5 Some chemistry involving esters
Esters are produced by the reaction of a carboxylic acid with an alcohol and result from the formation of a new bond (Reaction 2.1). For example, ethyl butanoate, the major constituent of artificial pineapple flavouring, is made from the reaction of butanoic acid with ethanol.
There is a certain logic to the naming of these compounds. Note the endings on the names: -ic for the carboxylic acid, -ol for the alcohol and -ate for the ester.
Because the other product is water, this type of reaction is known as a condensation reaction. (There are other condensation reactions that do not involve the formation of water, but they do involve two molecules joining together to form a larger molecule with the elimination of a different smaller molecule, e.g. ammonia or hydrogen chloride, instead of water. We will not be concerned with these other condensation reactions.) This condensation reaction is common to nearly all carboxylic acids, R1COOH, and alcohols, R2OH. So, we can write the general reaction, 2.2, where the abbreviation R1 represents the rest of the carboxylic acid molecule and R2 represents the rest of the alcohol molecule. Check through Reactions 2.1 and 2.2 and make sure you can follow the way in which some of the atoms move from one molecule to another as the reaction takes place. We have colour coded the reacting groups in Reactions 2.1 and 2.2 to help you see this change.
The use of the symbol R to represent the rest of the molecule is common in organic chemistry. It enables us to focus on the parts of the molecules that matter (the functional groups) without the formulae being cluttered up with the parts that are not reacting.
Reaction 2.3 shows the required carboxylic acid and alcohol.
An important thing about this reaction is that, although it shows an ester forming from a carboxylic acid and an alcohol, it could equally well have been written the other way round. It would then show the reaction of an ester with water, to produce a carboxylic acid and an alcohol. This is an example of a chemical reaction that can run in either direction, rather unsurprisingly known as a reversible reaction. They are very common and sometimes you see the use of the symbol
instead of =. The reverse of the condensation reaction above is known as a hydrolysis reaction, reflecting the fact that it is reaction with water. The word is derived from hydro (water) and lysis (splitting), so hydrolysis is literally splitting with water, which accounts for the fact that any hydrolysis reaction gives two products.