[CONTRIBUTION FROM
THE
DEPARTMENT O F CISEMISTRY, UNIVERSITY
OF
COLORADO]
Mechanisms of Elimination Reactions. XII. The Reaction of cis- and trans-p-NitroP-bromostyrene with Ethanolic A&ali1~2 BY S T A N L E Y J. CRISTOL, XRTIlUR BEGOON,3 W I L L I A M P.NORRISAND P A T R I C I A RECEIVED MAY6, 1954
s. RAMEY
cis and tmns-p-nitro-B-bromostyrene isomers, upon reaction with ethanolic sodium hydroxide, are converted t o p-nitro~)hei~ylacetyiene and 1,l-diethosy-2-p-nitrophenylethane, respectively. The formation of the acetylene from the cis haloolefin (trcins elimination) is 2300 times as rapid as the formation of the acetal from the trens haloolefin. Various reactioii l)atli r r i q ,1"19 JOURNAI., 7 6 , 3005 (18>41 (XI lleceased December 15, 1 ~ 1 . ~ 1 , ( 4 1 A. -r. D a n n , A . I I , , ~ , - ~ ~in^^ J JV. navies. J . C ~ P I I I ..sor- , iini fl928). f , j ) This
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(6) (a) L. P. Hammett, "Physical Organic Chemistry," McGrdw13111 Book Co., Inc.. New York, N. Y., 1940, chapter X I ; (b) ht. 1,. Render, THISJOURNAL, 73, 1626 (1951). 171 J. F. Bunnett and R . E. Zahler, Chcm. Reus., 49, 273 (1951). (8) J. D. Park, h l . L. Sharrah and J. R. Lacher, THISJ O U R N A L , 71, 23.37 ilOd91. (9) S . D . Ross, W. A. Leach and 1 . Kuntz, ibid., 7 4 , 2908 (19.52). (10) P. Friedlander and M. Lazarus, A n n , 229, 234 (1885), J Tliirlr hnd S. Haeckel, i b i d . , 326, 1 (1902); J. Meisenheimer and F . Iieim, B f r . ,38, 107 (1905); K . W. Rosenrnund, ibid., 46, 1034 ( 1 9 1 3 ) , .I. Loevenich. J . Koch and U. Pucknat. i b i d . , 63, I i X t i (1930); A . 1,ambert. C . W .Scaife and A . E. Wilder-Smith, 1.Chem. Soc., 1471 11817).
Sept. 20, 1954
Cis- AND ~ ~ ~ ~ ~ - ~ - N I T R O - ~ - B WITH ROM ETHANOLIC O S T Y R AELNK AEL I
4550
styrene have apparently not been studied. The TABLE I addition of ethanol to p-nitrophenylacetylene must DATAAND SECOND-ORDER RATE CONSTANTS FOR THE REof course involve addition to a p-nitrostyrene ACTION OF THE ISOMERS OF P-NITRO-6-BROMOSTYRENE WITH system (equation 3). 92.6 WT. % ' AQUEOUSETHANOLIC SODIUM HYDROXIDE Several experiments were carried out in the hope Temp., Halide, NaOH. 104k, .M M I./sec /mole Isomer OC. that reactions 1 to 3 could be demonstrated by trapping the presumable intermediate, 9-nitro- cis (H and Br 11.01 0 00826 0 0479 26.7 phenylacetylene. trans-9-Nitro-p-bromostyrene was trans) .00830 .0479 25.7 treated with a 10-fold excess of 0.15 M sodium hyAve. 26 2 droxide in ethanol containing a suspension of mer,00957 1 9 . 9 5 ,0431 95.3 curic oxide. The reaction mixture was stirred and ,00974 ,0431 99.8 heated for six days a t 43" (8 half-lives of the reac.00874 .0431 98.1 tion of the haloolefin with alkali). There was isoAve. 97.7 lated only 17% of bis-(p-nitrophenylethyny1)-mercury along with about 50% of the acetal. p-Ni.00745 .0180 195 25.17 trophenylacetylene gave a isyoyield of the dialkyl,01054 ,0360 181 mercury in a few minutes a t room temperature, .01019 ,0596 180 although the yield was lowered t o 44y0 by a sixAve. 188 hour reflux period in ethanol. The lower yield ob.00380 ,0315 373 29.96 tained with the haloolefin led us to abandon this .0215 365 ,00441 test; isolation of the dialkynylmercury does not establish the necessary proof for the existence of Ave. 369 the acetylene intermediate, even for a portion of ,00728 0859 0.108 the reaction, as i t is still necessary to show whether trans (H and 30.42 ,0859 ,171 ,00800 Br cis) or not the rate of the bromide-producing reaction Ave . .170 was influenced by the mercuric oxide (the trans halide was observed to be relatively inert toward ,0859 ,311 .00707 35.21 mercuric oxide in refluxing ethanol). ,0859 ,307 .00684 It was observed that thiophenoxide ion would Ave. ,309 compete successfully with ethoxide for p-nitro.00705 ,0859 ,581 40.17 phenylacetylene to give one of the geometric iso. 573 .0859 ,00704 mers of phenyl p-nitrophenylvinyl thioether, but Avc. ,577 experiments revealed also that the rate of producing bromide ion from the trans haloolefin in alkaline TABLE I1 solution was enhanced markedly by the presence of RATE CONSTANTSAND QUANTITIES 01' thiophenoxide ion. No evidence is available, there- SECOND-ORDER ACTIVATION FOR THE REACTIONS OF THE ISOMERS OF p fore, to show that the acetal arises, in the trans WITH SODIUM HYDROXIDE IN 92.6 system, via an elimination process, and the other NITRO-6-BROMOSTYRENE AND ISOPROPYL ALCOHOL AT 30" WT. % ETHYL reaction paths must be considered. io%, Enct., AS*, cal./ Determination of Reaction Rates.-The rates Isomer Alcohol l./sec./mole kcal./mole deg./mole were determined in ordinary commercial ethanol 3 70 23.9 +12 cis Ethyl (92.6 wt. yo),and the extent of reaction was esti17.1b - 3' Isopropyl 11400" mated by Volhard titration, substantially as 23.8 -4 tuum Ethyl 0.158 described previously.2,11 The data obtained are Isopropyl ,553" 21.3* -l(lh given in Table I . The rate runs were generally Extrapolated from the data given in ref. 2 . hRefereiicc run to i O % completion, although good straight lines were obtained to 80-907, completion. Ac- 2. tivation energies and entropies were calculated in coho12is strengthened by these results. A comparithe usual fashion" from the data in Table I, and son of the rates in isopropyl alcohol and in 92.6 w t . these and rate constants read off the activation yo ethanol indicates that in the trans elimination energy curves a t 30" are given in Table 11. Also case (cis isomer), a change from ethanol to isoproincluded in Table I1 are similar data for the known pyl alcohol increases the rate constant by about 30dehydrobromination2 of the isomeric p-nitro-@- fold, the increase in rate being due to a large debromostyrenes with sodium hydroxide in isopropyl crease in energy of activation which is partly c o n i alcohol. The rate constants for the isopropyl alco- pensated by a decrease in entropy of activation. hol solvent were extrapolated by an Xrrhenius plot For the trans isomer, the increase in rate is only from the data reported previously.2 3.5-fold, although again the decrease in activation The data in Table I1 are of interest. trans Elim- energy is partly compensated by an entropy decrease. The smaller increase in rate for the trans ination of the elements of hydrogen bromide'by alkali in ethanol from the cis haloolefin is 2300 isomer may be due to different effects of solvent times more rapid than whatever process is occurring upon the different mechanisms involved in cis and but is equally well explained in the trans haloolefin and is therefore much more trans rapid than cis elimination. Thus the discussion by the assumption that the reactions outlined in given previously for the system in isopropyl al- equations 4 or 5 are involved in the reaction of the trans isomer in ethanol, going a t a rate about ten S.J. Cristol, THISJOURNAL. 69, 338 (10.17); S. J . Cristol. N. l
