Loss of water from the tetrahedral intermediate is reversible too: just as ROH will attack a protonated carboxylic acid, H2O will attack a protonated ester. In fact, every step in the sequence from carboxylic acid to ester is an equilibrium, and the overall equilibrium constant is about 1. In order for this reaction to be useful, it is therefore necessary to ensure that the equilibrium is pushed towards the ester side by using an excess of alcohol or carboxylic acid (usually the reactions are done in a solution of the alcohol or the carboxylic acid). In this reaction, for example, no water is added and an excess of alcohol is used. Using less than three equivalents of ethanol gave lower yields of ester.

Alternatively, the reaction can be done in the presence of a dehydrating agent (concentrated H₂SO₄, for example, or silica gel) or the water can be distilled out of the mixture as it forms.

●Making esters from alcohols You have now met three ways of making esters from alcohols:

 • with acyl chlorides

 • with acid anhydrides

 • with carboxylic acids.

Try to appreciate that different methods will be appropriate at different times. If you want to make a few milligrams of a complex ester, you are much more likely to work with a reactive acyl chloride or anhydride, using pyridine as a weakly basic catalyst, than to try to distil out a minute quantity of water from a reaction mixture containing a strong acid that may destroy the starting material. On the other hand, if you are a chemist making simple esters (such as those in Chapter 2, p. 31) for the flavouring industry on a scale of many tons, you might prefer the cheaper option of carboxylic acid and a strong acid (e.g. H₂SO₄) in alcohol solution.

مواضيع ذات صلة

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making alcohols instead of ketones

Making ketones from esters: the problem

Acid chlorides can be made from carboxylic acids using SOCl2 or PCl5

Hydrolysing nitriles: how to make the almond extract, mandelic acid

Amides can be hydrolysed under acidic or basic conditions too

Base-catalysed hydrolysis of esters is irreversible

Acid-catalysed ester hydrolysis and transesterification

Acid catalysts can make bad leaving groups into good ones