Question

What does alcohol form by reaction with $SOC{l_2}$ ?

Answer 1

This question requires the study of organic reactions of an alcohol (either primary or secondary) with thionyl chloride ( $SOC{l_2}$ ). The various mechanisms that are involved in this process need to be carefully studied in order to understand the formation of alkyl halides as a product.

Complete answer:
Treatments with thionyl chloride and phosphorus tribromide, respectively, are the most frequent procedures for converting ${1^o}$ - and ${2^o}$ -alcohols to the corresponding chloro and bromo alkanes (i.e. replacement of the hydroxyl group). Because of the severe acidity of these hydrohalic acids and the carbocation rearrangements associated with their use, these reagents are often favoured over concentrated HX.
When it comes to converting an alcohol into a better leaving group, synthetic organic chemists have a few options. Using thionyl chloride or phosphorus tribromide, one popular technique is to convert the alcohol to an alkyl chloride or bromide.
$C{H_3}C{H_2}OH + SOC{l_2}\xrightarrow{{Pyridine}}C{H_3}C{H_2}Cl + S{O_2} \uparrow + HCl \uparrow$
In the reaction of thionyl chloride with chiral ${2^o}$ -alcohols, either inversion or retention has been reported. The intermediate chlorosulfite ester combines with a base like pyridine to generate a "pyridinium" salt, which then undergoes a rather clean SN2 reaction to form the inverted chloride.
The chlorosulfite interacts with configuration retention in ether and related solvents, likely via a close or intimate ion pair. This is referred to as an $S{N_i}$ reaction (nucleophilic substitution internal). The ion pair's carbocation partner may also rearrange.

Note:
Despite their widespread utility, phosphorus tribromide and thionyl chloride both have drawbacks. ${1^o}$ - and ${2^o}$ -alcohols that have been hindered react slowly with the former and may create rearrangement products.