3.3.9: Carboxylic Acids and Derivatives
Background
Carboxylic acids are compounds which contain the -COOH group (a carbonyl next
to an alcohol group). The naming system counts the number of carbons in the chain,
starting from the carbonyl carbon.
They are formed through the oxidation of alcohols using acidified potassium dichromate (VI), and by the
hydrolysis of nitriles.
Heat an alcohol under reflux with an excess of acidified potassium dichromate (VI):
alcohol + 2[O] → carboxylic acid + water
Add water to the C ≡ N bond and heat under reflux – this can be done using either an acid or an alkali:
• Acid hydrolysis produces a carboxylic acid and ammonium. The carboxylic acid can be distilled off
of the mixture.
nitrile + water + dilute acid → carboxylic acid + ammonium ions
• Base hydrolysis produces a carboxylate salt and ammonia. Add a strong acid to liberate the weak
acid from the salt, and distil off the acid.
nitrile + water + alkali → carboxylate salt + ammonia
carboxylate salt + strong acid → carboxylic acid
Carboxylic acids have relatively high boiling points because of their ability to form hydrogen bonds, as well
as Van-der-Waals forces and dipole-dipole interactions.
Small carboxylic acids are soluble in water because of their ability to hydrogen bond, but larger carboxylic
acids are not because the other end of the chain will interrupt the hydrogen bond interactions.
Carboxylic acids are acidic because of their ability to lose the H+ ion from
the -COOH group. When the H+ ion is lost and replaced by a metal in an
ionic compound, a carboxylate salt is formed. In a solution with water, a
hydrogen ion (proton) is lost to form a salt and the hydroxonium ion H3O+ -
the reaction is reversible. Carboxylic acids are weak because only a small
proportion of the molecules will dissociate at any time. The acidic properties
of carboxylic acids dictate their reactions with other substances:
carboxylic acid + metal → salt + hydrogen
carboxylic acid + metal hydroxide → salt + water
carboxylic acid + metal carbonate → salt + water + carbon dioxide
carboxylic acid + ammonia → ammonium salt
carboxylic acid + amine → ammonium salt
Esters
Esters are carbonyl compounds formed by the reaction of alcohols
with carboxylic acids using an acid catalyst. They contain the -
COOR’ group, where the H from -COOH has been replaced with an
alkyl group. The naming system describes the alkyl group first, then
the acid group.
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst –
usually concentrated sulphuric acid. The reaction is slow and reversible and is known as esterification.
The hydrolysis of esters returns them to a carboxylic acid and alcohol. The reaction with pure water is too
slow to be of use, so acids or alkalis are added to increase the rate of reaction.
Background
Carboxylic acids are compounds which contain the -COOH group (a carbonyl next
to an alcohol group). The naming system counts the number of carbons in the chain,
starting from the carbonyl carbon.
They are formed through the oxidation of alcohols using acidified potassium dichromate (VI), and by the
hydrolysis of nitriles.
Heat an alcohol under reflux with an excess of acidified potassium dichromate (VI):
alcohol + 2[O] → carboxylic acid + water
Add water to the C ≡ N bond and heat under reflux – this can be done using either an acid or an alkali:
• Acid hydrolysis produces a carboxylic acid and ammonium. The carboxylic acid can be distilled off
of the mixture.
nitrile + water + dilute acid → carboxylic acid + ammonium ions
• Base hydrolysis produces a carboxylate salt and ammonia. Add a strong acid to liberate the weak
acid from the salt, and distil off the acid.
nitrile + water + alkali → carboxylate salt + ammonia
carboxylate salt + strong acid → carboxylic acid
Carboxylic acids have relatively high boiling points because of their ability to form hydrogen bonds, as well
as Van-der-Waals forces and dipole-dipole interactions.
Small carboxylic acids are soluble in water because of their ability to hydrogen bond, but larger carboxylic
acids are not because the other end of the chain will interrupt the hydrogen bond interactions.
Carboxylic acids are acidic because of their ability to lose the H+ ion from
the -COOH group. When the H+ ion is lost and replaced by a metal in an
ionic compound, a carboxylate salt is formed. In a solution with water, a
hydrogen ion (proton) is lost to form a salt and the hydroxonium ion H3O+ -
the reaction is reversible. Carboxylic acids are weak because only a small
proportion of the molecules will dissociate at any time. The acidic properties
of carboxylic acids dictate their reactions with other substances:
carboxylic acid + metal → salt + hydrogen
carboxylic acid + metal hydroxide → salt + water
carboxylic acid + metal carbonate → salt + water + carbon dioxide
carboxylic acid + ammonia → ammonium salt
carboxylic acid + amine → ammonium salt
Esters
Esters are carbonyl compounds formed by the reaction of alcohols
with carboxylic acids using an acid catalyst. They contain the -
COOR’ group, where the H from -COOH has been replaced with an
alkyl group. The naming system describes the alkyl group first, then
the acid group.
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst –
usually concentrated sulphuric acid. The reaction is slow and reversible and is known as esterification.
The hydrolysis of esters returns them to a carboxylic acid and alcohol. The reaction with pure water is too
slow to be of use, so acids or alkalis are added to increase the rate of reaction.
