Sabatier and formic acid were esterified by titanium dioxide to the extent of 65 per cent. at 150°, the esterification was only 47 per cent. at 120°.

Mailhe have shown that various esters of fatty acids, e.g., ethyl and methyl formates, acetates, propionates, and butyrates, can be prepared by passing the vapours of a mixture of the alcohol and the acid in question over heated titanium dioxide.1 The yields obtained approximate to those given by the ordinary (Berthelot's) method of direct esterification after prolonged contact of the acid and alcohol. Thorium oxide is also effective, though less so than the titanium compound.

With titanium oxide heated to 280-300°, and using molecular proportions of alcohol and acid, the yield of methyl propionate obtained was 729 per cent., and of ethyl butyrate 71 per cent. The yield can be increased by using an excess of either the acid or the alcohol.

A lower temperature was used for obtaining the formic esters. Methyl, ethyl, propyl, butyl, isobutyl, and isoamyl formates were readily obtained by passing formic acid vapour, mixed with an excess of the alcohol in question, over titanium dioxide heated to 150°, or over thorium oxide at 200-220°. The catalytic effect decreases as the temperature is lowered. Thus in the case of ethyl formate, whilst equimolecular mixtures of ethyl alcohol and

1 Compt. rend., 1911, 152, 494, 1044.

Experiments with ethyl alcohol and various acids, using equi-molecular proportions of alcohol and acid in each mixture, indicated that in general the esterification was more complete the lower the molecular weight of the acid. Thus with titanium dioxide at 230°, acetic acid forms 67 per cent. of the ester, but isovaleric acid only 36 per cent. At 150°, formic acid gives 65 per cent., as noted above, but acetic acid only 20 per cent. As a catalyst, titanium oxide is more active than thorium oxide; moreover, its activity was found to persist undiminished during experiments extending over twenty days.

Primary alcohols are esterified more completely than secondary or tertiary alcohols, using the same acid for each.