Water scavenger, acetone dimethyl

Water scavenger, acetone dimethyl ketal as — 17, 231 ----, ethyl propionate as —... 

Methyl esters s. 18, 935 with acetone dimethyl ketal as water scavenger, f. methods, s. H. Liliedahl, Acta Ghem. Scand. 20, 95 (1966)... 

Although methanol is produced, the procedure is not restricted to the preparation of methyl esters in the presence of excess of another alcohol the reaction will lead to the formation of its ester, and if necessary things can be so arranged that the methanol produced volatilizes away, together of course with the acetone. Darkening may be prevented by the use of dimethyl sulphoxide [13]. Other water scavengers which have been used include 100% sulphuric acid [14] and graphite bisulphate [15]. 

Methyl, ethyl, benzyl, benzhydryl, p-nitrobenzyl, p-methoxy-benzyl, 4-picolyl, j3j -trichloroethyl, j3-methylthioethyl, /J-p-toluenesulphonylethyl, and -p-nitrophenylthioethyl esters may be prepared directly from the acid and alcohol. TTie most usual method [4, 5] consists of heating the acid and an excess of the alcohol with an acid catalyst (e.g., Fischer-Speier, hydrochloric or sulphuric acid). The extent of reaction is improved if the water formed is removed by azeotropic distillation with an inert solvent (benzene, carbon tetrachloride, or chloroform). Considerable variation is possible in the natvire of the acid catalyst thus phosphoric acid [6], aryl sulphonic acids [7, 8, 9], alkyl sulphates [10], and acidic ion-exchange resins [11] may be employed. Removal of the water by azeotropic distillation during the formation of methyl esters is difficult and Brown and Lovette [12] introduced the novel reagent acetone dimethyl acetal (7) for the direct formation of methyl esters. In the presence of a trace of methanol and an acid catalyst the reagent acts as a scavenger of water formed by esterification and liberates further methanol for reaction. 

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