933. Pyrolyses of ethyl cyanide, t-butyl cyanide, cumyl cyanide, and acetamide, and bond strengths in cyanides

Abstract

Pyrolyses of the above cyanides and acetamide have been carried out by the aniline-carrier technique. Ethyl cyanide decomposes by three reactions yielding ultimately hydrogen, methane, and ethylene as gaseous products; the decomposition to give a methyl radical is a first-order homogeneous reaction with (686-765') : log k (sec.-l) = 14.1 -(72,700/2.3RT). The analogous reaction for t-butyl cyanide has (602-652") : log k (sec.-l) = 15-2 -(70,200/2.3RT). The rate-determining step for the production of methane in the pyrolysis of cumyl cyanide is of the first-order and homogeneous with (520--624"): log h (sec.-l) = 12-3 -(54,100/2.3RT). The methane probably arises from bond rupture in the nitrile yielding a methyl radical, but it is possibly produced by direct molecular elimination. Acetamide decomposes mainly by the molecular elimination of ammonia for which the rate constant is (663-747") log k (sec.-l) = 14.7-(73,400/2.3RT). * Calculated from AH," (Me) = 33.9 and AH," (Pr') = 17.6 kcal. mole-' (ref. 13). t Ref. 3.Obviously the introduction of a CN group into a molecule considerably weakens the adjacent C-C bond and the effect is fairly consistent throughout the short series of compounds studied.

Very low-pressure pyrolysis of alkyl cyanides. III. tert-Butyl cyanide. Effect of the cyano group on bond dissociation energies and reactivity

“…As mentioned previously, DE's A factor is clearly too low for a four-center molecular elimination but combination of the reported rate constant at the mid temperature of their study with our predicted A factor gives E = 72. 4 kcal/mol which is reasonably close to our value. Figure 2 shows the comparison between our values and the experimental data of DE.…”

Bond Dissociation Energies by Kinetic Methods

“…The decomposition of ethylnitrile has been shown to occur (95) via the reaction (95) that this difference should be close to 13 kcal, in comparison with other (D(C-H) -Z)(C-CH3)) differences, for compounds in which the H atom and CH3 radical are attached to a CH2 group. It was further concluded (95) that the most probable source of error in these determinations wras in the value of Afff0(C2H6CN) used to calculate A#f°(CNCH2). Thus the value D(CNCH2-H) = D(CNCH2-CH3) + 13 = 86 kcal was recommended as the best available.…”

The Elimination Reactions of Some Alkyl Cyanides in the Gas Phase

“…The chromatograms showed essentially HCN and the corresponding olefine at the lower temperatures. Hunt et al (4) obtained methane, ethane, and hydrogen in addition to the olefines and HCN in the decomposition of ethyl cyanide. The presence of large amounts of H, in their products is also explained by molecular mechanism.…”

“…There was no autocatalysis by HCN and the mechanism was non-chain, as the presence of a chain sensitizer, cyanogen bromide had no effect on the rate of reaction. Hunt et al (4) investigated the effect of the CN group on the strengths of the adjacent C-C and C-H bonds by pyrolyzing ethyl (between 686 and 765 "C) and t-butyl cyanides (between 602 and 652 "C). They found that ethyl cyanide decomposed both by free radical and molecular mechanisms, with HCN and olefine as the main products of the molecular mechanism.…”