6.1.3(c)-(d) - Esterification and ester hydrolysis

Reading ester structures

An ester contains the group $\text{–}\mathrm{COO}\text{–}$ in the middle of its structure. Written as a general formula, an ester is:

The part before the carbonyl carbon, $\mathrm{RCO}\text{–}$, comes from the carboxylic acid or acid anhydride side. The part after the single-bonded oxygen, $\text{–}\mathrm{OR'}$, comes from the alcohol side. This is the most important map in the lesson because OCR questions often give a product or reactant structure and expect you to keep the connectivity correct.

For example, ethanoic acid and propan-1-ol form propyl ethanoate:

The $\mathrm{CH_{3}CO}\text{–}$ part comes from ethanoic acid, so the ester name ends in ethanoate. The $\mathrm{CH_{3}CH_{2}CH_{2}}\text{–}$ group comes from propan-1-ol, so the first word is propyl. A common error is to reverse these parts and write an ester with the oxygen connected to the wrong carbon chain.

Carboxylic acids with alcohols

Carboxylic acids react with alcohols to form esters in the presence of an acid catalyst. OCR gives concentrated sulfuric acid, $\ce{H2SO4}$, as a typical catalyst. This reaction is called esterification and is also a condensation reaction because a small molecule, water, is formed.

The double arrow matters: this esterification is reversible. In practical work, the mixture is commonly heated, often under reflux, so volatile reactants or products are not lost while the mixture is being heated. The acid catalyst speeds the reaction but is not used up overall.

Worked example:

Write the equation for making ethyl ethanoate from ethanoic acid and ethanol.

1. The acid-derived part is $\mathrm{CH_{3}CO}\text{–}$ from ethanoic acid.
2. The alcohol-derived part is $\text{–}\mathrm{OCH_{2}CH_{3}}$ from ethanol.
3. Water is the other product.

The condition to state is an acid catalyst, such as concentrated $\ce{H2SO4}$. If the question asks for how the reaction is heated, "heat under reflux" is a suitable supporting condition.

Reflux uses a vertical condenser open at its top: water enters the lower condenser connection and leaves the upper one, while condensed vapour returns to the reaction flask. Use electrical heating for flammable organic mixtures. Reflux retains volatile material; it does not separate the ester or change the equilibrium position. Excess alcohol or removal of a product can favour ester formation, whereas adding a catalyst only speeds approach to equilibrium.

Acid anhydrides with alcohols

OCR also requires esterification of acid anhydrides with alcohols. An acid anhydride has the general form $\ce{(RCO)2O}$, which means it contains two acyl groups joined through an oxygen atom.

When an acid anhydride reacts with an alcohol, the products are an ester and a carboxylic acid:

Compared with carboxylic acid esterification, this is a more reactive route and OCR product questions normally treat it as going in the forward direction. The key product pattern is still the same: the alcohol provides the group after the ester oxygen, and one acyl half of the anhydride becomes the carboxylic acid by-product.

Worked example:

Ethanoic anhydride reacts with propan-1-ol.

The ester is propyl ethanoate. The by-product is ethanoic acid. Do not add water as a product for this route; water is the by-product in the carboxylic acid plus alcohol route.

Read the anhydride connectivity as $\mathrm{R-C(=O)-O-C(=O)-R}$. The central oxygen joins the two acyl carbons. One alcohol molecule gives one ester molecule and one acid molecule, not two esters. This route is more reactive than the carboxylic acid route and does not require the same concentrated-acid catalyst.