EPOXIDATION OF ALKENESIN
GASPHASE
THE
3319
iodohydrin. The conclusion thus is that the Benson and Haugen calculation cannot be applied to fourcenter addition to a carbonyl bond, unless further modification would be done, such as enlarging the H I bond distance in the transition state more than in the transition state for additiqn to olefins (The ground sta,te bond length 0.40 A.2o). The rate constant For the elimination of water from
+
acetaldehyde iodohydrin can be calculated by the same method, using Haugen and Benson's c a l c ~ l a t i o n for s~~ addition of water to olefinic double bonds, which gives k = 1013.6-64.2/8, which is much smaller than the calculated rate constant for H I elimination. (24) G. R. Haugen and 9. W. Benson, Int. J . Chem. Kinet., 2, 235 (1970).
Epoxidation of Alkenes in the Gas Phase by D. J. M. Ray and D. J. Waddington* Department of Chemistry, the University of York, Heslington, York YO1 bDD, England
(Received April I d , 1972)
It is suggested that the retardation of acetaldehyde oxidation by alkenes is due to addition of peracetyl radicals to the alkene. This leads to the formation of the corresponding epoxide from the alkene. Addition of 2,3-dimethylbut-a-ene leads to long induction periods before acetaldehyde is oxidized and this may be due to interaction of peracetic acid with the alkene, leading to enhanced suppqeasion of chain branching. The addition reaction between peracetgl radicals and cis-but-2-ene is calculated as 2 X lo3 m3 mol-' sec-l a t 457 K.
I n a previous paper,' we reported that many aliphatic alkenes retard the gas-phase oxidation of acetaldehyde. By comparing the relative retardation coefficients of the alkenes and by considering the products formed from the alkenes, we suggested that the alkene reacted with acetylperoxy radicals (formed during the propagation reactions in acetaldehyde oxidation2), the resulting alkene-acetylperoxy radicals being stabilized by hyperconjugation. The addition reaction (reaction 1) is able to compete with the a1t)ernativereaction (reaction 2) However, 2-methyl-2-butene and 2,3-di~
03CCH3 GH&Oa.
+. >C=C
C-C
C-C
