The chlorides of these compounds are obtained when phosphorus
pentachloride is allowed to act upon the acids, and are as a rule
crystalline. For the purpose of synthesis they may be employed as
follows:

1. They readily form esters with alcohols, which on subsequent
saponification with alkali are converted into the esters of the free
hydroxybenzoic acids.

2. The chlorides interact energetically with esters of amino-acids, and
may be coupled with amino-acids in aqueous alkaline solution. On
subsequently removing the carbo-methoxy group derivatives of
hydroxybenzoic acids are obtained, _e.g._,

CH_3.CO_2.O.C_6H_4.CO.Cl + 2NH_2CH_2.CO.C_2H_5
= NH_2.CH_2.CO_2.C_2H_5 + HCl + CH_3.CO_2.O.C_6H_4 CO.NH.CH_2CO_2C_2H_5.
CH_3.CO_2.O.C_6H_4.CO.NH.CH_2.CO_2.C_2H_5 + 3NaOH
= Na_2CO_3 + C_2H_5OH + CH_3OH + HO.C_6H_4.CO.NH.CH_2.COONa.

3. In the presence of AlCl_3 the chlorides easily combine with benzene,
and on removing the carbomethoxy group unsymmetrical hydroxy derivatives
of benzophenone are formed:--

CH_3.CO_2.O.C_6H_4.CO.Cl + C_6H_6 = CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + HCl
CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + 3NaOH
= NaO.C_6H_4.CO.C_6H_5 + Na_3CO_3 + CH_3OH + H_2O

4. The chlorides may be coupled with free hydroxybenzoic acids, and on
removing the carbomethoxy group didepsides are obtained. Repetition of