) HCl (0. 10ml) , at 100deg. Rate of cyclohexanone formation for the system: Cyclohexanol (190mmol)» HhCl ^ H J ) (0.302mmol), FeCl (1.54mmol) HCl (0.10ml), at 100deg. Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), HhCl 3fl O (0.302mmol), F e C 1 - 1 A : (-6. 1?- 12.3mmol), B: (18.5mmol), C: (24.7mmol), D: (1.54mmol); HCl (0.10ml), at 100deg. Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl33H^O A: (0.604mmol), B: (0.302mmol), C: (0.118mmol); FeCl (9.25mmol), HCl (0.10ml), 6 at 100deg. P a g e Graph (9). , 30 Acidity scale. Graph (10). 38 Variation i n a c i d i t y for the systems: Cyclohexanol (190mmol), EhCl 33H aO (0.3o2mmol), FeCl^ (9.25mmol), A: HCl (0.10ml), B: HCl (2.0ml). Graph (11). 42 Bates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl^H^G (0. 302mmol), FeCl^ (9.25mmol) * HCl A: (zero), B: (0.10ml), C: (2.0ml); at 100deg. Graph (12). 43 Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl^H^O (0.302mmol) , FeCLj (1.54mmol), HCl A: (zero), B: (0.10ml), at 100deg. Graph (13). 45 Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), S h C l ^ H ^ (0. 302mmol) , F e d ^ (12.3mmol), HCl A: (zero), B: (0.10ml); at 100deg. x i i Page Graph (14) . 46 Graph (15). 4 9 Graph (16) . 50 Graph (17). 51 Bates of cyclohexanone formation for the systems: FeCl^ 49.25mmol), HCl (0.10ml), at 100deg. I n i t i a l water content A: (zero), B: (2.0ml), C: (1.0ml, with presaturation of oxygen with water). Bates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), BhCl33H^O (0. 302mmol), F e C l j (9.25mmol) , HCl (0,10ml), at 100deg. A: (no presaturation); B: (presaturation with cyclohexene)• Variation i n cyclohexene concentration f o r the systems: Cyclohexanol (190maol) , BhCl^H^O (0. 302mmol), F e C l 3 (9.25mmol), HCl (0.10ml), at 100deg. A: oxygen presaturatrd with cyclohexene; B; no presaturation. Bates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), BhCl,.3H 0 (0. 302mmol) , FeCl- (9.25mmol), HCl (0.10ml),; A: (at 150deg) , B: (at 100deg). x i i i Page Graph (18) . 53 Graph (19) . . 54 Graph (20). 57 Graph (21a) . 58 Graph (21b) . 58 Bates of cyclohexanone formation f o r the systems: Cyclohexanol (190mmol), RhCl^H^Q (0. 302mmol), FeCl 3 (6. 17mmcl) , HCl (0.10ml), A: (at 150deg), B: (at 100deg). Bates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl^H^O (0. 302mmol) , FeCl^ (12.3mmol), A: (at 150deg) B: (at lOOdeg). Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl^3HL0 (G.302maol), FeCl^ (12. 3mmol), HCl (0.10ml), at 100deg A: (phosphine absent), B: (0.308mmol PPh ) . Rate of cyclohexanone formation for the system: Cyclohexanol (190mmol), RhCl„3H 0 (0.302mmol), FeCl. (9.25mmol) , HCl (0.10ml), at 100deg. Rate of cyclohexanone formation for the system: Cyclohexanol (115mmol), Cyclohexanone (75mmol), RhCl 3H 0 (0.302mmol), Feel (9.25miaol), j ** 3 HCl (0.10ml), at lOOdeg. x i v P a g e G r a p h ( 2 2 ) . 60 E f f e c t o f p r e h e a t i n g R h C l 3H 0 w i t h c y c l o h e x a n o n e . G r a p h ( 2 3 ) . 62 D e p e n d e n c e on o x y g e n f l o w . G r a p h (21*) . 77 R a t e o f o x y g e n a b s o r b t i o n f o r t h e s y s t e m ; C y c l o h e x a n o l (100mmol) , R h C l 3H..0 ( 0 . 1 5 1 m m o l ) , F e C l (4 .63mmol) , a t 1 0 0 d e g . G r a p h ( 2 5 ) . 79 R a t e s o f o x y g e n a b s o r p t i o n f o r t h e s y s t e m s : A : C y c l o h e x a n o l (60mmol) , C y c l o h e x a n o n e ( 4 1 m m o l ) , R h C l 3H o (0 .151mmol) , F e C l ^ ( 4 . 6 3 m m o l ) , a t 1 0 0 d e g . B;C y c l o h e x a n o n e (102mmol) , R h C l 3H O (0 . 151mmol) , F e C l ^ ( 4 . 6 3 m m o l ) , a t 1 0 0 d e g . G r a p h ( 2 6 ) . 84 R a t e s o f c y c l o h e x a n o n e f o r m a t i o n f o r t h e s y s t e m s : A : C y c l o h e x a n o l ( 1 9 0 m m o l ) , B h C l 31^0 (0 . 302mmol) , F e C l 3 ( 9 . 2 5 m m o l ) , H C l ( 0 . 1 0 m l ) , R a p i d o x y g e n f l o w a t 1 0 0 d e g . B : C y c l o h e x a n o l ( 1 0 0 m m o l ) , E h C l 3H O (0 . 1 5 1 m m o l ) , F e C l ( 4 . 6 3 m m o l ) , * 3 C l o s e d s y s t e m a t 1 0 0 d e g . Hate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol}, BhCl 3H 0 (0. 302mmol) , CuCl (11.6mmol) , HCl (0.15ml) , at 100aeg. Bates of cyclohexanone formation for the systems; Cyclohexanol (190mmol), BhCl^H^O (0.302mmol), F e C l 3 (9.25mmol), HCl (0.10ml), at lOOdeg; o-xylene A: (210mmol), B: (630mmol). Bates of cyclopentanone formation f o r the system: Cyclopentanol |245mmol), ShCl^H^O, Feci (9.25mmol), HCl (0.10ml), at 100deg. xvi ABBREVIATIONS atm atmosphere b.p. boiling point deg degrees centigrade g gram hr hours i n inch or inches min minute or minutes ml m i l l i l i t r e mmol millimole m.p. melting point nm nanometres N,T. P. normal temperature and pressure R.T. room temperature vpc vapour phase chrcmatograph x v i i ACNOWLE DGBMENTS I would l i k e to thank Dr. B. B. Janes for being my supervisor. 1 CH&PTER (1) INTRODUCTION The homogeneous c a t a l y t i c oxidation of alcohols to the corresponding carbonyl compound has been studied using a variety of t r a n s i t i o n metal ca t a l y s t s . The reaction mechanism appears to involve one of two pathways. I t can proceed either by a c a t a l y t i c dehydrogenation to produce molecular hydrogen , cr by oxidation, employing oxidizing agents i n combination with t r a n s i t i o n metal catalysts . Chloro-complexes of Rh(llj) have been found to catalyse the dehydrogenation of iso-propyl alcohol to acetone *. Hydrogen gas i s evolved when iso-propyl alcohol i s refluxed at 83deg with rhodium t r i c h l o r i d e trihydrate, concentrated hydrochloric acid and lithium chloride., Rhodium metal i s deposited during the reaction, and the rate of dehydrogenation decreases as rhodium i s removed from solution. The mechanism suggested for t h i s dehydrogenation involves formation of an alkoxide, followed by the transfer of a hydride ion from the oj-carbon atom of the coordinated alkoxide to the rhodium, with l i b e r a t i o n of the ketone. The rhodium hydride intermediate then either reacts with a proton to form hydrogen or decomposes to give rhodium metal: - Cl^h-OCHMe.^ + H++ Cl" —} CljUh-H 3" + H e ^ O 3-Me.CHOH + RhCl J= CI Rh-OCHMe* 2 HCl + HRhCl v==? H~+ RhCl? 5 X * 5 " v o HBhCl > Rh 3 The rhodium chloride catalysed dehydrogenation of i s o - propyl alcohol has also been examined i n the presence of t i n (H) chloride 2 . I t was found that the dehydrogenation was homogeneously catalysed by rhodium-tin complexes without p r e c i p i t a t i o n of the metal. The suggested mechanism followed that above and involved formation of molecular hydrogen by reaction of a rhodium hydride intermediate with protons from the solvent. However, the pi-acceptor SnCl~ acts as a ligand to rhodium, and i s thought to s t a b i l i z e the hydride intermediate against decomposition to the metal. Reported conversions to acetone were low (<10%)./ Dehydrogenation of primary and secondary alcohols by homogeneous c a t a l y s i s to form aldehydes and ketones respectively has also been investigated more recently using ruthenium and osmium complexes of the type: { a (OCOR) (CO) (PPh ) ], (R=CF , C, F , c 6 F j - ) 3« The mechanism suggested again involves B elimination within an alkoxide to form the corresponding carbonyl compound and a hydride. Acid attack on the hydride l i b e r a t e s molecular hydrogen and regenerates the cat a l y s t . Reported yields appear to be again generally low {<1Q%) for oxidation of alcohols proceeding by t h i s dehydrogenation pathway. The second mechanistic pathway involves the use °f oxidizing agents i n conjunction with the t r a n s i t i o n metal catalysts. , Oxidation of alcohols using t e r t - b u t y l hydroperoxide 3 with either vanadium or ruthenium catalysts,* and with chloramine-T using ruthenium has been reported s. Oxidation of primary and secondary alcohols using ruthenium catalysts i n combination with amine N-oxides at room temperature generally gives the corresponding aldehyde or ketone in good y i e l d . (>90% i n some cases) , except i n the case of o l e f i n i c alcohols *. I h i l e the above procedures are obviously useful, they do require the consumption of expensive organic oxidants, and c l e a r l y the use of molecular oxygen i n combination with a c a t a l y t i c agent would be more desirable. Homogeneous c a t a l y t i c oxidation of secondary alcohols to ketones using molecular oxygen with PdCl -NaOAc as c a t a l y s t under mild conditions was recently published while the work described i n t h i s thesis was in progress 7. By measuring the oxygen uptake, together with vpc analysis, the stoichiometry of the reported reactions was shown to be: R«B'«CHOH • 1/20, > B«R**C=0 • H 0 Yields of ketone were generally over 90% except for o l e f i n i c alcohols (e.g. p-menth-8-en-ol) , which were thought to poison the catalyst by strong complexation. In a t y p i c a l example, trans 3-3-5-trimethylcyclohexanol (17mmol) was s t i r r e d with PdCl (0.170mmol) and NaOAc (8.5mmol) using ethylene carbonate as solvent at 38deg. The reaction mixture was maintained under oxygen at 1atm pressure. The conversion to ketone was 98% a f t e r 54hr. I t was found that the water, which was produced i n an eguimolar amount with the alcohol consumed, inhibited the reaction. The mechanism suggested for the c a t a l y t i c cycle involves complexation of the alcohol to Pd(||) and then deprotonation to give a Pd(||j alkoxide followed by the B-hydride transfer from C to Pd to y i e l d the ketone and a Pd (11) hydride complex. This i s then thought to be oxidized by oxygen to give water and regeneration of the active Pd(jj) species. The c a t a l y t i c oxidation of cyclohexanol to cyclohexanone, by the Bh ( I I I )-Fe ( I I I )-O^-catalyst system used i n the present work , could be visualised as proceeding by e i t h e r or both of the two mechanistic pathways described above. The f i r s t pathway involves oxidation of the alcohol by molecular oxygen, and i t s subsequent conversion to water: This mechanism would presumably involve i n i t i a l complexation of cyclohexanol with a Rh(|j|) species followed by release of a proton from the coordinated alcohol to give a fib (|||)-alkoxide. A B-hydride transfer from carbon to rhodium would y i e l d cyclohexanone and a Bh(||) hydride complex which could be oxidized by molecular oxygen to give water and an active Bh(|lj) species, although l i t t l e i s known about interaction of metal hydrides with oxygen 8 . This pathway, which w i l l i n future be referred t c as reaction ( 1 ) , i s i l l u s t r a t e d i n Fig ( 1 ) . 2Rh - H + Fig (1). Mechanism (1). » 2Rh + H 2 0 6 In the second mechanistic pathway, cyclohexanol i s c a t a l y t i c a l l y oxidized to cyclohexanone with evolution of hydrogen ( i . e . by dehydrogenation): This mechanism would correspond to that shown above, at least to formation of the fib. (11 |) hydride, but then reaction of the hydride complex with protons produces molecular hydrogen and the active Eh (M| ) species: Bh(||j)-H ^ £ fih (I II-) • In the case of a Rh ( | J | ) hydride, there i s also the p o s s i b i l i t y of the reduction of 8h{|||) to Sh(|) by reductive elimination of a proton: Hh(M|)-H } Rh H) •>• H* Further, there would then probably be production of rhodium metal formed by disproportionation of fih{|)9: 2fih(|) H*M0) •• fih <||) Loss of rhodium (HI) via these reactions would suppress the 7 c a t a l y t i c oxidation cycle. However, i n the presence of iron i n the f e r r i c state, the Bh{|) could presumably be reoxidized to £h(||U, the f e r r i c iron being reduced to the ferrous state: Bh(l) + 2 F e ( U i ) — ^ 2 F e | H ) + BhUIl) Regeneration of the iron ( H I ) by passing oxygen through the solution appeared l i k e l y since oxidation of F e ( l l ) by molecular oxygen i s known to occur i n agueous systems, and i s dependent on the a c i d i t y of the solution. 2H" + 2Fe(||) 2Fe{|||) • 2H^0 The o v e r a l l process for regeneration of the Bh(|||) can thus be represented as: Bh(|) • 2H* • 1/20^ ? B h ( I H ) • Ĥ O Although protons are liberated i n the f i r s t stage of t h i s mechanism, their concentration would be expected to be small, p a r t i c u l a r l y as protons are consumed in the re-oxidation of iron to the Fe{|||) state; t h i s suggested that t h i s mechanism might be f a c i l i t a t e d by the addition of concentrated hydrochloric acid to the system. The dehydrogenation mechanism i s i n future referred to as reaction (2). I t was i n i t i a l l y hoped that oxygen could be used as a direct oxidant for Bh{|) * ° , but i n the absence of Fe (lit) the 8 systems were found to be i n e f f i c i e n t . Use of mild conditions i s a feature of a homogeneous c a t a l y t i c process and can lead to greater product s e l e c t i v i t y which can have an important impact on energy and resource u t i l i z a t i o n . Homogeneous c a t a l y t i c oxidation of alcohols would be of particular significance i f i t proceeded by dehydrogenation because of the economic value of the hydrogen gas as a f u e l , e s p e c i a l l y i f the reaction produced a high yield of ketone at low concentration of catalyst. Preliminary unpublished results i n t h i s laboratory on the homogeneous c a t a l y t i c oxidation of cyclohexanol tc produce cyclohexanone using a combination of rhodium t r i c h l o r i d e trihydrate, f e r r i c chloride, and concentrated hydrochloric acid, with oxygen passing through the solution at 100-120deg and 1atm pressure had shown that a 30% conversion was attained after 6hr. However, no futher conversion was detected after an additional 12hr. This particular system was selected as the primary subject for detailed investigation and i s the main topic of t h i s thesis. Cyclohexanol was chosen for study because of the i n d u s t r i a l importance of cyclohexanone, which i s produced i n large quantities by heterogeneous c a t a l y t i c oxidative dehydrogenation of the alcohol at high temperature. Cyclohexanone i s used as a solvent for c e l l u l o s e acetate, n i t r o c e l l u l o s e , natural resins, v i n y l resins, crude rubber, waxes, f a t s , shellac and D.p.T. I t i s also converted i n t o adipic acid which i s used i n the manufacture of nylon and urethan foams. The approach to the detailed investigation was: (1) to find the dependence of the system on such factors as 9 rhodium concentration, f e r r i c chloride concentration, concentrated hydrochloric acid concentration, temperature etc. (2) to elucidate the stoichiometry of the reaction by which oxidation occurs. 10 CHAPTER (2) OXYGEN FLOW SYSTEMS 2.1 Experimental setup The cyclohexanol sample was placed i n a 100ml three-necked round bottomed flask immersed i n a constant temperature o i l bath. A gas i n l e t tube passed through one neck and was held i n place by a rubber septum. A condenser system was attached to the second neck, and the t h i r d e x i t was closed by a stopper, enabling samples to be withdrawn using a syringe whenever necessary. In the early stages of t h i s study, a major d i f f i c u l t y was found to arise from an i n e f f i c i e n t condenser system, which led to s i g n i f i c a n t loss of sample, carried out by the oxygen flow. The condenser system f i n a l l y used was f i v e feet i n length, the upper one t h i r d containing small glass beads. The coolant used was ice- c o l d water, which was siphoned through the condenser between two f i v e - g a l l c n reservoirs. The oxygen entering the flask was dried by being passed through a column containing d r i e r i t e . The gas was then presaturated with cyclohexanol and cyclohexanone by allowing the oxygen to bubble through a f l a s k containing egual volumes of the alcohol and ketone. The f l a s k containg the presaturation mixture was maintained at the same temperature as the oxygen leaving the condenser system. It was found that using t h i s arrangement for a t y p i c a l reaction system aft e r three hours of rapid oxygen flow, and 11 after allowing the condenser to drain, the loss i n weight was of the order of 5%. This could be accounted f o r by (a) cyclohexanol remaining i n the condenser (cyclohexanol i s a viscous l i q u i d at room temperature) and (b) loss due to conversion to the more v o l a t i l e cyclohexene. The oxygen flow was controlled by the valve on the storage cylinder and the flow rates could be measured by channeling a small portion of the flow through a bubbler located between the supply and the drying column. Using t h i s arrangement, flow rates could be reproduced by adjusting the regulator u n t i l the desired flow measured i n bubbles per second emerged through the bubbler. The term rapid oxygen flow used frequently i n t h i s section refers to a reproducible steady stream of oxygen through the reaction mixture, and i s at the same rate i n each case. The temperature of the reaction f l a s k could be maintained to within 1deg. Presaturation Before using t h i s experimental arrangement to study the conversion of cyclohexanol to cyclohexanone, i t was important to show that cyclohexanone was not transfered i n t o the reaction system from the presaturation f l a s k i n appreciable quantities. A sample of cyclohexanol (20ml, 190mmol) was placed i n the reaction f l a s k at lOOdeg. Usinq rapid oxygen flow through the system after 1/2hr analysis (vpc) showed the solution contained approximately 1% cyclohexanone. Very l i t t l e transfer of either cyclohexanol, or cyclohexanone, would be expected considerinq the low vapour pressures at 20deg (cyclohexanol mp 24deg, bp 161.5deg. Cyclohexanone mp -16deg, bp 156deg) . 12 Analysis of reaction mixtures. , Samples (1-2pl)were vithdrawn by syringe and analysed using a Hewlett Packard Besearch Gas Chromatography The column used was 15% FFAP on CHROM » (high performance), 80-100mesh. The column oven temperature was maintained at 90deg. The f a s t e s t chart speed (2in/min) was used to produce the largest possible areas. , Ratios of weights of components i n reaction systems were determined by photocopying the vpc readout, and then accurately weighing the appropriate sections of paper. The weight of a pure compound i s proportional to the area enclosed i n the vpc readout corresponding to that compound. Hence having established the relationships between rat i o s of areas and r a t i o s of weights of the corresponding compounds by i n j e c t i n g standard solutions:(cyclohexanol/cyclohexanone, cyclohexanol/water, cyclohexanol/cyclohexene)# the r a t i o s of weights of components in a reaction mixture could be determined. Purity of cyclohexancl Vpc analysis showed the cyclohexanol contained 1-2% cyclohexanone. This could not have been e a s i l y removed by d i s t i l l a t i o n as the alcohol and ketone have almost i d e n t i c a l b o i l i n g points. Traces of water and cyclohexene were also found to be present. As water was found to be a product of the reactions studied i t was not considered necessary to remove traces of water i n i t i a l l y present. 13 M i s c i b i l i t y of l i q u i d components Cyclohexanol, cyclohexanone and cyclohexene are miscible i n a l l proportions.. Dp to 2ml of water i s miscible with 20ml cyclohexanol or 20ml of a solution containing 6 0% cyclohexanol and 40% cyclohexanone. Mixtures of cyclohexanol and concentrated hydrochloric acid appear miscible i n a l l proportions. 2 .2 The cyclohexanol-rhodium t r i c h l o r i d e trihydrate concentrated hydrochloric acid system. The reaction mixture (cyclohexanol 2 0ml,190 m m oi; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; concentrated hydrochloric acid 0.10ml) was investigated at 100deg with rapid oxygen flow. The solution was i n i t i a l l y red as the rhodium t r i c h l o r i d e trihydrate dissolved. After 1hr the solution was dark reddish-brown and rhodium metal had been deposited as a black residue. T^e conversion t 0 cyclohexanone was 6% after 1/2 hr and 10% aft e r 1hr. The water content after 1hr was 0.3g,17mmol. No cyclohexene was produced. 2.3 The cyclohexanol-ferric chloride concentrated hydrochloric acid system. The reaction mixture (cyclohexanol 20ml,190mmol; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 0.10ml) was investigated at 100deg with rapid oxygen flow. After 1/2hr no conversion to cyclohexanone had occurred. The cyclohexene content was 9%.,The water content was 0.2g,11mmol. The solution was i n i t i a l l y dark green becoming dark brown after 1/2hr. 14 The system (cyclohexanol 20ml#190mmol; f e r r i c chloride 4..0g,24.'?maol; concentrated hydrochloric acid 0.10ml) was studied at 100deg with rapid oxygen flow. After 1/2hr no cyclohexanone had been formed..There was 20% cyclohexene present i n the mixture and 0,6g,33mmol water.. The system (cyclohexanol 20ml,190mmol; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 2.Qml) at 100deg with rapid oxygen flow produced no cyclohexanone a f t e r 1/2hr; there was 20% cyclohexene i n the reaction mixture and 1.1g,61mmol water. It therefore seems that no oxidation of cyclohexanol to cyclohexanone takes place i n the absence of rhodium t r i c h l o r i d e trihydrate., Cyclohexene i s produced by the interaction of cyclohexanol with f e r r i c chloride presumably by elimination of water: Increasing the f e r r i c chloride content from 1.5g to 4.0q appears to increase the amount of cyclohexene and water present in the reaction mixture. , 2.4 The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride-concentrated hydrochloric acid system The following section describes i n some d e t a i l the 15 oxidation of cyclohexanol to cyclohexanone using a combination of rhodium t r i c h l o r i d e trihydrate and f e r r i c chloride, and constitutes the major part of t h i s t h e s i s . 2.4a Dependence on i n i t i a l f e r r i c chloride concentration A series of experiments was ca r r i e d out at lOOdeg using rapid oxygen flow i n which d i f f e r e n t amounts of f e r r i c chloride were i n i t i a l l y added to the following system: (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; concentrated hydrochloric acid 0.10ml) ., The following i n i t i a l weights of f e r r i c chloride were used: 1.5g,9.25mmol; 2.0g,12.3mmol; 3.0g,18.5mmol; 4.0g,24.7mmol; 1.0g,1.54mmol; 0.25g,1.54mmol. In each case the solution was i n i t i a l l y dark green, becoming darker as the reaction progressed. After 3hr the solutions were dark brown i n colour, and contained some dark s o l i d matter. The variation i n composition was investigated f o r each mixture over a 3hr period by withdrawing samples at 1/2hr in t e r v a l s . Each reacting system was found to contain cyclohexanol, cyclohexanone, cyclohexene and water. The r a t i o of weights of the organic components present was determined and expressed as a percentage of the t o t a l . The water content was determined as the t o t a l weight and hence the t o t a l number of moles of water present. The rate of cyclohexanone formation over a 3hr period i s shown f o r each amount of f e r r i c chloride present i n Graphs (1)- (6). Inspection of these graphs shows that the i n i t i a l rate of reaction and the f i n a l y i e l d are both dependent on the weight of f e r r i c chloride used. Graphs (1),(2) and (5) corresponding to 1.5g, 2.0g and I.Og of f e r r i c chloride, respectively, are very 16 si m i l a r and i t appears that there exists an optimum range for f e r r i c chloride addition from 1.0-2. Og. In each case the reaction rate declines s t e a d i l y as time progresses, and the production of cyclohexanone after 3hr i s n e g l i g i b l e , at which time there i s approximately 40% cyclohexanone i n the mixture. When i n i t i a l guantities of f e r r i c chloride are i n excess of t h i s optimum range, the i n i t i a l reaction rate i s lowered and the cyclohexanone content after a 3hr period i s reduced. This i s i l l u s t r a t e d in Graphs (3) and (4) corresponding to 3.0g and 4.0g of f e r r i c chloride respectively. Graph (6), corresponding to 0.25g f e r r i c chloride, shows that for i n i t i a l weights of f e r r i c chloride substantially lower than 1.0g the i n i t i a l reaction rate i s reduced and the production of cyclohexanone appears to be terminated after 1 ..50.hr when the system contains only 20% ketone. For purposes of comparison, the r e s u l t s for these systems containing 0.25-4.0g f e r r i c chloride are shown together i n Graph (7). Besults for I.Og, 1.5g and 2.0g l i e within the shaded envelope. The system containing 1.5g f e r r i c chloride, i . e . i n the centre of the optimum range, w i l l hereafter be refered to as system (1). The experiment using system (1) was repeated under the same conditions as above to show the r e p r o d u c i b i l i t y of the re s u l t s . Graph (1a) shows the two sets of data corresponding to system (1) at 100deg and i t can be seen that the points axe reproducible to within 2%. The experiment using system (1) was repeated using 20.Og,200mmol cyclohexanol instead of 20ml,190mmol. After 3hr 17 TIME (HOURS) Graph (1). fiate of cyclohexanone formation for the system: Cyclohexanol (190mmol), fihCl^H^O (0.302amol), FeCl (9.25maol), HCl (0.10ml), at 100deg. 18 TIME (HOURS) Graph (1a). Bate of cyclohexanone formation for the system: Cyclohexanol '(190mmol), RhCl^H^O (0. 302amol), FeCl^ (9.25mmol), HCl (0.10ml)r at lOOdeg. Reproducibility of r e s u l t s . 19 60 + TIME (HOURS) Graph (2) . Rate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol), HhCl 3H 0 (0.302maol), FeCl (12.3mmol) HCl (0.10ol), at 100deg. 20 60 + Graph (3). Rate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol) , BhCl 311̂ 0 (0. 302mBol) , F e C l 3 <18.5mmol) HCl (0.10ml), at 100deg. 21 60 + 40 TIME (HOURS) Graph (4) . Sate- of cyclohexanone formation f o r the system: Cyclohexanol (190mmol), RhCl^H^O (0.302mmol) , Feci (24.7mmol) HCl (0.10ml), at 100deg. 22 60 + (HOURS) G r a p h (5) . B a t e o f c y c l o h e x a n o n e f o r a a t i o n f o r t h e s y s t e m : C y c l o h e x a n o l ( 1 9 0 m a o l ) , B h C l 3H 0 ( 0 . 3 0 2 a m o l ) , P e C l (6.1 7 o o o l ) H C l ( 0 . 1 0 a l ) , a t 1 0 0 d e g . 23 Graph (6) . Hate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol), BhCl 3H 0 (0. 302mmol), FeZl (1. 54mmol) HCl (0.10ml), at lOOdeg. 24 Graph (7). Sates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCljSH^O (0.302maol), FeClg A: (6.17- 12.3maol), B: (18.5aaol), C: (24.7aaol), D: (1.54maol); BC1 (0.10al) , at 100deg. 25 the mixture contained 40% cyclohexanone. The dark brown solution was f i l t e r e d . The f i l t r a t e weighed 19.9g and the s o l i d brown residue weighed 0.17g aft e r drying. The residue was insoluble i n acetone but p a r t i a l l y soluble i n concentrated hydrochloric acid to give a green-yellow solution and a dark residue. Water and cyclohexene The nature of the vpc readout made i t more d i f f i c u l t to determine accurately the amount or r a t i o of these components present as the chromatograph was operated under conditions most suited to give accurate alcohol/ketone r a t i o s . However, the measurements corresponding to water and cyclohexene are probably of s u f f i c i e n t accuracy to reveal trends i n their e f f e c t on the oxidation reaction. It was found for t h i s series of experiments that both the cyclohexene and the water content of the systems remained approximately constant during the course of each experiment. Results f o r t h i s s e r i e s are shown in Table (1). The importance of water and cyclohexene are discussed i n d e t a i l i n sections 2.4d and 2.4e. 2.4b Dependence on rhodium t r i c h l o r i d e trihydrate concentration a series of experiments was carried out i n which d i f f e r e n t i n i t i a l amounts of rhodium t r i c h l o r i d e trihydrate were introduced into systems containing f e r r i c chloride i n the optimum range. The experiments were carried out at 100deg with rapid oxygen flow, and each contained (cyclohexanol 20ml,190mmol; f e r r i c chloride 1,5g,9.25mmol; concentrated hydrochloric acid O.^Oml). 26 Table (1) Weight of f e r r i c chloride (g) (mraol) %Cyclohexene Water content (g) (mmol) 0. 2 5 1.0 1.5 2.0 3.0 4.0 1.54 6. 17 9.25 12. 3 18.5 24.7 1-2 4-5 6-9 11- 13 14-16 12- 14 <0.2 0.3 0.5-0.7 0.5-0. 7 0.5-0.7 1.0 <11 17 28-39 28-39 28-39 56 Table (1). Variation in water and cyclohexene content with f e r r i c chloride concentration for the system; Cyclohexanol (190mmol); BhCl,3H 0 (0. 302mmol) , HCl (0.10ml), at 100deg. < 27 Compositions of systems containing 0.050g,0.118mmol; and 0.256g,0.604mmol rhodium t r i c h l o r i d e trihydrate were studied. The variation of percentage cyclohexanone i n the reaction mixtures i s plotted in Graph (8), together with the corresponding data for system There appears to be a general trend of increased i n i t i a l reaction rate and increased conversion to ketone a f t e r 3hr as the i n i t i a l concentration of rhodium t r i c h l o r i d e trihydrate i n the system i s raised. However, the increase in the f i n a l y i e l d of cyclohexanone on doubling the i n i t i a l weight of rhodium t r i c h l o r i d e trihydrate from 0.J28g to 0.256g i s small. The variation i n rhodium t r i c h l o r i d e trihydrate content did not appear to greatly influence the cyclohexene content of the reaction mixtures. 2.4c Dependence on a c i d i t y Acidity scale. A scale of a c i d i t y in cyclohexanol was established by employing a combination glass electrode and pH meter. The a c i d i t y of a homogeneous mixture of 20ml cyclohexanol and 0.10ml concentrated hydrochloric acid was a r b i t r a r i l y fixed at 3,20, Further additional volumes of concentrated hydrochloric acid were added to the mixture and meter readings taken a f t e r thorough mixing, Measurements were taken at 20deg and are presented i n Table (2). ; Graph (9) i s a plot of the meter readings against the value Log {total volume HCl / t o t a l volume of solution). This gives a 28 60 + TIME (HOURS) -t-3 Graph (8). Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol) , RhCljiB^O A: (0.604mmol), B: (0. 302Bmol) , C: (0.118mmol); PeCl (9. 25mmol) , HCl (0.10ml), at 100deg. 29 Table (2) Total volume cone HCl added to 20ml alcohol V (HCl) (ml) Log V (HCl) V (Total) Meter reading PX 0. 10 2. 30 3.20 0. 20 2. 00 2. 975 0. 30 1. 83 2. 825 0. 40 1.71 2.75 0. 50 1.61 2.65 0.60 1.54 2. 60 0.75 1. 44 2. 525 2. 00 1. 04 2.30 5. 00 0. 70 1.95 20. 00 0.30 1.70 Table (2). A c i t i t y of cyclohexanol/concentrated HCl mixtures. 30 METER READING Graph (9). Acidity scale. 31 straight l i n e and thus the system i s behaving i n a manner si m i l a r to the HCl-water system, and the meter readings can be considered analogous to the pH scale of an aqueous system. These meter readings for cyclohexanol systems are hereafter referred to a s the pX. A l l readings we^e taken at 20deg. Osing t h i s scale of a c i d i t y , cyclohexanol has a pX of 7.20 and cyclohexanone 8.00. In order to have an approximate value of the a c i d i t y at the s t a r t of a t y p i c a l cyclohexanol oxidation, mixtures of varying amounts of f e r r i c chloride were added to 20ml,190mmol cyclohexanol and 0.10ml concentrated hydrochloric acid and each mixture heated to 100deg for approximately 1/2hr with rapid oxygen flow. The mixtures were then cooled to 20deg and the pX measured. In each case a residue was present., The r e s u l t s are shown i n Table (3). The effect of adding water to these systems was studied using the solution i n i t i a l l y containing 1.5g,9.25mmol f e r r i c chloride. Results are presented i n Table <4). The high a c i d i t i e s produced by the f e r r i c chloride-cyciohexanol systems presumably aris e from e q u i l i b r i a of the type: Fe(CfeH ÔH) ^ Fe{C H.OH) (C H 0) + mH+ or Fe{| I I ) • X C ^ O H Addition of f e r r i c chloride to cyclohexanone also produces solutions of high a c i d i t y . On mixing 1.50g,9.25mmol f e r r i c Table (3) Weight of f e r r i c chloride pX value (g) (mmol) 3 2 0 . 2 5 1.54 1.70 1.5Q 9 . 2 5 1.70 4 . 0 2 4 . 7 * 1.70 Table ( 3 ) . Acidity of cyclohexanol/FeCl mixtures. 3 3 Table(4) Total water added pX value (ml) (mmol) 0 0 1.70 0.50 28 2.45 1.00 56 3.00 Table (4). Effect of water on ac i d i t y of the system: Cyclohexanol (190mmol), FeCl (9.25mmol), HCl (0.10 ml). 34 chloride and 20ml,190mmol cyclohexanone a tarry brown residue was formed and the solution had a pX value of 0.0. The e f f e c t of adding water to t h i s solution i s similar to the cyclohexanol system as shown i n Table (5). The high a c i d i t y of the f e r r i c chloride-cyclohexanone mixture indicates a very strong int e r a c t i o n involving release of protons. Low concentrations of the enol form of cyclohexanone are known to e x i s t i n equilibrium with the keto form: The interaction of f e r r i c chloride with cyclohexanone could be explained i n terms of e g u i l i b r i a of the type: Fe (C tfl q0H) Fe(C,B OH) Or: X Interaction between the carbon-carbon double bond of the ring and the metal center i s also possible. Table (5) Total water added pX value (ml) (mmol) 0 0 0.0 0.50 28 1.40 1.00 56 2.4 0 2.00 112 3.00 Table (5). Effect of water on a c i d i t y of the system Cyclohexanone (190mmol), FeCl. (9.25mmol), HCl (0.10mol). 36 It can be r e a d i l y seen from the above data that water greatly reduces the a c i d i t y of f e r r i c chloride-cyclohexanol and f e r r i c chloride-cyclohexanone mixtures. I t i s reasonable to conclude that the interaction between the metal s a l t and the alcohol or ketone i s much weaker i n the presence of water (see 2.4d below). A series of experiments was carried out to determine how the a c i d i t y of a t y p i c a l reaction mixture varied during the conversion of cyclohexanol to cyclohexanone. The system studied in each case had composition (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302ramol; f e r r i c chloride 1.5g#9.25mmol; concentrated hydrochloric acid 0.10ml), i . e . System (1) at 100deg with rapid oxygen flow. Four different mixtures were heated at 100deg for 1/2hr,1.00hr#1.75hr, and 2.50hr respectively. Each was then cooled to 20deg and the a c i d i t y measured. The results are shown i n Table (6) and are also represented i n Graph (10). Analysis of these mixtures at the times indicated above give compositions i n good agreement with the expected values from Graph (1a).,The above measurements show a steady decrease in a c i d i t y as the % cyclohexanone increases (i.e. as the rate of oxidation decreases). The change i n a c i d i t y over a 2.50hr period i s equivalent to the change i n a solution containing 20ml concentrated hydrochloric acid in 2 0ml cyclohexanol to a solution containing less than 0.10ml concentrated hydrochloric acid i n 20ml cyclohexanol. F i n a l a c i d i t y measurements on the corresponding mixtures containing 3.0g, 18.5mmol f e r r i c chloride and 0.25g,1.54mmol 37 Table (6) Time pX value (hrs) 0 1.70 0.5 1.85 1.0 2.10 1.75 2.40 • 2. 50 3.95 Table ( 6 ) . Variation of a c i d i t y with time f o r the system: Cyclohexanol (190mmol) , BhCl^H^O (0.302mmol), FeCl^ (9.25mmol), HCl (0.10ml), at 100deg. 60 O pX VALUE TIME (HOURS) Graph (10). Variation i n a c i d i t y f o r the systems: Cyclohexanol (190mmol), BhCl, 3H 0 (0. 302maol), FeCl_ (9.25m mol) , A: BC1 (0.10ml), B: HCl (2.0ml). 39 f e r r i c chloride (i.e. After 3hr and 1.50hr, respectively, also give pX values i n the range 3.90-4.00. These r e s u l t s suggested the p o s s i b i l i t y of producing an increased y i e l d by addition of a large amount of concentrated hydrochloric acid to the system i n an attempt to maintain a high a c i d i t y over a longer period. The system (cyclohexanol 10ml,95mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 10.0ml) was studied at 100deg with rapid oxygen flow. The i n i t i a l a c i d i t y (pX=1.70) f e l l to pX=2.0 after 1hr, s l i g h t l y below the value f o r system (1) after the same time period (see Graph 9).,- The conversion to cyclohexanone however was only U% and the cyclohexene content 28%. The system (cyclohexanol 20ml,190mmolj rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; concentrated hydrochloric acid 2.0ml) at 100deg with rapid oxygen flow was also studied over a 3hr period . The results are presented in Table (7). This system appears to maintain a constant a c i d i t y (pX=2.30) throughout the reaction. The rate of cyclohexanone formation i s shown plotted in Graph (10). Inspection shows t h i s plot i s almost l i n e a r . However the slope i s considerably less than the reaction rate for system (1), at the corresponding pX value (see Graph 10). These r e s u l t s i n d i c a t e that other factors besides the a c i d i t y have a s i g n i f i c a n t e f f e c t on the conversion of cyclohexanol to cyclohexanone, and i n p a r t i c u l a r the water and cyclohexene content of a particular mixture. It i s also apparent that addition of concentrated hydrochloric acid to a system containing f e r r i c chloride i s 40 Table (7) Time % Cyclohexanone % Cyclohexene Water content pX (hrs) (q) 0.5 2 14 1.0 2.30 1.0 4 17 1.5 6 21 2.0 8 22 1.5 2.5 10 3.0 10 2.30 Table (7). Variation i n cyclohexanone content, cyclohexene content, water content, and a c i d i t y for the system: Cyclohexanol (190mmol), RhCl^H^O (0. 302mmol) , F e C 1 ^ (9. 25nmol) , concentrated HCl (2.0ml) at 100deg. 41 unnecessary to produce an a c i d i c medium as reguired for mechanism (2). Indeed, addition of concentrated hydrochloric acid in quantities between 0.10ml and 10.0ml to a f e r r i c chloride-cyclohexanol mixture probably produces a lowering of pX value because of the water introduced. The addition of 0.10ml concentrated hydrochloric acid to the mixtures studied i n 2.4a would therefore appear to be unnecessary. I t may be predicted therefore that the difference between mixtures studied i n 2.4a and the corresponding mixtures in which no concentrated hydrochloric acid i s added, would be small, as the amount of water contained i n 0.10ml concentrated hydrochloric acid i s small. The system (cyclohexanol 20ml, 190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0. 302mmcl; . f e r r i c chloride 1.5g,9.25mmol) at 100deq with rapid oxygen flow was studied and compared with the correspondinq system i n which 0. 10ml concentrated hydrochloric acid was added. The results are plotted i n Graph (11). I t appears that the i n i t i a l reaction rate i s somewhat hiqher when the concentrated hydrochloric acid i s omitted, but the f i n a l y i e l d i s very s i m i l a r {2% higher). Comparison of the system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128q,0.302mmol; f e r r i c chloride 0.25g,1.54mmol;) at 100deq with rapid oxyqen flow, with the correspondinq system i n i t i a l l y containinq 0.iQml concentrated hydrochloric acid (Graph 12), i l l u s t r a t e s the same trend - a somewhat hiqher i n i t i a l reaction rate but a very s i m i l a r f i n a l y i e l d of cyclohexanone. The system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e 50 + Graph (11). Rates of cyclohexanone formation f o r the systems Cyclohexanol (190aaol), RhClg3H.^0 <0. 302aaol), PeClg (9.25maol) HCl A: ( z e r o ) , B: ( 0 . 1 0 a l ) f C: ( 2 . 0 a l ) ; at lOOdeq. 43 Graph (12). Batss of cyclohexanone formation f o r the systems: Cvclchexanol (190mmol), R h C l ^ H j O (0.302amol), F e C l 3 ( 1 . 5 4 B B O 1 ) , HCl A: ( z e r o ) , B: (0.10ml), at 100rleq. 44 trihydrate 0.128g,0.302mmol; f e r r i c chloride 2.0g, 12.3mmol) with rapid oxygen flow at 100deg i s almost i d e n t i c a l with the corresponding system with 0.10ml concentrated hydrochloric acid introduced i n i t i a l l y (Graph 13). 2.4d Dependence on water content In the previous section i t was suggested that addition of large quantities of concentrated hydrochloric acid to a reaction mixture may cause a supression of the oxidation of cyclohexanol to cyclohexanone, partly because of the introduction of water into the system. Table (1) shows that f o r the reaction mixture (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g>0.302mmol; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 0.10ml) at 100deg and rapid oxygen flow the water content remains approximately constant at approximately 0.6g,33mmol.J The eff e c t of increasing t h i s water content by i n i t i a l addition of 1.0g,56mmol water, and also presaturating the oxygen with water at 20deg^ was studied for the same system. The production of cyclohexanone over a 2hr period i s shown i n Graph (14). At the end of t h i s period i t can be seen there was about 4% cyclohexanone present, at which time the water content had risen to about 2g,112mmol. Another experiment was carr i e d out i n which 2.0g,112mmol water was i n i t i a l l y added to System (1) at 100deg with rapid oxygen flow (no presaturation). The production of cyclohexanone over a 4.50hr period i s shown i n Graph (14). After 1.25hr the water content had f a l l e n to 1g,56mmol. This was apparently the re s u l t of removal of water by the flow. As can be seen the H5 Graph (13). Bates of cyclohexanone formation f o r the systems: Cyclohexanol (190mmol), RhCl^H^O (0. 302mmol) , FeCl (12.3aiDol), HCl A: (zero), B: (0.10ml); at lOOdeg. 50 40-1 H Z o < u 30H X o >• u 204 2: u u OS u CL. 10 + TIME (HOURS) Graph (14). Pates of cyclohexanone formation for the systems Cyclohexanol (190mraol) , RhCl^H^O (0. 302mmol), FeCl 3 (9.25mmol) HCl (0.10ml), at 100daq. I n i t i a l water content ft: (zero), B (2.0ml), C: (1.0ml, with presaturation of oxyqen with water). 47 r e a c t i o n t h e n p r o c e e d s t o w i t h i n a few % o f t h e y i e l d e x p e c t e d f o r s y s t e m ( 1 ) . T h e f i n a l a c i d i t y was p X = 3 . 7 0 , T h e e f f e c t o f a d d i t i o n o f w a t e r t o t h e t y p e s o f s y s t e m s s t u d i e d t h u s a p p e a r s t o c a u s e a s u p p r e s s i o n o f t h e i n i t i a l r a t e o f p r o d u c t i o n o f c y c l o h e x a n o n e , e v e n when t h e a c i d i t y i s m a i n t a i n e d a t a h i g h l e v e l . I t w o u l d a p p e a r t h a t when a r e a c t i o n m i x t u r e s i m i l a r t o s y s t e m (1) c o n t a i n s i n e x c e s s o f 1g ,56m m o l w a t e r t h e s u p p r e s s i o n o f t h e o x i d a t i o n r e a c t i o n b e c o m e s v e r y s i g n i f i c a n t . T h e e f f e c t o f w a t e r on c o n v e r s i o n t o c y c l o h e x a n o n e may p a r t l y e x p l a i n t h e t r e n d i n s y s t e m s c o n t a i n i n g i n e x c e s s o f 2 g , 1 2 m m o l f e r r i c c h l o r i d e ( s e e s e c t i o n 2 . 4 a ) . I n s y s t e m s o f t h i s t y p e * t h e o x i d a t i o n o f t h e a l c o h o l i s s u p p r e s s e d , a n d t h i s may i n p a r t b e due t o a n i n c r e a s e d w a t e r c o n t e n t . , 2 . 4 e D e p e n d e n c e on c y c l o h e x e n e C y c l o h e x e n e i s a p r o d u c t o f t h e r e a c t i o n s y s t e m s b e i n g d e s c r i b e d . I t c a n be i d e n t i f i e d by i t s r e t e n t i o n t i m e on t h e g a s c h r o m a t o g r a p h a n d i t s c h a r a c t e r i s t i c u n p l e a s a n t s m e l l i n t h e o x y g e n f l o w l e a v i n g t h e c o n d e n s e r s y s t e m . I t i s p r e s u m a b l y p r o d u c e d by e l i m i n a t i o n o f w a t e r f r o m t h e a l c o h o l u n d e r a c i d i c c o n d i t i o n s : T a b l e (1) shows t h a t f o r s y s t e m (1) t h e c y c l o h e x e n e c o n t e n t 48 remains constant at approximately 6-9% during the r e a c t i o n . An experiment was c a r r i e d out i n which system (1) was i n v e s t i g a t e d a t 100deg but the r a p i d oxygen flow was pres a t u r a t e d with cyclohexene at 25deg. Graph (15) shows the v a r i a t i o n i n the cyclohexanone c o n c e n t r a t i o n i n the mixture over a 2hr p e r i o d . The r a t e of o x i d a t i o n f o r the same system with no p r e s a t u r a t i o n with cyclohexene i s a l s o shown.:The v a r i a t i o n i n cyclohexene content f o r these two experiments i s shown i n Graph (16). I t can be seen when t h e r e i s no p r e s a t u r a t i o n , the cyclohexene content f a l l s s t e a d i l y from about 9% to about 6% , whereas t h e r e i s a steady accumulation of cyclohexene i n the mixture when the oxygen i s p r e s a t u r a t e d with the alkene. when p r e s a t u r a t i o n was employed the r e a c t i o n system was found t o gain i n weight by about 5g a f t e r 2hr which i s approximately c o n s i s t e n t with a cyclohexene content of 30% (vpc). I t can be r e a d i l y seen t h a t i n c r e a s i n g the cyclohexene content of the r e a c t i o n mixture above about 10% has a d e f i n i t e s u ppressing i n f l u e n c e on the co n v e r s i o n of a l c o h o l t o ketone . T h i s i n f l u e n c e may be important i n the s e r i e s o f experiments d i s c u s s e d i n s e c t i o n 2.4a, and may p a r t l y account f o r the suppression o f r e a c t i o n r a t e i n systems c o n t a i n i n g 3.0g,18.5mmol, and 4.0g,24.7mmol, f e r r i c c h l o r i d e , both of which appear t o c o n t a i n more than 10% cyclohexene. The high percentages of cyclohexene produced i n the systems c o n t a i n i n g 2ml and 10ml concentrated h y d r o c h l o r i c a c i d d i s c u s s e d i n s e c t i o n 2.4c (14-22% and 28% r e s p e c t i v e l y ) may a l s o c o n t r i b u t e t o the i n h i b i t i o n of the o x i d a t i o n process. I t i s p o s s i b l e t o s p e c u l a t e about the mechanism by which 4 9 60 + 0 1 2 3 TIME (HOURS) Graph (15). Rates of cyclohexanone formation f o r the systems Cyclohexanol (190mmol), RhCl^3H^0 (0. 302»»ol) r FeCl3 (9.25mmol), HCl (0.10ml), at 100deg. A: (no p r e s a t u r a t i o n ) ; B: ( p r e s a t u r a t i o n with c y c l o h e x e n e ) . 50 60+ 40 u z u U 30 s o -J >< u 20 H Z u u as u a. io TIME (HOURS) Graph (16). Variation in cyclohexene concentration for the systems: Cyclohexanol (190mmol) , BhCl„3H 0 (0. 30 2mmol) , HCl (0.10ml), at lOOdeg. A: oxygen presaturatrd with cyclohexene; B: no presaturation. FeCl^ (9.25mmol), 51 the cyclohexene i n h i b i t s the conversion of cyclohexanol to cyclohexanone, and t h i s may be the resu l t of coordination of the o l e f i n at a rhodium centre. 2.4f Temperature Dependence A l l experiments described so far have been carried out with the reaction mixture at 100deg. The e f f e c t of increased and decreased temperature on cyclohexanone production was investigated by performing experiments using system (1) with rapid oxygen flow at 50deg and at 150deg. The re s u l t s for these temperatures are plotted in Graph (17) together with r e s u l t s for 100deg. At 50deg the conversion proceeds very slowly indeed, having reached 4% after 4hr. At 150deg the average reaction rate during the f i r s t 1/2hr i s almost doubled, but the oxidation appears to cease after 1hr, when the conversion has reached 35%. The system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; f e r r i c chloride 1.0g6.17mmol; concentrated hydrochloric acid 0.10ml) shows a s i m i l a r e f f e c t on comparing data at lOOdeg and I50deg with rapid oxygen flow (Graph 18). Similarly for the system (cyclohexanol 20ml,19bmmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; f e r r i c chloride 2.Og,12.3mmol) with rapid oxygen flow the i n i t i a l reaction rate i s s i g n i f i c a n t l y higher at 150deg compared to 100deg , but the f i n a l y i e l d of cyclohexanone i s s l i g h t l y reduced (Graph 19). 52 Graph (17). Bates of cyclohexanone formation for the systems: Cyclohexanol (190maol), BhCl 3H 0 (0. 302aaol) , PeCl (9.25aaol), i Z 3 HCl (0.10ml),; A: (at 150deq) , B: (at lOOdeg) . 53 60 + TIME (HOURS) Graph (18). Rates of cyclohexanone formation for the systems: Cyclohexanol (190mmol), RhCl^H^O (0.302aaol), FeCl (6.17mmol)r HCl (0.10ml), A: (at 150aeq), B: (at 100deq). 54 50+ TIME (HOURS) Graph (19). Bates of cyclohexanone foraation f o r the systeas: Cyclohexanol (190amol), BhCl 3H 0 (0.302aaol), FeCl (12.3aaol). 3 x 3 A: (at 150deg) B : (at 100deg). 55 2.4g Effect of addition of a phosphine It has been found that phosphines frequently have profound effects on reactions catalysed by rhodium • complexes and can coordinate to both Rh(|) and Rh(ll|) centres. An approximately 1:1 molar r a t i o of triphenylphosphine, (PPhj) to rhodium t r i c h l o r i d e t r i h y d r a t e was produced by adding 0.10g,0.380mmol triphenylphosphine to the system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0. 128g,0.302mmol; f e r r i c chloride 2.Qg,12.3mmol; concentrated hydrochloric acid 0.10ml) with rapid oxygen flow at 100deg. The r e s u l t s are plotted i n Graph (20) together with those for the same system i n the absence of the phosphine. It can be seen that addition of a phosphine does not appear to have a s i g n i f i c a n t e f f e c t . 2.4h Attempt to detect hydrogen It was found that hydrogen could be detected using the gas chrcmatograph employing the same column at room temperature. Hydrogen gave a pen response in the dir e c t i o n opposite to that for ether substances, in particular oxygen and nitrogen. Rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; and f e r r i c chloride 1.5g,9.25mmol; were placed i n the reaction flask. The apparatus was flushed with helium gas and 20ml,190amol cyclohexanol injected into the flask. The reaction mixture was heated to 100deg with no gas flow through the system. Ho evolution of gas was seen, and analysis of a sample of gas after 1/4hr showed no hydrogen present. I f reaction (2) were operating, conversion to cyclohexanone with evolution of hydrogen would be expected to occur u n t i l a l l the f e r r i c chloride had been reduced to the 56 50+ 0 1 2 3 TIME (HOURS) Graph (20) . Rates of cyclohexanone formation for the systems: Cyclohexanol (190maol), RhCl^H 0 (0.302mmol), FeCl (12.3mmol), HCl (0.10ml), at 100deq A: (phosphine absent), B: (0.308aool 57 ferrous state. Vpc analysis of the solution showed that no conversion to cyclohexanone had occurred. 2.4i Attempts to increase the f i n a l y i e l d of cyclohexanone The most successful experiments so f a r described produce a mixture containing approximately 40% cyclohexanone. Variations in i n i t i a l f e r r i c chloride concentration , rhodium t r i c h l o r i d e trihydrate concentration, or temperature appear unable to increase t h i s y i e l d , although higher temperatures produce t h i s f i n a l composion more r a p i d l y . / An experiment was car r i e d out i n which a mixture containing {cyclohexanol 12ml,115mmol% cyclohexanone 8mi,75mmol) was heated to 100deg with rapid oxygen flow. The composition was ascertained by vpc analysis after ten minutes when ( f e r r i c chloride 1.5g,9.25mraol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; concentrated hydrochloric acid 0.10ml) were added.,A mixture having t h i s composition has a pX =1.20 at room temperature, in between that for the corresponding systems containing pure cyclohexanol and pure cyclohexanone. The f i n a l a c i d i t y of the reaction mixture after 2hr was pX=2.40. The f i n a l reaction mixture contained 4% cyclohexene and less than 0.1g,6mmolwater. The variation i n cyclohexanone content over the 2hr period i s shewn i n Graph (21b). Graph (21a) shows the change i n composition f o r system (1) at 100deg with rapid oxygen flow over a 3hr period. It can easily be seen that there i s very l i t t l e , i f any, further production of the ketone i n either system containing over 40% cyclohexanone, whether at high a c i d i t y TIME (HOURS) Graph (21a). Rate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol), BhCl 3H0 (0. 302mmol) , FeCl (9.25maol), HCl (0.10ml), at 100deg. Graph (21b). Rate of cyclohexanone formation for the system: Cyclohexanol (115mmol), Cyclohexanone (75mmol), RhCl 3H 0 (0. 302iauiol) , FeCl (9.25mmol) , HCl (0.10ml), at 3 i 3 100deg. 59 (1.20-2.40) or at low a c i d i t y (4.0,corresponding to system 1.) It seems reasonable to conclude that the probable reason for the oxidation reaction ceasing af t e r reaching a 40% cyclohexanone content i s that there i s an i n h i b i t i n g i n t e r a c t i o n between the ketone and either the f e r r i c species or the rhodium species present., 2.4J The inte r a c t i o n between rhodium t r i c h l o r i d e trihydrate and cyclohexanone Rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol, was shaken with cyclohexanone 1.0ml,10mmol, u n t i l i t dissolved to produce a red solution. After leaving overnight, vpc analysis showed only cyclohexanone. The mixture was heated i n a small glass v i a l at 100deg for 1/2hr when the solution became dark brown, with red o i l y droplets. This mixture was added to the system (cyclohexanol 18ml,115mmol; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 0.10ml) and heated at 100deg with rapid oxygen flow. The composion of the mixture changed from 4% cyclohexanone (vpc) after f i v e minutes to 14% cyclohexanone a f t e r 1.50hr. (Graph 22). These results suggest that the interaction between a rhodium species and cyclohexanone may be the cause of a decrease in the rate of conversion as the cyclohexanone content i n the mixture builds up. Such an inte r a c t i o n may also be responsible for the apparent l i m i t of conversion at approximately 40%. 60 2.4k Dependence on oxygen flow A l l systems described so far have employed a constant rapid oxygen flow. In reaction (1) the molecular oxygen, having oxidized the alcohol to the ketone i s converted to water. In reaction (2) , the oxygen i s primarily used to re-oxidize iron from the ferrous state back to the f e r r i c state. An experiment was carr i e d out using system (1) at lOOdeg without rapid oxygen flow, but with a i r i n the apparatus above the solution. After 2hr a 6% conversion to the ketone was found. Following rapid oxygen flow through t h i s mixture at 100deg for 3hr, a conversion to 2536 cyclohexanone was found (Graph 23), This reduced y i e l d suggests that on standing, at raised temperature i n the absence of oxygen flow, some int e r a c t i o n occurs which reduces l a t e r c a t a l y t i c a c t i v i t y , i n the presence of rapid oxygen flow. An experiment, carried out using system (1) at 100deg, substituting a rapid flow of dry nitrogen for oxygen, produces a 5% conversion to cyclohexanone after 1/2hr, and no further conversion aft e r 1hr. The a c i d i t y of the reaction mixture after 1hr had f a l l e n to pX=1.95, in d i c a t i n g that the iron was probably s t i l l present i n the f e r r i c state. I f the en t i r e 1.5g,9.25mmol, f e r r i c chloride were converted to the ferrous state, and the oxidation were proceeding exclusively by the route involving re-oxidation of Rh(l) to Rh ( l l l ) by iron, a conversion of about 5% would be expected (9.25mmol cyclohexanol= 1.0g (approximately) i . e . 5% conversion) I f reaction (1) was operating exclusively ( i . e . oxidation of cyclohexanol by molecular oxygen producing water), no conversion 50+ Graph (22). Effect of preheating RhCl.3H 0 with cyclohexanone 50 Graph (23). Dependence on oxygen flow. 63 to the ketone would be expected i n the nitrogen flow experiment. However, the r e s u l t s of t h i s experiment are by no means d e f i n i t i v e as the o r i g i n a l solution was not degassed and may have contained oxygen. Also, the nitrogen used may have contained traces of oxygen, enough to produce a conversion i n the order of 5%. The experiment was not repeated taking measures to completely exclude oxygen because i t was decided that futher investigations should be carried out employing a closed system. 64 2.41 Attempts to determine the f e r r o u s / f e r r i c r a t i o Reaction (2) involves the conversion of f e r r i c to ferrous as rhodium i s re-oxidized. A possible cause of the l i m i t of conversion to 40% cyclohexanone was at one time thought to be the complete conversion of almost a l l the ir o n present to the ferrous state with the re-oxidation by molecular oxygen proceeding very slowly. This idea was supported by a c i d i t y measurements. Considering system (1) the complete reduction of f e r r i c chloride 1.5g,9.25mmol, to the ferrous state would produce ferrous chloride 1.17q,9.25mmol, (assuming ferrous chloride was the species present). A mixture of (ferrous chloride 1.17q,9.25mmol; cyclohexanol 12ml,115mmol; cyclohexanone 8ml,75mmol) has a pX value of 4.00, the same as system (1) after 3hr of rapid oxygen flow at 100deg. The idea of conversion to ferrous chloride i s also supported by the depositing of a s o l i d brown material on the side of the reaction f l a s k i n many experiments, apparently s i m i l a r in nature to the l i g h t brown colour of anhydrous ferrous chloride. I t was thought that knowledge of the f e r r o u s / f e r r i c r a t i o during, and at the end of, a t y p i c a l oxidation reaction would be very useful and might indicate something of the mechanism involved., A standard quantitative method to determine the concentration of ferrous iron present i n an aqueous solution by t i t r a t i o n involves i t s oxidation to the f e r r i c state by usinq potassium dichromate as oxidant and N-phenylanthranilic acid as indicator. To employ t h i s type of method, i t would of course be ess e n t i a l to remove the alcohol and ketone from a reaction 65 mixture, as both are oxidizable by potassium dichromate, and the i r presence would lead to erroneous results. I t was found that after leaving a reaction mixture on a rotary evaporator f o r several hours at lOOdeg an o i l y dark brown residue remained. Heating to 150deg and pumping on a vacuum l i n e l e f t a tarry residue. Thus i t proved very d i f f i c u l t to remove the organic components, again indicating a strong i n t e r a c t i o n between them and the iron species present. It was therefore decided to employ a spectophotometric method f o r analysis. It i s well known that potassium thiocyanate, KSCN, produces a dark red colouration i n aqueous solution i n the presence of f e r r i c i r o n ; Fe(SCN) and/or Fe(SCN) may be extracted into e t h e r 1 1 . This i s a sensitive q u a l i t a t i v e test for f e r r i c i r o n , and has also been used previously i n quantitative estimations. Different weights of f e r r i c chloride were mixed with cyclohexanol 20ml,190mmol, at room temperature. The mixture was then diluted with a 40:60 mixture of acetone and water and potassium thiocyanate 6g,62mmol, added. The mixture was shaken to produce a dark red solution, and made up to 1 l i t r e , using the same solvent. After thorough shaking, 10ml of t h i s solution was diluted to 250ml. The absorbances of the diluted soluted sotutions were measured i n the range 46 0-480nm using a Cary 14 machine. The absorbances measured are given i n Table (8). These r e s u l t s indicate there i s a d i r e c t relationship between the weight of f e r r i c chloride used and the absorbance. A solution was s i m i l a r l y prepared using ferrous chloride 1.17g,9.25mmol. The diluted s o l u t i o n had an absorbance maximum of 0.16 i n the range 460-480nm. , Table (8) Weight of f e r r i c > Absorbance chloride (460-480nm) (g) (mmol) 1.50 9.25 2.16 1.00 6.17 1.36 0.50 3.08 0.69 Table (8). Absorbance of solutions after addition of KSCN cyclohexanol/FeCl systems. 67 The technique appeared to be of use in determining approximately the f e r r i c content of a reaction mixture. I t was necessary to determine whether inte r a c t i o n between f e r r i c chloride and cyclohexanol at elevated temperatures would r e s u l t in a change i n the measured absorbance. F e r r i c chloride 1.5g,9.25mmol, was heated with cyclohexanol 20ml,190mmol, at 100deg with rapid oxygen flow for 1hr. The mixture was made up as above and the absorbance measured. Insoluble p a r t i c l e s were present i n the red solution, which had an absorbance of 0.75, instead of the expected 2.16. A s i m i l a r experiment using cyclohexanone instead of the alcohol also produced s o l i d p a r t i c l e s i n the red solution and an absorbance maximum of 0*75 i n the region 46 0-480nm. When ferrous chloride 1.I7g,9.25mmol, was heated i n cyclohexanol at 100deg with rapid oxygen flow f o r 1»r the ac i d i t y was found to be pX=1.70, and the colour of the solution was s i m i l a r to that of f e r r i c chloride in cyclohexanol. When the solution was made up as above and the absorbance measured, the maximum absorbance in the region 460-480nm was found to be 1.00. It can be concluded that t h i s technique i s not useful for quantitative analysis of f e r r i c i r o n because of inte r a c t i o n between f e r r i c chloride and cyclohexanol at high temperature. i I t also seems probable (but not certain) that ferrous chloride i s converted to f e r r i c chloride on passing molecular oxygen through a mixture of ferrous chloride and cyclohexanol at 100deg. A possible alternative method was considered to be a spectrophotometry analysis for ferrous iron, based on formation 68 of the compound potassium ferrous f e r r i c y a n i d e . , I t has long been known that on treating a solution containing Fe{|1) ions with hexacyanoferrate(||) a precipitate c a l l e d Turnbull's blue i s produced. Standard solutions were made up by adding known weights of ferrous chloride to cyclohexanol 20ml,190mmol, d i l u t i n g the solution with water and then adding potassium ferricyanide 3.1g,9.4mmol. The solutions were then diluted to 1 l i t r e . F i l t r a t i o n showed a l l the blue compound had dissolved. This intense blue solution was diluted by a factor of 50 and the maximum absorbance measured using a Cary 14 instrument i n the region of 690nm. The results are shown in Table (9)., The r e s u l t s indicate that the absorbance measured i s proportional to the o r i g i n a l amount of ferrous chloride used. A solution was made up by mixing f e r r i c chloride 1.5g,9.25mmol, and cyclohexanol 20ml,190mmol; at room temperature, d i l u t i n g the mixture with water and adding potassium ferricyanide 3.1g,9.4mmol. A green solution was formed which was made up to 1 l i t r e . A portion was diluted by a factor of two, and the maximum absorbance measured i n the region of 690nm. , Another solution was made up by f i r s t heating f e r r i c chloride 1.5g,9.25mmol, and cyclohexanol 20ml,190mmol, with rapid oxygen flow at lOOdeg for 1hr. This mixture was diluted with water, and potassium ferricyanide 3.1g,9.4mmol, added, to give a blue-green solution which contained insoluble p a r t i c l e s . This solution was made up to 1 l i t r e , a portion diluted by a factor of 50 and f i l t e r e d . The maximum absorbance was measured Table (9) Weight of ferrous chloride Di l u t i o n Absorbance <g) (mmol) (690nm) 1. 17 9.25 50 1 . 30 0.585 4.68 50 0.65 Table (9) . Absorbance of solutions after addition of K {CN) cyclohexanol/FeCl systems. Table (10) Weight of f e r r i c chloride Temp Di l u t i o n absorbance (g) (mrnol) (deg) (690nm) 1.50 9.25 R.T. 2 0.90 1.50 9.25 100 50 0.44 Table (10). Absorbance of solutions after addition of K (CN), to 3 6 cyclohexanol/FeCl systems. 71 in the region of 690nm, and the results are presented i n Table (10). These r e s u l t s show that the method may be of p r a c t i c a l use in an approximately determination of the ferrous i r o n content i n systems containing i r o n and cyclohexanol which have not been heated. However, as i n the thiocyanate case already discussed, strong interaction between f e r r i c chloride and cyclohexanol at high temperature invalidates the method as regards analysis of the types of reaction mixture under consideration., 72 CHAPTER (3) CLOSED SYSTEMS 3.1 Experimental setup The reaction mixture under investigation was placed i n a 25ml round-bottomed f l a s k containing a magnetic s t i r r e r bar. The fl a s k was immersed i n an o i l bath maintained at lOOdeg, and was connected to a condenser through which tap water flowed. The top of the condenser was connected to a manometer by means of thick walled rubber tubing. The manometer consisted of a uniform glass tube approximately 100cm i n length and 1cm i n cross sectional area, joined at the lower end to a c o i l of tygon tubing of smaller diameter held i n position i n a large beaker. The glass tube had previously been calibr a t e d by f i l l i n g with water from a burette. The ether end of the tygon tubing was connected to a narrower piece of uniform glass tube, approximately 20cm i n length, held i n a v e r t i c a l position adjacent to the lower part of the 100cm tube. The manometer was f i l l e d with clean mercury such that the mercury l e v e l was v i s i b l e i n the upper part of the short glass tube., The system could be evacuated and f i l l e d with oxygen or nitrogen at 1atm pressure. Gas uptake by a rapidly s t i r r e d reaction mixture could be measured by allowing the mercury l e v e l in the iQOem tube to r i s e a ce r t a i n distance as measured by a scale , then r e f i l l i n g the system with gas to 1atm pressure. Normally the mercury l e v e l was allowed to r i s e 5.0cm, then returned to i t s o r i g i n a l position by 73 rapid addition of oxygen u n t i l a pressure of lata was restored . This cycle could then be repeated as many times as necessary. Under these conditions the pressure under which a reaction occurred would vary between 76.0 and 60.0cm Hg. At the end of such a series of uptake and r e f i l l i n g cycles, the shorter glass tube could be raised and the tygon tubing uncoiled u n t i l the mercury l e v e l s i n the manometer were equivalent. The volume of gas absorbed at 1atm corresponding to the 5.0cm change in mercury l e v e l could thus be calculated, and hence the t o t a l volume of gas absorbed during the course of the experiment. The water of the gas i n the system was estimated to be 25deg. /Hence, the volume of gas absorbed at N.T.P could be found. The reaction mixture were i n i t i a l l y degassed, then the system f i l l e d to a pressure of 1atm. Absorbtion due to the s o l u b i l i t y of the gas i n cyclohexanol alone was measured, so that the volume of gas involved i n a chemical oxidation process could be calculated. The oxygen and nitrogen used in these closed system experiments was dried by passing through columns containing d r i e r i t e and phosphorus pentoxide. 3.2 The cyclohexanol-rhodium t r i c h l o r i d e trihydrate-concentrated hydrochloric acid system 3.2a Reaction under oxygen The reaction mixture (cyclohexanol I0.0g,100mmol; rhodium t r i c h l o r i d e trihydrate 0.Q64g,0.151mmol; concentrated hydrochloric acid 0.10ml) was investigated at 100deg under 74 oxygen. The solution was i n i t i a l l y red as the rhodium t r i c h l o r i d e trihydrate dissolved, becoming dark reddish-brown with time. No cyclohexene was produced. After 4.25hr the conversion to cyclohexanone was found to be 8% (vpc). The oxygen absorption corresponded to a 1% conversion on the basis of reaction (1). The water content at the end of the reaction was found to be 0.2g,11mmol; which agrees reasonably with the amount expected assuming reaction (1) i . e . 0. 14g,8mmol« In the absence of concentrated hydrochloric acid a cyclohexanone content of 6% (vpc) was found a f t e r 4.50hr, and the oxygen absorption corresponded to a 5% conversion. 3.2b Reaction under helium The system (cyclohexanol 10g,100mmol; rhodium t r i c h l o r i d e trihydrate 0.064g,0.151mmol; concentrated hydrochloric acid 0.10ml) was investigated at 100deg under helium., The solution was i n i t i a l l y red, becoming dark brown after f i v e minutes. After 2hr no conversion to cyclohexanone could be detected, (vpc), and no evolution or absorption of gas had occurred. At t h i s time the solution was dark brown and rhodium metal had been deposited as a black residue. 3.3 The cyclohexanol-ferric chloride system The reaction mixture (cyclohexanol 10.Qg,1OOmmol;ferric chloride 0.75g,4.63mmol;) was studied at 100deg under oxygen 75 After ten minutes the solution was dark green and the mixture was found to contain 9% cyclohexene and 0.2g,11mmol, water. There was no conversion to cyclohexanone. The water conversion corresponds to the expected value of 0.16g,9mmol, assuming i t i s produced from cyclohexanol by elimination in the formation of cyclohexene. After 2.50hr the cyclohexene content had risen to 14% and the water content to 0.4g#22mmol# rather higher than the expected value of 0.25g,14mmol. At thi s time the solution was dark brown and s o l i d p a r t i c l e s were present. There was no conversion to cyclohexanone, but there had been some oxygen absorption beyond that expected to saturate the cyclohexanol. The rather higher than expected water content and oxygen absorption may be due to some side reactions (see section 3.4). The reaction mixture (cyclohexanol 10.0g,100mmol; f e r r i c chloride 2.0g,12.3mmol) was also investigated at 100deg under oxygen. After ten minutes the dark green solution was found to contain 14% cyclohexene and 0.5g,28mmol, water which i s higher than the expected value of 0.25g,14mraol. No conversion to cyclohexene was detected. After 2hr the mixture was dark brown and contained 18% cyclohexene and 0.6g,33mmol# water. As above, there was a small amount of oxygen absorption beyond that expected to saturate cyclohexanol. 3.4 The cyclohexanol-rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride system 3.4a Reaction under oxygen The reaction mixture (cyclohexanol 10.Og,100mmol; rhodium 76 t r i c h l o r i d e trihydrate 0.064g,0.0151mmol; f e r r i c chloride 0.75g,4.63mmol) was studied at lOOdeg under oxygen. This mixture corresponds to the optimum conditions as determined by the oxygen flow experiments. Concentrated hydrochloric acid was not added as t h i s appeared unnecessary. The ;results are shown i n Graph (24), which i s a plot of oxygen absorption against time. After f i v e cycles (136 minutes), the conversion to cyclohexanone was found to be 29% (vpc) and that expected on the basis of oxygen absorption 32%. The mixture contained 8% cyclohexene. After nine cycles (469 minutes) the conversion to cyclohexanone was found to be 40% (vpc) and the oxygen absorption corresponded to a 50% conversion. At t h i s time the oxygen absorption had not ceased. The water content was estimated to be 0.9g,50mmol, and the system had gained i n weight by 0.8g. The expected water content on the basis of the reactions; i s 0.B6g,47.78mmol. These reactions weight of 0.7g. would predict a gain i n 77 10 TIME MINS. Graph (24). Rate of oxygen absorbtion for the system: Cyclohexanol (100mmol), RhCl 3H 0 (0.151mmol), FeCl (4.63mmol), at lOOdeg. 78 Vpc analysis of 2ul of reaction mixture was compared with 2ul of a (6:4) mixture of cyclohexanol and cyclohexanone. The r a t i o of the t o t a l areas of vpc readout for the experimental and known mixtures was 1.07. The reaction mixture was allowed to absorb oxygen f o r another 20hr at a pressure of approximately 1atm. The mixture then contained 37% cyclohexanone and the above r a t i o had decreased to 0.875. I t therefore appears that, as the reaction mixture was s t i l l absorbing oxygen but no futher conversion to the ketone i s seen, another oxidation process i s occurring. As no additional products are detected using vpc analysis, i t i s possible that the products of t h i s oxidation process are either l i g u i d s of high b o i l i n g point or s o l i d s . The reaction mixtures (cyclohexanol 6.0g,60mmol; cyclohexanone 4.0g#41mmol; rhodium t r i c h l o r i d e trihydrate 0.064g,0.151mmol; f e r r i c chloride 0.75g,4.63mmol) and (cyclohexanone 10,Og,102mmol; rhodium t r i c h l o r i d e trihydrate 0.O64g,0.151mmol; f e r r i c chloride were also studied at 100deg under oxygen. The res u l t s are shown i n Graph (25) which shows that both systems absorb oxygen. The cyclohexanol-cyclohexanone system did not appear to have changed in composition aft e r 6hr of oxygen absorption (vpc). The most probable explanation of t h i s oxygen absorption, with no product detected by vpc analysis, appears to be conversion of cyclohexanol and cyclohexanone to adipic acid: 7 9 lOT 8T 6 + TIME MINS. Graph (25). Rates of oxygen absorption for the systems: A: Cyclohexanol (60mmol) , Cyclohexanone (4 1mmol), RhCl33Ha<9 (0.151mmol) , FeCl ̂ (4.63maol) , at 100deg. B: Cyclohexanone (102amol), HhCl 3H 0 (0.151mmol), FeCl (4.63omol), at 100deg. 80 a Adipic acid i s a s o l i d at normal temperature (mp 151deg) and soluble i n cyclohexanol, Ypc analysis of a solution of adipic acid i n cyclohexanol showed no peak other than that due to cyclohexanol, as was expected. The l i t e r a t u r e contains a number of references to the oxidation of either cyclohexanone or cyclohexanone-cyclohexanol mixtures by a i r or oxygen at normal pressure i n the temperature range 60-120deg, Acetic acid i s usually employed as solvent, together with cyclohexane i n seme cases. The most common catalyst used i s manganous acetate, in conjunction with other t r a n s i t i o n metal s a l t s . Yields of adipic acid of about 70% are commonly reported a f t e r several hours. Other acids are sometimes reported as by-products of the oxidation process, e.g. g l u t a r i c , s u c c i n i c , v a l e r i c and c a p r y l i c acid 1 2 . For example, an eguimolar mixture of cyclohexanol and cyclchexanone with an equal weight of cyclohexane can be oxidized by oxygen gas using a c e t i c acid as a solvent and a mixture of Mn, Co and Cu s a l t s as a catalyst at 65-90deg l 3 . Good yiel d s of adipic acid are obtained a f t e r 12hr. In another example, a cyclohexanol-cyclohexanone cyclohexane mixture was oxidized by oxygen at 60-100deg using acetic acid as solvent and a mixture of cobaltous acetate and 81 vanadyl acetylacetonate as cat a l y s t *•-*. Adipic acid was obtained in good y i e l d . Cyclohexanone, by i t s e l f , can be oxidized i n acetic acid at 80deg by a stream of a i r using manganese acetate as c a t a l y s t to give adipic acid i n 60-70% y i e l d 1 S . An attempt was made to i s o l a t e adipic acid from a reaction mixture by extraction with aqueous sodium hydroxide solution, followed by a c i d i f i c a t i o n . No adipic acid was recovered, perhaps because because of stronq complexation between iron and the acid. In the closed system experiment usinq cyclohexanol, a 40% y i e l d of cyclohexanone was obtained usinq vpc analysis of the mixture. The oxygen absorption, however, corresponded to a 50% conversion. This could be explained by assuming that approximately 2.5% conversion of cyclohexanol to adipic acid had taken place: COOH cooH 82 This small conversion to adipic acid i s also consistent with the r a t i o values obtained on vpc analysis of 2ul of reaction mixture and known solution as described above. The decrease i n r a t i o from 1.07 to 0.875 afte r passage of oxygen for a long period can be explained by conversion of reactants to the dicarboxylic acid which i s not detected by vpc. The r e s u l t s for the rapid oxygen flow system and the closed system are shown together in Graph (26) f o r the optimum reaction mixture at 100deg. It can be seen that the rate of reaction i s suppressed in the closed system. This can presumably be explained by the steady increase i n water content in the closed system as the reaction progresses. An experiment employing a slow flow of oxygen through the optimum reaction mixture produced a conversion of 30% cyclohexanone after 3hr, very s i m i l a r to the results for the closed system. 3.4b Reaction under nitrogen The system (cyclohexanol 10.Og,100mmol; rhodium t r i c h l o r i d e trihydrate 0.064g,0.151mmol; f e r r i c chloride 0.75g,4.63mmol) was investigated under nitrogen at 100deg. After 5hr there had been no appreciable evolution or absorption. Vpc analysis showed there had been no conversion to cyclohexanone. 3.4c Effect of cyclohexene The system (cyclohexanol 8.0g,80mmol; cyclohexene 2.0g24mmol; rhodium t r i c h l o r i d e trihydrate 0.064g,0.151mmol; f e r r i c chloride 0.75g,4.63mmol) was studied under oxygen at 83 100deg. After 4hr the conversion to cyclohexanone was found to he 4%. (vpc), and the oxygen absorption corresponded to US..The cyclohexene conversion had increased to 32%. This compares with a conversion to cyclohexanone of 33% after the same period i n the absence of the i n i t i a l l y introduced cyclohexene. This r e s u l t i s i n good agreement with the studies of the e f f e c t of cyclohexene in the oxygen flow systems. , A TIME (HOURS) Graph (26). Rates of cyclohexanone formation for the systems: A Cyclohexanol (190mmol), RhCl^H^O (0.302mmol), FeCl 3 (9.25mmol) HCl (0.10ml), Rapid oxyqen flow at lOOdeq. B: Cyclohexanol (100mraol), RhCl 3H 0 (0.151mmol) 3 2. FeCl„ (4. 63mtnol) , Closed system at 100deq. 85 CHAPTER (4) RELATED SYSTEMS 4.1 Use of other metal s a l t s Other systems were studied i n which various metal s a l t s were substituted for f e r r i c chloride (e.g mercuric chloride, cupric chloride, chromic c h l o r i d e ) . A l l apeared to give i n f e r i o r r esults when compared to the f e r r i c chloride systems already discussed. For example, the system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g, 0.302mmol; cupric chloride 1.50g,11.6mmol; concentrated hydrochloric acid 0.15ml) at 100deg with rapid oxygen flow gives a 13% y i e l d of cyclohexanone after 1.50hr. (Graph 27). 4.2 The cyclohexanol-rhodium t r i c h l o r i d e trihydrate- p- toluene sulphonic acid monohydrate system The exact function of the f e r r i c chloride i n the systems described i n d e t a i l i n chapters 2 and 3 has not ibeen elucidated. The change i n a c i d i t y as the reaction progresses was described in section 2.4c. I t was suggested that the decline i n the reaction rate with time might be related to the appreciable decrease i n a c i d i t y from pX=1.70, for a t y p i c a l reaction mixture, to pX-3.90 af t e r a 40% conversion to cyclohexanone. Attempts to produce a constant high a c i d i t y during the reaction by addition of large amounts of concentrated hydrochloric acid were unsuccessful i n increasing the f i n a l y i e l d or i n i t i a l 86 60 Graph (27). Rate of cyclohexanone formation f o r the system: Cyclohexanol (190mmol), R h C l ^ H ^ (0. 302mmol) , CuCl^( 11. 6mmol) , HCl (0.15ml), at 100aeq. 87 reaction rate because of the water introduced i n t o the system. Agueous systems containing p-toluene sulphonic acid monohydrate are known to have high a c i d i t y . An attempt to produce a highly a c i d i c medium, i n the absence of water* was made by dissolving p-toluene sulphonic acid monohydrate i n cyclohexanol. p-Tcluene sulphonic acid monohydrate 0.5g,2.63mmol, was added to cyclohexanol 20ml,190mmol, and the solution heated u n t i l the s o l i d dissolved. After cooling to room temperature, the resulting solution had a pX value of 2.20. A solution of 5.0g,2€.3mmol, p-toluene sulphonic acid monohydrate i n 20ml,190mmol, cyclohexanol was s i m i l a r l y prepared, and was found to have an a c i d i t y of 2.05. The system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; p-toluene sulphonic acid monohydrate 0.50g,2.63mmol) was investigated at 100deg with rapid oxygen flow. The conversion to cyclohexanone was found to be <5% a f t e r 1hr. The system (cyclohexanol 20ml,190mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmol; p-toluene sulphonic acid monohydrate 5.0g,26.3mmol) was also investigated at 100deg with rapid oxygen flow. After 2hr there had been no conversion to cyclohexanone and the reaction mixture contained a large amount (>50%) cyclohexene. These r e s u l t s suggest that the function of the f e r r i c chloride might not s o l e l y be to produce a high a c i d i t y i n the reaction medium. However, no d e f i n i t e conclusions can be drawn on the basis of the results for systems containing p-toluene 88 sulphonic acid monohydrate as t h i s may i t s e l f coordinate with an active rhodium species in some manner, which might i n h i b i t e f f i c i e n t c a t a l y s i s . 4.3 Addition of a solvent I t was thought that addition of a solvent to a reaction mixture might improve the yi e l d of cyclohexanone. I t was decided that the solvent added should be miscible with cyclohexanol and cyclohexanone, be in e r t to chemical reaction under the conditions used (i.e. not contain oxidizable functional groups), and have a b o i l i n g point i n the same range as cyclohexanol and cyclohexanone. The solvent selected was o-xylene {bp 144deg). This also had the advantage that i t s vpc peak did not overlap with those of the alcohol and the ketone., The effect of addition of 20ml,210mmol o-xylene to system (1) was studied at 100deg with rapid oxygen flow. The results are shown in Graph (28), which appears to have a s l i g h t S-shape. After 1.50 hr the conversion to cyclohexanone was 40%, compared to 31% for system (1) at 100deg. However, the rate of conversion rapidly declines when the cyclohexanone content reaches 42% after 2hr. Thus a given % conversion to the ketone appears to be attained more rapidly in the presence of an egual volume of o- xyiene but the f i n a l composition remains the same (40% cyclohexanone). An experiment using 60ml,630mmol o-xylene instead of 20ml was carried out. The results are shown i n Graph (28). I t can be seen that the S-shape i s very pronounced. The % conversion to cyclohexanone after 2hr has declined from 42% to 30% on t r i p l i n g TIME (HOURS) Graph (28). Bates of cyclohexanone formation for the systems Cyclohexanol (190mmol), R h C l ^ O (0. 302mmol), F e c l 3 (9.25mmol) HCl (0.10ml), at 100deq; o-xylene A: (210mmol), B: (630mmol). 90 the amount of o-xylene. 4.4 Oxidation of cyclopentanol The system (cyclopentanol 20ml,245mmol; rhodium t r i c h l o r i d e trihydrate 0.128g,0.302mmcl; f e r r i c chloride 1.5g,9.25mmol; concentrated hydrochloric acid 0.10ml) was studied at 100deg using rapid oxygen flow. The presaturation mixture contained an egual volume of cyclopentanol and cyclopentanone. The conversion to cyclopentanone was followed over a 2.50hr period and the resu l t s a plotted i n Graph (29)., I t can be seen that the sit u a t i o n i s very s i m i l a r to the cyclohexanol case. 91 60+ TIME (HOURS) Graph (29). Pate of cyclopentanone for.ation for the system: Cyclop.nt.nol f 2 . 5..ol) . BhCl 33H 2 0 (0.302»*ol>, FeCI3(9.25**cl> HCl <0.10»1), at 100deq. 92 CHAPTER 5 SUMMARY OF RESULTS 5.1 Reaction mixtures containing the components: (cyclohexanol, rhodium t r i c h l o r i d e trihydrate, f e r r i c chloride and concentrated hydrochloric acid) were studied in the presence of oxygen. The nature of the reaction in which cyclohexanol was oxidized to cyclohexanone was investigated* No conversion to cyclohexanone occurred i n the absence of rhodium t r i c h l o r i d e trihydrate. Some degree of conversion was found i n the absence of f e r r i c chloride. The optimum conditions f o r conversion to cyclohexanone were produced by using a combination of rhodium t r i c h l o r i d e trihydrate and f e r r i c chloride. Under such conditions the rate of conversion to the ketone declined steadily u n t i l the mixture contained H0% cyclohexanone. 5.2 For a fixed amount of cyclohexanol and rhodium t r i c h l o r i d e trihydrate there i s an optimum amount of f e r r i c chloride necessary to achieve the maximum y i e l d in the shortest period at a given temperature . Addition of f e r r i c chloride i n excess of this optimum amount tends to suppress the rate of conversion to the ketone. This can probably be explained by the additional production of water and cyclohexene (see below). The optimum range for addition of f e r r i c chloride i s between 3 and 6mmol per 100mmol cyclohexanol. 93 5.3 Using a cyclohexa.nol/.ferric chloride r a t i o i n the optimum range at a given temperature, increasing the rhodium t r i c h l o r i d e trihydrate concentration beyond a ce r t a i n l e v e l does not s i g n i f i c a n t l y increase the f i n a l y i e l d of cyclohexanone or the reaction rate. 5.4 The oxidation reaction occurs under a c i d i c conditions. This a c i d i t y i s the r e s u l t of the i n t e r a c t i o n between f e r r i c chloride and cyclohexanol (and cyclohexanone). The a c i d i t y greatly decreases as the reaction progresses. Addition of concentrated hydrochloric acid to the mixture i s unnecessary and may i n h i b i t the oxidation reaction i f added i n large amounts due to the water introduced (see below). 5.5 cyclohexene i s produced i n a side reaction together with water. This i s presumably the r e s u l t of cyclohexanol undergoing an elimination reaction under a c i d i c conditions. Using the optimum cyclohexanol/ferric chloride r a t i o at lOOdeg, the cyclohexene content of the reaction mixture i s less than 1035, Introduction of cyclohexene i n amounts i n excess of 20% greatly suppresses the conversion to cyclohexanone* This i s presumably due to strong complexation of the o l e f i n with an active rhodium species, 5.6 Water i s produced during the c a t a l y t i c oxidation i n amounts greater than can be accounted for by the production of cyclohexene. This additional water content of the reaction 94 mixture i n a closed system i s i n good agreement with that predicted by reaction (1). The presence of water in the reaction mixture tends to supress the oxidation of cyclohexanol to cyclohexanone. 5.7 Using the optimum r a t i o of components, very l i t t l e conversion occurs i n the presence of oxygen at temperatures below 50deg. Increasing the temperature from 100deg to 150deg increases the rate of oxidation but does not increase the f i n a l y i e l d of the ketone. 5.8 Addition of a phosphine to a system containing the optimum r a t i o of components i n the presence of oxygen, i n an attempt to s t a b i l i z e possible Rh(|) intermediates, has l i t t l e s i g n i f i c a n t e f f e c t . 5.9 No hydrogen evolution could be detected when a system containing the optimum r a t i o of components was heated at 100deg under helium. This suggests reaction (2) may not be involved. 5.10 No gas evolution occurs when a reaction mixture i s heated at 100deg under nitrogen. No conversion to cyclohexanone could be detected. Reaction (2) would enable some conversion to the ketone to occur u n t i l a l l the iron present i n the f e r r i c state had been reduced to the ferrous state. Evolution of hydrogen would also occur. 95 5.11 Oxygen i s necessary for conversion of cyclohexanol to cyclohexanone to occur. Gas absorption i s detected when a mixture containing an optimum r a t i o of components at lOOdeg i s studied i n a closed system containing oxygen., This suggests reaction (1) i s occurring. Reaction (2) would give r i s e to a net evolution of gas. The measured volume of oxygen absorption i s i n good agreement with that predicted on the basis of reaction (1). 5.12 Using an optimum r a t i o of components in the presence of oxygen the conversion to cyclohexanone i s limited to approximately 40%. This l i m i t i s probably due to an i n t e r a c t i o n between cyclohexanone and some active rhodium species e s s e n t i a l for c a t a l y t i c oxidation. Oxygen absorption continues after conversion to cyclohexanone has ceased. This can be accounted for by assuming another oxidation process i s occurring, probably to produce adipic acid. 96 CHAPTER (6) DISCDSION AND CONCLUSIONS The oxidation of cyclohexanol to cyclohexanone i n the presence of oxygen using a combination of rhodium t r i c h l o r i d e trihydrate and f e r r i c chloride as c a t a l y s t has been shown to have the stoichiometry: A plausible mechanism f o r t h i s reaction has been discussed in the introduction. The function of the f e r r i c chloride has not been d e f i n i t e l y established. As well as being a potent oxidant f o r Rh(|), i t may be required to produce a highly a c i d i c medium i n the absence of water. However, the r e s u l t s f o r experiments discussed i n section 4.2, employinq p-toluene sulphonic acid monohydrate, i n combination with rhodium t r i c h l o r i d e trihydrate, tend to suggest thi s might not be the case. I t i s also possible that i t may contribute to the s t a b i l i z a t i o n of a Rh{l) intermediate amd prevent p r e c i p i t a t i o n of rhodium metal, as i n the case of the rhodium t r i c h l o r i d e trihydrate-stannous chloride system. ., The complex formed between Rh(j) and tin{|\) chloride has been formulated as: 1 6 97 Cl.Sn C l SnCl CI Sn C l SnCl \/\/ Rh Rh /\/\ The c o n v e r s i o n to cyclohexanone appears to be U n i t e d to approximately 4OX f o r the rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c c h l o r i d e system. T h i s i s p o s s i b l y the r e s u l t of i n t e r a c t i o n between cyclohexanone i n the e n o l i z e d form and the a c t i v e rhodium s p e c i e s r e q u i r e d f o r c a t a l y t i c a c t i v i t y . The complexation between the e n o l and rhodium might be expected t o be s t r o n g e r than the complexation between the a l c o h o l and rhodium because of the p o s s i b i l i t y of i n t e r a c t i o n between the double bond and the metal c e n t r e : OH 98 Interaction between a rhodium centre and acetone i n the enolised form has also been proposed 1 7 . Evidence for strong i n t e r a c t i o n between rhodium and a carbon-carbon double bond i s provided by the observation that addition of cyclohexene to the rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride-cyclohexanol system can greatly i n h i b i t c a t a l y t i c oxidation. Strong complexation of o l e f i n s has also been found to poison the c a t a l y s t in the palladium chloride-sodium acetate- alcohol systems.. The rate of dehydrogenation of isopropyl alcohol by rhodium t r i c h l o r i d e trihydrate - l i t h i u m chloride - stannous chloride-concentrated hydrochloric acid was found to be reduced as the concentration of acetone b u i l t up i n the mixture but was restored when the acetone was d i s t i l l e d o f f . This was attributed to a competing hydrogenation of the acetone back to the alcohol. Unfortunately i n the present studies, i t i s not possible to separate cyclohexanone from cyclohexanol by simple d i s t i l l a t i o n because of the s i m i l a r i t y of t h e i r b o i l i n g points. I t would be of inte r e s t to discover whether the o r i g i n a l c a t a l y t i c a c t i v i t y of the rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride, combination could be restored after removal of the ketone from a reaction mixture containing 60% cyclohexanol and 40% cyclohexanone. water i s produced during the c a t a l y t i c oxidation of cyclohexanol to cyclohexanone by the rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride system i n the presence of oxygen. The accumulation of water i n h i b i t s the c a t a l y s i s , as has been found in the palladium chloride-sodium acetate and palladium chloride- 99 cupric n i t r a t e systems. This could r e s u l t from competition between the water and the alcohol ligands for the rhodium. I t i s d i f f i c u l t to make a meaningful comparison between the various c a t a l y t i c systems for oxidation of secondary alcohols to ketones because d i f f e r e n t alcohols have been studied and at varying temperatures. Also, the effect of var i a t i o n i n concentration of catalyst on the oxidation process has not been published i n most cases. However, an attempt has been made to compare the ' c a t a l y t i c e f f i c i e n c y * of a number of systems, and the data are presented in Table (11). The concentration of the c a t a l y t i c species has been calculated for each system as mmol of ca t a l y s t per 100mmol of alcohol. The yi e l d refers to the % of ketone present i n the reaction mixture at the time indicated. The y i e l d for system <m) has been calculated on the basis of the report The ' c a t a l y t i c efficiency» has been calculated i n terms of the r a t i o s mmol ketone/(mmol catalyst) and mmol ketone/(mmol c a t a l y s t ) / ( h r ) . The % yiel d s f o r systems involving dehydrogenation appear to be low (3-10%). Whether higher y i e l d s of ketone can be obtained using these c a t a l y t i c dehydrogenation methods does not appear to have been investigated. Both c a t a l y t i c e f f i c i e n c y r a t i o s for rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride-oxygen systems compare favourably with the r a t i o s for other c a t a l y t i c systems. The r a t i o mmol ketone/(mmol catalyst) for the rhodium t r i c h l o r i d e t r i h y d r a t e - f e r r i c chloride-oxygen system at 100deg appears to increase as the concentration of rhodium decreases. 100 Comparison with the palladium chloride-sodium acetate-oxygen system shows that both r a t i o s are higher for the rhodium t r i c h l o r i d e trihydrate - f e r r i c chloride -oxygen combination, p a r t i c u l a r l y when the time factor i s considered. However, i t should be noted that c a t a l y t i c oxidation with the pd system occurs at lower temperature. It i s not clear why dehydrogenation of iso-propyl alcohol occurs using a combination of rhodium t r i c h l o r i d e trihydrate, lithium chloride and concentrated hydrochloric acid i n the presence of oxygen, whereas no dehydrogenation of cyclohexanol was detected using rhodium t r i c h l o r i d e trihydrate/concentrated hydrochloric acid, i n the presence of oxygen. This could possibly be explained i n terms of oxidation potentials for the conversion of alcohols to the corresponding carbonyl compounds as discussed i n reference (18). In t h i s publication i t was suggested that the greater d i f f i c u l t y of oxidation of cyclohexanol, compared to other secondary alcohols was related to i t s ring structure. ,. 101 Table (11). Catalyst Oxidant Alcohol a) BhCl 3/FeCl 3 11 0 (1 atm) b) c) d) e) f) BhClj g) PdCl /Cu(NO.) 0,(3atm) Cyclohexanol i i Cyclopentanol Cyclohexanol Temp deq 100 100 150 100 100 100 90 Time hr 2. 5 0.5 1.0 3.0 2.5 1.0 2.0 h) PdCl , NaOAc O (1atm) i) j) BuCl_(Pd ) N-methyl- morpholine N-oxide Trans 3,3,5, trimethyl cyclohexanol d-carverol k) BhCl^/LiCl/ Dehydrogenation Isopropyl 1) BhCl^/LiCl/ HCl/SnCl TL m) Bu(OCOCF^) CO (Prf ) 'I X * • sec- hexanol 38 38 B.T. 83 83 143 56 34 2.0 6.0 98 0.5 Table (11)., Comparison of data f o r c a t a l y t i c oxidation of alcohols. 102 Table (11) . Continued mmol c a t a l y s t / y i e l d mmol ketone/ mmol ketone/ (100mmol alcohol) ^ketone (mmol catalyst) {mmolcatalyst) / hr a) * o. 15 42 280 112 b) * 0. 15 14 92 184 c) * 0. 15 40 268 268 d)* 0.059 27 458 153 e) * 0. 124 38 306 152 f ) * 0. 15 10 67 67 4 2 0.5 0. 25 w 88 i ) 7 1.0 94 94 2.8 j ) f e 0.42 94 223 112 k)' 0.035 10 280 47 l ) 1 1. 15 9 8 0.8 0.033 ....... 3 91 182 * present work 103 REFERENCES 1. H. B. Chairman, j . chem. Soc. , (B) , (1967), 629. 2. H. B. Charman, J. Chem. Soc. (B), (1970) , 584. 3. A. Dobson and S.D. Robinson, Inorganic Chemistry, (1977), Vol 16, No 1, 137. 4. K. Akashi, A. O. Chong, K., Oshima and K. B. Sharpless, unpublished re s u l t s . (Quoted in re f . 6) 5. K. Oshima and K. B. Sharpless, unpublished r e s u l t s . (Quoted in r e f . 6) 6. K. B. Sharpless, K. Akashi and K. Oshima, Tetrahedron l e t t e r s , (1976), No. 29, 2503.. 7. T. F. Blackburn and J . Schwartz, J. Chem Soc. Chem Comm., (1977) , 157. 8..fi. C. Hui and B. R. James, Can J. Chem. (1974), 52, 348. C. A. Reed and H. P. Roper, J. Chem Soc. Dalton (1973), 1014. H. R. Symes and H. L. Roberts J. Chem. Soc. (A), (1968), 1450., 9. K. Thomas, J. A. Osborn, A. R. Powell, and G. H. Wilkinson, J. Chem. Soc. (A), (1968), 1801. B. R. JAMES, CHEM. IN CANADA, (1975), 27, (9) , 27. 104 10. B. B. James and M. Kastner, Can. J . Chem. (1972) , 50, 1698. 11. A. G. Haddock and L. 0. Medeiros, J . Chem. Soc. A. (1969) , . 1946. 12. H . Masuda and N. Ohtu, Kogyo Kagaku Zasohi, (1969), 72(11), 2381. (through Chem Abstracts (1970), 72, 78509) V. P. Ivanov, M. S. Furman, A. D. Shestakova and I . L. Arest- Yakubovich, Inform. Soobich. Gos. Nauch.-rlssled. Proekt. Inst. Azotn. Prom. Prod. Org. Sin., (1966), No 17, Pt 1, 5. (through Chem Abstracts (1968), 68, 70951) 13. loa Gosei Chem Industry Co L t a , Fr. Pat. 1,488,079. (through Chem Abstracts (1968), 68, 68475) 14. W. Horris, Ger. Offen. 2,037,189. (through Chem Abstracts (1971) , 74, 140987) 15. V. P. Ivanov, M.S. Furman, A. D. Shestakova and I. L. Arest-Yakubovich, Inform. Soobich. Gos. Nauch-Issled. Proekt. Inst. Azotn. Prom. Prod. Org. Sin. (1966), No. 17, Pt 1, 5. 16. A. G. Davies, G. Wilkinson and J. ,-F. Young, J . Amer. , Chem. Soc. , (1963) , 85, 1692. 17. M. Gargano, P. Giannoccaro and M.,Rossi, J . Organometallic Chem., (1977), 129, 239. 105 18. H. Adkins, R..H. Elofson, A. G. Rosson and C. C. Robinson, J. Amer. Chem. Soc. (1949) , 71, 3622. 19. fl. G. Lloyd, J. Org. Chem., (1967), 32 (9), 2816. $SIG

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