UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

The thermal decomposition of methyl vinyl ether Harris, Jack Edward 1942

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1942_A7 H2 T4.pdf [ 6.02MB ]
Metadata
JSON: 831-1.0062070.json
JSON-LD: 831-1.0062070-ld.json
RDF/XML (Pretty): 831-1.0062070-rdf.xml
RDF/JSON: 831-1.0062070-rdf.json
Turtle: 831-1.0062070-turtle.txt
N-Triples: 831-1.0062070-rdf-ntriples.txt
Original Record: 831-1.0062070-source.json
Full Text
831-1.0062070-fulltext.txt
Citation
831-1.0062070.ris

Full Text

THE THERMAL DECOMPOSITION OF METHYL VINYL ETHER A thesis submitted i n p a r t i a l f u l f i l -ment of the requirements for the degree of Master of Applied Science. by, Jack Edward Harris at THE UNIVERSITY OF BRITISH COLUMBIA 1941 - 1942. ACKNOWLEDGEMENT I wish to thank Dr. Wm. Ure for introducing me to th i s fascinating f i e l d of the study of Kinetics, and for his patient guidance and help during the time spent i n experimental determination of the data herein and i t s subsequent mathematical treatment. CONTENTS Page INTRODUCTION 1 EXPERIMENTAL 2 Preparation of Methyl Vinyl Ether 2 F i r s t Generator 3 Second Generator 3 Removal of Gaseous Samples 8 Reaction Chamber and Pressure Measuring Apparatus 8 Temperature Control 10 TREATMENT OF DATA - PART I. 13 Pressure - Time Graphs 13 Order of the Reaction 15 Reaction Rate Constant 18 Energy of Activation . 2 1 TREATMENT OF DATA - PART II» 24 Effect of the Ratio of Fin a l I n i t i a l Pressures 24 The Reaction Rate Constant 23 Energy of Activation 25 COMPLICATING FACTORS 31 Extent of surface 31 Condition of Surface 31 Effect of Wall Reaction on Rate Constant 33 SUMMARY OF RESULTS 3& MECHANISM OF DECOMPOSITION - ' 37 REFERENCES v 38 Appendix 39 INTRODUCTION This work has been done to supplement and continue that of Gordon Henry Turner as reported by him i n his thesis "The Thermal Decomposition of Dimethyl Acetal", now on f i l e i n the l i b r a r y of the University of B r i t i s h Columbia. For an outline of previous studies leading to this present work, 1 refer the reader to the above mentioned thesis. There he w i l l f i n d a l l the details necessary for the understanding and appreciation of t h i s thesis. For the most part I have referred to Turner's thesis, when explanations of the different phases of this present study,are contained therein. Turner found that dimethyl acetal decomposes to give methyl alcohol and methyl v i n y l ether. I t was thus considered desirable to study the thermal decomposition of methyl v i n y l ether i n the hope that i t would throw more l i g h t on the decomposition mechanism of the acetal. Although t h i s work has confirmed assumptions made i n explaining the decomposition mechanism of dimethyl acetal, i t has not been completed to the writer's sat-i s f a c t i o n . There are s t i l l complicating factors which could bear further investigation. The reader's attent-ion w i l l be drawn to these as they are mentioned i n the following pages. EXPERIMENTAL Preparation of Methyl Vinyl Ether, This preparation was carried out after the method of Dr. Wm. Chalmers ( 1 ) . CH2OHCH2OCH5 — (PBr^j —^ -CHgRrCH^ OCH^ .-CH2BrCH20CH3 — (NaOH) CH2CHOGH3. An excess of the methyl ether of ethylene glycol (8-10 moles) was mixed with two moles of anhydrous ethyl ether and placed i n a three-necked f l a s k provided with a reflux condenser, s t i r r e r and dropping funnel. The f l a s k was cooled i n ice and water and two moles of phosphorus t r i -bromide were added dropwise with vigorous s t i r r i n g . The mixture was then warmed on a water bath for several hours and allowed to stand over night. Methyl bromo-ethyl ether was separated from the above .by repeated d i s t i l l a t i o n . The methyl v i n y l ether was made by the reaction between the methyl bromo-ethyl ether and sodium-hydroxide. This was accomplished i n generators attached d i r e c t l y to the apparatus used i n studying the thermal, decomposition. These generators are shown i n figures 1 and 2, and are described i n the paragraphs immediately following. F i r s t Generator. The ether used i n making runs l a , 2a, 3a> 4a, and j?a was made i n the generator shown i n figure 1, The generator G and the column P were f i l l e d with sodium hydroxide pe l l e t s . With stopcocks SI, S3, S3, S6 and S? closed, and S2, S4, S? and S8 open, and whole apparatus as seen i n t h i s diagram was evacuated through the vacuum line•V, which leads through a mercury diffusion pump to a Cenco Hi-vac. Then stopcocks S4, S7 and S8 were closed. About 3 0 m l . methyl bromo-ethyl ether were introduced into the generator through the funnel F and stopcock SI. On warming the generator on a water bath, methyl v i n y l ether d i s t i l l e d off and passed through the column P into a' gas holder D (approx. 6 0 0 m l . capacity). The apparatus was so designed that any of the bromo-ether leaving G would condense i n the condenser C and run back into P, where i t would be i n contact with sodium hydroxide. After the reaction i n G was complete, S3 was opened u n t i l mercury from the reservoir Rl had ris e n above S2. Then both S2 and S3 were closed and the generator, column and condenser were removed by disconnecting the ground glass joint J. The gas contained i n D now consisted of methyl v i n y l ether together with any permanent gases (which might have been formed from impurities i n the bromo-ether or from the bromo-ether i t s e l f ) and possibly a trace of the bromo-ether. The Vinyl ether was frozen out by immersing the tube T l i n l i q u i d a i r and r a i s i n g the leveling bulb LI to increase the pressure i n D. The permanent gases were pumped off through S4 and S8» Then with only 34 and S? open, the l i q u i d a i r was removed. Before a l l of the ether had evaporated into the storage bulb B, S7 was closed and S8 opened to allow the residue to be pumped off. Thus only the middle f r a c t i o n of the gases which had been generated i n G were stored, and the l i g h t e r and heavier fractions were discarded. Second-/. Generator. In spite of the precautions taken i n the above method of preparation, i t was thought that a more e f f i c i e n t type of generator could be devised. Therefore the generator shown i n Figure 2 was b u i l t . As before, the whole apparatus was evacuated, and a l l stopcocks closed with the exception of S2. The traps Y and Z were immersed i n dry ice and l i q u i d a i r respectively. Then, with cold water flowing through the condenser C and hot water through the jacket H, about 3 0 m l . methyl bromo-ethyl ether were introduced from F into the sodium hydroxide packed generator G-. The design of th i s generator was such that any bromo-ether which evaporated while the pressure i n the apparatus was s t i l l low would be i n contact with sodium hydroxide for a longer period of time before passing into the condenserj and that which condensed i n C could run down to the heated part of the generator. What l i t t l e bromo-ether passed through the condenser was caught i n the dry ice trap. Thus only the very low boiling point constituents found their way to the l i q u i d - a i r trap. After the reaction was complete, S2 was sealed with mercury and the generator, condenser and dry-ice trap were removed. Then S4 and S8 were opened and the permanent gases pumped off. On the removal of the l i q u i d a i r , the f i r s t f r a c t i o n of methyl v i n y l ether evaporating was stored i n B. The remainder was taken from the trap to T l by immersing this tube i n l i q u i d a i r . Then, on the removal of the l i q u i d a i r from T l , part of the ether was allowed to evaporate into D and the rest was sealed off i n T l . The ether stored i n D was used i n a l l of the b-runs, and that stored i n B was used i n the c-runs. When a decomposition run was to be made, the ether was taken to the reaction chamber through S£. After the run had been completed, titee products of the decomposition were removed through S3 and stored i n the previously evacuated storage bulbs b l , yz, b3 and b4. Removal of Gaseous Samples,, To remove gaseous samples for analysis, the appropriate stopcocks were opened and the gas to be analysed was pumped . off through S9 to a burette - a Topler pump being used for th i s pumping. The samples taken fromthe burette were stored over mercury and. microanalysed (2). Reaction Chamber And Pressure Measuring Apparatus. . Figure 3 shows the method of accomplishing the thermal decomposition of the methyl v i n y l ether at constant volume, and of measuring the pressure at progressive stages through-out the decomposition. The furnace F, enclosing the decomposition chamber X and the platinum resistance thermometer PR, was brought to the desired temperature, and the whole apparatus as seen i n figure 3 was evacuated through S 3 , SI2 and S13. Then a l l other stopcocks were closed and S14 was opened u n t i l mercury from the reservoir R2 had risen into the U-tube U to the l e v e l H. I t w i l l be seen that manipulation of 310, S l l and S14 leading to the a i r leaks A l , A2 and A3, and through YB to a vacuum bottle, made i t possible to control the mercury lev e l i n either or both of the arms of the U-tube. 9 Ether was l e t into X through Sj> u n t i l the level i n U showed that the desired i n i t i a l pressure for the reaction had been reached. Then Sjp was closed and a i r allowed into SV (a s t a b i l i z i n g volume) through A l and A2 u n t i l the l e v e l i n U was brought back to H. The time which elapsed between the closing of and the re-establishing of the l e v e l H was recorded together with the pressure read from the manometer M« A i r was l e t into Sv at regular intervals; and, each time the l e v e l H was re-established by the pressure building up i n X, the time and pressure were recorded. I t w i l l be seen that each time the l e v e l H was established the pressure i n Sv was the same as the pressure i n X, and therefore the pressure measured by M was id e n t i c a l with the pressure i n X. Other parts of the apparatus not explained above are leveling bulbs to manipulate mercury for sealing stop-cocks, c a p i l l a r y tubes to retard the flow of gases, and traps to catch wild mercury, l i q u i d products etc. Temperature Control. Figure 4.is a wiring diagram showing the methods used to control and measure the temperature. The furnace F l was coupled i n series with a variable resistance R l , an ammeter A and a 110V-j>amp variac V. Across Rl was a secondary c i r c u i t consisting of another variable resistance R2 and a photoelectric relay (PEC) i n series. The double throw switch SF was placed i n the secondary c i r c u i t to accomodate another and similar arrangement for another furnace. The platinum resistance thermometer PR1 was wired across the center poles of the double throw switch SS -another double throw switch SP being placed i n this l i n e to acoompodate the platimum resistance thermometer used i n conjunction with the other furnace. The side of the ..; switch SS led to the wheatstone bridge WC and from there to the galvanometer Gl used with the photoelectric relay. The other side of SS was led to the wheatstone bridge WM and from there to a second 'galvanometer G2. With the proper adjustment of Rl and R2 any desired temperature i n the furnace could be maintained by the correct setting of WC, and t h i s temperature could be measured on WM by means of the l i g h t L2 and the scale Sc used i n conjunction with G2. The above method of controlling and measuring the temperature i n the furnace was found to be entirely satisfactory. At no time during a run was the temperature found to vary more than 0.035<?C on either side of the recorded temperature of the run. TREATMENT OF THE DATA. — PART 1. . Pressure - Time Graphs. Experimental t o t a l pressures were plotted as ordinates against time as abscissae. From the curves so obtained, "smoothed" values or the t o t a l pressures corresponding to equal increments of time were tabulated and used i n the following calculations. Graph 1. shows the type of curves obtained when t o t a l pressures were divided by corresponding i n i t i a l pressures and plotted against time. Dividing by the i n i t i a l pressures f a c i l i t a t e s comparisons of f r a c t i o n a l increases i n pressure from one time to another during a given run, or from one run to another f or a given time. Each curve w i l l be seen to turn upwards at the beginning. The length of time for which t h i s was noted varied from curve to curve and seemed to depend upon the temperature and i n i t i a l pressure and upon the condition of the reaction chamber. For r e l a t i v e l y high temperatures and for high i n i t i a l pressures the effect was nearly indisoernable because of the more rapid increases i n t o t a l pressure due to the main reaction. I t i s d i f f i c u l t , therefore to calculate the dependence of th i s "induction" period upon temperature and (ore) pressure. Experiments with a i r proved that i t was not due to the heating of the ether from room temperature to the temperature of the run. This takes an immeasurably short time. Order of the Reaction. Graph I I . shows J . 2 — plotted against time, where Pf "Pt p Q i s the i n i t i a l pressure, p^ i s the f i n a l pressure and P t i s the t o t a l pressure at time t . The i n i t i a l pressures were calculated from the f i n a l pressures by dividing by 2.2 . This value was determined by introducing methyl v i n y l ether into the cold reaction chamber, measuring the pressure, heating u n t i l the decomposition was complete and then measuring the f i n a l pressure. The value 2.2 i s an average of several runs made i n t h i s manner. The fact that the curves i n Graph I I . approach l i n e a r i t y shows the decomposition reaction to approximate an overall second order rate. I f the reaction were second order the ratios \^ -/^ . t t l / 2 and , 3/4 should be 1/3 and 3 respectively! where t i / 4 , •fcl/2 * l / 2 and ^3/4 are the times necessary for decomposition of 1/4, 1/2 and 3/4 of the ether respectively. Table 1 shows that the above ratios would indicate an approximately second order reaction. From th i s we may conclude that the main reaction i n the thermal decomposition of methyl v i n y l ether i s d e f i n i t e l y second order. i e f O Jo / o o > Run: 2b " 433°C 7 ... . ^. Jo Jo / c .• /o Run l b > 423 °C -b /o Run 3b • - VI > 2o : / °/ •< • 4I3°U . : 1 •j •' ; . :• 1 / ( / Q/ o/ Run 4b 403 °G i • 1 9 of / / < / / G / o/ o / / • i • Run 6c > 390°Q I 5 I •ClT-•«< O / V . • •. -GRAPH I I . - - • . — •• i X (< • • • • i °0 1 0 .2 time © J i n minutes 0 ;4 0 TABLE 1 Run T°C t l / 4 \/2 t l / 2 l a 400 5 a 423 .41 2 . 7 0 2a 423 .37 3 . 9 9 3a 430 .35 2 . 8 8 4 a 439 .37 3.13 4b 403 . 3 5 3 . 0 3 3b 413 .36 3.24 5b 414 .39 2 . 6 0 l b 423 .36 3 . 2 5 2h•, 433 .36 3.24 6c 390 .31 3c 403 • A9 3.14: 5c 414 ! .36 3 . 5 3 1c 414 . 4 4 3 . 5 3 2c 423 .28 2.99 4c 453 ,33 3.13 7o 413 .41 2 , 9 5 averages .35 3 , 1 5 Reaction rate constants (kg) were calculated for each run by use of the formula-- §=L (. ) ——(appendix (1)) t p f - p t p f - p Q; where S i s the r a t i o of f i n a l to i n i t i a l pressures deter-mined as described above. Values for kg calculated i n t h i s manner did not show any definite dependence upon temperature. This was assumed to be p a r t i a l l y due to what I have, for convenience, termed the "induction" period. As w i l l be seen i n the following example (chosen to show th i s e f f e c t ) , the determination of an exact constant for the rate of the main reaction was almost impossible - at least by t h i s method. Since values of kg so calculated were i n most cases constant over considerable periods of time, i t was decided that the main decomposition reaction was d e f i n i t e l y second order. Since averaging values such as those i n table 2 would not give good re s u l t s , the reaction rate constants used for further calculations were obtained from the formula -k 2 »• ^ 2 » 2 — ( a p p e n d i x (2)) t i / g P f The constants obtained i n t h i s manner are tabulated i n table 3. These values showed better agreement for runs at the same temperature and showed a temperature relationship TABLE 2. Run 1c 414°C min. P cm. fc2z zl03 0 7.00 2 7 .55 5 .0 . 4 8.20 5.0 6 8 . 8 5 6.7 8 9.50 7.1 10 10.15 8.5 12 1 0 . 7 0 9.4 14 1 1 . 0 5 9 . 5 16 11 .30 9.4 18 11.50 9.1 20 l l . 6 o 8.7 22 11.75 8.4 24 11 .90 8.3 26 12 .05 8 .3 28 12 .15 8.2 30 1 2 . 5 0 • 8.2 32 12.40 8.1 34 1 2 . 5 0 8.1 36 12 .60 8.1 38 1 2 . 7 0 8.1 40 - 12.85 8 .2 Pf * 15.40 effect of induction period TABLE 3 Run T°G Po cm t l / 2 mm fc2 Remarks l a 400 13 .30 _~ 5a 423 6 . 3 2 4.40 .0359 p^ too low. 2 a 423 1 0 . 9 5 8.65' .0105 3 a 430 9.40 2 . 9 0 .0273 4 a 439 9.40 2.48 .0428 proven unreliable . 0 0 8 1 by Po/Pf-Pt v s *• 4b 403 1 2 . 9 0 9 .52 3b 413 1 7 . 2 0 5 .27 .0110 5b 414 5 . 8 5 9 . 2 5 .0185 Po too low. l b 423 1 9 . 6 0 2.78 .0183 2b 433 2 0 . 1 0 1 .85 .0269 6c 390 •21.10 2 2 . 1 0 „0022 3 a 4o3 1 5 . 0 5 2 1 . 6 5 .0031 5c 414 17.40 7.68 .0075 l e 414 7 . 0 0 1 5 . 3 0 .0093 2c 423 1 5 . 8 0 8 .10 .0112 • 4c 433 1 0 . 5 0 4 , 7 5 .0200 7c 413 6.O7 14 .95 .0111 as seen i n Graph I I I . I t w i l l be noticed that the points i n t h i s graph congregate i n zones, and that the averages of points within each zone show a definite trend from zone to zone. Energy of Activation. The energy of activation (E) for a second order reaction i s given by the equation -k 2 = AT 1/ 2: e" |§r from which we may derive* E = 2 .303R logio k 2 _ RT_ — (appendix(3)) "T*T ~~ 2 . . d-^r-and since RT i s r e l a t i v e l y small compared with E, • E * - 2 .303R d l Q g l O k2 = const. d-L. . • T Thus plo t t i n g l o g 1 0 k 2 a g a i n s t _ l _ should give a straight l i n e . ^ This however was not the case. But on plotting logj_0 kg against T the straight l i n e i n Graph IV. was obtained - indicating the relationship — PTL E = 2 .303R £J^glp_kg_ 2 --.-(appendix ( 4 ) ) d T • O.IT2 - 0.992 T = 48 ,300 cals at 430°0 and 44 ,600 cals at 400°C. 1 1 l! 22t i . Am > 4>>U 4.4.0 • • • A • C jC /@ r • i: 4p (J a; L C / CO W/ o] o a ; ' 1roC A~\ Pi a > ! ;• ob 'O jC "b 'I 41U c - / b !o o ol GRAPH i : a — — a R b ~ — b R CI. HIS. ins. 400 7 ' T] / v ? -;-<§>-fc> -b< le double < xrylng widf j-e-n—:i-nc-l-ud-f sircles ar< sly from t. jd—i-n—t-he—i i averages le trend hi , The pts. ive not pyo 7 on XV Hi ago a • I 1' ( )• ; ; , - . . . . .. kgXlO 2 I 2^  ... . . . . . . . | 440 \ GRAPH I ••• • Y. • • • . • " :-I 4p0 \ o i l. 420 - T , e G \ 3 \ \ |;| . . lit • 410 , t : ' x • ii I Z O O • < l f ; c -1 >- • ' " 3 - ' C J c °S10k2 N < » J c y L • ' ". J c • J i The fact that the overall reaction was proven to approximate a second order rate would indicate that, although this l a s t equation may give values of the activation energy which are of the right order of magnitude, the equation i t -s e l f could hardly be expected to hold over any reasonably large range of temperatures. ': TREATMENT OF THE DATA — PARTII. Effect of the Ratio of Final to I n i t i a l Pressures, The dependence, or apparent dependence of the activation energy upon temperature was thought to be due to the condition of the reaction chamber. I t w i l l be remembered that the ratios of f i n a l to i n i t i a l pressures were taken as 2,2 for every run. I f , however, the pressure - time curves are extrapolated to zero time, the actual values of thi s r a t i o very and 2,2 i s just an average. I t i s -time that the difference between these values and the average was never more than -give percent, but the following calculations show that their use i n place of the average gives a constant value for the activation energy while having l i t t l e effect upon the other data. Using the individual values of Pf and the P 0's from Po extrapolation of the pressure - time curves, i t was found that the curves i n Graphs I and I I were not noticeably shifted, and that the approximation of an overall second order rate was indicated as before. Table 4. shows the values of s=p f/p 0 and the effect they have on. previously calculated data. The blank spaces i n table 4 correspond to curves which could not be extrapolated to zero time. The Reaction Rate Constant. Table j>. gives the values of kg calculated by use of the formula kg = l/pot^yg. (appendix (2)) This table includes only runs for which the pressure -time curves could be extrapolated to zero time. With the exception of kg for run 4b , the values i n columns a and b check quite closely, but i t w i l l be seen that those i n column b show more regularity than those i n column a. Energy of Activation. Under th i s heading i n part I i t was explained that, for a second order reaction, the energy of activation i s given by the equation <i( loglG-k?) • E - -2.3Q3R a ) l t ~ constant. and p l o t t i n g l o g 1 Q k g against 1/T should give a straight l i n e . Graph V. shows th i s condition f u l f i l l e d for the reaction rate constants i n column b, table 3» The slope of the l i n e i n Graph V. i s - 3 , 0 0 0 , giving E = 24 ,900 calories. This value would appear to be extremely low and i s d e f i n i t e l y not i n agreement with that previously calculated. TABLE A NOTE - s<=2.2 for (a) columns and. i s variable for (b) columns. Rum Tec: P o c m pfem s Jl/4 t3/4 ^1/2 (a) m (a&b) (A) C>) ( (b) (a) (b) (a) (b) 6e l a 4b 3c 3b 5b l c 5c 7c l b 2a 5a 2c 3 a 4e 2b 4a 390 400 403 403 413 414 414 414 413 423 423 423 423 430 433 433 439 21.10 13.30 12 .90 15 .05 17.20 5.85 7.00 17.40 6.07 19.60 10.95 6,32 15.80 9.40 1 0 . 5 0 20.10 9.40 20'. 95 12.40 16.80 5 .90 17.10 19.33 10.40 19.22 : 9 .80 46.40 29.25 28.40 33.10 37.85 12.85 15.40 38 .30 13.35 43.12 24.10 1 3 . 9 0 3 4 . 7 5 2 0 . 7 0 23.10 44.22 20.65 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2,2 2.2 2.2 2.2 2.2 2.22 2 . 2 9 2.23 2,18 2.24 2.24 2.22 2 .30 2 .09 22.10 * 9.52 21.65 5 .27 9 .25 1 5 . 3 0 7.68 14 .90 2.78 8,65 4.40 8.10 2.90 4 . 7 5 1.85 2.48 21.15 5.93 T£7$i 1 2 . 6 0 7.27 2.79 4 . 5 0 1.75 3 . 2 0 .31 .35 .19 .36 .39 .44 .36 .41 .36 .37 .41 .28 .35 .33 .36 .37 .31 ; "36 .28 ; .30 .35 .34 .34 .35 .33 3 . 0 3 3.14 3.24 2.60 3 .53 3.53 2.95 3 . 2 5 3 . 9 9 2.70 2.99 2.88 3.13 3.24 3.13 2.57 2.51 3 . 0 0 3 . 7 2 3.21 3.23 3 .28 3 . 3 4 averages .35 .33 3 . 1 5 3 .11 TABLE 5. 2 . 2 for (a) columns and i s variable for (b) columns. Run T°C kg (a) (b) 6c 390 .0022 .0022 4b 403 . 0 0 8 1 .0136 3b 413 .0110 . 0 0 9 3 3c 414 .0073 .0081 l b 423 .0183 .0185 4© 433 .0200 .0214 2:b 433 .0269 .0298 4 a 439 /0428 .0339 S8 i ,..0027-.0026' -*0025" 'j 0024 O 'i I T°C .... .. •: .. o / Q/Q - • i i • •• • • : • GRAPH V. • • ' . . ,. < • ; - - • t •> ^ -N C H C • og k 10 2 k To obtain the best value for the energy of activation i t was decided to use a l l of the individual values for kg and solve f o r the slope $S l o g l c f e L by the method of least squares. Thus ^ i o g 1 0 k 2 - m r i • b, where the slope m = d{1%]0&L d(^( Y l r ( ^ i o £ l 0 ^ 1 T 2 /-)_T ' Z j f ^ = - 8 , 5 0 0 Prom th i s we have: s - - 8 , 5 0 0 x( - 2 . 3 0 3 ) ( 1 . 9 8 5 ) - 3 8 , 8 0 0 calories. Although pl o t t i n g the reaction rate constants f i r s t calculated against l/T does not show any definite trend (Graph VI.), the activation energy calculated from these by the above method i s obviously the best value obtainable from the data i n hand. On consideration of the work of Turner and of the reaction rates i n th i s thesis, i t w i l l be seen that 5 8 , 8 0 0 calories i s reasonable for the energy of activation i n the thermal decompdjsition of methyl v i n y l ether. j 1 : \ ' sc -j ' J ' ' ' 00° f) The ob lia.ue_li.ne by the met 1510^ 2.1 _ _ s show the straight l i n e I 00/>fi trend a l i o d hod of lee 8 , 5 0 0 st squares 1 • ... . . . . . . . . . . O ' ( D . _.0.02-4_ O CD o o f. I 1, o • C -' GO |i O o • ...... -QQ-2rJ) o .x" . ' . ... ! . . . . . . . GRAPH YI. • r J c -1 f\ c - c o g 1 0 k 2 !>' 1 > C « -J < f\ c » % - -« J COMPLICATING FACTORS. -In addition to the runs previously discussed, four runs were made with a reaction chamber packed with glass tubing to give a surface 6 .25 times as great as that of the unpacked chamber. The information gained from these runs threw more l i g h t onto factors such as:-(I) The effect of extent of surface. ( i i ) The effect of condition of surface. To study these effects, four runs (2a,2c,8c, and 11c) a l l made at 423°C were considered as follows. Extent of Surface. Graph VH. shows that there i s a reaction dependent upon the extent of surface which tend to suppress the building up of pressure due to the main reaction. Thus we have the branching of the two sets of ,-lines - the lower branch In each set showing the result of adding to the extent of surface i n contact with the decomposing ether. This may be taken as an indication of a free radical mechanism (3). Condition of Surface. The condition of the walls of the reaction chamber i s by f a r the most important factor. Reference to Graph VII. shows that the extent of surface does not have much effect upon the general shape of the pressure - time curve, while the number of runs previously made i n a chamber (whether --1 2.25 1 1 i • • I Run 8 c 3 Run 2 £ t 4 2 3 ec. L 423 °C. -2 Run 11c 4 Run 2 c i 423°c; : 423°C _4_ ' -1-T75 3 = = = = ===a 1 - ~ " 1 .50 Pt/P 0 C ffiAPH V I I . . ^ ; ... ..... . 1 .25 -l.do c 1 0 a time i n mj ) .nutes. 3 0 4 w to packed or not) Is of great importance i n this respect. On comparing the values of t, . (table 6 . ) we fi n d 1/2 a good index to the dependence on wall conditioning of what must be the combining of free radicals on the walls.(4). I f we assume a chain mechanism involving free radicals, we'-may conclude that the combining of these radicals on the chamber walls terminates the chain and slows up the decom-position. As more and more runs are made i n a given chamber, the walls become conditioned i n such a manner that the free r a d i c a l combination i s slowed up. This leads to a faster overall decomposition rate. Effect of Wall Reaction on Hate Constant. The effects of both extent and condition of surface are seen i n table 7. Increased surface decreases the rate constant as seen by comparing the constants for 8c and 11c with those for 2'c and 2a respectively. Conditioning of the surface also increases the rate constants as seen by comparing kg for 8c with that for 11c. The constants for 2c and 2a do not show th i s effect. This may be due to thei r not being accurate enough and to the extent of surface of the unpacked chamber not being great compared with the i n i t i a l pressures. L i t t l e has been said and l i t t l e can be said about what has been previously termed the "induction period". A study of the pressure - time curves shows that t h i s period at the TABLE 6 Runs at 423°C. Run no. of previous runs * l / 2 kg Remarks. 8c 0 20.38 .0029 packed with glass tubing. . 2c ' 1 P. 10 .0112 11c 3 ' 8.55 .0069 packed with glass tubing 2a 8.65 .0105 3a 5 4.40 .0359 i* 2.78 .0185 TABLE 7 Runs at 423°G. Run Packing No. of. previous runs 8c yes 0 .0029 2a no 1 .0112 11a yes .0069 2a no 3 .0105 beginning of each run varies with i n i t i a l pressure, condition of surface, temperature and extent of surface. These factors are written i n the order of th e i r effect -i n i t i a l pressure being most important. SuMftARY. OF RESULTS ,The following i s a tabulated summary of the results of this work on the thermal decomposition of methyl v i n y l ether. 1. The overall rate of decomposition i s second order. 2. An increase i n surface lowers the rate. 3. "Conditioning" the walls increases the rate. 4. There are three main factors -(a) the main decomposition. (b) the induction period dependent upon -I. i n i t i a l pressure. I I . condition of surface. I I I . temperature of decomposition. IV. extent of surface. (c) a free r a d i c a l mechanism dependent upon -I , condition of surface. I I . extent of surface. 5 . The activation energy i s approximately | ^ ? 0 0 0 calories per mole. MECHANISM OF DECOMPOSITION I t i s d i f f i c u l t to propose a mechanism for the decomp-osi t i o n u n t i l analyses of the intermediate and f i n a l produots have been completed. Following, however, are equations giving what appear to be l i k e l y steps i n the decomposition -(1) CH3-0-CH=CH2 CHj + CH2C0H. —-5^" 2CH^ + CO. (2) CH^ + CH3-0-CH=CH2 — C H 4 + CH2-0-CH=CH2. CH4 + CO * C 2H 5. (3) CgH^ + CH^-0-CH=CH2 — j ~ C 2H 6 + CH2~0-GH=CH2. ' C2H£ + GO -t GgH^. (4) C2H^ + CHj —*« C^Hg. Equation (4) i s presumably the reaction terminating the chain. This would be dependent upon the extent and condition of the surface of the reaction chamber (3) REFERENCES. (1) Preperation of Ethers of Vi n y l Alcohol - Win. Chalmers. Canadian Journal of Research 7 , 464-71 ( 1932 ) . (2) The Thermal Decomposition of Dimethyl Acetal - G. H. Turner. - a thesis, U.B.C. ( 1 9 4 1 ) . (3) The Aliphatic Free Radicals. - F.O.Rice and K.Rice.-book, Johns Hopkins (1935)• (4) On the Polymerization of Ethylene and Propylene by Free Alk y l Radicals. - Beech and Rust (Shell Devel-opment, J . Chem. Phys.j 9 - 480 ( 1 9 4 1 ) . (3) Reference ( 3 ) . APPENDIX Symbols used i n the following derivations are as l i s t e d below - • p Q = i n i t i a l pressure. p f = f i n a l pressure* p^ = t o t a l pressure at time t . p^l= pressure of undecomposed ether at time t . s = r a t i o of f i n a l to i n i t i a l pressure. * l / 4 » - t i / 2 and t 3 / 4 = 1/4, 1/2 and 3/4 l i f e respectively. ' (1) To obtain the reaction rate constant from the pressure - time data. dt V ' = k 0dt. P d tr + _ 1 1 I T \ P f - sp 0. giving P t * P t t + s(p 0-p tT). •n m Pf~Pt ' f?) but s-1 and at zero time J)Q • » o « • • * • * •• » • i j5 ) from equations, (1), (2) and (3) kj,t—5=5L - s - l Pf~Pt Pf-Po thus k? - ^ — ( ^ r - - ™ T H « (2) To obtain the reaction rate constant from the half l i f e . From part; (1) p t . -for t l / 2 then P-fct " l/2po and p t = Vt±^. P t l / 2= Pf- ( ^ l ] | P o = (( 1-^(|-) )bf• ...simly; p t i / ^ ((1-5=1(1) P t 5 A - (( 1 ^ ( 1 ) ))pf. Thus, from the calculated values for the t o t a l pressures at t 1 y 4 , t 1 y 2 and t^/4 the values, of t-j^/4, t-jy 2 a n d- ^3/4 can be read from the pressure - time curves. " dt k2P 1 To get the reaction rate constant i n terms of the half l i f e , we integrate the above equation as p goes from p 0 to o 1 —p Q and % goes from 0 to t-jy 2. k 2 = Po°l/2 but p G » P-^ , therefore s k„ = 2 " P r t x/g and f o r s=2.2 „ „ v - 2.2 k 2 ~ r-r V l / 2 (3) The activation energy. For a second order reaction 1/2 _ E kg = AT e RT F 1 l n k 2 ~ - £7=r + In A + j i n T (1) d i f f e r e n t i a t i n g ( l ) with respect to l/T ' ; d(lnkg) _ E T d(|) ~ ~ * 2 ' E = - R i U n k g i _ RT. T = - R 2 H 2 i i o ^ i ( 2 » 3 0 3 ) -T (4) Differentiating (1) i n part (3) with respect to E = d(LMk:)PRT2 RTg = 2.303- R AilO£ 1 0k 2l T 2„ HT. dT d 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0062070/manifest

Comment

Related Items