Carboxylic Acids and Esters

Carboxylic acids

Carboxylic acids are weak acids with the –COOH functional group. They are named by adding the suffix ‘oic acid’. The functional group is always found at the end of the molecule.

Since carboxylic acids are weak acids, they partially dissociate into hydrogen ions and carboxylate ions.

The –COOH group is polar due to the presence of the electronegative oxygen atoms.

This means that electrons are pulled further towards the two oxygens, making them slightly negative. Carbon and hydrogen are therefore electron-deficient and have a slight positive charge.

Like other polar molecules, carboxylic acids can form hydrogen bonds with water. This enables the smaller carboxylic acids, such as methanoic acid and ethanoic acid, to dissolve easily in water.

As the chain length increases (and therefore the non-polar portion of the molecule increases) solubility decreases.

Reactions of carboxylic acids

Carboxylic acids react with metal hydroxides (alkalis) and metal oxides to form a salt and water.

They react with metal carbonates to form a salt, water and carbon dioxide.

Carboxylic acids also react with reactive metals to form a salt and hydrogen. This is a redox reaction in which the metal is oxidised and hydrogen is reduced.

Acyl chlorides

Acyl chlorides are compounds that contain the –COCl functional group. They are named using the suffix –oyl chloride e.g. ethanoyl chloride. The acyl chloride functional group is always found at the end of the molecule and the compound is named with carbon of the acyl group as carbon number 1.

Reactions of acyl chlorides

Acyl chlorides react with water, alcohols, ammonia and amines. These reactions happen readily and easily take place at room temperature and pressure. They are all nucleophilic addition-elimination reactions in which chlorine is substituted by oxygen or nitrogen. White fumes of hydrogen chloride will be observed.

Acyl chlorides react vigorously with cold water to form a carboxylic acid and hydrogen chloride.

They also react vigorously with alcohols at room temperature to form an ester and hydrogen chloride.

Acyl chlorides react violently with ammonia at room temperature to produce a primary amide and hydrogen chloride.

Acyl chlorides react violently with amines at room temperature to form a secondary amide and hydrogen chloride.

Esters are usually made by reacting an alcohol with a carboxylic acid, but they can also be made by reacting an acyl chloride with phenol. The reaction can take place at room temperature and pressure, but the reaction proceeds slowly. That said, it’s quicker than reacting phenol with carboxylic acids, so if you want to make an ester containing a benzene ring, acyl chlorides are your best bet.

Esters

If you react an alcohol with a carboxylic acid, you’ll form an ester. Esters are a homologous series which contain the functional group ‘-COO’ and can be identified by their distinctive fruity smell. They also evaporate easily (we say they are volatile) which makes them a useful group of compounds to use in perfumes and food flavourings.

Let’s look at the reaction between ethanol and ethanoic acid to produce the ester, ethyl ethanoate. This reaction requires an acid catalyst, which is usually concentrated sulfuric acid.

The removal of water creates the -COO ester bond, joining the carboxylic acid and alcohol molecules together to form the ester.

The reaction is reversible so the ester needs to be separated from the mixture as soon as it is formed. For smaller esters, this can be done by warming the mixture and distilling off the ester, since the ester will be more volatile (evaporates more easily) than the other components of the mixture. For larger esters, you’ll need to heat it under reflux and separate the product by fractional distillation.

To name esters you use the alcohol as the first part of the name and the carboxylic acid as the second part.  For example, the ester butyl propanoate would have been formed by reacting butanol and propanoic acid together.

Esters can also be made by reacting an alcohol with an acid anhydride. An acid anhydride is made by reacting two carboxylic acids together.

The acid anhydride reacts with the alcohol when it is gently heated. An ester and a carboxylic acid are formed which are then separated by fractional distillation. A catalyst is not necessary for preparing esters in this way.

The third, and final, way to make an ester is by reacting an acyl chloride with an alcohol. The products are an ester and fumes of hydrogen chloride.

Hydrolysis of esters

Esterification is a reversible reaction, which means that it can also go in the opposite direction i.e. the ester reacts with water to form a carboxylic acid and alcohol. Hydrolysis of esters can be carried out using either an acid or a base.

Acid hydrolysis involves a dilute solution of acid, such as dilute sulfuric acid or dilute hydrochloric acid and heating it with the acid under reflux. Water reacts with the ester to produce the carboxylic acid and alcohol from which it was formed.

Alkali hydrolysis involves refluxing the ester with dilute alkali, such as dilute sodium hydroxide. This time, you’ll get a carboxylate salt and an alcohol.