The photolysis of 3,4‐dimethylcyclopentanone

Abstract

The photolysis of trans-3,4-dimethylcyclopentanone has been studied in the gas phase, principally at 313 nm. However, a few experiments have also been performed using laser sources at 308 and 325 nm. Additionally, experiments were also carried out using the cis isomer. The major products produced by all three wavelengths and in the temperature range 100 to 150°C were propene, 1,2-dimethylcyclobutane, carbon monoxide, and 3,4-dimethylpent-4-en-a1. The formation of the cyclobutane was stereospecific and the effects of temperature, pressure, and wavelength on the relative product yields could be rationalized in terms of a mechanism involving the formation of a vibrationally excited triplet state which could yield both hydrocarbons and the aldehyde and a nonexcited triplet yielding only the aldehyde. Some high intensity experiments with an exciplex laser at 308 nm gave results which could be due to the occurrence of some two photon absorption by the cyclopentanone or absorption by an excited intermediate. The results are compared with those previously reported for other substituted cyclopentanones.

A study of product ratios in the direct (313 nm) and Hg‐sensitized (254 nm) photodecomposition of 2‐methylcyclohexanone in the gas phase

“…The C C6H& Products ratio in Tables 1-111 clearly shows the increasing tendency for hydrocarbon formation under conditions of lower pressure, higher temperature, and increasing excitation energy, consistent with decarbonylation being the higher energy dissociative pathway [1, 4,5]. The "energy content" of the species undergoing reaction also affects the t-A/c-A ratios, which are discussed first.…”

“…Temperature and wavelength effects are largely absent in the fragmentation to cyclization ratios for l,.l-biradicals arising from the photolysis of the methyl-substituted cyclopentanones referenced earlier [5,. * The literature on cyclopentanone itself is extensive, and we merely note that Mok [271 concluded that the ethylene/cyclobutane ratio was generally independent of all variables, with a mean value of 3.05 over the temperature range 26"-106"C, at 313 nm.…”

“…The mechanism developed by Frey and co-workers [5,[18][19][20]251 for medium-ring cycloalkanone (encompassing methyl-substituted cyclopentanones and cycloheptanone) photolysis, at several wavelengths and including Hg-photosensitized decomposition, is as follows: where S , and TI are the first excited singlet and triplet states of the ketone and RCHO denotes an aldehyde product molecule. The asterisk signifies vibrational excitation.…”

“…The photodecomposition is discussed in terms of the established triplet state and biradical intermediate mechanism for mediumring (C,-C,) cycloalkanones in the gas phase, in parallel with two other more recent studies [4,5] on cyclohexanone (CHI a n d 3,4-dimethylcyclopentanone (3,.…”

The photolysis of trans‐3,4‐dimethylcyclopentanone. II. Short Wavelengths

“…Recently we have shown that the direct photolysis of cyclopentanone with light of relatively short wavelength can be interpreted by a mechanism involving only singlet states [14]. Earlier work [7] on trans-3,4-dimethylcyclopentanone (Part I of this series) at longer wavelength was shown to involve both singlet and triplet states. We undertook the present study as a continuation of that work.…”