Dibenzoyl peroxide, homolysis

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Homolytic cleavage of most a bonds may be achieved if the compound is subjected to a sufficiently high temperature, typically about 200 °C. However, some weak bonds will undergo homolysis at temperatures little above room temperature. Bonds of peroxy and azo compounds fall in this category, and such compounds may be used to initiate a radical process. Di-tert-butyl peroxide, dibenzoyl peroxide... 

Dibenzoyl peroxide is an important compound because it can act as another initiator of radical reactions we ll see why later. It undergoes homolysis simply on heating. 

Addition of aliphatic aldehydes to perfluoroalkenes and cycloalkenes under y-ray irradiation or with heating in the presence of dibenzoyl peroxide gives partially fluorinated ketones by homolysis of the formyl C H bond for example, formation of 1,1,1,2,3.3-hexafliiorohep-tan-4-one. ... 

A fourth class of radical-forming reaction is homolytic cleavage. For an example, we can go back to dibenzoyl peroxide, the unstable compound we considered earlier in the chapter because it readily undergoes homolysis. 

In radical reactions, bonds break homolytically with one electron going one way and one the other. The radicals that are formed have an odd number of electrons, one of which must be unpaired. This makes them very reactive and they are not usually isolated. Even strong bonds can break into ions provided they are polarized, but to make radicals we need weak symmetrical bonds such as 0-0, Br-Br or I-I. Dibenzoyl peroxide, the Ph(C02)2 catalyst in this reaction, readily undergoes homolysis like this—the one-electron movements are represented by fish-hook arrows having one barb and odd electrons on atoms are represented by dots. 

A fourth class of radical-forming reaction is elimination. For an example, we can go back to dibenzoyl peroxide, the unstable compound we considered earlier in the chapter. The radicals formed from dibenzoyl peroxide by homolysis are themselves unstable and each can break down by cleavage of a C-C bond, generating CO2 and a phenyl radical. This is a radical elimination reaction, and is the reverse of a radical addition reaction. 

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