Alkenes, radical halogenation halogens

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

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. 

In these reactions (Scheme 3.1), the first electron addition is to the alkene giving a radical-anion. This interacts with the alkyl halide to transfer an electron, in a process driven by simultaneous cleavage of the carbon-halogen bond. The alkyl radical formed in this manner adds an alkene radical-anion [25]. Aluminium ions generated at the anode are essential to the overall process. They coordinate with the intermediate carbanion, which then interacts with the second halogen substituent in an Sn2 process to form the carbocycle. 

Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

Cyclohexene was shown to react partly by a radical mechanism when chlorinated in the absence of oxygen even in the dark.247 The reaction is slightly less stereoselective than the ionic process (96% trans-1,2-dichlorocyclohexane vs. 99%). Isomeric butenes under similar conditions give products nonstereoselec-tively.248 Branched alkenes are less prone to undergo free-radical halogenation. 

Benzylic halides can be prepared by the same radical-halogenating agents that give allylic halides from alkenes. These include Cl2, Br2, N-bromo-succinimide (Section 14-3A), S02C12, and im-butyl hypochlorite (see Exercise 4-18) ... 

Photoinduced free-radical halogenation is very useful for the functionalization of electron-deficient alkenes such as vinyl halides. Among halides, the addition of bromine is by far the most useful reaction, as shown in Scheme 3.25. In the first example, a polyfluoroalkene was brominated in a high yield by using a 300 W light bulb as the light source. Of note, the synthesis of l,2,4-tribromo-l,l,2-trifluorobutane (38) was carried out under solvent-free conditions in a near-quantitative yield and on a multigram scale (Scheme 3.25a) [67]. l,2-Dibromo-l,l,2-trichloroethane (39) was likewise obtained in about 250 g amounts by the solar lamp irradiation of neat trichloroethylene to which bromine was continuously added (Scheme 3.25b) [68],... 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

The action of light on an R2PX molecule in the presence of an activated alkene as halogen acceptor can lead to R2P radicals, namely,... 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

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