Radicals radical halogenation

N Bromosuccimmide provides a low concentration of molecular bromine which reacts with alkenes by a mechanism analogous to that of other free radical halogenations... 

The comparative ease with which a benzylic hydrogen is abstracted leads to high selectivity m free radical halogenations of alkylbenzenes Thus chlorination of toluene... 

Sections Free radical halogenation and oxidation involve reactions at the benzylic 11 12-11 13 carbon See Table 112... 

Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

Mechanism and selectivity of free-radical halogenation with cyclic N-haloimides 99MI19. 

Preparing Alkyl Halides from Alkanes Radical Halogenation 33B... 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives. Acyl halides can be a brominated or chlorinated by use of NBS or NCS and HBr or HCl. The latter is an ionic, not a free-radical halogenation (see 14-2). Direct iodination of carboxylic acids has been achieved with I2—Cu acetate in HOAc. " ° Acyl chlorides can be a iodinated with I2 and a trace of HI. Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium A-isopropylcyclohexylamide in THF and treatment of this solution at -78°C with... 

The first step involves a reduction of the diazonium ion by the cuprous ion, which results in the formation of an aryl radical. In the second step, the aryl radical abstracts halogen from cupric chloride, reducing it. The CuX compound is regenerated and is thus a true catalyst. 

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