Halides, alkyl, from alcohols

This reaction can occur as S l with a tertiary alcohol or SN2 with other alcohols. 

Alcohols can also be converted to alkyl halides by phosphorus halides such as PCI PBr3, and PI via an 5 2 mechanism resulting in poor yields with tertiary alcohols. Another reagent for producing alkyl halides from alcohols is thionvl chloride, OCl2, resulting in sulfur dioxide and hydrochloric acid. 

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Preparation of Alkyl Halides from Alcohols and Hydrogen Halides... 

PREPARATION OF ALKYL HALIDES FROM ALCOHOLS AND HYDROGEN HALIDES... 

We 11 begin with the preparation of alkyl halides from alcohols by reaction with hydro gen halides... 

Chemical reactivity and functional group transformations involving the preparation of alkyl halides from alcohols and from alkanes are the mam themes of this chapter Although the conversions of an alcohol or an alkane to an alkyl halide are both classi tied as substitutions they proceed by very different mechanisms... 

A more efficient method of preparing alkyl halides from alcohols involves reactions with thionyl chloride (SOCL). 

Although a substantial number of reactions are described in the text, they belong to a relatively modest number of mechanistic types. The preparation of alkyl halides from alcohols and HX, the cleavage of ethers, and the preparation of amines from alkyl halides and ammonia (and many other reactions) all, for example, occur by a nucleophilic substitution mechanism. The following is a brief review of the main mechanistic pathways discussed in the text. 

Suggest reagents that could be used to prepare these alkyl halides from alcohols ... 

As already known, a halogen atom at position 4 in the pyridazine ring is more reactive towards nucleophiles then one at position 3. This has been confirmed also with 3,4-dichloro-5,6-diphenylpyridazine and various nucleophiles (81MI2). The high reactivity of 6-halo-1,3-dimethylpyridaz-inium halides or phosphorodichloridates has been used to prepare alkyl halides from alcohols (80S746). 

Other substitution reactions we ve seen include some of the reactions used for preparing alkyl halides from alcohols. We said in Section 10.7, for example, that alkyl halides can be prepared by treating alcohols with HX—reactions now recognizable as nucleophilic substitutions of halide on the protonated alcohols. Tertiary alcohols react by an S>jl pathway involving unimolecular dissociation of the protonated alcohol to yield a carbo-cation, whereas primary alcohols react by an 8 2 pathway involving direct bimolecular displacement of H2O from the protonated alcohol (Figure 11.23). 

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