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Reaction of rhodium III chlorides with ethylene in aqueous HC1 solution. Kastner, Michael Robin 1970

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REACTION OF RHODIUM ( I I I ) CHLORIDES WITH ETHYLENE IN AQUEOUS HC1 SOLUTION BY. MICHAEL ROBIN KASTNER B . S c . ( H o n s ) , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1968 A T H E S I S SUBMITTED I N P A R T I A L FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e D e p a r t m e n t o f CHEMISTRY We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE U N I V E R S I T Y OF B R I T I S H COLUMBIA J u n e , 1970 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study . I f u r t h e r agree t h a t permiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Department of The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date ABSTRACT In the presence of i r o n (III) and other oxidants, HC1 solutions of RhCl3'3H20 under mild conditions catalyze the oxidation of ethylene to acetaldehyde. The k i n e t i c s of the reaction measured by gas-uptake techniques ind i c a t e the presence of both ethylene dependent and independent steps. Hydroxy species such as RhCl 5(OH) 3" and RhCl^(OH) (H 2 0 ) 2 ~ , although present i n very small concentrations, are s i g n i f i c a n t l y r e a c t i v e towards ethylene. A mechanism based on that postulated f o r a s i m i l a r Pd(II) system i s presented. This involves the rearrangement of a rhodium (III) hydroxy IT - ethylene complex to a a - complex, followed by the production of acetaldehyde and rhodium ( I ) . However, unlike i n the Pd(II) system where the rate determining step i s the conversion of the IT - to a -C 2 H 4 complex, the rate determining step i n the Rh(III) system i s thought to involve the production of the TT - complex. Iron (III) oxidizes Rh(I) back to Rh(III), giving the net r e a c t i o n : CzWh + H 20 + 2Fe(III) -fiM111)) CH3CHO + 2H + + 2Fe(II) i i TABLE OF CONTENTS ABSTRACT i LIST OF TABLES . . i v LIST OF FIGURES . . ........ i v ABBREVIATIONS • v ACKNOWLEDGMENTS ........... v . CHAPTER 1. INTRODUCTION 1.1 Ob j e c t i v e s . • • • -L 1.2 Mechanism of Hydrogen A c t i v a t i o n . 3 1.3 Rh(I)- and ( I I I ) - Catalyzed Polymerization Reactions of Ol e f i n s and Acetylenes........ i ; , • 4 1.4 Rh(I)- and ( I I I ) - Catalyzed Isomerizations of O l e f i n s 7 1.5 C a t a l y t i c Reactions i n G e n e r a l • . . . . . . . . . — '. 9 1.6 Reactions of Ethylene with Rh(III), Ru(II) and Pd(II). ......... 10 CHAPTER 2. APPARATUS AND EXPERIMENTAL PROCEDURE 2.1 Constant Pressure Gas Uptake Apparatus 14 2.2 Gas Uptake Experimental Procedure 16 2.3 Reaction Product A n a l y s i s . . . . ; , 17 2.4 Instrumentation ; 17 2 . 5 Materials 17 CHAPTER 3. RESULTS 3.1 Stoichiometry and Products. 19 3.2 Iron(III) Dependence... 22 3.3 Rhodium (III) Dependence 23 3.4 Ethylene Dependence... . 23 i i i 3 . 5 Proton and Chloride Dependences . 2 3 3 . 6 A c t i v i t y of Rhodium (III) Aquochloro Species 28 3 . 7 Other Oxidizing Agents. ; 32 CHAPTER 4 . DISCUSSION 4 . 1 A c t i v i t y of the Pentachloro Species . 3 3 4 . 2 A c t i v i t y of the Tetrachloro Species 39 4 . 3 Inverse Chloride Dependence..........; 41 4 . 4 The Production of Acetaldehyde 4 3 4 . 5 Use of Oxidants other than Fe (III) ... r • • • • • • • • • 48 4 . 6 Ethylene Uptake Rates i n 6M Acid» 48 4 . 7 The Ethylene : Iron(III) Stoichiometry 49 CHAPTER 5 . SUMMARY AND SUGGESTIONS FOR FURTHER WORK 52 REFERENCES . , , ..... 54 i v LIST OF TABLES TABLE 1 Stoichiometrics of Ethylene: Iron ( I I I ) : Rhodium ( I I I ) . ..... 20 TABLE 2 Summary of K i n e t i c Data at 80° and 1 atm C2ttk 22 23 TABLE 3 . Summary of Ethylene Dependences TABLE 4 Summary of Proton and Chloride Dependences..... 25 TABLE 5 Rates of Ethylene Absorption f o r R h C l n ( H 2 0 ) + 29 TABLE 6 Summary of Proton and Ethylene Dependences f o r RhCls 2 -.. 30 TABLE 7 Summary of Proton and Ethylene Dependences f o r RI1CI4 32 TABLE 8 Summary of Rates with Various Oxidizing Agents 32 TABLE 9 Temperature Dependence of Rate Constants k & and k^.. 38 LIST OF FIGURES FIGURE 1 Constant Pressure Gas Uptake Apparatus 15 FIGURE 2 Uptake Plot of Ethylene Absorption... 19 FIGURE 3 V i s i b l e Spectra of Reaction Solution.........../... 20 FIGURE 4 Dependence of Linear Rate on Rh(III)..... 23 FIGURE 5 Dependence of Linear Rate on [ C 2 H 4 ] 26 FIGURE 6 Dependence of Linear Rate on [H +] at 3MC1~ ; 26 FIGURE 7 Dependence of Linear Rate on [H +] at 2MC1~ 27 FIGURE 8 Dependence of Linear Rate on [Cl~] ... 27. FIGURE 9 Uptake p l o t of Ethylene Absorption i n 6MH+. 28 FIGURE 10 Dependence of RhCl 2, - Rate on [C 2H!j at three a c i d i t i e s . . . . . . . . , 31 FIGURE 11 Dependence of RhCl^ Rate on [C 2Hit] at two a c i d i t i e s 31 FIGURE 12 Dependence of C2Uk Independent Rate on [H +], f o r RhCl2," ... 34 FIGURE 13 Dependence of C ^ Dependent Rate on [ H + ] , f o r RhCl 2;" 34 FIGURE 14 Arrhenius Plot f o r r e a c t i o n of RhClf" with C 2 H 4 . . 38 FIGURE 15 Dependence of C2Hi,. Dependent Rate on [H +], f o r RhCltt" 39 V ABBREVIATIONS IR: Infrared NMR: Nuclear magnetic resonance ESR: Electron spin resonance (S): Solvent molecule P : P a r t i a l pressure, mm. [ ]: Concentration in. _1 moles l i t e r DMA: NjN'Dimethylacetamide ACKNOWLEDGMENTS I wish to acknowledge the supervision of t h i s research made pos s i b l e by the extreme generosity of Dr. BRIAN R. JAMES. Also I am indebted to Miss Judy Copeland and Miss Rosalind Amies f o r typing t h i s t h e s i s . ( v i Do you think that I'm crazy? & Out of my mind? - M Do you think that I creep i n the night And sleep i n a phone booth? u< CHAPTER 1. INTRODUCTION 1.1 Objectives In the l a s t ten years, many platinum metal complexes i n s o l u t i o n have been found to catalyze a wide v a r i e t y of reactions. Notable among these i s the a c t i v a t i o n of hydrogen f o r the reduction of inorganic and organic substrates. One of the e a r l i e s t reactions i n v e s t i g a t e d was the chlororhodium (III) - catalyzed hydrogen reduction of i r o n (III) i n aqueous 3M hydro c h l o r i c a c i d at 8 0 ° 1 . The k i n e t i c s from gas uptake pl o t s were of the form -d[H 2]/dt= k [ H 2 ] [ R h ( I I I ) ] , and the rate determining step was thought to involve the h e t e r o l y t i c s p l i t t i n g of molecular hydrogen: R h H I + H 2 Jli^ Rh I HH-+H + (1) k - l . The metal hydride thus formed then reduces the Fe(III) substrate i n a rapid step: TT T TTT ^ 2 T T T T T Rh H"+2Fe £+. t Rh +2Fe +H+ (2) In equations (1) and (2), the CI" and H 20 ligands were omitted but from l a t e r spectrophotometric i n v e s t i g a t i o n s 2 , the rhodium complex i n 3M HC1 i s l i k e l y present as [ R h ( H 2 0 ) C l 5 ] 2 ~ . Subsequently, James and Rempel 3 showed that only the anionic species [ R h C l 6 ] 3 ~ to [Rh(H 20)2Cl l t]~ are e f f e c t i v e c a t a l y s t s , with the a c t i v i t y decreasing with decreasing number of chloride ligands. Direct evidence f o r an equilibrium o f the type shown i n equation (1) was obtained f o r the corresponding ruthenium (III) system by i s o t o p i c exchange experiments using deuterium 1*! 5. It was pointed o u t 3 that reaction (2) could involve a Rh(I) species rather than a Rh"'''^ !-!" species since i n the absence of substrate, metal was r a p i d l y produced and t h i s was thought to be due to the following f a s t reactions R h m H -y Rh r+H + ( 3 ) 2Rh T Rh 0+Rh n (4) The Rh''" state could be s t a b i l i z e d i n the presence of o l e f i n s as a Rh* (o l e f i n ) complex 3. Wilkinson and coworkers 6 found that a simple bubbling of hydrogen through neutral aqueous solutions of RhCl3 gave a ra p i d r e a c t i o n characterized by colour changes i n v o l v i n g the production of [ R h ^ O ^ C l ^ * from III polymeric species. Again a Rh H- intermediate was postulated: . Rh(H20)3Cl3+H2 5^Rh(H 20)3Cl 2H+H ++Cl- (5) Rh(H 20) 3C1 2H+H 20 -»• Rh(H 20) ( +Cl 2 ++H-; (6) H++H- -> H 2 (7) This c a t a l y t i c s u b s t i t u t i o n reaction was l a t e r observed using ethylene 7 or carbon monoxide 8 instead of hydrogen, and a more d e t a i l e d i n v e s t i g a t i o n of t h i s ethylene r e a c t i o n i n t h i s l a b o r a t o r y 9 i n d i c a t e d that Rh(I) intermediates were involved. The postulated mechanism i s ou t l i n e d as Rh I I I+C 2H t t -> RhI+CH3CHO (8) C l n R h I + R h I I I C l 3 + [ C ^ R h 1 CI R h I H C l 2 ] -> R h I C l 2 - + R h I I I C l 2 + + ( n - l ) C l - (9) and i s considered i n more d e t a i l i n sec t i o n 1.6. 10 At the completion of t h i s work on ethylene a paper by Rund appeared which also concluded that c a t a l y t i c s u b s t i t u t i o n at a Rh(III) center occurs through such Rh*-*—Rh* bridged intermediates. On considering reactions (1) and (2), and the p o s s i b i l i t y of the reduction step going v i a Rh(I) intermediates, a corresponding system using C2Htt instead of. H 2 was studied i n the present work. Russian workers have b r i e f l y mentioned the reac t i o n of ethylene with R h C l g 3 - i n the presence of Cu(II) s a l t s 1 1 . K i n e t i c studies of t h i s type are also useful i n determining the factors involved i n the a c t i v a t i o n of a v a r i e t y of gaseous molecules such as H 2 3 and CO 8 by a si n g l e t r a n s i t i o n metal complex. This thesis describes studies on the Rhodium (III) - catalyzed ethylene reduction of i r o n (III) . (Chapter 3). In general, r e a c t i o n (3) and the reverse r e a c t i o n are thought to be important for c a t a l y s i s . b y rhodium ( I I I ) 1 2 . Some pertinent l i t e r a t u r e reports w i l l now be summarized concerning hydrogen a c t i v a t i o n (section 1.2), catalyzed reactions i n v o l v i n g o l e f i n s and acetylenes (1.3-1.5), and reactions of ethylene with platinum metal complexes (1.6,1.7). 1.2 Mechanisms of Hydrogen A c t i v a t i o n Homogeneous hydrogenation reactions have been the subject of many r e v i e w s 1 3 - 1 9 . I t i s well established that there are three basic-processes whereby a metal ion i n a complex can ac t i v a t e molecular hydrogen. These are: (a) h e t e r o l y t i c s p l i t t i n g , which involves no change i n the metal oxidation s t a t e : ML +Ho^ML ,H-+H++L n z ^ n-1 In t h i s e s s e n t i a l l y s u b s t i t u t i o n a l process, r e a c t i v i t y i s governed by the s u b s t i t u t i o n l a b i l i t y of the complex ML n > the s t a b i l i t y of the hydride formed, and the presence of a s u i t a b l e base to s t a b i l i z e the released proton. (b) homolytic s p l i t t i n g , which involves an oxidation number increase of 1: 2ML +Ho 5^ 2ML ,H+2L n z ^ n-1 (c) dihydride formation, which involves an oxidation number increase of 2: ML +H2 ML H 2 n z n z In cases (b) and .(c), r e a c t i v i t y w i l l also be dependent on the ease of metal oxidation. A l l the hydride intermediates are thought to con-t a i n hydrogen as an anionic ligand. The l a b i l i t y and thermodynamic s t a b i l i t y of these hydrides help to determine the c a t a l y t i c a c t i v i t y of the metal complex. As mentioned i n s e c t i o n 1.1, the Rhodium (III) chloride - catalyzed hydrogen reduction or i r o n ( I I I ) 1 i n aqueous acid s o l u t i o n i s thought to involve h e t e r o l y t i c s p l i t t i n g ; a s i m i l a r mechanism has been invoked f o r a c t i v a t i o n of hydrogen by Rh(III) species i n a p r o t i c polar s o l v e n t s 2 0 . Rh(I) complexes activa t e H 2 through dihydride f o r m a t i o n 2 1 . 1.3 'Rh(i) - and Rh(III) - catalyzed polymerization reactions of o l e f i n s and acetylenes Cramer 2 2 has i n v e s t i g a t e d i n d e t a i l the mechanism of the rhodium (III) chloride - catalyzed ethylene dimerization i n ethanolic HC1 s o l u t i o n . I n i t i a l l y , when ethylene i s only slowly absorbed, the active anion [ R h ^ C l 2 ( C 2 H t t ) 2 ] " i s thought to be formed. This anion can be formed d i r e c t l y i f [ R h ^ C l ( C 2 H [ + ) 2 ] 2 i s d i s s o l v e d i n ethanolic HC1 , no induction period i s then observed i n the dimerization r e a c t i o n . Cramer has postulated the following four-step mechanism. The b i s ethylene dichlororhodium ( I ) anion A i s r a p i d l y converted by r e a c t i o n with HC1 i n t o an e t h y l ethylene rhodium (III) complex, B: C H 2 C H 3 ( 1 0 ) CI CH 2 ( S ) i s a m o l e c u l e o f w a t e r , e t h a n o l , o r c h l o r i d e . B_ t h e n d e c o m p o s e s b y a s l o w r e a r r a n g e m e n t t o a n n - b u t y l r h o d i u m ( I I I ) c o m p l e x , £ : ( S ) C H 2 C H 2 C H 2 C H 3 (11) T h r o u g h l o s s o f HC1 a n d ( S ) , C d e c o m p o s e s r a p i d l y t o g i v e a 1 - b u t e n e C H C H 2 C H 3 r h o d i u m ( I ) c h l o r i d e c o m p l e x , D_: C I -HC1 (S) C I ( 1 2 ) D F i n a l l y , 1 - b u t e n e a n d a m o l e c u l e o f (S) i n £ a r e r a p i d l y d i s p l a c e d b y e t h y l e n e , a n d t h i s r e p r o d u c e s t h e i n i t i a l b i s e t h y l e n e d i c h l o r o r h o d i u m ( I ) a n i o n A . D 2 C H 2 C H 2 ^ A + C H 3 C H 2 C H = C H 2 + ( S ) ( 1 3 ) T h e a c t u a l d i m e r i z a t i o n p r o c e s s i n t h e s e c o n d s t e p o c c u r s as a r a t e d e t e r m i n i n g i n s e r t i o n r e a c t i o n o f t h e c o o r d i n a t e d e t h y l e n e b e t w e e n t h e C H 2 C H 3 l i g a n d a n d t h e r h o d i u m c e n t e r . A l l t h e r h o d i u m c o m p l e x i n t e r -m e d i a t e s seem t o b e p r e s e n t as a n i o n s . S i n g e r a n d W i l k i n s o n 2 3 s h o w e d t h a t b i s ( t r i p h e n y l p h o s p h i n e ) c h l o r o r h o d i u m ( I ) i n n o n p o l a r s o l v e n t s w i l l d i m e r i z e m o n o s u b s t i t u t e d a - h y d r o x y a c e t y l e n e s : 2 R 2 C ( 0 H ) C = C H -v R 2 C ( 0 H ) C H = C H - C = C - C ( 0 H ) R 2 I n c o n t r a s t , o t h e r m e t a l c o m p l e x e s u s u a l l y r e s u l t e i t h e r i n t r i m e r i z a t i o n to aromatic compounds, or polymerization of the acetylene. The mechanism has been construed as follows. The a-hydroxy acetylene reacts with the solvated species RhCl(PPh3) 2(S) to give a square planar Rh(I) species with trans PPh 3 groups, A. i Ph3P C + - Rh I y R 2 C - 0 H <S> P P h 3 ,P C I p h 3 p HC III C I R 2 C - 0 H Rh 1 * C I (14) PPhc Then oxidative c i s addition of a second acetylene, by edge displacement of the c h l o r i d e , gives complex B_. CR III C A + RCEECH H C CI (15) P P h , I R 9C H V B The a c e t y l i d e group t r a n s f e r from the metal to a carbon atom of the l a s t -coordinated acetylene, v i a a four center t r a n s i t i o n state C_, leads to the penta coordinate species D i n which the dimer i s bound by a a bond: B ;CR HC I ^ ' I I I _ ^Rh H P P h : R 9 C C I I l i -H Ph 3P I C I R2C0H ( 1 6 ) P P h , L a s t l y , a hydride t r a n s f e r v i a a three-center t r a n s i t i o n state produces the dimer and regenerates RhCl(PPh 3)2(S) again. H H C S ) I I D + R C H C - C = C - C R 2 (17) OH The hydroxy group i s necessary f o r dimerization, and t h i s may be to f u l f i l the s t a b i l i z a t i o n of complex B_ through acetylene coordination. Very recent reports of the Rh(I) - catalyzed polymerizations o f o l e f i n s include the production of pentamers of allene and c y c l i c allene using [ R h C l ( C 2 H 4 ) 2 ] 2 2 t f > a n d the d i - and t r i m e r i z a t i o n of norbornadiene by triphenylphosphine complexes of rhodium (I) c h l o r i d e 2 5 . 1.4 Rh(I)- and Rh(III) - catalyzed isomerizations of o l e f i n s Harrod and C h a l k 2 5 found that o l e f i n double bond migration i s catalyzed by rhodium (III) c h l o r i d e . This was i n t e r p r e t e d as a r e v e r s i b l e addition of a hydridometal complex to the o l e f i n : Along with t h i s isomerization have been detected exchanges of the metal-and carbon hydrogens from deuterated metal complexes. From the v a r i e t y o f conditions used, and the degree of s t e r e o s p e c i f i c i t y produced, these authors concluded that t h i s r e a c t i o n i s s e n s i t i v e to the metal, coordinated ligands, and the source of hydride which o r i g i n a t e s from the o l e f i n i t s e l f or from added co-catalysts such as alcohols. RhCl 3 was considered by Rinehart and L a s k y 2 7 to involve a i r - a l l y l i n t e r -mediate. In general terms, a s i n g l e migration may be represented as RCH = C H - C H 3 ^ RCH2-CH-CH3 ^±RCH 2-CH=CH 2 (18) M MH The isomerization of 1,3 to 1,5 cyclo-octadiene catalyzed by RCH=CH-CH3 M CH R d f ^ ^ C H 2 MH RCH2-CH=CH2 M (19) In the cyclooctadiene isomerization, the proposal i s w r i t t e n 2 7 5 Rh or Rh H (20) Rh A t h i r d mechanism i n v o l v i n g carbene intermediates has been postulated by D a v i e s 2 8 f o r some Pd(II) - catalyzed isomerizations: RCH2CH=CH2 1 R C H 0 C - C H 3 II Pd RCH=CH-CH3 (21) Pd  Pd Cramer 2 9 has shown that l i n e a r butenes are isomerized by soluble rhodium c a t a l y s t s obtained by a fast anaerobic reaction of [ R h C l ( C 2 H i t ) 2 ] 2 or Rh(acac)(C 2 H ^ ) 2 with HC1. These i n a c t i v e Rh(I) complexes are converted in t o a c a t a l y t i c a l l y active equilibrium mixture containing Rh(III): ( S ) Rh iCl 2(S) [RhCl(C 2H^) 2]2 CH 2 = CH 2 ' + (S) / R h I I I c i 3 ( S ) 2 A+HC1 v CH 2CH 3 -(S) (22) (23) The o v e r a l l r e a c t i o n may be written s i m i l a r to that f o r the ethylene dimerization given on pages 4 and 5. (a) S t a r t i n g with the product from reaction of a square planar Rh(I) com-plex and the o l e f i n , the Rh(I) i s oxidized by HC1 to Rh(III); f o r s i m p l i c i t y a l l three ligands on Rh*L 3 ( o l e f i n ) are written as chl o r i d e s : RCH2CH=CH2 + HC1 ^  RCH2CH=CH2 4- T ir in R h C l 3 Rh C I 4 H (24) (b) A rearranges v i a an a l k y l intermediate B: A ^ + C 1 ^ R C H 2 C H C H 3 V " C 1 ^ RCH=CHCH3 - n I i n +n • • i n L A Rh C l 5 Rh 1 Cli+H (25) B C (c) C_ i s reduced to a Rh(I) complex through loss of HC1: C ==i RCH=CHCH3 +HC1 D (d) S o l v o l y s i s of D breaks o f f the isomerized coordinated o l e f i n D + (S) ^  RCH=CHCH3+^*013(S) (27) An extensive review of o l e f i n isomerization generally covering the l i t e r a t u r e up to 1966 has been given by O r c h i n 3 0 . 1.5 C a t a l y t i c reactions i n General James 1 2 i n h i s extensive review on rhodium chemistry concludes that the Rh(III) - catalyzed hydrogenations, polymerizations, and isomerizations of o l e f i n s involve some or a l l of the following steps: (a) the r e v e r s i b l e oxidation o f Rh(I) by a proton to give a Rh(III) hydride (b) the l a b i l i t y of hydrogen i n Rh(III) a l k y l - and o l e f i n complexes (c) the importance of appropriate a u x i l i a r y ligands i n the c a t a l y t i c e ffectiveness of rhodium (d) the i n s e r t i o n of a coordinated o l e f i n between an a l k y l group or hydride, and the metal ion to which i t i s attached. Point (d) exemplifies an addition r e a c t i o n of t r a n s i t i o n metal ions i n general that produces an intermediate ir-complex, which subsequently rearranges to add the elements of the o r i g i n a l complex across the o l e f i n ' s double bond: M-R +R2C=CR2+R2C:|CR2+R2(r- 9R2->MCR2-CR3 (28) M-R * ' M R 10 R = H, a l k y l , growing chain. This type of mechanism i s an example of the wide class of i n s e r t i o n reactions and has been suggested f o r numerous c a t a l y t i c reactions, including hydrogenation, hydration, hydroformylation, isomerization, and p o l y m e r i z a t i o n 1 3 - 1 5 > 1 7 > 2 0 > 3 1 - 3 3 . 1.6 Reactions of Ethylene with Rh(III), Ru(II) and Pd(II) Rhodium (III) chloroaquo complexes i n aqueous s o l u t i o n , RhCl n(H 20)^ ^,were studied by Wolsey, Reynolds and Kl e i n b e r g 2 and character-ized spectrophotometrically. Neutral aqueous solutions of R h C l 3 ' 3 H 2 0 at ambient temperatures consist of monomeric and polymeric s p e c i e s 3 4 - 3 6 . E q u i l i b r a t i o n at room temperature f o r several weeks or at 80° f o r several days produces a mixture of RhCl 3(H 20)3 and [ R h C l 2 ( H 2 0 ) 4 ] * s p e c i e s 9 . It was found, however, that i f ethylene i s bubbled through a neutral aqueous so l u t i o n o f R h C l 3 . 3 H . 2 O at room temperature and one atmosphere, the r e a c t i o n rate due to e q u i l i b r a t i o n i s a u t o c a t a l y t i c giving a rapid production of mainly [RhCl 2 (H 20 ) i j + within f i v e minutes 9. The autoc a t a l y s i s was shown not to be due to any aquochloro species of Rh(III), and i t was suggested that hydroxy species were aiding the production of small amounts of Rh(I) species through a mechanism u s u a l l y postulated f o r the ethylene reduction of metal i o n s 3 7 * 3 8 . OH OH I T T T I Cn 2 _ Rh + CzH^ Rh 1| -»• Rh-CH2CH20H -> Rh +CH3CH0+H+ (29) CH2 -II CH 2 The a u t o c a t a l y t i c nature o f the s u b s t i t u t i o n r e a c t i o n y i e l d i n g [RhCl 2 (H 20) 4 ] + was explained as follows. A chloride-bridged intermediate 1 °» 3 9 » 1 + 0 i s formed by rea c t i o n of a Rh(III) species with the Rh(I) chloro species produced i n equation 29 with the bridging c h l o r i d e o r i g i n a t i n g from the s u b s t i t u t i o n i n e r t Rh(III) c h l o r i d e complex: C l - R h 1 - — C I — - R h 1 1 1 C l 2 (30) Ele c t r o n t r a n s f e r then occurs, and since Rh(I) i s l a b i l e while Rh(III) i s III I s u b s t i t u t i o n i n e r t , equation 30 could y i e l d Rh C l 2 and Rh CI species. The Rh(I) species then reacts with more Rh(III) species. Due to reaction 29 continuing, the concentration of Rh(I) species b u i l d s up, but must remain small such as < 10-6M 9 > 1 0 as none i s detected spectrophotometrically, and no metal production v i a disproportionation was observed. Also no ethylene uptake or acetaldehyde was detected. A recent report 1* 1 has ind i c a t e d that aqueous solutions of some Rh(III) complexes contain small but k i n e t i c a l l y s i g n i f i c a n t amounts of Rh (I) species. K i n e t i c studies i n s o l u t i o n of the reaction of C2H4. with metal species to form complexes are usually complicated due to subsequent decomposition of the complex. One r e a c t i o n studied that involves the production of a 1:1 u-complex without such decomposition i s that of C2R\. with chlororuthenate (II) species i n aqueous HC1 solutions as reported by James and Halpern 1* 2. The reaction showed an inverse dependence on chl o r i d e , and a z e r o - t o - f i r s t order dependence i n ethylene with decreasing ethylene concentration. The mechanism postulated involved formation of a Ru**(C^^) complex through an i n i t i a l d i s s o c i a t i o n r e a c t i o n : . R u H C l =F==^ R u H C l .+C1- (31) n k-1 R u ^ l ^ + C ^ J l 2 > R u 1 1 ^ ^ ( 3 2 ) No such r e a c t i v i t y toward ethylene was noted f o r solutions containing Ru(III) or Ru(IV). K i n e t i c studies s i m i l a r to those of the above reaction were found by James and Rempel 3 8 f o r the ethylene reduction of RhCl 3 i n dimethyl-acetamide solvent. This again suggested a two-step d i s s o c i a t i o n mechanism s i m i l a r to equations 31 and 32. These were followed by f a s t e r steps such as [ R h I I I C l n _ 1 ( C 2 H t t ) ] ( : 4 " n ) + + H 2 0 + [Rh ICl n_ 1] ( 2" n ) ++CH 3CH0+2H + (33) [ R h 1 ^ ^ ] ^ " " ^ ^ ^ -> [ R h C l n _ 1 ( C 2 H t f ) 2 ] C 2 _ n ) + (34) As i n the corresponding system f o r Rh(III) i n neutral aqueous s o l u t i o n (equation 29) the acetaldehyde production was again explained by the decomposition of a a-bonded hydroxy species. No s p e c t r a l or gas uptake data were found to i n d i c a t e a f a s t i n i t i a l e q u i l i b r a t i o n such as Rh I I ICl n+C 2H [ + :?=^ R h I I I C l n _ 1 ( C 2 H i t ) + C l " (35) as found f o r the Pd(II) system. 3 7 Such a rea c t i o n followed by a rate-determining d i s s o c i a t i o n of the complex .can give the same k i n e t i c s as those observed. The mechanism of the Pd(II) oxidation of ethylene has been studied i n d e t a i l and i s summarized i n several r e v i e w s 4 3 - 4 6 . H e n r y 3 7 found that i n aqueous HC1 s o l u t i o n the o v e r a l l r e a c t i o n i s C2Ui+ +PdCl 2 +H20 -> CH3CHO +Pd° +2HC1 (36) Gas uptake measurements i n d i c a t e d a rate law of the form _d = k [ P d c i , g - ] [ c 2 i M d t L 2 4 J [H+][C1 -] 2 U / J The Pd(II) i s present i n i t i a l l y as P d C l ^ 2 - ; a l l reactions below w i l l be written using four-coordinate Pd(II) species. The chloride dependence of (37) and a r a p i d i n i t i a l uptake suggested that the f i r s t steps were the i n i t i a l formation of a n-complex PdCla 2~+C ?Hu -^L» PdClgCoH^+Cl- (38) followed by the loss of another chloride to give an aquated complex: ' 13 PdCl 3C 2H 4-+H 20 . P d C l 2 ( H ^ C ^ + C l - • • (39) Of several subsequent reactions that have been suggested which lead to the formation of acetaldehyde, H e n r y 3 7 concluded that the inverse acid dependence r e s u l t s from i o n i z a t i o n of the coordinated water molecule PdCl 2(H 20)C 2H [ ++H 20 i L PdCl 2(0H)C 2H i + _+H 30 + (40) F i n a l l y a slow rearrangement of the n-ethylene complex to a o-complex PdCl 2(0H)C 2H I t-+H 20 P d C l 2 (H 20) (CH 2CH 20H)~ (41) i s followed by a ra p i d decomposition which y i e l d s the products PdCl 2(H 20) (CH 2CH 20H)~ - 1 ^ 4 . Pd°+2C1-+H 30 ++CH 3CH0 (42) Weiss'*7 found that added CuCl 2 or F e C l 3 favoured the homogeneous production of CH3CH0 from and P d C l 2 , since Cu(II) and Fe(III) halides oxidize Pd metal back to Pd(II). These oxidations are more e f f e c t i v e than using molecular oxygen. However, oxygen w i l l r e a d i l y oxidize Cu(I) or copper metal i n ac i d chloride s o l u t i o n s , and the following sequence below has been used f o r the qua n t i t a t i v e oxidation of ethylene by oxygen to acetaldehyde: C 2H[ t+PdCl 2+H 20 CH 3CHO+Pd°+HCl \ (43) Pd°+2CuCl 2 + PdCl 2+2Cu° (44) 2Cu+2HCl+J202 •> 2CuCl 2+H 20 (45) C 2H 1 ++%0 2 + CH3CH0 (46) CHAPTER 2. APPARATUS AND EXPERIMENTAL PROCEDURE 2.1 Constant Pressure Gas Uptake Apparatus Most of the k i n e t i c data were found by measuring gas uptake at constant pressure i n the apparatus shown i n Figure 1. The reaction f l a s k A, indented f o r increased surface area, could be cli p p e d to the metal p i s t o n and shaken by a Welsh v a r i a b l e speed e l e c t r i c motor; e f f i c i e n t s t i r r i n g ensured that the re a c t i o n was not d i f f u s i o n c o n t r o l l e d . The heating bath B of Dow Corning 550 f l u i d s i l i c o n e o i l , was thermostated with a Jumo thermo-regulator and merc-to-merc r e l a y control c i r c u i t . The o i l bath was contained by a four l i t e r beaker i n s u l a t e d i n a plywood box. Polystyrene sections were f i t t e d over the top of the o i l bath, and by e f f i c i e n t s t i r r i n g and 25-watt elongated l i g h t bulb heaters, the temperature could be maintained to within 0.05°C. The reaction f l a s k was connected by a length of pyrex glass tubing with two spring c o i l s and a tap C, to a c a p i l l a r y manometer D con-t a i n i n g a l i q u i d of n e g l i g i b l e vapour pressure. D was connected to a gas burette E, containing mercury and a tube N of known diameter. The manometer and burette were immersed i n a rectangular perspex water bath at 25° thermostated as above. The gas burette was connected by means of an Edwards high vacuum needle valve M, to a simple gas handling system with mercury manometer F, and connections reading to a Welsh Duo Seal rotary vacuum pump G and gas i n l e t Y. A Lab Chron 1402 timer was used f o r recording r e a c t i o n times. 8 ™ ijduiijUiilLaJixiUiJjaA v :i'iiiin , . J 111.1 " i1: t : , i . I I ' I , , • • FIGURE 1 . Constant Pressure Gas Uptake Appa r atus 16 2.2 Gas Uptake experimental procedure Known amounts of reactant solutions were pipetted i n t o the reaction f l a s k A ( i n figure 1) which was then f i t t e d with the s p i r a l tubing containing tap C. The f l a s k and tubing were attached at p o s i t i o n 0 and the re a c t i o n s o l u t i o n was degassed under vacuum a f t e r f r e e z i n g i n l i q u i d nitrogen. Reactant gas at a s l i g h t l y greater pressure than the solvent vapour pressure at the re a c t i o n temperature was then admitted through tap Y to the reaction v e s s e l , and then tap C was closed. The f l a s k and s p i r a l tubing were then disconnected from 0 and attached to the gas burette at H, with the f l a s k being placed i n the o i l bath B and attached to the motor driven shaker I. The remainder of the system beyond tap C was then evacuated and reactant gas was introduced up to C through tap Y to a pressure s l i g h t l y less than the desired r e a c t i o n pressure; tap C was opened and the pressure adjusted to the desired r e a c t i o n pressure through Y. A k i n e t i c run was s t a r t e d by simultaneously c l o s i n g taps K and L, and s t a r t i n g the timer and shaker. As the reaction proceeded, gas was absorbed i n t o s o l u t i o n and the o i l l e v e l i n the l e f t limb of the manometer D began to r i s e . The o i l l e v e l s were balanced to the i n i t i a l r e action pressure by admitting gas i n t o the burette through the needle valve M. This caused the mercury l e v e l i n tube N to r i s e , and the l e v e l was measured by means of a Pye cathetometer. This height change of mercury was a d i r e c t measure of the volume of gas absorbed i n ml. at 25° at the reaction pressure, and was r e a d i l y converted to moles per l i t e r of s o l u t i o n . 2.3 Reaction Product Analysis To analyze the products at the end of a gas uptake experiment, the reaction was stopped and the reaction mixture quenched i n l i q u i d nitrogen. The organic products were d i s t i l l e d o f f d i r e c t l y from the reaction f l a s k and then run through a gas chromatograph. Peak areas were, compared with standard solutions of any suspected products, acetaldehyde, ethylene g l y c o l , and a c e t i c a c i d . S i m i l a r analyses of reaction products were done using NMR. Product y i e l d s of the ethylene oxidation r e a c t i o n are discussed i n Chapter 3. S o l i d inorganic residue products were analyzed and characterized by i n f r a r e d (nujol mull). 2.4 Instrumentation V i s i b l e and u l t r a v i o l e t absorption spectra were recorded on a Perkin Elmer 202 spectrophotometer. Matched s i l i c a c e l l s of 1 mm path length were used. Infrared spectra were recorded on a Perkin Elmer 137. L i q u i d organic products were analyzed by use of a Beckman GC-2A chroma-tograph with a dinonyl phthaiate column, and by a Varian T-60 NMR spectrometer. An AEI M S 9 mass spectrometer was used. 2.5 Materials Rhodium (III) was obtained as RhCl33H 20 from Platinium Chemicals. Ethylene was obtained as C P . grade from Matheson Co. Acetaldehyde from Eastman Organic.Chemicals was d i s t i l l e d immediately before use each time. Inorganic s a l t s used were FeCl 36H 20 AR grade from Fisher S c i e n t i f i c , Fe(C10 l t) 3 6H20 from A l f a Inorganics, L i C l and LiClOi* 3H 20 AR grades from 18 A l l i e d Chemical Co. Because of the hygroscopic nature of the f e r r i c c h l o r i d e , f e r r i c solutions were standardized by reduction to the ferrous state using a Jones Reductor, and then oxidized back to the f e r r i c state using standard dichromate s o l u t i o n s . A s o l u t i o n made up by weighing out 0.20 M FeCl 3-6H 20 was analyzed to be 0.19 M. CHAPTER 3. RESULTS 3.1 Stoichiometry and Products ^s,_ When excess FeCl 3-6H 20 was' added to a s o l u t i o n of 0.1M RhCl 3-3H 20 i n 3M HC1 and then exposed to ethylene at one atmosphere and 80o, ethylene was absorbed. Solutions with d i f f e r i n g c h l o r i d e - and hydrogen ion concen-t r a t i o n s were used and the reac t i o n rate exhibited inverse c h l o r i d e - and hydrogen ion dependences. For k i n e t i c convenience, a s o l u t i o n of 2.0M L i C l and 0.1M HC1 was used for the i n i t i a l d e t a i l e d s t u d i e s . The manner of ethylene absorption i n t o s o l u t i o n i s shown i n Figure 2. Such uptakes 8 7 6 . 5 4 sc CM 3 U i — i CM O 2 t~H 1 0 0 1 2 3 4 5 6 7 8 Time x 10" 3, s. FIGURE 2. Uptake p l o t of ethylene absorption. Rh(III) = 0.1M, Fe(III) = 0.19M, Temperature = 80°, solvent = 2M L i C l , and 0.1M HC1, C ^ = 1 atm. were l i n e a r up to points which corresponded to the consumption of ethylene and i r o n (III) i n the mole r a t i o range 1:3.7+0.5 as shown i n Table 1. At the l i m i t i n g value of the l i n e a r uptake, where rhodium metal s t a r t e d to appear, the p l o t i n Figure 2 showed a sharp upward break. This w i l l be c a l l e d the end of the reaction because at t h i s point, the spectrum of the reaction s o l u t i o n showed a complete loss of i r o n (III) chloride as seen by the loss of absorption at 422 nm,". (See Figure 3.) The r e s u l t i n g TABLE 1. Stoichiometrics of Ethylene : Iron (III) at several. [Rh(III)]. Ethylene = 1 atm, solvent = 0.1M HC1, 2M L i C l , Temperature = 80° 10h moles C 2H 4 a 10 4 moles F e C l 3 10 4 moles RhCl 3 2.2 7.6 4.0. 1.0 3.8 2.0 ; • 1 . 0 3.8 2.0 1.1 3.8 2.0 1.2 3.8 1.0 0.9 3.8 1.0 1.1 3.8 2.0 a. A l l uptake experiments y i e l d l i n e a r ethylene dependences up to the point where rhodium i s p r e c i p i t a t e d as the metal. The ethylene stoichiometry corresponds to the l i m i t of the l i n e a r uptake. to 350 400 450 500 550 wavelength, nm FIGURE 3. V i s i b l e spectra of r e a c t i o n s o l u t i o n . Temperature = 80°, solvent - 2M L i C l and 0.1M HClj , 0.1M RhCl 3; , 0.1M RhCl 3 + 0.19M F e C l 3 ; , 0.1M RhCl 3 + FeC.l 3 at end of rea c t i o n with ethylene, corresponding to 6000 s on Figure 2. i r o n (II) has p r a c t i c a l l y no absorbance between 350 and 550 nm as was shown by the spectrum of a 0.19M s o l u t i o n of FeCl 2'4H 20 i n the same solvent. With a t o t a l of 2.1M c h l o r i d e , the predominant rhodium species i s . R h C l 5 ( H 2 0 ) 2 ~ 2 . The v i s i b l e spectrum of Rh(III) remained e s s e n t i a l l y unchanged over the l i n e a r region (see Figure 3) and corresponded to that of the pentachloro species. At a c i d i t i e s of more than 0.1M at t h i s chloride concentration, reproducible l i n e a r uptakes were found. At lower a c i d i t i e s , HCIO^ rather than HC1 was used and uptake p l o t s were not as reproducible. At the end of the r e a c t i o n , when a l l the 0.19M ir o n (III) was consumed, the organic products which were separated by d i s t i l l a t i o n to 205° and analyzed by gas chromatography and NMR, consisted only of water and acetaldehyde. Comparisons with standard solutions showed no presence of a c e t i c a c i d , ethylene g l y c o l , or paraldehyde. The d i s t i l l a t e from a reaction using 0.1M D C 1 i n D 20 which consumed 0.05M C2H.4 y i e l d e d by NMR a doublet . at 8.1 T. Further additions to the NMR tube of each of the l i q u i d s above showed that only CH 3 C H 0 was present. Comparison with standard solutions of C H 3 C H O i n 0.1M D C 1 (in D 20) showed the rea c t i o n s o l u t i o n to contain 0.043M C H 3 C H O . The inorganic residue a f t e r d i s t i l l a t i o n was shown by IR to con-t a i n no carbonyls. As was pointed out e a r l i e r , the end of the rea c t i o n was s i g n i f i e d by the loss of Fe(III) chlorides although the C 2 H i + uptake corresponded to reduction at only about one h a l f of the i r o n (III) since C 2 H i + i s a two-equivalent reducing agent. To check that the i r o n (III) had been converted to i r o n (II) and not to some other i r o n (III) species that had no absorption at 422 nm, the i r o n (II) produced was estimated as follows. A f t e r an ethylene uptake experiment was done using 0.19M F e ( I I I ) , the C H 3 C H O was removed by bubbling nitrogen through the s o l u t i o n . Then by t i t r a t i n g p o t e n t i o m e t r i c a l l y with K 2Cr 207, the concentration of Fe(II) was determined to be 0.18M. This stoichiometry problem w i l l be discussed l a t e r . 3.2 Iron (III) Dependence As a n t i c i p a t e d from the l i n e a r uptake p l o t s , no dependence of the rate on [Fe(III)] was found (See Table 2). This i n i t i a l F.e(III) concen-TABLE 2. Summary of K i n e t i c Data at 80° and .1 atm C 2H 4. Solvent = 0.1M HC1, 2M L i C l [FeCl 3] [RhCl 3] 10 6v, Ms-1 a 0.0 ; 0.10 36 s- 1 b 0.095 0.10 12 C 0.19 0 . 0 . 0.0 0.19 0.0015 0.4 0.19 0.01 0.7 0.19 0.03 3.3 0.19 0.05 4.5 0.19 0.10 8.2 d 0.38 0.10 8.1 0.19 0.15 13.6 0.38 0.175 14.9 a. v i s the l i n e a r rate of ethylene uptake. b. Ethylene i s consumed i n a f i r s t order rate. c. Rhodium metal p r e c i p i t a t e s early i n uptake. d. Using 0.1M DC1/D20, v = 7.9 x 10~6Ms~.*. t r a t i o n was always kept i n excess, u s u a l l y double the Rh(III) concentration. In the absence of F e ( I I I ) , or i f Fe(III) was not present i n excess, rhodium metal, a heterogeneous c a t a l y s t was slowly produced. When free rhodium metal was added to a reac t i n g s o l u t i o n , an increase i n the l i n e a r rate was noticed. 23 3.3 Rhodium (III) Dependence • A s e r i e s of ethylene uptake experiments at various Rh(III) con-centrations showed that the r e a c t i o n i s f i r s t order i n Rh(III) (see Table 2 and Figure 4). [Rh(III)] FIGURE 4. Dependence of the l i n e a r rate on [Rh(III)], using C2H 4 at 1 atm, 80° 2M L i C l , 0.1M HC1, and 0.19M F e C l 3 . For Rh(III) = 0.175M, FeCl3 = 0.38M. v i s the l i n e a r rate of ethylene consumption. 3.4 Ethylene Dependence The k i n e t i c dependence on ethylene was found by i n v e s t i g a t i n g several experiments at d i f f e r e n t ethylene pressures each at 75, 80, and 85°. The data and c a l c u l a t i o n s f o r these uptakes are given i n Table 3. A p l o t of l i n e a r rates versus c a l c u l a t e d ethylene molarity i s given i n Figure 5. 3.5 Proton and Chloride Dependences In an e f f o r t to e s t a b l i s h the dependences of the rate on [H +] and on [ C l ~ ] , ethylene uptakes were studied at several acid and chl o r i d e con-centrations. The r e s u l t s are summarized i n Table 4. A p l o t of rate versus [ H + ] _ 1 at constant [Cl~] was found to be reasonably l i n e a r as shown TABLE 3 . Summary o f E t h y l e n e D e p e n d e n c e s . R h ( I I I ) = 0 . 1 M , F e ( I I I ) = 0 . 1 9 M , L i C l = 2 M , HC1 = 0 . 1 M , T e m p e r a t u r e s = 7 5 , 8 0 , 8 5 ° 3mm. P 1 ' ,mm. a 1 0 6 v , M s - x b 1 0 t t [ C 2 H t + ] C 330 35 2 . 2 0 . 7 6 410 115 3 . 6 2 . 5 1 5 2 5 2 3 0 4 . 6 5 . 0 2 6 4 5 350 5 . 1 7 . 6 5 760 4 6 5 7 . 5 1 0 . 1 5 8 5 0 5 5 5 8 . 5 1 2 . 1 0 3 8 0 20 2 . 9 0 . 4 0 4 1 0 50 3 . 3 1 . 0 5 3 0 170 5 . 1 - : 3 . 4 6 5 0 2 9 0 6 . 8 5 . 8 760 4 0 0 8 . 1 8 . 0 8 5 0 4 9 0 1 0 . 0 9 . 8 9 5 0 5 9 0 1 0 . 5 1 1 . 8 4 7 5 35 3 . 3 0 . 6 4 5 4 0 100 4 . 6 1 . 8 1 6 0 5 165 6 . 5 3 . 0 760 3 2 0 9 . 1 5 . 8 8 5 0 4 1 0 1 1 . 1 7 . 4 a . The p a r t i a l p r e s s u r e s o f C 2 H i + a r e b a s e d o n t h e v a p o u r p r e s s u r e s o f t h e 0 . 1 M HC1 s o l u t i o n b e i n g 2 9 5 , 3 6 0 , a n d 440 mm a t 7 5 , 8 0 , a n d 8 5 ° , f r o m r e f e r e n c e 4 8 . b . T h e s e a r e n o t o b s e r v e d r a t e s b u t t r u e r a t e s , a n d e q u a l t o ( o b s e r v e d r a t e ) x ( t o t a l p r e s s u r e , P,mm)/(760 mm). c . T h e s o l u b i l i t y o f C 2 H i + i s a n e x t r a p o l a t e d v a l u e f r o m r e f e r e n c e 4 9 . T A B L E 4 . Summary o f P r o t o n a n d C h l o r i d e D e p e n d e n c e s , a t 8 0 ° , 1 a t m C 2 H 4 , Q . 1 9 M F e C l 3 , 0 . 1 M R h C l 3 • [HC1] [ H C 1 0 4 ] . [ L i C l ] [ L i C l O i J [ T o t a l C l " ] a 1 0 6 v , M s " 6 . 0 6 . 0 0 . 0 3 . 0 -• . • 3 . 0 0 . 5 7 0 . 1 0 . 1 1 0 . 8 0 . 0 5 2 . 0 2 . 0 5 1 0 . 2 0 . 1 0 . 5 0 . 6 9 . 3 0 . 1 2 . 0 2 . 1 8 . 2 0.1 2 . 0 2 . 0 1 2 . 8 0 . 1 2 . 9 3 . 0 6 . 3 0 . 1 3 . 0 3 . 1 5 . 8 0 . 2 2 . 0 2 . 2 5 . 3 , 0 . 2 2 . 8 3 . 0 3 . 4 . 0 . 5 c. o T rs 1 . 1 ' 1 . 0 2 . 0 3 . 0 0 . 9 1.0 2 . 0 2 . 0 6 . 4 0 . 1 1 . 0 1 . 0 1 . 1 9 . 7 0 . 1 1 . 0 2 . 0 1 . 1 9 . 3 0 . 1 2 . 0 1 . 0 2 . 1 7 - 1 o . i 3 . 0 0 . 0 3 . 1 5 . 8 0 . 0 5 0 . 1 0 . 1 5 4 . 4 b 0 . 1 0 . 1 0 . 2 0 3 . 0 b 0 . 2 0 . 1 0 . 1 1 . 6 5 b 2 2 . 0 1 6 . 0 a . T o t a l c h l o r i d e c o n c e n t r a t i o n e x c l u d e s c h l o r i d e f r o m 0 . 1 M RI1CI3 a n d f r o m 0 . 1 9 M F e C l 3 . U s i n g t h e e q u i l i b r i u m c o n s t a n t s f o r f e r r i c c h l o r i d e c o m p l e x e s i n 1M H C 1 0 4 , K j = 4 . 2 , K 2 = 1 . 3 , K 3 = 0 . 0 4 ( r e f e r e n c e 5 0 ) , i t c a n b e s h o w n t h a t 0 . 1 9 M F e C l 3 i n s o l u t i o n y i e l d s 0 . 2 4 M f r e e c h l o r i d e . b . 0 . 2 M F e ( C 1 0 4 ) 3 u s e d r a t h e r t h a n 0 . 1 9 M F e C l 3 . 12 10 i tn 2 > ID o 0 8 5 ° / / 8 0 ° / ^LT 7 5 ° y y o 0 10 12 2 4 6 8 FIGURE 5 . D e p e n d e n c e o f t h e l i n e a r r a t e o n [ C 2 H , . ] , u s i n g 0 . 1 M R h C l 3 , 2M L i C l , 0 . 1 H C 1 , 0 . 1 9 M F e C l 3 a t 7 5 , 8 0 ^ a n d 8 5 ° i IA > ID o FIGURE 6 . D e p e n d e n c e o f t h e l i n e a r r a t e o n [H ] a s H C 1 , u s i n g 0 . 1 M R h C l 3 , 0 . 1 9 M F e C l 3 , 1 atra C 2H4, 8 0 ° . T o t a l [ C I " ] u s i n g L i C l ^ 3 M . i n F i g u r e 6 a n d 7 . P l o t s o f ( r a t e ) " 1 v e r s u s [ H + ] , a n d o f ( r a t e ) - 1 v . [ H + ] ~ w e r e n o t l i n e a r . G r a p h s o f r a t e v e r s u s [ C I " ] " 1 , o f ( r a t e ) " 1 v e r s u s [ C l ~ j , a n d o f ( r a t e ) " 1 v e r s u s [ C I ] " 1 w e r e p l o t t e d f o r a c o n s t a n t [ H + ] o f 0 . 1 M , a n d 0 I 1 1 1 1 1 0 5 10 15 2 0 [ H + ] - l FIGURE 7 . D e p e n d e n c e o f t h e l i n e a r r a t e o n [ H + ] a s H C 1 , u s i n g 0 . 1 M R h C l 3 , 0 . 1 9 M F e C l 3 , 1 a t m C2Hi+, 8 0 ° . T o t a l [ C I - ] u s i n g L i C l ^= 2 M . • ' ' w r-t I s 0 0 1 1 1 1 1 1 \ 1 1 L - i 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 [ C I ] " 1 0 1 2 3 4 [ C I ] FIGURE 8. D e p e n d e n c e o f t h e l i n e a r r a t e o n [ C l ~ ] a s 0 . 1 M H C l + L i C l , u s i n g 0 . 1 M R h C l 3 , 0 . 1 9 M F e C l 3 , 1 a t m C2Uk, 8 0 ° . O, r a t e v e r s u s [ C I ] " 1 ; A r a t e - 1 v e r s u s f C l ] ; • , r a t e - 1 v e r s u s [ C I ] - 1 a p p e a r i n F i g u r e 8 . T h e s e do n o t t a k e i n t o a c c o u n t c h a n g e s i n c o n s t a n t i o n i c s t r e n g t h ( a d j u s t e d w i t h L i C l O ^ ) b e c a u s e r a t e s w i t h o r w i t h o u t a d d e d L i C l O ^ a r e q u i t e s i m i l a r ( s e e T a b l e 4 ) . A d d i t i o n o f L i C l O ^ u p t o 2 . 0 M h a d l i t t l e e f f e c t o n t h e r a t e o f r e a c t i o n c a r r i e d o u t a t 0 . 1 M H + a n d 1 . 1 M C I " , i n d i -c a t i n g t h a t v a r i a t i o n s i n i o n i c s t r e n g t h w e r e u n i m p o r t a n t . 3 . 6 A c t i v i t y o f R h o d i u m ( I I I ) A q u o c h l o r o S p e c i e s T h e r e s u l t s p r e s e n t e d s o f a r i n F i g u r e s 4 - 8 r e f e r t o s o l u t i o n s c o n t a i n i n g m a i n l y t h e R h C l 5 ( H 2 0 ) 2 - s p e c i e s ; t h e a c t i v i t y o f o t h e r a q u o -c h l o r o s p e c i e s was t h e n s t u d i e d . A s e r i e s o f t h e s e c o m p l e x e s i n s o l u t i o n was made b y v a r y i n g c h l o r i d e a s HC1 a n d a d j u s t i n g t h e t o t a l a c i d i t y t o 6M 2 ( w i t h HCIO^ . A l i s t o f t h e r e a c t i o n r a t e s f o r t h e s p e c i e s [ R h C l n ( H 2 0 ) ^ ] L i s g i v e n i n T a b l e 5 . T h e l a r g e d e v i a t i o n i n t h e r a t e s u s i n g R h C l 5 ( H 2 0 ) 2 ~ a n d R h C l i i ( H 2 0 ) 2 ~ i s d u e t o t h e f o r m o f t h e e t h y l e n e u p t a k e p l o t a s s h o w n i n F i g u r e 9 . T h i s w i l l b e d i s c u s s e d i n C h a p t e r 4 . T h e s e s p e c i e s w e r e a l s o FIGURE 9 . U p t a k e p l o t o f e t h y l e n e a b s o r p t i o n . R h ( I I I ) a s R h C l 4 ~ = 0 . 0 7 2 5 M , F e ( C 1 0 4 ) 3 = 0 . 0 9 5 M , H + = 6M a s 0 . 1 7 M HC1 + 5 . 8 3 M H C I O ^ , . 8 0 ° , C 2 H 4 = 1 a t m . TABLE 5 . R a t e s o f E t h y l e n e A b s o r p t i o n a t 1 a t m , 8 0 ° , H + = 6M a n d 0 . 1 M . A . [ C I ] c o n t r o l l e d b y [ H C I O 4 + H C l ] = 6M M a i n S p e c i e s [ R h ( I I I ) ] [ H C l ] [ F e ( I I I ) ] 1 0 6 v , M s - 1 R h C l g 3 - 0 . 1 6 0 . 1 9 F e C l 3 0 R h C l 5 ( H 2 0 ) 2 - 0 . 0 7 2 5 2 0 . 1 F e ( C 1 0 i t ) 3 2 . 4 + 1 . 1 R h C l 4 ( H 2 0 ) 2 " 0 . 0 7 2 5 0 . 1 7 0 . 1 F e ( C 1 0 i t ) 3 7 . 2 + 1 . 6 R h C l 3 ( H 2 0 ) 3 0 . 0 7 2 5 0 . 0 3 5 0 . 1 F e C C 1 0 4 ) 3 0 . 8 R h C l 3 ( H 2 0 ) 3 0 . 0 7 2 5 0 . 0 3 5 O . l F e ( 0 1 0 4 ) 3 0 . 9 - • - 0 . 0 3 5 0 . 1 F e ( C 1 0 4 ) 3 0 . 8 R h C l 2 ( H 2 0 ) l ) + 0 . 0 7 2 5 0 . 0 2 0 . 1 F e ( C 1 0 4 ) 3 0 . 8 5 R h C l ( H 2 0 ) 5 2 + 0 . 0 7 2 5 0 . 0 1 O . l F e ( C 1 0 4 ) 3 0 . 8 R h ( H 2 0 ) s 3 + .. 0 . 0 7 2 5 0 . 0 0 0 • - 3 o.s B . [ C I " ] c o n t r o l l e d b y L i C l . H C I O 4 = 0 . 1 M M a i n S p e c i e s [ R h C l 3 ] [ L i C l ] [ F e ( I I I ) ] 1 0 6 v , M s ~ ; 1 R h C l 5 ( H 2 0 ) 2 - 0 . 1 2 0 . 1 F e ( C 1 0 4 ) 3 1 2 . 8 R h C l 4 ( H 2 0 ) 2 ~ 0 . 1 0 . 2 0 . 1 F e ( C 1 0 4 ) 3 4 2 . 5 f o u n d t o b e t h e m o s t a c t i v e c a t a l y s t s ; t h e e t h y l e n e d e p e n d e n c e s a t v a r y i n g a c i d i t i e s w e r e s t u d i e d f o r t h e s e s y s t e m s . T h e d a t a a r e s u m m a r i z e d i n T a b l e s 6 a n d 7 . P l o t s o f r a t e v e r s u s e t h y l e n e m o l a r i t y f o r d i f f e r e n t a c i d i t i e s a l l a t 8 0 ° a p p e a r i n F i g u r e s 10 a n d 1 1 . TABLE 6 . Summary o f P r o t o n a n d E t h y l e n e D e p e n d e n c e w i t h 0 . 1 M R h ( I I I ) a s R h C l 5 ( H 2 0 ) 2 - ; 8 0 ° , 0 . 1 9 M F e C l 3 , 2M L i C l [HC1] R , m m p i a C2Hi + ,mm l O ^ M s -1 10 4 [ C 2 H l t ] b 0 . 0 5 4 1 0 48 5 . 4 0 . 9 6 0 . 0 5 5 6 0 198 7 . 3 3 . 9 6 0 . 0 5 7 6 0 3 9 8 1 0 . 2 7 . 9 6 0 . 1 0 3 8 0 20 2 . 9 0 . 4 0 . 1 0 4 1 0 5 0 3 . 4 1 . 0 0 . 1 0 5 3 0 170 5 . 1 3 . 4 0 . 1 0 6 5 0 2 9 0 6 . 7 5 . 8 0 . 1 0 7 6 0 4 0 0 8 . 2 8 . 0 0 . 1 0 8 5 0 4 9 0 1 0 . 0 9 . 8 0 . 1 0 9 5 0 5 9 0 1 0 . 5 1 1 . 8 0 . 2 0 4 1 0 5 3 1 . 4 1 . 0 6 0 . 2 0 5 6 0 2 0 3 3 . 3 4 . 0 6 0 . 2 0 7 6 0 4 0 3 5 . 3 8 . 0 6 a . T h e p a r t i a l p r e s s u r e s o f C 2 H 4 a r e b a s e d o n t h e v a p o u r p r e s s u r e s o f t h e 0 . 0 5 , 0 . 1 0 , a n d 0 . 2 0 M HC1 s o l u t i o n s b e i n g 3 6 2 , 3 6 0 , a n d 3 5 7 mm a t 8 0 ° . b . T h e s o l u b i l i t y o f C 2 H 4 a t 8 0 ° i s a n e x t r a p o l a t e d v a l u e f r o m r e f e r e n c e 49 . 0 I i i i i i i 0 2 4 6 8 10 12 -. . lonczH,,] FIGURE 10 D e p e n d e n c e o f t h e l i n e a r r a t e o n [ C 2 H i J a t t h r e e a c i d i t i e s a s H C 1 , u s i n g 0 . 1 M R h ( I I I ) a s R h C l 5 2 _ , 0 . 1 9 M F e C l 3 , 2M L i C l , 8 0 ° 5 0 4 0 - 30 - x / ^ / i/> r - H 10 n r i i i . . . 0 2 4 6 8 10 1 0 1 + [ C 2 H l f ] FIGURE 11 D e p e n d e n c e o f t h e l i n e a r r a t e o n [ C 2 H 1 J a t t w o a c i d i t i e s a s H C 1 0 4 , u s i n g 0 . 1 M R h ( I I I ) a s R h C l ^ - , 0 . 2 M F e C C l O ^ ) 0 . 2 M L i C l , 8 0 ° TABLE 7 . Summary o f P r o t o n a n d E t h y l e n e D e p e n d e n c e s w i t h 0 . 1 M R h ( I I I ) a s R h C l 4 ( H 2 0 ) 2 " ; 8 0 ° , 0 . 2 M F e ( C 1 0 1 + ) 3 , 0 . 2 M L i C l , [ H C 1 0 J P,mm H ^ a l O ^ M s " 1 10 - [ C ^ ] b ' C 0.1 410 50 7 1.0 0.1 560 200 23 4.0 0.1 760 400 43 8.0 0.2 410 53 4.2 1.06 0.2 560 203 15 4.06 0.2 760 403 23 8.06 a , b . S e e T a b l e 6. c . A s s u m i n g t h a t t h e s o l u b i l i t y o f e t h y l e n e i s t h e same i n HCIO4 a s i n H C l f o r a g i v e n s e t o f c o n d i t i o n s , a n d t h a t t h e t w o m e d i a h a v e s i m i l a r v a p o u r p r e s s u r e s . 3 . 7 O t h e r O x i d i z i n g A g e n t s S e v e r a l o t h e r o x i d a n t s b e s i d e s i r o n ( I I I ) w e r e i n v e s t i g a t e d f o r t h e r h o d i u m ( I I I ) - c a t a l y z e d o x i d a t i o n o f e t h y l e n e . L i n e a r r a t e s w e r e o b s e r v e d f o r r e a c t i o n s u s i n g t h e s e o x i d a n t s a n d a r e g i v e n i n T a b l e 8, a l o n g w i t h i r o n ( I I I ) f o r c o m p a r i s o n . T h e r a t e s f o r t h e r e a c t i o n s w i t h C u ( I I ) a n d C r ( I V ) w e r e i n d e p e n d e n t o f o x i d a n t c o n c e n t r a t i o n o v e r t h e r a n g e 0.2 - 0.4M. TABLE 8 . Summary o f r a t e s w i t h v a r i o u s o x i d i z i n g a g e n t s , w i t h 0 . 1 M H C l , 2M L i C l , 0 . 1 M R h C l 3 , 8 0 ° , 1 a t m CzHh. o x i d i z i n g a g e n t p 1 ) T m 1 0 6 r a t e , M s " 1 0 . 1 9 M FeCl3«6H20 400 8.2 100 3.4 0.2 M euCl2*2H20 400 1 4 « 3 0.2 M K 2Cr 20 7 400 9 - 7 3 0 0 mm. 0 2 100 .1.2' CHAPTER 4 . D I S C U S S I O N 4 . 1 A c t i v i t y o f t h e P e n t a c h l o r o S p e c i e s T h e r a t e v e r s u s ^ H ^ ] p l o t i n F i g u r e 5 s h o w s a n i n t e r c e p t a t I^H^] = 0 s o t h e r e i s a c o n t r i b u t i o n t o t h e o v e r a l l r a t e , w h i c h i s 2 -i n d e p e n d e n t o f ^ 2 ^ 4 ' S i n c e t h e s t a r t i n g c a t a l y t i c s p e c i e s i s R h C l ^ , we h a v e i m m e d i a t e l y t h e e x p r e s s i o n r a t e = - d j C ^ ] = k j R h C l , . 2 " ] + ^ [ R h C ^ 2 " ] [ C ^ ] ( 4 7 ) d t w h e r e k a a n d k^ a r e p s e u d o f i r s t a n d s e c o n d o r d e r r a t e c o n s t a n t s c o n t a i n i n g a l l d e p e n d e n c e s e x c e p t t h o s e o f r h o d i u m a n d e t h y l e n e . k v a l u e s u s e d s u b s e q u e n t l y a r e f o r 8 0 ° u n l e s s o t h e r w i s e s t a t e d . I n T a b l e 5 , t h e o n l y o t h e r a c t i v e c a t a l y t i c r h o d i u m c h l o r o -2 -s p e c i e s b e s i d e s R h C l ^ seems t o b e R h C l ^ , s o we m i g h t a s s u m e t h a t t h e 2 -C 9 H ^ i n d e p e n d e n t s t e p i n v o l v e s a d i s s o c i a t i o n f r o m R h C l j . t o R h C l ^ . V.'e now h a v e f o r t h e t w o r a t e - d e t e r m i n i n g s t e p s 2 - k a -R h C l 5 — - — > R h C l 4 + CI ( 4 8 ) ? V R h C l 5 + C 2 H 4 — - — > p r o d u c t s ( 4 9 ) U s i n g F i g u r e 1 0 , f o r [ H + ] = 0 . 1 a n d [ R h ( I I I ) ] = 0 . 1 , t h e i n t e r c e p t g i v e s k = 2 . 8 x 10 5 s a n d t h e s l o p e g i v e s k, = 6 . 7 x 10 2 M _ 1 s T h e R h C l ^ m u s t t h e n r e a c t w i t h ^2^4 * n a f a s t e r s t e p t h a n t h e d i s s o c i a t i o n g o v e r n e d b y k . A l s o , t h e r h o d i u m s p e c i e s f r o m t h e p r o d u c t s m u s t f i n a l l y 2 -b e r e g e n e r a t e d a s R h C l j . a f t e r t h e f a s t e r s t e p s t o e x p l a i n t h e o b s e r v e d l i n e a r r a t e s i n ( 4 7 ) . 2 -E x p e r i m e n t s u s i n g R h C l j - a s a r e a c t a n t show t h e f o l l o w i n g . ( a ) t h e C^H^ i n d e p e n d e n t r a t e i s a c i d d e p e n d e n t b e c a u s e t h e i n t e r -c e p t s i n F i g u r e 10 a r e n o t e q u a l . A n i n v e r s e a c i d d e p e n d e n c e i s o b s e r v e d , a n d a p l o t o f t h e s e i n t e r c e p t s v e r s u s [ H + ] * g o e s e s s e n t i a l l y t h r o u g h t h e 34 o r i g i n a s s h o w n i n F i g u r e 12 b e l o w . T h i s s u g g e s t s t h a t b y f a r t h e s o o i - H V) I 4-> !/> CX, S CO O CJ i - l •> f-l ^ CO • O C - H H - r i [1, 10 15 20 FIGURE 1 2 . D e p e n d e n c e o f t h e C^H^ i n d e p e n d e n t r a t e o n [ H + ] a s H C l , u s i n g 0 . 1 M R h C l g 2 " , 0 . 1 9 M F e C l 3 , 2M L i C l , 8 0 ° . m a j o r c o n t r i b u t i o n t o k i n ( 4 8 ) i s o n e i n v o l v i n g h y d r o x y s p e c i e s : a R h C l 5 ( 0 H ) 3 " + H 2 0 — - > R h C l 4 ( 0 H ) ( H 2 0 ) 2 - + C I ( 5 0 ) B u t we c a n n o t c o m p l e t e l y e x c l u d e t h e p o s s i b i l i t y t h a t a s m a l l e r c o n t r i b u t i o n i n v o l v i n g p u r e l y a q u o s p e c i e s a l s o o c c u r s : k. R h C l 5 ( H 2 0 ) 2 " + H o 0 — — -> R h C l 4 ( H 2 0 ) 2 + C I ( 5 1 ) T h i s a c i d i n d e p e n d e n t s t e p r e q u i r e s t h a t F i g u r e 12 s h o w s a p o s i t i v e i n t e r -c e p t o n t h e o r d i n a t e a x i s . ( b ) T h e r a t e v e r s u s C 2 H 4 p l o t s i n F i g u r e 10 i n d i c a t e t h a t t h e C 2 H ^ d e p e n d e n t r a t e i s e s s e n t i a l l y a c i d i n d e p e n d e n t b e c a u s e t h e s l o p e s a t . t h e d i f f e r e n t a c i d i t i e s a r e p r a c t i c a l l y e q u a l . A p l o t o f t h e s l o p e s o f F i g u r e 7 10 - t h a t i s , t h e c o n t r i b u t i o n b y t h e C 2 H ^ d e p e n d e n t s t e p t o t h e o v e r a l l r e a c t i o n i n ( 4 7 ) - v e r s u s [ H + ] - 1 i s s h o w n i n F i g u r e 1 3 ; t h e i n t e r c e p t ^ - . 5 . 9 x s •. o us r - l <H I o V ) n CO > PH to o • O r-H O H » H 0 10 15 20 [HT 1 FIGURE 1 3 . D e p e n d e n c e o f t h e C 2 H ^ d e p e n d e n t r a t e o n [H ] a s H C l , u s i n g 0 . 1 M R h C l 5 2 " , 0 . 1 9 M F e C l 3 , 2M L i C l , 8 0 ° . - 3 - 1 - 5 - 1 10 s a n d t h e s l o p e v a l u e d 9 x 10 Ms . T h u s m a i n l y a q u o s p e c i e s a r e i n v o l v e d b u t t h e r e a p p e a r s t o b e a s m a l l c o n t r i b u t i o n f r o m h y d r o x y s p e c i e s : R h C l 5 ( H 2 0 ) 2 " + C 2 H 4 — - * p r o d u c t s (521 3 - k 4 R h C l 5 ( 0 H ) + C 2 H 4 -—> p r o d u c t s ( 5 3 ) E q u a t i o n ( 4 7 ) c a n now b e e x p a n d e d t o i n c l u d e t h e a c i d d e p e n d e n c e . r a t e = { k j I C 5 + k 2 > [ R h C l 5 ( H 2 0 ) 2 _ ] + { k 3 + k 4 K a 5 } ' [ R h C l 5 ( H 2 0 ) 2 " ] [ C ^ ] ( 5 4 ) [ H + ] [ H + ] w h e r e K 5 = [ R h C l , . ( O H ) 3 " ] [ H +]/[Rhci_ ( H _ 0 ) 2 " ] . U s i n g t h e d a t a a t [ H + ] = 0 . 1 a n d [ R f i C l g ( H 2 0 ) 2 " ] = 0 . 1 , F i g u r e 10 g i v e s r a t e = 2 . 8 x 1 0 - 6 + 6 . 7 x l O " 3 ^ ^ ] M s " 1 ( 5 5 ) F o r e x a m p l e , when [ C 2 H 4 ] = 8 x 10 ^ ( f o r 1 a t m ) , t h e r a t e i s c a l c u l a t e d t o b e 8 . 2 x 10 Sis 1 w h i c h i s c o n s i s t e n t w i t h t h e same v a l u e g i v e n i n T a b l e 6 . E q u a t i o n ( 4 7 ) c a n a l s o b e e x p a n d e d a s t h e p l o t o f r a t e v e r s u s [ H + ] 1 s h o w n i n F i g u r e 7 . r a t e = ' { k ^ k ^ C ^ ] } [ R h C l 5 Ob/)) 2 " ] + ( k ^ ^ [ C ^ ] } K a 5 [ R h C l 5 ( H 2 0 ) 2 " ] ( 5 6 ) [ H + ] W i t h [ R h C l 5 ( H 2 0 ) 2 " ] = 0 . 1 a n d [ C 2 H 4 ] = 8 x 10 ^, t h e r e l a t i o n s h i p b e c o m e s ' r a t e = 4 . 5 x 1 0 - 6 + 3 . 1 x 1 0 " 7 M s " 1 [ H + ] T h e r e a c t i o n r a t e i n v o l v i n g r h o d i u m a q u o s p e c i e s i s g i v e n b y t h e v a l u e 4 . 5 x 10 Sis 1 i n ( 5 7 ) . F r o m ( 5 6 ) , t h i s r a t e r e s u l t s f r o m t h e k 2 a n d k^  t e r m s , n a m e l y 4 . 5 x l O - 6 = { k 2 + k 3 [ C ^ ] } [ R h C l 5 ( H 2 0 2 ~ ] ( 5 8 ) F i g u r e 12 i n d i c a t e s t h a t t h e k 2 p a t h i s n o t i m p o r t a n t . T h e r e f o r e , 36 n e g l e c t i n g k 2 i n ( 5 8 ) g i v e s k 3 = 5 . 6 'x 10 2 M 1 s 1 . T h e i n t e r c e p t i n F i g u r e 13 g i v e s a d i r e c t m e a s u r e m e n t o f k 3 a s 5 . 9 x 10 2 M J s l . T h e e t h y l e n e d e p e n d e n t c o n t r i b u t i o n g i v e n b y ( 5 4 ) a n d ( 5 5 ) b e w r i t t e n a s 6 . 7 x 1 0 " 3 = ( k 3 + k 4 K 5 } [ R h C l 5 ( H 2 0 ) 2 " ] ( 5 9 ) 1 . 1 x 1 0 " 3 s _ 1 . T h e c o n t r i b u t i o n t o t h e e t h y l e n e d e p e n d e n t r a t e f r o m h y d r o x y s p e c i e s i s g i v e n b y ( 5 6 ) a n d ( 5 7 ) : 3 . 1 x l 0 - 7 = { k 1 + k 1 + [ C 2 H t t ] } K [ R h C l 5 ( H 2 0 2 " ] ( 6 0 ) U s i n g t h e k. K 5 v a l u e a b o v e , ( 6 0 ) g i v e s k x K 5 = 2 . 2 x 1 0 ~ 6 M s " 1 . L a s t l y , t h e e t h y l e n e i n d e p e n d e n t r a t e u s i n g ( 5 4 ) a n d ( 5 5 ) c a n b e w r i t t e n a s v a l u e s o f k± a n d k^ a r e o f t h e - o r d e r o f 2 . 2 x 1 0 2 s 1 a n d 1 . 1 x 1 0 5 M 1 s 1 . • I n a n y c a s e t h i s i n d i c a t e s t h a t i n t h e e t h y l e n e i n d e p e n d e n t s t e p s , t h e h y d r o x y s p e c i e s a r e r o u g h l y 1 0 8 m o r e r e a c t i v e t h a n a q u o s p e c i e s . T h e r e a c t i o n s i n v o l v e d a r e s h o w n i n ( 5 0 ) a n d ( 5 1 ) , a n d t h e c o n c l u s i o n i s t h a t t h e h y d r o x y c o m p l e x R h C l5 ( O H ) 3 u n d e r g o e s a q u a t i o n w i t h t h e l o s s o f a c h l o r i d e some 1 0 8 t i m e s f a s t e r t h a n t h e a q u o c o m p l e x R h C l 5 ( H 2 0 ) 2 . T h i s f i n d i n g i s p r o b a b l y q u i t e s i g n i f i c a n t . T h e s t r o n g l a b i l -i z i n g e f f e c t o f t h e h y d r o x y g r o u p i n o c t a h e d r a l c o o r d i n a t i o n c h e m i s t r y i s 51 w e l l k n o w n . I n t h e w e l l - s t u d i e d c o b a l t ( I I I ) s y s t e m b e l o w , t h e r a t e 2 . 8 x l 0 " 6 = { k x K 5 + k 2 } [ R h C l 5 ( H 2 0 ) 2 " ] ( 6 1 ) C o ( e n ) 2 L C l + + H 2 0 > C o ( e n ) 2 L ( H 2 0 ) 2 + + C l (62) - 4 constant f o r L = cis-OH i s some 10 times that for the r e a c t i o n with L = c i s H^O^. This has been r a t i o n a l i z e d i n terms of the strong ir-donor a b i l i t y of -OH to s t a b i l i z e a t r i g o n a l bipyramidal t r a n s i t i o n state v i a the i n i t i a l d i s s o c i a t i o n of the c h l o r i d e i n (62) — that i s , an S ^ l mechanism. Cobalt (III) s u b s t i t u t i o n reactions generally occur through a d i s s o c i a t i v e mechanism. Corresponding rhodium (III) systems have been l i t t l e studied, but the data have generally been interpreted i n 52 53 terms o f more S^2 character although the evidence i s not strong ' . The most r a t i o n a l explanation f o r the OH e f f e c t i s that the mechanism of (5.0) follows that of some Cobalt (III) systems and goes through an i n i t i a l S^l d i s s o c i a t i o n of a c h l o r i d e ligand. 54 Robb and Steyn. have determined the value of d i r e c t l y and give a value of 8 x 10 ^s * at 30° i n p e r c h l o r i c - h y d r o c h l o r i c a c i d media of constant i o n i c strength y = 4.00. Considering the completely d i f f e r e n t nature of the studies and the uncertainty of the C.J-1^  s o l u b i l i t y i n the present work, the agreement i s reasonable. The present k 2 value could i n f a c t be subject to considerable error since i t s estimation i n (61) depends on the d i f f e r e n c e between two quite s i m i l a r numbers 2,8 x 10 ^ and 2.2 x 10 ^. The former i s the intercept of Figure 10 which should be reasonably c o r r e c t ; the l a t t e r i s estimated through equations (58) to (60) with some uncertainty. The data also shows that C^H^ reacts 10^-10^ times more r a p i d l y 3- 2-with RhCl 5(0H) than with RhCl 5(H 20) . The same hydroxy species has 8 6 8 been shown to be 10 - 10 times as r e a c t i v e as the aquo species towards, carbon monoxide. The second order r a t e constant f o r r e a c t i o n (63) has been determined as 0.2 M~*s _ 1 at 80° i n 3M HCl which i s comparable to 2-R h C l 5 ( H 0 ) + CO RhCl 4(C0)( H o)" + C 1" ( 6 3 ) •1 - 1 t h e v a l u e o f 0 . 0 5 6 M s f o r k^ d e t e r m i n e d i n t h i s w o r k f o r t h e c o r r e s p o n d i n g e t h y l e n e r e a c t i o n ( 5 2 ) . M e a s u r e m e n t s o v e r t h e r a n g e 7 5 - 8 5 ° y i e l d e d t h e t e m p e r a t u r e d e p e n d e n c e o f t h e r a t e c o n s t a n t s f o r t h e t w o - t e r m l a w i n ( 4 7 ) a n d ( 5 4 ) f r o m t h e s l o p e a n d i n t e r c e p t s o f F i g u r e 5 . CSee T a b l e 9 . ) TABLE 9 . T e m p e r a t u r e d e p e n d e n c e o f r a t e c o n s t a n t s k a n d k, f o r t h e r e a c t i o n o f 0 . 1 M R h C l , . 2 - w i t h CJi.. F e C l _ = 0 . 1 9 M , RC1 = 0 . 1 M , L i C l = 2M. T ° C 1 0 5 k , s _ 1 a 1 02 k b , M " 1 s " 1 75 80 85 2 . 4 2 . 8 3 . 2 5 . 6 6 . 7 1 1 . 2 T h e k v a l u e s c a n n o t b e d e t e r m i n e d w i t h a n y a c c u r a c y b u t t h e d a t a s u g g e s t c l t h a t t h e y v a r y l i t t l e w i t h t e m p e r a t u r e . T h e t e m p e r a t u r e d e p e n d e n c e o f k w i l l b e m a i n l y g o v e r n e d b y v a r i a t i o n i n k i n ( 5 4 ) , s u g g e s t i n g a l o w a c t i v a t i o n e n e r g y f o r ( 5 0 ) w h i c h c o u l d b e c o n s i s t e n t w i t h t h e m e c h a n i s m d i s c u s s e d a b o v e . J 3 O t>0 o 2 . 7 9 FIGURE 1 4 . A r r h e n i u s p l o t f o r t h e r e a c t i o n o f 0 . 1 M R h C l 2 -w i t h C 2 H 4 . F e C l 3 = 0 . 1 9 M , HC1 = 0 . 1 M , L i C l = 2M. 39 F r o m t h e A r r h e n i u s p l o t i n F i g u r e 1 4 , t h e e n t h a l p i e s o f a c t i v a t i o n a r e AH = 5 . 8 k c a l . , a n d AH, = 1 6 . 9 k c a l . A s s u m i n g t h a t v a r i a t i o n o f k a t ) c w i t h t e m p e r a t u r e i s m o s t l y d u e t o v a r i a t i o n i n k ^ , a n d k^ t o k ^ , t h e e n t r o p i e s a r e A S ^ — - 3 2 e . u . , a n d A s ^ J ^ - 1 7 e . u . 4 . 2 A c t i v i t y o f t h e T e t r a c h l o r o S p e c i e s . E x p e r i m e n t s u s i n g R h C l ^ a s a r e a c t a n t show t h e f o l l o w i n g : ( a ) T h e r e i s no a p p r e c i a b l e £^^4 i n d e p e n d e n t r a t e a s s h o w n b y t h e a b s e n c e o f a n i n t e r c e p t i n F i g u r e 1 1 . T h a t i s , a s t r i c t l y f i r s t o r d e r d e p e n d e n c e o n C^H^ i s o b s e r v e d . A n y p o s s i b l e s m a l l i n t e r c e p t w o u l d 2 -l i k e l y b e d u e t o t h e p r e s e n c e o f s m a l l a m o u n t s o f R h C l ^ i n t h e r e a c t a n t . T h e o n l y r e a c t i o n o f R h C l ^ t h a t n e e d b e c o n s i d e r e d i s o n e i n v o l v i n g C 2 H 4 . ( b ) T h e s l o p e s o f t h e r a t e v e r s u s C^i^ p l o t s i n F i g u r e 11 a t t h e t w o a c i d i t i e s show t h a t t h e r a t e i s c o n s i d e r a b l y a c i d d e p e n d e n t . + - 1 F i g u r e 15 s h o w s t h e s l o p e s o f F i g u r e 11 v e r s u s [H ] . O n l y t w o s e t s o f d a t a e w> O - H m 4-1 • - H O I to 10 > p, CM O • O r-H i-< • 1—I I/) 1—I 10 [HY 1 FIGURE 1 5 . D e p e n d e n c e o f t h e C ~ H . d e p e n d e n t r a t e o n [H ] a s H C I O ^ , u s i n g 0 . 1 M R h C l 4 , 0 . 2 M F e C C l O ^ , 0 . 2 M L i C l , 8 0 ° . - 2 - 1 w e r e m e a s u r e d ; t h e i n t e r c e p t v a l u e d 1 . 8 x 10 s . a n d t h e s l o p e v a l u e 4:4 x 1 0 " 3 M s _ 1 , T h u s a s b e f o r e we c o n s i d e r r e a c t i o n s o f C _ H . w h i c h 2 4 i n v o l v e b o t h a q u o a n d h y d r o x y s p e c i e s : k 5 R h C l 4 ( H 2 0 ) + C 2 H 4 — - > p r o d u c t s ( 6 4 ) R h C l 4 ( 0 H ) ( H 2 0 ) 2 " + C 2 H 4 & — » p r o d u c t s ( 6 5 ) T h e r a t e l a w f o r t h e r e a c t i o n o f R h C l 4 s p e c i e s w i t h e t h y l e n e i s o f t h e f o r m r a t e = { k c [ R h C l 4 ( H 2 0 ) 2 " ] + k d [ R h C l 4 ( O H ) ( H 2 0 ) 2 " ] } [ C 2 H 4 ] ( 6 6 ) w h e r e k a n d k , a r e p s e u d o f i r s t a n d s e c o n d o r d e r r a t e c o n s t a n t s , c d r E q u a t i o n ( 6 6 ) c a n b e w r i t t e n a s r a t e , . = { k 5 + k 6 K a 4 > [ R h C l 4 ( H 2 0 ) 2 ] [ C 2 H 4 ] ( 6 7 ) w h e r e K & 4 = [ R h C l 4 (OH) ( H 2 0 ) 2 ~ ] [ H + ] / [ R h C l 4 ( H 2 0 ) ~ ] . E q u a t i o n ( 6 7 ) i s a n a l y s e d d i r e c t l y b y u s i n g t h e i n t e r c e p t a n d s l o p e v a l u e s o b t a i n e d a b o v e f r o m F i g u r e 1 5 : 1 . 8 x 1 0 _ 2 s " 1 = k 5 [ R h C l 4 ( H 2 0 ) 2 ] ( 6 8 ) g i v e s k^. = 1 . 8 x 10 * M * s a n d 4 x l O ^ M s " 1 = k 6 K a 4 [ R h C l 4 ( H 2 0 ) 2 " ] ( 6 9 ) g i v e s k , K 4 = 4 x 10 2 s * . E q u a t i o n ( 6 7 ) i s a l s o a n a l y z e d b y u s i n g 0 3 t h e d a t a a t [ H + ] = 0 . 1 a n d [ R h C l 4 ( H 2 0 ) 2 ~ ] = 0 . 1 w i t h t h e s l o p e o f F i g u r e 11 r a t e = 5 . 7 x 1 0 _ 2 [ C 2 H 4 ] M s " 1 ( 7 0 ) T h e d a t a a r e i n t e r n a l l y c o n s i s t e n t s i n c e ( 6 8 ) a n d ( 6 9 ) g i v e a s u m m a t i o n o f 5 . 8 x 1 0 " W 1 f o r ' { k 5 + k 6 K a 4 / [ H + ] > u s i n g [ H + ] = 0 . 1 a n d [ R h C l 4 ( H 2 0 ) 2 ~ ] = 0 . 1 , a n d ( 6 7 ) a n d ( 7 0 ) g i v e 5 . 7 x l O ' ^ ' s " 1 f o r t h e same q u a n t i t y . T h e a c i d i t y o f R h C l 4 ( H 2 0 ) 2 ~ w i l l b e somewhat h i g h e r t h a n t h a t o f R h C l r ( H _ 0 ) 2 ~ a n d K 4 i s l i k e l y t o b e a b o u t 1 0 _ 6 M . T h u s k ' w i l l b e o f t h e 5 2 a 6 4 - 1 - 1 o r d e r o f 4 x 10 M s A n a l y s e s o f t h e r a t e d a t a a t 8 0 ° i n S e c t i o n s 4 . 1 a n d 4 . 2 a r e s u m m a r i z e d i n t h e f o l l o w i n g S c h e m e : 41 products + C 2 H 4 RhCl 5(H 20) -CI RhCl l t(H 20 ) 2 2 K5 products +C 2 Hij R h C l 5 ( O H ) 3 " + H + -CI RhCli>(OH) (H 20) 2"+H + +C 2 H i | products +C2H4 products kiK =2.2x10 6 M s _ 1 1 a k 2 = 6x10" 6s - 1 k 3 =5.6xl0" 2M" 1s" 1 khK 5 = l . l x l 0 " 3 s _ 1  H a k 5 =1.8xlO~ 1M" 1s" 1 k 6K l +=4xl0" 2s" 1  b a K 5=10"8M,K 1+=10"6M, a 'a ' kl =2.2xl0 2s" 1 kh = l . l x l 0 5 M _ 1 s _ 1 kfi =4xl0 4M" 1s" 1 It now seems c l e a r that rate paths showing no dependence 2-on concentration are indeed p o s s i b l e f o r the RhCl,- species since k 5 [ C 2 H 4 ] > k 2 and k ^ - f C ^ ] >> k 1K &. That i s , as mentioned on page 33 the RhCl^ must react with C 2H^ i n a f a s t e r step than the d i s s o c i a t i o n s governed by k = k.K-*V[H+] +kn. (71) 4.3 Inverse Chloride Dependence The inverse c h l o r i d e dependence which was seen i n Table 4 but which di d not analyze f o r any of the simple dependences shown i n Figure 8, could a r i s e i n a number of p o s s i b l e ways. Here are three. (a) I f the composition of the rhodium chloro species changes i n s o l u t i o n over the c h l o r i d e concentration range studies (0.6 - 3.1 M) then the dependence could a r i s e from the d i f f e r i n g r e a c t i v i t i e s of the species. The a c t i v i t y of the complexes decreases i n the order - 2- 3-RhCl 4 > RhCl 5 > Rh C l & , and t h i s could q u a l i t a t i v e l y explain the dependence. The v i s i b l e spectra of the i n i t i a l solutions are a l l 2-e s s e n t i a l l y that of the RhCl,- species but contributions from quite small 3-amounts of RhCl^ at lower c h l o r i d e and RhCl^ at higher c h l o r i d e could be s i g n i f i c a n t . 42 (b) Notable changes i n gas s o l u b i l i t y i n chloride-perchlorate 3 8 media have been observed f o r and CO over s i m i l a r concentration ranges ' and changes i n ethylene s o l u b i l i t y with the change i n media here could be important although the data i n Table 4 i n d i c a t e that a d d i t i o n of LiClO. has l i t t l e e f f e c t on the measured r a t e . 4 (c) It i s p o s s i b l e that the c h l o r i d e dependence a r i s e s from a genuine k i n e t i c dependence f o r r e a c t i v i t y i n v o l v i n g the pentachloro species. As was seen, the r e a c t i v i t y a r i s e s mainly from reactions (50) and (52) with a smaller c o n t r i b u t i o n from (53). An inverse c h l o r i d e dependence could a r i s e i f the back-reaction of (50) becomes s i g n i f i c a n t . Reaction (50) would have to be written as: RhCl 5(OH) 3"+H 20 — - ^ RhCl 4(0H)(H 20) 2"+C1" (72) N k - l The c o n t r i b u t i o n to the rate i n (54) from (65) then becomes k k [RhCl (0H) 3"][C H J 2 £_i_ (73) k_x[Cr] + k 6 [ C 2 H 4 ] This c o n t r i b u t i o n however was independent of [C 2H 4] at 2.1 M c h l o r i d e which means that k 6 [ C 2 H 4 ] » k [ C l ~ ] . Thus at lower [Cl~] no e f f e c t on the r a t e would be observed and indeed the ra t e changes very l i t t l e from 0.6 to 2.1 M c h l o r i d e (Table 4).. I t i s p o s s i b l e that at higher [CI ] the k j l C l ] term w i l l become more s i g n i f i c a n t and an inverse dependence observed. But i t must be remembered that at much higher [CI ] , 3-the i n a c t i v e RhCl^ may be present. In any case the o v e r a l l r e a c t i o n r a t e , neglecting k 2 terms from (51),is given by r a t e = k ^ j R h C l ^ O H ) 3 " ] [ C ^ ] + k 3[RhCl 5(PL/)) 2~] [ C ^ ] + k j R h C l ^ O H ) 3 " ] [ C ^ ] k . J C l " ] + k 6 [ C 2 H 4 ] (74) 43 The co n t r i b u t i o n due to the l a s t two terms at [H +] = 0.1 i s known to be 5.3 x 10 Sis Thus a c h l o r i d e dependence would be given by k (rate - 5.3 x 1 0 - 6 ) " 1 = ~ 1 t C 1 J =- + - = — (75) k 1k 6[RhCl 5(OH) ] [ C 2 H 4 ] k^RhCl^OH) ] and a pl o t of (rate - 5.3 x 10 ^) * versus [CI ] should be l i n e a r . However the data do not analyze f o r a l i n e a r p l o t , i n d i c a t i n g that the back rea c t i o n given by k ^ i s not s i g n i f i c a n t . The independence of the rate ' given by (73) on [C 2H 4] a t 2.1M CI does suggest that an'increase to 3.0M CI would have very l i t t l e e f f e c t on the rate. The chloride dependence then i s not resolved, but contributions from the t e t r a - and hexachloro complexes are suspected. 4.4 The Production of Acetaldehyde The k i n e t i c data show that both aquo and hydroxy complexes of rhodium (III) react with ethylene. For the ethylene independent path f o r the pentachloro species, the r e a c t i o n sequence may be written as follows (neglecting the c o n t r i b u t i o n from (51)) : ' ' • « K — RhCl c(H_0) — ^ RhCl c(OH) + H (76) o k RhCl 5(0H) 3" + H 20 — - — > RhCl 4(0H)(H 20) 2"+C1~ (50) ? k RhCl 4(OH)(H 20) + C 2H 4 f a g ° > products (65) The ethylene dependent path involves (52) and (53) followed by f a s t e r -decomposition to products: R h C l 5 ( H 2 0 ) 2 " + C 2H 4 k 3 > products (52) 3- 4 RhCl 5(OH)° + C 2H 4 > products (53) 44 37 38 On analogy with the oxidation of ethylene by other metal ions ' , (52) may be written i n more d e t a i l as follows. This was discussed on page 10 R h C l 5 ( H 2 0 ) 2 " + C 2H 4 — > Cl 4Rh(H 20) " H > Cl 4Rh(0H) 2" > t t C 2 H 4 C 2 H 4 — Cl 4Rh-CH 2CH 2OH > C H ^ C H ^ H — — > RhCl 4 3~+CH 3CHO (77) >RhCl. i . - 4 Reaction (53) w i l l produce the hydroxy intermediate A d i r e c t l y . Migration of the hydroxy ligand r e s u l t s i n formation of the a - bonded Rh(III) hydroxy ethyl intermediate J 3 . Decomposition of J3 can not involve a carbonium ion formed with the production of Rh(I) because no ethylene g l y c o l i s formed by . 37 re a c t i o n with the solvent . Rather, the rhodium center leaving with two more electrons a s s i s t s a 1,2 hydrogen s h i f t which d i r e c t l y y i e l d s 37 acetaldehyde and a proton . Since t h i s redox step w i l l be much f a s t e r than e a r l i e r steps, t h i s proton l i b e r a t e d w i l l not be be accounted f o r i n the inverse acid dependence. In Scheme (77), a coordinated CI must be l o s t on r e a c t i o n with C 2H 4 since a coordinated H 20 i s necessary f o r subsequent r e a c t i o n to products. Such a r e a c t i o n mechanism also r e a d i l y accounts f o r the complete 3-i n a c t i v i t y of the hexachloro species RhCl^ , and gives convincing evidence that a coordinated H 20 or OH i s necessary f o r the formation of the 43 hydroxy ethyl intermediate. It has been postulated.by.some authors that attack by a non-coordinated hydroxide i s involved. Reactions (64) and (65) w i l l follow a very s i m i l a r path to that outlined i n (77) although i t i s not immediately obvious whether C 2H 4 w i l l replace a coordinated H 20 or CI . For example, + c r 2- 6 2 2-RhCl.(OH)(H-0) + C H , > Cl.Rh(OH) or Cl,RhfOH)(H_0) (78) 4 2 2 4 4^ 3^ • 2 C 2 H 4 C 2 H 4 • An important conclusion i n these studies i s that solutions containing RhCl^O^O^ species are considerably more r e a c t i v e towards 2-C^H^ than are solutions containing RhCl^fH^O) at 0.1M a c i d i t y and 1 atm t o t a l pressure. This i n i t i a l l y seemed unusual i n that the series of aquochloro complexes becomes more l a b i l e with increasing number of coordin-3 ated chlorides . The major c o n t r i b u t i o n to the rate f o r t e t r a c h l o r o 2-system i s (65) i n v o l v i n g RhCl^(OH)(H^O) and C f o r which the data 4 -2-1 show k^K =• 4 x 10 s : the major co n t r i b u t i o n to the rate for the 6 a 2-pentachloro system i s (52) i n v o l v i n g RhCl^O^O) and C 2H 4 ^ o r w n i - c n - 2 - 1 - 1 k. = 5,6 y 1 fl • M ^ - Thus a l t h o u g h hvrlroxv snec.ies are present i n verv small amounts i n both systems at 0.1M .'acidity, more k i n e t i c a l l y s i g n i f i c a n t amounts are present i n the t e t r a c h l o r o system and t h i s i s reasonable since 4 5 K- w i l l be > K . The rates of r e a c t i o n between C„H. and the hydroxy a a 2 4 ' ' 4 - 2 - 1 5 species are governed by the values krK = 4 x 10 s and k.K = i ^ 1 6 a 4 a -3-1 + . 1.1 x 10 s (the c o n t r i b u t i o n to the r a t e i s given by kK[Rh(III)]/[H ] ) , 4 5; and at any given a c i d i t y the k,K term w i l l be > k.K . The actual r a t e . 6 ' 6 a 4 a constants are probably quite s i m i l a r and i n f a c t the present data suggest 3_ that k^ i s somewhat larger than k ^ — that i s , RhCl^(OH) i s more r e a c t i v e than RhCl 4(0H) ( H 2 0 ) 2 " i s towards C ^ . . At very high a c i d i t i e s ( t h e o r e t i c a l l y at [H +] * = 0), the C^H^ independent step f o r the pentachloro system becomes n e g l i g i b l e (Figure 12) and the r e a c t i o n r a t e w i l l be governed purely by the rate - 2 - 1 - 1 2-constant k^ = 5.6 x 10 M s f o r the r e a c t i o n between RhCl^fH^O) and C 2 H 4 . Secondly, the rate f o r the RhCl^ system w i l l be governed purely -1 .1 by the rate constant k 5 = 1.8 x 10 s for the r e a c t i o n between RhCli t(H20)2 and C^H^. The rates would thus be quite s i m i l a r although 2 . as mentioned above one might have expected the R I 1 C I 5 aquo species to be somewhat more a c t i v e . The e n t i r e r e a c t i o n , neglecting the very small contributions from k 2 and k$ i n (51) and (64), may now be written as a c a t a l y t i c system f o r the production of acetaldehyde using rhodium (III) aquopentachloro species. H 20, H + and CI as reactants and products are not shown. (79) K 5 • ' . • ^ RhCli tCH 2CH 20H 4 ; RhCl^ ( C 2 H i t ) (OH) The rhodium (I) species i s immediately oxidized back to rhodium (III) by i r o n . ( I I I ) . Since the t o t a l c h l o r i d e concentration remains constant, the 2_ RhCl5 species are regenerated as required by the k i n e t i c and spectrophotometrie data. When a l l the i r o n (III) has been consumed, or i f no i r o n (III) on other oxidant i s present, the rhodium (I) r a p i d l y disproportionates to rhodium metal: 2 Rh(I) — - » R h ( 0 ) + Rh(II) (80) This r e a c t i o n has always been written to explain the production of rhodium 3 8 9 metal from rhodium (I) e s p e c i a l l y i n aqueous solutions ' ' but no q u a n t i t a t i v e data have been a v a i l a b l e . However, recent work 5 5 i n v o l v i n g the oxidation of ethylene by Rh(III) i n the absence of any reoxidant such as Fe(III) has shown that rhodium metal i s produced as well as a pale yellow s o l u t i o n which gives an ESR si g n a l thought to be due to a paramagnetic Rh(II) species. This disproportionation to a possible monomeric Rh(II) species i s being further studied. p -When ethylene was oxidized with RhCl 5 (again using 2M L i CI and 0.19M F e C l 3 ) i n 0.1MDC1 i n D 20, NMR measurements showed that no deuterium' was incorporated into the acetaldehyde molecule. That a l l of the hydrogen atoms of the acetaldehyde molecule have come from the ethylene, i s consistent with the hydrogen s h i f t postulated i n (77). A s i m i l a r r e s u l t has been 37 observed f o r the Pd(II) oxidation of CgH^ i n D 20 . For the present Rh(III) r e a c t i o n , the small isotope e f f e c t k^/k^ = 1.04 shown i n Table 2 i s s i g n i f i c a n t . The corresponding isotope e f f e c t measured i n the Pd(II) 37 system was 4.0; t h i s was a t t r i b u t e d to the d i f f e r e n c e i n K values i n .... a D 20 and H 20, and showed that the i o n i z a t i o n to a hydroxy species was p r i o r to the rate determining step. The isotope e f f e c t of 1.04 i n the present work ind i c a t e s that the i o n i z a t i o n i s subsequent to the rate determining" step at the conditions of measurement, and t h i s i s consistent with the 2_ i n t e r p r e t a t i o n that the reac t i o n between RhCl 5(H 20) and C ^ ^ i n (52) p r i o r to i o n i z a t i o n i s the major contribution f o r these conditions. The present study gives no data on the f a s t reactions between Rh(I) and F e ( I I I ) . Both of these species are s u b s t i t u t i o n l a b i l e , assuming that Fe(III) i s i n the high spin state. In the reac t i o n medium there are aquo, hydroxy and chloride ligands, a l l of which are s u i t a b l e 56 f o r bridging so that an inner or outer sphere r e a c t i o n i s poss i b l e . The mechanism of electron t r a n s f e r may be s i m i l a r to that suggested f o r Rh(I) Rh(III) reactions i n v o l v i n g chloro-bridged intermediates (page 11). Spectrophotometric and gas uptake measurements showed no evidence f o r a rapid i n i t i a l e q u ilibrium process to form an ethylene n-complex v i a a r e a c t i o n such as K ^ R h C l 5 ( H 2 0 ) 2 + C 2H 4 ^ RhC^ ( C 2 H 1 J (H 20) + C l " (81) 37 as was postulated i n the corresponding Pd(II) system . I f K i s very small, such a process followed by a slow decomposition o f the Tr-complex could equally well explain most of the k i n e t i c data f o r the ethylene dependent path. However, such a mechanism should give r i s e to a rate i n v e r s e l y proportional to chl o r i d e at least f o r that due to the ethylene dependent path. The chlo r i d e dependence.found argues against t h i s . Also the measured isotope e f f e c t as discussed above argues against such a pre-equilibrium. In s i m i l a r systems i n v o l v i n g reactions of rhodium chlorides with C 2 H 4 i n DMA,and with CO i n aqueous HC1, no ra p i d pre-e q u i l i b r i a were found. 4.5. Use of Oxidants Other Than F e ( I I I ) . Table 8 shows that some v a r i a t i o n i n the l i n e a r uptake rates i s observed on using d i f f e r e n t oxidants to regenerate the Rh(III) c a t a l y s t . Since no Rh metal production was observed i n these systems, the Rh(I) must be oxidized r a p i d l y back to Rh(III) and thus the measured C 2 H 4 l i n e a r uptake rates are expected to be independent of the nature of the oxidant used. The v a r i a t i o n i n rates upon using F e ( I I I ) , Cu(II) and Cr(VI) i s not great and i s probably not s i g n i f i c a n t . The r e s u l t f o r 0 2 does seem low but t h i s system should be furth e r investigated as described i n Chapter 5. Experimentally t h i s system requires the use of uptake measurements from a gas-mixture (ethylene and oxygen) but t h i s procedure i s not well established f o r the usual apparatus at present. 4.6 Ethylene Uptake Rates i n 6M Acid. Figure 9 shows that i n 0.17 M HC1 and 5.83 M H C I O 4 , l i n e a r uptake rates are not observed. An"induction period before the reduction of Fe(III) took place, and a reduction of Rh(III) to the metal, was observed.. The f a c t that rhodium metal i s produced may i n d i c a t e that Fe(III) can less e a s i l y oxidize any low-valence Rh intermediate such as Rh(I). For instance, the reduction p o t e n t i a l of the Fe(III) - Fe(II) couple might be lower i n higher acid concentration. 57 However, Connick and McVey found that the opposite i s true f o r t h i s and other couples i n HCIO4. but that the couple i s lower i n HOIO^ than i n HCl. . y .. The "induction period" does consist of a small rate of ethylene uptake and t h i s i s observed f o r a l l species i n HC10if, such as RhCl with n ^ 3 and even f o r a s o l u t i o n containing no rhodium (III) n • (Table 5). This could r e s u l t from a slow hydration r e a c t i o n of the ethylene. No such r e a c t i o n i s apparent i n 6M HCl. Thus rhodium chloroaquo species RhCl n with n - 3 are e f f e c t i v e l y i n a c t i v e as c a t a l y s t s f o r the r e a c t i o n described i n t h i s t h e s i s , and t h i s i s reasonable because these species are progressively more s u b s t i t u t i o n i n e r t that R h C l 5 2 ~ and. RhClit". In R h C l 6 3 " there i s no p o s s i b i l i t y of a coordinated H2O or OH as required f o r the subsequent i n s e r t i o n r e a c t i o n (29). 4.7 The Ethylene: Iron(III) Stoichiometry. In answer to the C^Hi^FeCIII) stoichiometry problem from section 3.1, there seemed a f t e r much d e l i b e r a t i o n to be several p o s s i b i l i t i e s . The f a c t i s that the observed C 2HL f:Fe(III) molar consumption r a t i o of roughly 1:4 i s too low i f C2.H4 i s acting as a two-equivalent reducing agent-. One answer i s that Fe(III) chlorides were being removed by some way other than reduction by C 2H t (. F i r s t , there might be a f u r t h e r reduction by the CH 3 CH0 produced i n s o l u t i o n . This would produce CH3COOH but none was found i n the reaction products;... also CH3CHO did not react with Fe(III) under the re a c t i o n conditions. The p o s s i b i l i t y that CH3CHO reduced Fe(III) i n a c a t a l y t i c system-through Rh(III) was also investigated. Indeed, under N 2 at 80°Rh(III) was reduced to metal (presumably v i a Rh(I)) by CH 3 CH0 and the p o s s i b i l i t y of (82) and (83),as , shown below, occuring seemed l i k e l y : Rh(III) + CH3CHO -> Rh(I) + CH3COOH (82) Rh(I) + 2Fe(III) — — - » Rh(III) + 2Fe(II) (83) However, as mentioned e a r l i e r , no CH3COOH was detected and also a re a c t i o n of CH3CHO with the usual mixture of Rh(III) and Fe(III) gave no changes whatsoever. That i s , neither Rh(III) nor Fe(III) was consumed. It i s not at a l l c l e a r why the presence of Fe(III) should i n h i b i t a rea c t i o n such as (82). . Second, i t was considered that Fe(III) chlorides might be removed h v r o m n 1 R - y - i no- w i t h othc 1" 1 i or.cmrl<; r ) - r o r 1 i i r e r l r h j - H n a the r f i a r t i n n , Rut a c r f l - i n no acetates were found. T h i r d , Fe(III) might have formed bridged species with Rh(III) or with i t s e l f but no evidence f o r e i t h e r of these reactions was found s p e c t r o s c o p i c a l l y . _____ No decarbonylation of the acetaldehyde by the rhodium complexes was observed. Such decarbonylation to give metal carbonyls has been observed f o r low. valent platinum metal complexes inc l u d i n g Rh(I) species-(see, for example reference 21). The wide v a r i a t i o n i n C2H4 stoichiometrics f o r several i d e n t i c a l experiments (Table 1) portends to an argument that the problem may not be a purely chemical one such as anunforeseen side r e a c t i o n , but rather a more ph y s i c a l one. The C^ H t j W a s apparently consumed i n a smaller quantity than that corresponding to the reduction of the Fe(III) c h l o r i d e s . I f some of the CH3CHO product (which corresponded to the amount of C 2 H 4 absorbed into solution) was present i n the gas phase, then the apparent o v e r a l l uptake of C 2 H 4 would be low since the k i n e t i c technique measures volume changes i n the gas phase. In s p i t e of the alle g e d i n f i n i t e s o l u b i l i t y of CH3CHO i n hot aqueous s o l u t i o n s 5 8 , the presence of C H 3 C H O vapour i s thought to explain the observed low C 2 H 4 . consumption. Some experimental data support t h i s reasoning. • A mass spectrum of the gas above s o l u t i o n at the end of an uptake experiment at 80° showed the presence of CH3CH0, and the vapour pressure of the r e a c t i o n solvent + 0.05 M CH3CH0 at room temperature was found to be roughly twice that of the reac t i o n solvent alone. (As seen i n Figure 2, the C^Hi,. consumption at the end of a r e a c t i o n i s 0.05 M instead of the expected 0.1 M). : 62 ' • L i t e r a t u r e data give a value f o r the p a r t i a l pressure p 1 of acetaldehyde at 82° as 380mm over a 0.55M aqueous acetaldehyde s o l u t i o n . A 0.05M s o l u t i o n assuming Henry's Law would have a P 1 of 35mm. The volume of the apparatus open to acetaldehyde i s about 50ml mainly at room temperature, and assuming Boyle's Law, t h i s would contain 0.9 x 10 4 moles of gaseous acetaldehyde. The amount of acetaldehyde present i n the 2ml of reactant s o l u t i o n w i l l be 1.0 x 10 14 moles. These c a l c u l a t i o n s show that f o r the present apparatus, one h a l f of the acetaldehyde produced w i l l be i n the gas phase. This causes the apparent ethylene uptake to be about one h a l f the true uptake and explains the stoichiometry problem. It should be noted also that since the measured rates are therefore one h a l f the true rates, the calculated rate constants are apparent ones and should be doubled to give the true values. CHAPTER 5. SUMMARY AND SUGGESTIONS FOR FURTHER WORK The r e a c t i o n of rhodium (III) chlorides with ethylene and i r o n (III) i n aqueous HCl at 80 d and 1 atm was shown to c a t a l y t i c a l l y produce acetaldehyde. Rhodium (I) i s a l i k e l y intermediate, rather than a rhodium (III) hydride which was suggested f o r a s i m i l a r r e a c t i o n i n v o l v i n g oxidation of hydrogen to protons 1. The k i n e t i c s of the Rh(III) r e a c t i o n with C 2 H 4 were studied and proved to be much more complex than a n t i c i p a t e d . A f i r s t order ' dependence on Rh(III) was observed but the o v e r a l l r e a c t i o n involved contribu-. tions from an ethylene independent path as well as the expected ethylene dependent one. Studies on the acid dependence of the con t r i b u t i n g paths have shown that hydroxy species although present i n very small concentrations are s i g n i f i c a n t k i n e t i c a l l y . -The reactions involved i n the rate determining steps are summarized below: K 5 R h C l 5 ( H 2 0 ) 2 " - RhCl 5(OH) 3"+H + RhCl 5(OH) 3~+H 20 k l a RhClt^OH) (H 20) 2~+C1~ RhCl 5(H 20) 2"+H 20 —^2—» RhCl ( +(H 20) 2+Cl" RhCl 5(H 20) 2"+C 2H 1 + k3 > RhCl^ C C z H i J (H20)~+C1~ k • RhCl 5(OH) 3~+C 2H 1 + — * R h C l ^ ( C ^ ) (0H)2~+C1~ R h C l ^ H ^ ) ^ . ======= RhCl^fOH) (H 20) 2"+H + RhCl ^ ( H ^ ^ C ^ ^ 5 > RhCl 4 ( C ^ ) (H 20) +H20 or RhCl 3(C 2H t t) ( H 20) 2+C1" k 6 RhCli+COH) ( H 20) 2"+C 2 H l t > RhClh ( C ^ ) (OH) 2 " + H 20 or R h C l 3 ( C 2 H i J ( O H ) ( H 20)~+Ci Values of the rate constants k 2, k3, ks and the composite constants klK 5 , k 4K 5 , kgK 4 at 80° have been determined, a a a R e l a t i v e l y f a s t subsequent decompositions o f the C 2 H 4 TT- com-plexes are assumed to occur v i a a mechanism we l l - e s t a b l i s h e d f o r the 37 corresponding Pd(II) system . For example, RhCli f(C 2H l t) (OH) 2" > RhCli^CHgCH^H) 2" o- T 3_ . RhCl 4(CH 2CH 2 0 H ) 2 . > Rh CI4 +CH3CHO+H . Rh(I) i s r a p i d l y oxidized back to Rh(III) by the Fe(III) present, and the net r e a c t i o n i s C 2H 4+H 2 0 + 2 F e(III) — R h ( H I ) — » CH 3CHO+ 2 H + + 2 F e(II) The reported r e s u l t s are considered s i g n i f i c a n t since they would suggest that more e f f i c i e n t rates of conversion of C 2H 4 to C H 3 C H O would be observed i n less acid conditions where higher concentrations of Rh(III) hydroxy complexes would be present. The oxidative a b i l i t y of 2_ Cr 207 decreases at lower a c i d i t i e s , and Fe(III) and Cu(II) r e a d i l y give hydroxy complexes which may be less e f f i c i e n t oxidants. But 0 2 i s p a r t i c u l a r l y appealing f o r use as an oxidant of Rh(I) since i t suggests that C 2 H 4 / O 2 mixtures might be oxidized d i r e c t l y to C H 3 C H O . This would give a process somewhat simpler than the Wacker Process where 0 2 and Cu(II) are needed to regenerate Pd(II) from Pd metal. Oxygen i s probably not very e f f i c i e n t as an oxidant at ordinary pressures i n aqueous s o l u t i o n because of i t s low s o l u b i l i t y , and i t s oxidation power does decrease with decreasing a c i d i t y 5 9 . However, the use of higher 0 2 pressures and some other 6 0 6 1 solvent system such as DMA (where the O 2 s o l u b i l i t y i s higher ' and Rh(I) i s 38 more stable ) might prove rewarding. REFERENCES 1. J . F. Harrod and J . Halpern, Can. J . Chem. 37, 1933 (1959). 2. W. C. Wolsey, C. A. Reynolds and J . Kleinberg, Inorg. Chem. 2_, 463 (1963). 3. B. R. James and G. L. Rempel, Can. J . Chem. 44, 233 (1966). 4. J . F. Harrod, S. Ciccone and J . Halpern, Can. J . Chem. 3_9, 1372 (1961). 5. J . Halpern. and B. R. James, Can. J . Chem. 44, 671 (1966). 6. R. D. G i l l a r d , J . A. Osborn, P. B. Stockwell, and G. 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