Halides, alkyl from radical halogenation

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

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. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Alkyl halides are important starting materials in the synthesis and manufacture of a variety of organic compounds. Free-radical halogenation (Scheme 4.12) is an important methodology for the synthesis of alkyl halides from alkanes. Brominations (X = Br) tend to be very selective because of the... 

Radical substitution reactions can also be used to remove functional groups from molecules. A useful reagent for this (and, as you will see, for other radical reactions too) is tributyltin hydride, Bu3SnH. The Sn-H bond is weak and B SnH will react with alkyl halides to replace the halogen atom with H, producing BL SnHal as a by-product. 

A halogen atom abstracts hydrogen from the alkane (RH) to form an alkyl radical (R ). The radical in turn abstracts a halogen atom from a halogen molecule to yield the alkyl halide (RX). 

Preparing Alkyl Halides from Alkanes Radical Halogenation 335 Table 10.1 A Comparison of the Halomethanes... 

For the lanthanide metals, Sm, Eu, and Yb, which have a readily accessible ( + 2) oxidation state, oxidative addition reactions of M(1I) complexes with halogens and organic halides are dominated by the atom transfer or free radical mechanism (cf. 5.S.2.9.1.) in which two metal ions are each oxidized to the -I- 3 state. Numerous examples illustrate the ability of cyclopentadienyl or indenyl Ln(II) complexes (Ln = Sm, Eu, Yb) to abstract halogen atoms from molecular halogens ", halogenated solvents sueh as CH2CI2 and and alkyl halides . An archetypieal example is ... 

There are three ways alkyl halides can be named. They are all correct naming conventions, and the compounds may be listed under any one of the possibilities. When researching the compounds in reference books, you may have to look under the alternate names to find information on the compound. The first naming convention is one in which the radical is named first, the ine is dropped from the halogen, and an ide ending is added. For example, if the compound has one carbon, the radical for one carbon is methyl. If there is chlorine attached to the methyl radical. 

Several functional group exchange reactions produce alkyl halides. The most general one is the formation of an alkyl halide (R-X) from an alcohol using various reagents. Alkyl halides are also formed from alkenes, as shown, and from alkanes via radical halogenation. 

The path for the formation of the Grignard reagent is believed to involve transfer of an electron from the sea of electrons present in the metal into the lowest unoccupied molecular orbital (LUMO) of the alkyl (aryl) halide. The radical anion/ surface-metal cation can continue with the transfer of a second electron or, depending on steric and electronic circumstances, an alkyl (aryl radical) and halogen anion can be produced. Coupling and elimination products occasionally accompany reduction and those products can be rationalized as being derived from free radicals. The halide anion remains associated with the surface as it does with most reactions in solvents that cannot support ionic materials. The alkyl radical would be free from that constraint. 

A classic example of how reactivity is related to selectivity is concerned with the free radical halogenation of alkanes by CI2 and Br2. In this free radical chain reaction, the step that sets the position of the halogen in the alkane is a hydrogen atom abstraction step. The carbon based radical created in this first propagation step then abstracts a halogen atom from the CI2 or Br2, giving the alkyl halide (see below). In free radical halogenation by either CI2 or Br2, tertiary alkyl halides are created preferentially to secondary, which in turn are formed preferentially to primary alkyl halides. This reflects the fact that the order of radical stability decreases from tertiary to secondary to primary. Yet, the extent of the selectivity for tertiary over secondary over primary is quite different for chlorination and bromination. 

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