Halogenation of alcohols

Alcohols are converted to alkyl halides using a number of methods. All methods involve activating the OH group to make this into a better leaving group (see Section 5.3.1.4). 

The hydroxide ion is relatively unstable and hence is a bad leaving group 

The mechanism of these reactions depends on whether a primary, secondary or tertiary alcohol is used. (Section 5.3.1 discusses the mechanisms.) 

The OH group can be converted into a better leaving group, namely water, by protonation. As water is neutral, it is more stable than the hydroxide ion and hence is a better leaving group. 

Reaction with phosphorus trihalides (PBr3, PC13) 

In a concerted reaction, all bond making and bond Ixealdng occurs in one step 

Halogen can be introduced into a molecule is several ways. An important method involves replacement of the OH unit of an alcohol with halogen. The hydrogen atom of an aUtyl fragment, particularly an allylic or benzylic fragment can be replaced with halogen by a free-radical mechanism. Both of these transformations will be examined in this section. 

In Section 2.7.B, the reaction of a tertiary alcohol with HBr or HCl gave a halide via a carbocation intermediate. This is a very general reaction in which alcohols react with mineral acids (HCl, HBr, and HI) to give the corresponding alkyl halide.Wg gajj explain the process by a simple example in which 1-methyl-l- 

Chapter 2. Acids, Bases, Functional Group Exchanges 

The bromination method mentioned above that uses triphenylphosphine and either CBr4 or elemental bromine is a particularly mild method for this transformation. Cyclopropylcarbinyl alcohols are acid sensitive substrates, for example, usually giving ring-opening products or cyclobutane derivatives on treatment with acid (see above). S,158 pjjg reaction of 147 with PPh3 and bromine gave a 79% yield of 148. S9.160 3 35 

PhNHMe ZnBr2( Ph3P, Et02CN=NC02a (CF3CO)20 / LiBr 

PI12PF3 Chem Pharm Bull 16 1784 (1968) (1°, 2° alkyl 1° benzylic) 

PhNEt2BH3,12 NaBH4,12 MeSiCl3) Nal (Me3Si)20-P205, Nal Me3SiI 

PhNHMe PdCl2(FhCN)2 ZnCl2) PPh3, Et02CN=NC02Et 

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Experiments conducted using aqueous chloride salts as pretreatment agents prompted Lewandowski and Ollis to adjust this mechanism to reflect the possible role of aqueous protons (H ) in the pretreatment process [69], Based on kinetic studies of the halogenation of alcohols, they suggested that acidic conditions were required to displace surface hydroxyl groups prior to chloride addition to the catalyst ... 

TH EMES Displacements of Sulfonates Halogenation of Alcohols Deoxygenation C-Branching Chain Extension... 

Scheme 6.4 Phosphane-assisted halogenation of alcohols and carboxylic acids.

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