Rates of gas-phase decarboxylation of seven a-dirnethylated Byunsaturated acids have been measured at a series of temperatures. Simple changes in alkylation alter the rate by up to 200-fold. The activation parameters for these reactions suggest that the reaction is only slightly affected by the electric effect of the alkyl groups : these groups exert a much larger effect by straining the double bond.WHILE some of the finer details remain to be examined, the gas-phase decarboxylation of &unsaturated acids has been shown to be a no-mechanism " reaction.2 Typically, the rates of such reactions are insensitive to substitution.3 We have observed, however, that two differently substituted phenylbut-3-enoic acids de-TABLE 2 carboxylate at rates differing by three orders of magnitude.1 This large rate difference was attributed to I changes of the conjugation and substitution of the double columns, and was >99%. The physical constants were as in Table 2. N.m.r. spectroscopy showed that acid (11) had the trans-configuration, as did the material recovered from partial decarboxylation. The stereochemistry Of (Iv) is not known. M. p. B. p. nD20 Lit. 82-83"/ 1.4284 * €3. p. 185"/760 mm. 10 mm.
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