4.41C-4.43C - Ester formulae, uses and preparation practical

Reading ester names

An ester name has two parts. The first part comes from the alcohol. The second part comes from the carboxylic acid, but the ending changes from -oic acid to -oate.

For this Pearson specification, propanol means propan-1-ol and butanol means butan-1-ol unless a question tells you otherwise. So ethanol plus propanoic acid forms ethyl propanoate, and methanol plus ethanoic acid forms methyl ethanoate.

The name order is easy to remember:

1. alcohol part first
2. acid part second
3. acid ending changes to -oate

Be careful: the name order is not always the same as the way a structural formula is written. For example, CH3COOCH2CH3 is ethyl ethanoate. The formula starts with the ethanoate part, but the name starts with the ethyl part.

Writing structural formulae

The general pattern for making an ester is:

In structural formulae, a carboxylic acid can be shown as RCOOH, where R is the rest of the carbon chain. An alcohol can be shown as R'OH, where R' is the alkyl group from the alcohol. The ester is RCOOR'.

The oxygen next to the alcohol group stays in the ester. The water comes from the OH of the carboxylic acid and one H from the alcohol.

Here are three structural-formula examples:

The acid part is the part ending in COO in the ester formula. The alcohol part is the carbon group after that oxygen. This is why CH3CH2COOCH3 is methyl propanoate, not propyl methanoate.

Displayed formulae and reversing

A displayed formula shows every atom and every bond. For an ester, the most important section is the ester link:

The carbon in C=O is the carbonyl carbon. It is double-bonded to oxygen and single-bonded to the oxygen that links to the alcohol group. On the other side, most non-methanoate esters have the carbonyl carbon bonded to a carbon group; in methanoate esters, it is bonded to H. The single-bonded oxygen has two single bonds and no hydrogen attached in the ester.

For CH3CH2COOCH3, the displayed formula must show:

• a CH3CH2 group joined to the carbonyl carbon
• the carbonyl carbon double-bonded to O
• the carbonyl carbon single-bonded to another O
• that single-bonded O joined to CH3

Compare CH3COOCH2CH3, where the carbonyl carbon is joined to CH3, with HCOOCH2CH3, where the carbonyl carbon is joined to H. Both still contain the ester link -COO-.

To work backwards from an ester formula, split it around the ester group:

Add H back to the acid part to get the carboxylic acid: CH3CH2COOH. Add OH to the alcohol carbon group to get the alcohol: CH3OH, CH3CH2OH, CH3CH2CH2CH2OH, and so on.

The common displayed-formula mistakes are losing the oxygen between the two carbon groups in -COO-, drawing a carboxylic acid -COOH instead of the ester -COO-, or adding an impossible extra H to a carbonyl carbon that already has four bonds.