Test of Precision.-In order to test the precision attainable, five separate determinations were made for toluene. These yielded values for Henry's law constant a t -78.51" of 298, 299, 299, 300, 299 mni. Representative data for four of these determinations are illustrated in Fig. 8. It was therefore concluded that the Henry's law constant could be obtained with a precision of 1 part in 300. Although refinements in technique might have improved the precision, we found it ample to distinguish between the different compounds whose basicities were of interest. Materials.-The trimethyl- and tetramethylbenzenes were generously supplied by Professor Lee Irvin Smith of the University of Minnesota. The remaining compounds were commercial products, the purest available. Depending upon the quantity available, the compounds were distilled either through a Todd Micro Column (rated efficiency: 50 theoretical plates) or a helices packed colurnri (rated
efficiency: 75 theoretical plates). Only materials which boiled constantly and exhibited constant refractive indices were utilized. The bromobenzene was a Bureau of Standards sample for styrene determinations by the freezing point method and was used without further purification. Physical data for these compounds are summarized in Table X.
Acknowledgment.-We wish to acknowledge our indebtedness to the Standard Oil Company (Indiana) for financial support of this investigation and to Professor L. I. Smith for his generous samples of the higher methylbenzenes. N'e are also indebted to hlr. Lionel Doniash and Miss Martha Havill for assistance in calculating the dissociation constants for the aromatic-hydrogen chloride complexes. 1,AFAYETI.II:. I N O I 4 V A
[CONTRJEitJTIOV FROM TJIE DEPARTMENT O F CICEMISTRY, LTNIVBRSITY OF CALIFORNIA AT
DAVIS]
The Thermal Racemization of Dimethyl ( - )-Bromosuccinate BY LAWRENCE J. ANDREWSAND
JAms
E. HARDWICKE
RECEIVED JANUARY 7, 1952 Contrary to a previous report it has been found that no extensive racemization occurs when optically active bromosuccinic acid is converted to its dimethyl ester with diazornethane. The ester does, however, lose activity when subjected to repeated distillation under reduced pressure. To demonstrate the sensitivity of this ester t o thermal racemization a series of kinetic measurements of the rate of loss of optical activity of this compound a t several temperatures has been made. Kinetic measurements of the dehydrobromination reaction, which occurs simultaneously with racemization, have also been made. Similar kinetic studies have also been carried out using optically active methyl esters of a- and 8-bromobutyric acids. The results are of interest as regards the mechanisms of the racemization and dehydrobromination reactions.
It is known that certain esters of optically active are of interest in connection with a discussion of bromosuccinic acid undergo slow autoracemization possible mechanisms for the racemizations. The Kinetic Measurements.-In the course of a t room temperature.' It has also been demonpreliminary measurements it was noted that a prestrated that in acetone solution dimethyl (-)bromosuccinate can be racemized in the tempera- cipitate was formed in samples of the dimethyl ture range of 0 to 52' by the addition of inorganic bromosuccinate which had been heated for long bromides2 However, little is known concerning periods of time. This precipitate, m.p. 100-101", the thermal racemization of pure esters of this type. was readily identified by mixed melting point Interest in this problem has arisen as an out- determination as dimethyl fumarate. Walden * growth of attempts to check the report3 that a also recognized that this ester showed a tendency racemic ester is formed when (-) -bromosuccinic to form dimethyl fumarate on distillation, and 01acid is treated with diazomethane. All samples son and Long4have observed in studying the halide of dimethyl bromosuccinate which have been ion catalyzed racemization of ( - ) -bromosuccinic prepared by this method in this Laboratory have acid the simultaneous occurrence of a dehydroretained a high degree of optical activity compar- bromination reaction. To study the racemization rates two series of able to that of samples prepared by esterification with methanol in the presence of anhydrous hy- measurements were made at each temperature indrogen chloride. I t has, however, been noted that vestigated. In the first series the loss in optical the optical rotation of the ester diminishes appreci- activity of samples of the pure bromoester as a ably as the illaterial is subjected to repeated dis- function of the heating time was determined. In a second set of measurements weighed samples ti llation. A series of measurements of the kinetics of of the ester were heated over various time intervals racemization and dehydrobromination of dimethyl and were then analyzed for hydrogen bromide. The following reactions have been considered ( - )-bromosuccinate at temperatures in the vicinity of 100" has been made to establish that the in deriving a rate law for the interpretation of the racemizations resulting from distillation of the ester data taken in following the loss of optical activity are thermally induced. In addition similar studies of the bromoester ki have been made with the methyl esters of optically l Z d (1) active CY- and P-bromobutyric acid. These results P. Walden, Bcr., 31, 1416 (1898). (2) (a) T. Wagner-Jauregg, Monalsh., SS, 791 (1929): (b) R. Kuhn and 1.. Wagner-Jauregg, Nafurwiss., 17, 103 (1929); ( c ) E. Bergrnann, Ilelu. Chim. Arlo, 2 0 , 590 (1937); (d) A. R.Olson, I,. D. Frashier a n d 1'. J. Spieth. J. Phys. b Colloid. Chcm., 55, 860 (1951). (3) R. Bergmariii and Y.Spriuzak. T H r s JUURNM., 60, 1998 (1938).
kz ka
(1)
1-u -(4) A
k, d
----f
(2)
u
K. Olson and P. A Long, ibid., 56, 1294 (1934).
(3)
July 20, 1952
THERMAL RACEMIZATION OF DIMETHYL ( - )-BROMOSUCCINATE
where 1 and d represent the enantiomorphic forms of the bromoester, U is the ester dehpdrobromination product and kl = k l . If the observed losses in optical activity result only from reactions (1)-(3), the rates of disappearance of the 2- and d-isomers and of appearance of unsaturate are given by equations (4)-(6).
+ +
-d(E)/dt = h(l) - h ( d ) ka(l) -d(d)/dt = k i ( d ) - k l ( 0 Md) d(U)/dt = kaI(l) (d)l
+
(4) (5) (6)
The optical rotation of the heated samples a t any time t is given by equation (7) in which the subscripts refer to times t and zero and C is a constant. at
= C[(Z)t - ( 4 t l = C[-2(d)t
+ + ( l ) o - (UXI (40
(7)
Using equations (4)-(7) it can be shown that
-
* [y+ dt
= C
= a
3583
TABLE I1 THERATESOF DEHYDROBROMINATION OF THE ESTERSAT 130.7" Time, min.
Bromoester, millimoles
HBr, millimole
476 1290 2740 4490
Bromosuccinate 0.676 0.066 ,548 .143 .861 .445 .682 .397
653 1422 2133 3582 5372
&Bromobutyrate 1.093 0.145 1.672 ,427 1.294 ,509 1.085 .574 1.323 .go1
+
[2k~ k s ] (8)
k: X IO:, min. -
21.5 23.4 26.6 19.4 Av. 22.7 21.8 20.7 23.4 21 .o 21.3 Av. 21.6
TABLEI11 SUMMARY OF RATECONSTANTS FOR Loss
or that
OF ACTIVITY AND DEHMROBROMINATION OF THE BROMOESTERS
where kobsd is the rate constant calculated fro:*) the observed changes in rotation. The constant k3 can be determined from the dehydrobromination rate measurements using the
Temp., OC.
min.
-1
kil X los, min. -1
kl x 104, min.- 1
110.0 130.7
Bromosuccinate 4.09 2.03 7.61 4.63 35.1 22.7
1.94 3.57 16.4
110.0 130.7 140.0
Bromobutyrate 0.79 2.72 5.75 21.6 12.6 38.8
0.26 1.79 4.36
100.0
(10)
where a represents the number of moles of bromoester at zero time and x represents the moles of hydrogen bromide a t time t . The data were interpreted using equations (9) and (10) on the assumption that the volume of the samples did not change during the course of the reaction. A similar interpretation was applied to the results obtained in studies of the rates of loss of optical activity and of dehydrobromination of methyl (+)-P-bromobutyrate. Typical data for individual runs are given in Tables I and 11, and the rate constants are summarized in Table 111.
kobad X 10'.
To check on the nature of the dehydrobromination product of methyl P-bromobutyrate, which was presumed to be methyl crotonate, fifteen grams of the bromoester was heated at atmospheric pressure for 17 hours at 160-180°. During this time only about 10% of the theoretical quantity of hydrogen bromide was evolved, and only small amounts of low boiling ester could be isolated by distillation of the heated material. However, an appreciable quantity of crotonic acid precipitated from the TABLEI cooled high boiling residue. It seems likely that THERATES OF LOSS OF OPTICAL ACTIVITY OF DIMETHYL the methyl crotonafe produced by the dehydro( -)-BROMOSUCCINATE AND METHYL (+)-@-BROMObromination reaction was subject to attack by the BUTYRATE AT 130.7" evolved hydrogen bromide to form crotonic acid and Ti,me, kob8d.X 1'3'. min. a0 min. -1 methyl bromide. In the kinetic studies on the dehydrobromination Bromosuccinate reactions the rates were followed by titrating the 0 -31.8' *.. contents of the reaction tubes for acidic substances. 120 -20.2 37.5 Therefore the occurrence of an ester cleavage re250 -14.5 31.3 action, producing crotonic acid a t the expense of - 7.02 38.5 392 hydrogen bromide, should not affect the accuracy 486 - 6.40 33.0 of the dehydrobromination rate constants reported Av. 3 5 . 1 for methyl 0-bromobutyrate. It is noteworthy 0-Bromobutyrate that no solid substances were observed to precipi0 +11.1" ... tate from the cooled methyl P-bromobutyrate 149 10.2 5.84 rate samples. 282 9.52 5.36 Relatively few rate samples were run a t any in356 9.03 5.72 dividual temperature because of the labor involved 402 8.65 6.14 in preparing the active esters in large quantity. 1370 5.09 5.67 All reactions, however, were followed to better than 50% completion. Some difficulty was encountered Av. 5 . 7 5 Observed rotatioii in 0.6-din. tube based 011 thc average in making these measurements because of darkening of individual rate samples and because, iii the case of several readings. 0
LAWRENCE J. ANDREWSrIND
3.34
of the bromosuccinate, of precipitation of dimethyl
fumarate in some of the reaction tubes before polarimeter readings could be made. As can be seen from Tables I and I1 the rate constants in any one run show appreciable variations, although no marked trends, indicative of other than first order kinetics, appeared. Under the conditions of these experiments the dehydrobroinination reactions apparently were irreversible. The susceptibility of methyl (+)-a-bromobutyrate to racemization on prolonged heating was also investigated. This compound, however, showed no change in optical rotation when heated a t 1.X’ for 24 hours. -4 few semi-quantitative measurements on dimethyl ( - )-chlorosuccinate were also made. This compound gave approximately the same k u h 3 d and K B values ( 3 X 10Vtinin-l) a t 130”)indicating that the loss of activity could be attributed, a t least in large part, to the dehydrochlorination reaction. Heavy precipitation of the dimethyl fumarate in the reaction tubes even in early stages of the reaction prevented a more extended study of the kinetics of loss of activity of this ester. It was observed in establishing the most satisfactory experimental conditions for the kinetic measurements that the reaction rate of the bromosuccinate was more rapid when reaction tubes which had not been scrupulously cleaned were used. For example, samples sealed in tubes which had been cleaned only by washing with water gave kobar, values a t 100” of 1.2 x I O d 3 inin.-’ as compared to the value of 4.09 X IO-* iiiin:-‘ obtained by using tubes which had been cleaned with hot concentrated nitric acid. It was believed that these variations in kob.d values might be attributed to the inclusion in the reaction tubes of varying trace amounts of catalytic iiiaterial. However, a series of experiments in which occasional rate samples were intentionally contaminated revealed no substance which had any pronounced catalytic effect. Thus samples of bromosuccinate or @-bromobutyrate which had been treated with anhydrous hydrogen bromide lost activity a t about the same rate as did untreated samples.; Addition of glass wool, powdered Pyrex or powdered soft glass to the reaction tubes, had no noticeable influence on the rate of reaction of the bromosuccinate. The energies oi activation, E , as defined by equation (11) for the racemization and dehydrobromination reactions of the bromosuccinate and 8-brotnobutyrates were estimated from plots of log K; m i d log ka against thc corresponding lf’7‘ values. k
= .le-l:,lfT
(11,
JAMES E.
Vol. ’74
TABLEI\’ ACTIVATION E ~ R G I EAND S FREQUENCY FACTORS FOR RACEMIZATIONS ASD L)EHYDROBROMINATIONS E,
Ester
Process
kcal. mole -1
Bromosuccinate Bromobutyrate Bromosuccinate Bromobutyrate
Kacetnizatiori Racemization Uehydrobrorninatioti Uehydrobromiiiation
21.4 28.8 23.2 28.8
‘THE
A,
sec. - 1
1.0 x 1.3 x 1,3 X 1.3 X
107 10‘0
IO7 1Olo
a uniniolecular reaction.6 However the magnitude of the calculated A values is markedly influenced by relatively small variations in the calculated energies of activation. Because of errors inherent in the measurement of the rates it seems likely that the reported A values may deviate from the correct valucs by at least 3. power of ten. The kinetic studies on the broiiiosuccinate indicate that repeated distillation of this ester should produce racemizations oi the order of magnitude which has been observed experimentally. The Reaction Mechanisms.-The kinetic data do iiot provide sufficient evidence to establish conclusively the reaction mechanisms for the observed racemizations and dehydrobrominations. Certain of the results, however, are of value in a discussion of possible reaction paths. For example, it is noteworthy that the 6-bromobutyrate racemizes and dehydrobrominates in a manner similar to the bromosuccinate while the cr-bromobutyrate is inert to these thermally induced reactions. It thus appears that in order that reaction occur the halogen atom must be located on a carbon beta rather than alpha to the carbalkoxy group. I t seems highly unlikely that the observed reactions of the bromosuccinate proceed through the enol form li()
0
,c-c
-CH--C’ ‘OR
HO
BI
It is also significant that the frequency factors for the reactions of the bromosuccinate and /?-bromobutyrate reactions are sufficiently small as to be indicative that the reacting molecules suffer a restriction in the number of degrees of freedom in attaining the activated complex configuration. This is typical of many reactions which are intramolecular in type.’ One possible reaction mechanism which might account both for the racemization and dehydrobromination reactions of the P-bromobutyrate is rapid CHI -CII-CI-I~ - C -0CHj
‘The resultant values along with the calculated frequency factors, A , are reported in Table IV. It should be noted that these frequency factors are much smaller than those generally expected for 1 5 ) R. K u h n a n d 1’. W&gner-Jauregg, ref. 2 ( a , b ) , have suggested t h a t traces of hydrogen bromide are responsible for the “autoracemiza tion” of t h e hromosuccinate. It is somewhat surprising t h a t the prese n t experimental results offer no evidence for such a catalysis. in view of the fact t h a t lithium bromide so effectively promotes racemization of the ester in acetone solution Id I t might be possible to ioterprel the present results if information concerning the solulJility and dissociat i o n constant of hydrogen bromide i n the Imre ester a t temperatures i’ 1 0 0 ” ai.iilablt:
HAKDWICKE
Br
I cversible
_ _ --+ f--
OK CH~-CII-CH=C-OCHJ
+ Br-
OH ( 6 ) S Glasstone, K. J . J.aidler and H. Eyring, “ T h e Theory of Rate I’rocrsses,” hIcCrrdw-Hili Book C o m p a n y , Inc , New York, N. Y . .
lY4l. (7)
i d ) J. IC. Kiiicaid and 1 ). S. ’I‘arbell, IS J O U R N A L , 61, 3083 (lU3’X: [ t i ) I?. G Poster A . C Coiie atid 1;. Daniels, ibid , 69, 1895 !194i)
July 20, 1952
THERMAL RACEMIZATION OF DIMETHYL ( - ) -BROMOSUCCINATE
3585
The rate determining step in this sequence is pic- distilled to yield 27.8 g. of ester (b.p. 103-106' at 7 mm.) of [ a ] 1 663.2'. ~ Samples of ester prepared in this manner lost tured as the formation of a carbonium ion from the optical activity when subjected to redistillation in much the enol form of the ester. The resultant carbonium same manner as was observed with samples prepared by the ion could then recapture bromide ion or lose a pro- diazomethane procedure. One sample of the ester, [a]2sD -65.6', was dissolved in ton to form, respectively, either of the enantioan ether solution of diazomethane. After this solution morphic forms of the bromoester or the unsaturated had stood for four hours in the refrigerator, it was concenester. It should also be noted that this cation trated on the steam-bath at reduced pressure. The undismight yield crotonic acid by reaction with bromide tilled residue gave [ C Y ] ~ D-63.4'. Therefore the ester is ion. The activated complex in the slow step of the not appreciably racemized by contact with diazomethane. Dimethyl ( - )-Chlorosuccinate .-( - )-Chlorosuccinic acid reaction might be so constituted that the enolic prepared from L(+)-aspartic acid by the method of hydrogen would assist in the rupture of the carbon- was H ~ l m b e r g . ~The product gave m.p. 175-176' and [ a I a 5 ~ bromine bond by a process comparable to hydrogen -20.9' (0.0614 g./cc. in water). A solution of 5.3 g. of the acid in 20 ml. of ether and 15 ml. of methanol was cooled to bond formation as indicated below. 5' and treated with excess diazomethane in ether. After several hours the resultant solution was concentrated and the residual oil was distilled. The 4.2 g. of product had I the following properties: b.p. 99-100' (11 mm.); [ c Y ] ~ ~ D Br C-OCHs -42.3"; calcd. C1, 19.64; found C1, 19.55. Methyl ( +)-a-Bromobutyrate.-Racemic a-bromobutyric acid was prepared by the procedure of Fischer and Mouney"' 0 rat '0 and was resolved by methods described previously.11 After two resolutions the acid (b.p. 90-93' at 10 mm.) gave ~ (0.5-dm. tube). The acid was esterified using Evidence for the existence of the 0-H-Br bond has a 2 6 +8.83' been obtained in studies of o-bromophenol.* This the diazomethane procedure described for preparation of di(-)-bromosuccinate. The product had the followcyclic structure for the activated complex is in methyl ing properties: b.p. 48-51' (15 mm.); n 2 ' ~1.4497; d26, accord with the frequency factors observed for 1.4022; [ a I 2 6f13.5'; ~ calcd. Br, 44.2; found, Br, 44.3. Methyl ( +)-P-Bromobutyrate.---Bromobutyric acid was these reactions. A similar process could be written to account for the reactions of the bromosuccinate. prepared and resolved according to the directions of Olson and Miller." The crude product, a Z 6+17.3 ~ (0.5-dm. The thermodynamic data for the bromobutyrate, tube), was esterified with diazomethane in ether solution. at least, do not contradict the supposition that The resulting methyl ( +)-@-bromobutyrate had the followboth the racemization and dehydromination re- ing properties: b.p. 71' (15 mm.); [ a ] " ~f10.85°; n% 1.3858; calcd. Br, 44.15; found Br, 44.10. actions proceed through the same activated com- 1.4524; The Thermal Decomposition of Methyl 6-Bromobutyrate. plex. It is entirely possible that the two reactions -For this study a sample of the inactive ester (b.p. 111of the bromosuccinate also proceed through a 113' (93 mm.), n2sD 1.4520) prepared from P-bromobutyric common activated complex since the differences acid and methanol using anhydrous hydrogen bromide as a in the reported activation energies may result catalyst, was employed. The procedure was closely similar to that used in the methylation of bromosuccinic acid. entirely from experimental error. Fifteen grams (0.083 mole) of methyl P-bromobutyrate was heated at 160-180' on an oil-bath for 17 hours. Evolved gases were trapped in a known volume of standard sodium Experimental hydroxide. Analysis of the trap contents indicated that ( -)-Bromosuccinic Acid.-This was prepared from L- only about 10% of the theoretical quantity of hydrogen (+)-aspartic acid according to the method of Holmberg.9 bromide was evolved. The product had the following properties: m.p. 170-171'; The heated sample was subjected t o distillation a t atmos[ o L ] ~ ~-39.6' D (0.184 g./cc. in water); neut. equiv. calcd., pheric pressure. A small low boiling fraction weighing 98.5; found, 98.6. 0.37 g. was collected from 120-155'. This material had a The Reaction of (-)-Bromosuccinic Acid with Diazo- characteristic ester-like odor and gave ?223D 1.4330. It abmethane.-To 7 g. of ( -)-bromosuccinic acid was added sorbed bromine from carbon tetrachloride solution and was excess diazomethane (prepared from nitrosomethylurea) in water insoluble. This material was undoubtedly methyl ether at 0'. The resultant solution was allowed to stand crotonate. for 30 minutes and was then concentrated on the steamFrom the cooled residue in the distilling flask a large bath a t reduced pressure. The residual yellow oil ( [ a ] 2 6 ~ quantity of crystalline material precipitated. The crystals -82.2') was distilled under reduced pressure. The 6.6 g. were filtered (weight 2.2 g.) and recrystallized from Skellyof dimethyl ( -)-bromosuccinate thus obtained had the solve C. The recrystallized material was identif$d as following properties: b.p. 108-110' (10 mm.); [ a I z 6 ~crotonic acid in terms of it: melting point, 68-70 , and -55.8'; dm4 1.501; n% 1.4610; calcd. Br, 35.5; found mixed melting point, 69-71 , with an authentic sample. Br, 35.2. After a second distillation (103-104' a t 7 mm.) The isolated crotonic acid gave a neut. equiv. of 85.7; theothe ester showed a specific rotation [a]2 6 -48.6'. ~ retical for CsH&OOH, 86.1. A sample of the ester prepared by reversing the order of The possibility that hydrogen bromide liberated in the mixing of reactants in a procedure similar to that used by thermal decomposition of the bromoester might have cleaved Bergmann and Sprinzaka was still highly optically active. some of the unreacted material to form 8-bromobutyric acid This ester sample (b.p. 103-108' at 7 mm.) gave [ ( Y ] ~ ~ D was considered. Any attempt to separate small amounts of -54.1 and Br, 35.3. These procedures were used several this acid from the mixture of crotonic acid and methyl ptimes t o prepare small samples of the ester. The lowest bromobutyrate seemed futile. It is significant that the unspecific rotation recorded for these samples was [ C ~ ] ~ Dpleasant odor of the bromoacid could not be detected in the -39.1'. reaction product. Also it should be noted that no further Esterification of ( -)-Bromosuccinic Acid with Methanol. hydrogen bromide elimination occurred after nine hours of -A solution of 43 g. of the acid in 200 ml. of methanol was heating the bromoester, whereas crotonic acid precipitation cooled to 0' and saturated with anhydrous hydrogen chlo- was not observed until after a 17-hour heating period. ride. The resultant solution was kept a t refrigerator tem- These facts suggest that once methyl crotonate develops perature for two days and was then concentrated under re- to an appreciable amount in the reaction mixture, the libduced pressure a t 30". The residue was washed with so- erated hydrogen bromide is consumed to form crotonic acid. dium bicarbonate and water. dried over calcium sulfate and CHa-CH-
CH
\\
/
(8) 0.R. Wulf, U. Liddel and S. B. Hendricks, THIS JOURNAL, 68, 2289 (1930). (9) B. Holmberg, Bcv., 6OP 2194 (1927).
(10) E. Fischer and A. Mouneyrat, i b i d . , 33, 2387 (1900). (11) (a) P. A. Levene, T. Mori and L. A. Mikeska, J. B i d . Chcm., T S , 342 (1927); (b) P. A. Levene and M. Kuna, i b i d . , 141, 391 (1941). J O U R N A L , 60, 2687 (1938). (12) A. R. Olson and R. J. Miller, THIS
A consideration of the mechanism of the ester cleavage reaction suggests that methyl crotonate would be much morc susceptible than methyl 8-bromobutyrate t o reaction with hydrogen bromide. The Kinetic Studies.-For the optical rotation studich samples of the pure bromoesters were sealed in small Pyrex glass tubes. The tubes had previously been exposed for 24 hours t o hot concentrated nitric acid, washed with distilled water, dried and stored in a desiccator over phosphorus pentoxide. The rate samples were immersed in the vapor of a refluxing liquid of appropriate boiling point. Substatices such as water, toluene, chlorobenzene and 71butyl ether were used. The tubes were removed from time to time, quenched in ice water and opened. The optical rotations of the contents were determined using a 0.S-dm. inicropolarimeter tube. Samples were ordinarily allowed to equilibrate to bath tcmperature for one hour before the first sample was withdrawn. The optical rotation of this sample was used as cy0 in the calculation of rate constants, KOhpd, reported in Table I. To follow the kinetics of dehydrobrotnination weighed samples of the bromoesters were heated in the Pyrex tubes as described above. Tubes rernoved for analvsis were broken in a beaker of ice-water, and the contents of the
beaker were titrated with 0.1 ii sodium hydroxide to the phenolphthalein end-point using a microburet. The reaction tubes were again allowed to equilibrate to hath temperature for at least one hour before withdrawal of thc first sample. The time of withdrawal of this samplc was chosen as zero time in calculating rate constants. Analyses of samples withdrawn subsequently were corrected on the assumption that all samples underwent the same per cent. dchyclrohromination during the cquilibratioii period. To establish that unreacted bromoester did not consume standard base under the conditions of the titration approximately 0.2 g. of the pure esters were treated with cold aqueous hydrochloric acid in arnounts approximating the hydrogen bromide content of some of the rate samples. By titration with sodium hydroxide the added hydrochloric acid could bc accounted for with an accuracy of &0.5%.
Acknowledgment.-The authors wish to express their appreciation to Professor A. I-?. Olson and Jlr. Frances J. Spieth for their suggestions concerning the preparation of this manuscript. I>,n\is, CALIFORNIA
[CONTRIBUTION FROM THE DEPARTMEN r O F CHEMISTRY AND LABORATORY FOR XUCLEAR SCIENCE MASSACIKSETTS ISSTITUTE O F TECHNOLOGY]
AND
ENGINEERING,
Rearrangement of 2-Methylbutane-l-C14over Aluminum Bromide1*' B Y JOHN D. ROBERTSAND GEORGE I
