Unimolecular decompositions of brominated and chlorinated hydrocarbons

The following direct conclusions may be drawn from examination of the two tables (i) the Arrhenius A values are grouped around sec (n) there 

In 1950 Barton and Head proposed that unimolecular dehydrochlorination proceeds via a cyclic four-membered transition state of the following type  

BROMINE COMPOUNDS, UNIMOLECULAR DBHYDROBROMINATIONS ARRHENIUS PARAMETERS 

Compound Method Temp, range (°C) A (sec ) E (kcal.mole Ref. ) 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

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