THIONYL CHLORIDE

(1) CONVERSION OF ALCOHOL INTO ALKYL CHLORIDE (DARZEN’S METHOD): Alkyl chlorides can be prepared from corresponding alcohols by the reaction with SOCl₂. The reaction take place by SN_i mechanism (retention in configuration) but if we use base such as pyridine C5H5N then inversion takes place and reaction occurs by normal SN2. SOCl2 is preferred over other reagents such as PCl₅ and PCl₃ because in case of SOCl₂ by-products are gases (SO₂ and HCl and escape from the reaction vessel but in case of PCl₅ and PCl₃ the by-products are POCl₃ and H₃PO₃) and these are non-volatile so purification of alkyl halide requires separation.

(2) CONVERSION OF CARBOXYLIC ACID INTO ACID CHLORIDE: Acid chlorides can be obtained by reacting carboxylic acid with SOCl₂.

(3) CONVERSION OF PRIMARY AMIDES INTO NITRILES: Primary amides when heated with SOCl₂ they undergo dehydration to yield nitriles. For dehydration of primary amides, we can also use PCl₅, P4O₁₀, POCl₃ C₆H₅SO₂Cl and Al₂O₃.

(4) BECKMANN REARRANGEMENT (CONVERSION OF KETOXIMES INTO AMIDES): Ketoximes (RR’C=NOH) when treated with acidic reagents such as SOC_l2, they rearrange to substituted amides (RCONHR’ or R’CONHR). The group that migrates is generally the one anti to -OH of oxime but when attached alkyl groups of oximes are both bulky then mixtures of two amides are possible (RCONHR’ and R’CONHR). The oximes of cyclic ketones give ring enlargement. Among other reagents used are PCl5, Concentrated H₂SO₄, HCOOH, Liquid SO₂, HMPA, P₂O₅-Methanesulphonic acid, HCl-HOAc-AC₂O, Silica gel and polyphosphoric acid.