10.12: Summary
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Alkyl halides are not often found in terrestrial organisms, but the kinds of reactions they undergo are among the most important and well-studied reaction types in organic chemistry. In this chapter, we saw how to name and prepare alkyl halides, and we’ll soon make a detailed study of their substitution and elimination reactions.
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 N-bromosuccinimide (NBS) to give the product of allylic bromination. The NBS bromination of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance.
Alcohols react with HX to form alkyl halides, but the reaction works well only for tertiary alcohols, R3COH. Primary and secondary alkyl halides are normally prepared from alcohols using either SOCl2, PBr3, or HF in pyridine. Alkyl halides react with magnesium in ether solution to form organomagnesium halides, called Grignard reagents (RMgX), which are both nucleophilic and strongly basic.
Alkyl halides also react with lithium metal to form organolithium reagents, RLi. In the presence of CuI, these form diorganocoppers, or Gilman reagents (LiR2Cu). Gilman reagents react with organohalides to yield coupled hydrocarbon products.