Carbonyl Group

Carboxylic Acids and Corboxylate Salts

Carboxylic Acids and Corboxylate Salts

Carboxylic acids share the general formula:

C2H2nO2

And contain the following functional group:

As they are able to form hydrogen bonds with water, most carboxylic acids consisting of short chains are very soluble in water. However, in non-polar solvents they tend to be less soluble.

Reactions of carboxylic acids with bases

Carboxylic acids are weak acids which form carboxylate (or alkanoate) salts that they partially dissociate in water:

R-COOH(aq) + H2O(l) ? RCOO(aq) + H3O+(aq)

Carboxylic Acids Carboxylic acids are capable of acting like acids because of the relatively high stability of the R-COO anion. Known either as a carboxylate or alkanoate ion, the anion has the following structure:

The ion’s stability is due to its delocalised system. However, these acids are weak and so will not dissociate fully in water.

If a stronger base, for example hydroxide ions or carbonate ion, are present then the acid will completely dissociate.

With sodium hydroxide

When carboxylic acids react with sodium hydroxide, sodium salt and water are produced:

R-COOH(aq) + NaOH(aq) ? R-COONa+(aq) + H2O(l)

With sodium carbonate

When carboxylic acids react with sodium carbonate, sodium salt, carbon dioxide and water are produced:

2R-COOH(aq) + Na2CO3(aq) ? 2R-COONa+(aq) + CO2(g) + H2O(l)

This is used as the standard test for carboxylic acid because, if present, in the presence of sodium carbonate a colourless gas which turns lime water milky will be produced.

Reaction of carboxylate salts and acids

Carboxylate salts are salts of weak acid which means that they possess basic properties. Many have a pH of between 8 and 9:

R-COOH(aq) + H2O(l) ? R-COOH (aq) + OH(aq)

With strong acids, like hydrochloric acid, they react completely to produce the original carboxylic acid:

R-COOH(aq) + H+(aq) ? R-COOH (aq)

Reaction of carboxylic acids with alcohols

A condensation reaction occurs and an ester is produced when a carboxylic acid and an alcohol are heated with concentrated H2SO4 under reflux:

R1-COOH + R2OH ? R1-COOR2 + H2O

This reaction is called esterification and is an example of condensation in which two or more organic molecules combine to create one large molecule and, in the process, release a small molecule like HCl or H2O. For example:

ethanoic acid + ethanol ? ethyl ethanoate + water

In such a reaction:

  • H2SO4 behaves like a catalyst
  • the alcohol behaves like a nucleophile
  • the lone pair on the O atom attacks the carboxylic acid’s positive centre

The reaction is reversible. It is also able to reach equilibrium and has an equilibrium constant of approximately 4 which indicates that around 70% of the reactants are converted into products once equilibrium has been reached.