Kinetics of the Pyrolysis of Organic Iodides

Publisher Website
Google Scholar
Abstract
An analysis of the data on iodide pyrolysis is made in the light of some recent work together with the newly proposed atom mechanisms for the rapid reactions of RI+HI→RH+I2. It is shown that two mechanisms seem to be rate limiting in the decompositions of alkyl iodides: RI→HI+olefin,I+RI→I+HI+olefin.These are then followed by very rapid reaction of RI+HI, which maintains the HI at a very low stationary state concentration. Mechanism A appears to govern the pyrolyses of i-PrI, EtI, t-BuI and CH3COI, while mechanism B holds for n-PrI, i-BuI and n-BuI. The pyrolysis of sec-BuI appears to involve both A and B as does that of 1,2-C2H4I2. For φCH2I, the mechanism is: I+φCH2I⇋φCH2+I2 (equilibrium),2 φCH2→(φCH2)2 (slow);and for allyl I it is: I+allyl I⇋allyl+I2 (equilibrium),allyl+allyl I→biallyl+I (slow),2 allyl→biallyl (slow). The I2 in the system brings about a rapid conversion of biallyl to cyclohexene via abstraction of an allylic H atom. It is shown that the racemization of optically active sec-BuI is not a Walden inversion but a simple reversible abstraction of I: I+RI*⇋ lim 21R+I2,followed by racemization in the back reaction. Where possible, the data are analyzed quantitatively, and Arrhenius parameters are assigned to the different steps. These are discussed in the light of current data on molecular structure and bond energies. A unique and somewhat puzzling feature of reaction B is that it appears to go as a concerted step, so that the activation energy is precisely equal to the endothermicity of the reaction. By detailed balancing, the reverse reaction would have no activation energy. This does not occur for alkyl radical attack on n-PrI but appears to be unique to I atoms. If it does occur, it is the first example of a concerted reaction involving free radicals.

This publication has 17 references indexed in Scilit: