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 these operations yields tri- and tetradepsides. Preparation of Didepsides A simple application of these syntheses is offered by _p_-hydroxybenzoic acid. When the chloride of its carbomethoxy derivative is allowed to interact with _p_-hydroxybenzoic acid in aqueous alkaline solution, in the cold, the alkali salt of carbomethoxy-_p_-hydroxybenzoic acid is formed:--[Footnote 1: _Ber._, 1909, 42, 216.] CH_3.CO_2.O.C_6H_4.CO.Cl + NaO.C_6H_4.COONa = CH_3.CO_2.O.C_6H_4.CO_2.C_6H_4.CO_2.Na + NaCl. Being sparingly soluble, the salt in this case is readily deposited as crystals, but is readily converted into the free acid by hydrochloric acid. In most other cases, however, the alkali salts are easily soluble and the aqueous solution is then directly acidified with a mineral acid. The chlorides, being for the most part solids, the mode of procedure is as follows:--the hydroxybenzoic acid required for coupling is dissolved in normal or double-normal alkali (the volume calculated per molecule acid), a little acetone added, and the mixture well cooled; a further molecule of 2N caustic soda and the chloride (I molecule) dissolved in dry acetone are added in small portions, whilst stirring, to the mixture. In spite of the low temperature the coupling proceeds quickly and the sparingly soluble product can in most cases be precipitated from the solution by acidifying and diluting with water. In case of more easily soluble coupling products the acetone is driven off under reduced pressure or the liquid acidified and diluted, and the substance extracted with ether. Instead of alkali, dimethylaniline may be employed, with the exclusion of water as a solvent for the purpose of coupling. Another suitable method of obtaining _o_-didepsides is that of treating _o_-hydroxybenzoic acids with phosphorus trichloride and dimethylaniline (_e.g_., synthesis of disalicylic acid, Boehringer & Sons).[Footnote: Ger. Pat., 211,403.] The carbomethoxy derivatives of the depsides are as a rule crystalline substances of distinct acidic character, and decompose alkaline carbonates.