June 5, 1954
DEBROMINATION OF P-BROMOSTYRENE AND ~-NITRO-/~-BROMOSTYRENE 3005
AFo - TAS". we can estimate AF'. AHo and AS" for this equilibrium. The results are presented in Table VIII.lS TABLE VI1 The data of Table VI permit us to draw some RATES AND EQUILIBRIA FOR T H E ANILINE-2,4-DINITROconclusions as to the effect of solvent on the extent CHLOROBENZENE REACTION IN ABSOLUTEETHANOL of complexing between aniline and 2,4-dinitroTe,mp.. kx, K, C. 1. mole-' hr.-1 1. mole-1 chlorobenzene. Complexing is most extensive in the most polar solvent, ethanol, and is either absent 24.4 0.29 0.45 or occurs to only a slight extent in the least polar 35.0 .46 .I9 solvent, ethyl acetate. I n a mixture of these two 45.0 .77 .10 solvents, the equilibrium constant has the expected From the slope of a plot of In K vs. 1/T, we can intermediate value. The observed effect is that estimate AHo for the equilibrium between the com- predicted by both Weiss4" and h l ~ l l i k e n for ~~ plex and aniline and 2,4-dinitrochlorobenzene. US- complexing of the charge-transfer type and lends ing the expressions AF' = - RT In K and AH' = support to their theories. Acknowledgment.-It is a pleasure to acknowlTABLE VI11 edge helpful and stimulating discussion with Prof. ENTROPY, FREEENERGY A N D HEATCONTENT OF THE ANIC. Gardner Swain of the Massachusetts Institute I , I N E - 2 , ~ - ~ I N I T R O C H L O R O B E N Z E N E hf01,ECULAR COMPLEX of Technology and with Drs. Fielding Brown and E QUI LI BRIUM George J. Kahan of these laboratories. Temp., AHO, AFO, A.9,
along with our results a t 24.4', are given in Table VII.
O C .
24.4 35.0 45.0
kcal. mole--1
-13.1 -3.i -13.1
cal. mole--1
+ 472 + io20 + 1400
[CONTRIBUTION FROM THE
cal. mole--'
-45.8 -46.0 -45.9
(13) It is also possible to plot both In ka a n d In k i v s 1 / T and then to calculate the heat, entropy and free energy ot activati t hki and
icn NORTli
ADAMS,MASSACHUSETTS
DEPARTMENT O F CHEMISTRY,
UNIVERSITY O F COLORADO]
Mechanisms of Elimination Reactions. XI. Alkaline Dehydrobromination of Isomers of P-Bromostyrene and of p-Nitro-0-bromostyrene' s2
BY STANLEY J. CRISTOLAND WILLIAMP. NORRIS RECEIVED NOVEMBER 23, 1953 Reaction-rate constants and quantities of activation for the dehydrobromination of the cis and trans isomers of 8-bromostyrene and of p-nitro-8-bromostyrene with sodium hydroxide in isopropyl alcohol have been measured. The effect of the p-nitro substituent was found to be significantly greater in elimination of cis elements of hydrogen bromide than elimination of trans groups, The data are discussed in terms of a concerted process for lrans elimination and a multiple-stage, carbanion-intermediate process for cis elimination.
It has been previously proposed2-* that a t least two mechanisms exist for base-promoted dehydrohalogenation of alkyl halides (and in general, for bimolecular elimination reactions). One of these has been described as a concerted process in which the removal of the proton by base is believed to be synchronous with the formation of the multiple bond and loss of halide ion. The second process has been termed "multiple-stage" and is presumed to involve a rate-determining removal of a proton by base yielding a carbanion, which then loses a halide ion to give the olefinic product. Elimination of trans groups has been assumed to use the concerted process, whereas cis elimination has been assumed to involve the multiple-stage process. Isotopic evidence that a carbanion intermediate is involved in elimination from @-benzenehexachloride has been described recently.2 A consequence of this dual mechanistic scheme (1) This paper was presented in part before the Division of Organic Chemistry at the Spring 1953 meeting of the American Chemical Society in Los Angeles, California. (2) Previous paper in series: S. J. Cristol and D. D. Fix, THIS JOURNAL, 71, 2647 (1953). (3) (a) S. J. Cristol, i b i d . , 69, 338 (1947); (b) S. J. Cristol, N. I,. Hause and J. S. Meek, i b i d . . 73, 674 (1951); (c) S. J. Cristol and A. Begoon, ibid., 74, 5025 (1952). (4) S. I. Miller and R. M . Noyes, { b i d . , 74, 629 (1952).
(see below for extended discussion) is that the appropriate placing of electron-attracting or electronwithdrawing groups on the 0-carbon atoms should have a greater effect upon the multiple-stage process, where the carbanion is being formed, than on the concerted process, where much of the negative charge is being dispersed to the departing halogen rather than to the &carbon atom. Accordingly data on elimination from the cis and trans isomers of P-bromostyrene and p-nitro-0-bromostyrene should be a test of this theory. Product Study.-As ethanol was found t o be an unsuitable solvent for the alkaline dehydrobromination of trans-p-nitro-0-bromostyrene, due to the formation of the ethyl acetal of p-nitrobenzaldehyde rather than the p-nitrophenylacetylene desired,> the first problem was to find a solvent in which one could prove that elimination occurred. As sodium t-butoxide in t-butyl alcohol ordinarily is sluggish in addition to multiple bonds,6 this was first studied. It was found that trans-P-nitro-& bromostyrene (the slower eliminating isomer) gave an 88% yield of p-nitrophenylacetylene upon (5) S. J. Cristol, A. Begoon, W. P. Norris and P . S. Ramey, unpublished work. (6) P. Beyerstadt and S. M. McElvain. TAIS J O U R N A L , S8, 529 (1936); 69, 2266 (1937).
3006
STANLEY J. CRISTOL AND WILLI.~M P. NORRIS
treatment with this reagent. The reaction of the cis isomer was too fast to measure conveniently, however, in this solvent a t room temperature, and t-butyl alcohol cannot be used as a solvent a t low temperatures because of its melting point of about 25'. Sodium hydroxide in isopropyl alcohol was next tried as a reagent. The reactions were found to go a t convenient rates. trans-@-Bromostyrene gave 80% of the theoretical phenylacetylene (isolated as the mercuric derivative). trans-p-NitrophenylP-bromostyrene gave considerably less than the theoretical amount of p-nitrophenylacetylene when treated with this reagent, but the fact that elimination was the initial step in the reaction could fortunately be demonstrated. The dehydrobromination of 232 mg. of this material was allowed to proceed to 54% completion, and the products were separated by chromatographic analysis. Ninety-eight per cent. (of theoretical) of the unreacted e-nitro-/3-bromostyrene was isolated, along with 20% (of theoretical) of 9-nitrophenylacetylene and 39% (calculated as diisopropyl acetal of p-nitrobenzaldehyde) of non-crystalline material. The nature of this oil was not investigated. As noted above, in ethanol the corresponding ethyl acetal is p r ~ d u c e d . ~A major share of the approximately 40% discrepancy in recovery can probably be attributed to loss of the fairly volatile p-nitrophenylacetylene under the conditions of the product isolation. A control experiment was then run to determine to what extent P-nitrophenylacetylene reacted with sodium hydroxide and isopropyl alcohol when exposed for the same length of time as in the dehydrobromination. Accordingly, 80 mg. of p nitrophenylacetylene (the amount which would theoretically be formed in the dehydrobromination above) was exposed to the same concentration of alkali in isopropyl alcohol for the time calculated by integration to give the same exposure. Thirty per cent. of the original p-nitrophenylacetylene was recovered and a 30% yield (based upon the isopropyl acetal) of an oil was obtained also. The similarity in recovery in the two experiments indicates that the major initial reaction product is p-nitrophenylacetylene and that this solvent is satisfactory for elimination studies. cis-p-Nitro-/3-bromostyrene gave excellent yields of P-nitrophenylacetylene with sodium hydroxide in 95y0ethanol a t room t e m p e r a t ~ r e ,and ~ i t is safe to assume that this would also be true in isopropyl alcohol. cis+Bromostyrene was converted to the expected elimination product, phenylacetylene. Measurement of Reaction Rates.-The rates were determined in isopropyl alcohol which was dried over calcium oxide before use, and the extent of reaction was determined by Volhard titrations for halide ion, substantially as described previously.3 A refrigerated thermostatted bath was used for temperatures below 20' and temperature control was *0.03'. A thermostatted waterbath was used between 20 and 45' and temperature control was rt0.02'. =Ibove 45' a thermostatted oil-bath was used and temperature control varied from rt0.03 to +0.05', with poorer control at the higher temperatures. cis-a-Bromostyrene
Vol. 76
and cis- and trans-9-nitro-P-bromostyrene were run in volumetric flasks; trans-P-bromostyrene was run in sealed Pyrex test-tubes. The calculation of rate constants was done as described p r e v i ~ u s l y . ~ Corrections of rate constant for expansion or contraction of solvent7 were made when appropriate. The rate data are given in Table I. TABLEI SECOND-ORDER RATECONSTANTS FOR THE DEHYDROBROMIS A T I O T O F VARIOUS Cis- A N D tranS-P-BROMOSTYRESES WITH SODIUM HYDROXIDE IS ISOPROPYL ALCOHOL 10'k, Tyw., C.
Halide,
2.02
0.003433 .003449 .003464
10.30
,003489 .003503
22.15
,003499 .003438
trans-p-Xitro-@- 48.04 bromostyrene
,009987 ,01004
Compound
cis-p-Xitro-abromostyrene
cis-@-Bromostyrene
trans-@-Bromost yr e n c
M
50,12
.01005 .01004
61.40
,009899 ,01089 .01000
22.15
.01196 .01111
32.37
,01184 ,01212
43.04
,01055 ,01127 .009571
97.57
,01098 , 0 1000 ,01007 , 0 1024
107.42
,01018 ,01024 ,01025 ,01007
118.27
,01045 ,009747 ,01015 ,01033 . 01041 II09389
These solutions also contained 0.1870 M.
SaOH,
M
1 /mole
f
sec
0.01311 607 ,007678 025 .007678 632 Av. 62 1 ,007604 1730 ,007664 1730 Av. 1730 ,007565 5250 .007505 5230 AV. ,5240 ,1262 2 48 ,1922 2 45 AV. 2 46 4 54 ,2011 ,2011 4 62 Av. 4 58 ,1849 15 3 ,1849 15 2 ,09645 10 8 AV. 13 S ,1048 2 85 ,1048 2 83 Av. 2 83 ,1036 9 29 9 l
