Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

INFLUENCE OF THE CATALYST 145
form the ester more rapidly than does the organic acid itself. The
hydrolysis is also accelerated to the same extent as is the esterification,
so that exactly the same equilibrium is attained as in the absence of
sulphuric acid, for instance, by heating the components to a high temperature
in a sealed tube. The influence of the catalyst (the sulphuric
acid) consists, therefore, only in an increase in the velocity of the reaction ;
the position of the equilibrium is left unchanged. This important law
holds for all reactions which are catalytically accelerated.
It has been shown that the catalytic action of acids is proportional
to their strength, of which their degree of ionisation is an expression.
Conversely, the strength of an acid can be determined by measuring
the rate of hydrolysis of an ester (usually methyl acetate) in aqueous
solution in the presence of the acid concerned.
An understanding of the theoretical principles which have been but
briefly discussed in this section is indispensable to anyone who does not
wish to practise organic chemistry merely as a culinary art.
Other Methods for the preparation of Esters.—Esters are extremely
easily formed from silver salts by the action of alkyl iodides :
R.COOAg + I.R' —>• R.COOR' + Agl.
The same object is attained by treating alkali salts with a dialkyl
sulphate. Usually in this case only one alkyl group takes part in the
reaction in accordance with the equation :
R.COONa + (CH3)2SO4
> R.COOCH3 + CH3.SO4Na .
The alkyl group of the salt of the alkylsulphuric acid can also be
made available for esterification if the temperature is raised sufficiently.
The formation of esters from acid chlorides or anhydrides need
only be recalled here. This method also has practical importance.
For the esterification of difficultly accessible acids the elegant diazomethane
method (p. 273) is most appropriate ; it usually proceeds very
smoothly.
The lower members of the series of esters are colourless liquids
having pleasant fruity odours. The higher members, as well as most
esters of aromatic acids, are crystalline substances. The boiling points
of esters containing alkyl groups of low molecular weight (CH3, C2H5,
C3H7) are lower than those of the corresponding acids :
CH3.COOCH3 Boiling point 57°
CH3.COOC2H5 „ „ 78°,
CH3.COOH „ „ 118°.
It is worthy of note that the melting points of the methyl esters are
generally higher than those of the corresponding ethyl esters; thus,
to take a well-known example, dimethyl oxalate (melting point 54°)
is a solid whilst the diethyl ester is a liquid.
The esters are often prepared as an end in themselves, and are used