Carboxylic acids: properties and reactions

In a nutshell

Carboxylic acids have a carboxyl ( $-COOH$ ) functional group. They are weak acids and are able to undergo multiple reactions.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

$RCOOH \rightleftharpoons RCOO^- \,+\,H^+ \\ \ \\carboxylic \space acids\, + carbonate \rightarrow salt \, +\,water \,+\, carbon \space dioxide \\ \ \\carboxylic \space acids\, + metal \rightarrow salt \, +\,hydrogen \\ \ \\carboxylic \space acids\, + alkali \rightarrow salt \, +\,water \\ \ \\carboxylic \space acids\, + metal\space oxide \rightarrow salt \, +\,water$

Carboxyl functional group

Carboxylic acids have a carboxyl ( $-COOH$ ) functional group. They are always found at the end of the carbon chain ( $C_1$ ) and have the suffix '-oic acid' when naming them.

Example

Chemistry; Organic chemistry II; KS5 Year 12; Carboxylic acids: properties and reactions — 1. Ethanoic acid; 2. Benzoic acid

Weak acids

Carboxylic acids are weak acids because they partially dissociate in water.

$RCOOH \rightleftharpoons RCOO^- \,+\,H^+$

The equilibrium lies to the left as most of the carboxylic acid does not dissociate.

Hydrogen bonds

Due to the polarised nature of the carboxyl group, hydrogen bonds are formed between the partially negative oxygen atoms and partially positive hydrogen atoms. This allows smaller carboxylic acids to be soluble in water as they are able to form hydrogen bonds with water molecules.

Chemistry; Organic chemistry II; KS5 Year 12; Carboxylic acids: properties and reactions — 1. Dashed lines represent hydrogen bonding

Reactions of carboxylic acids

Here are a few reactions of carboxylic acids you need to know.

Reaction with carbonates

$carboxylic \,acids\, + carbonate \rightarrow salt \, +\,water \,+\, carbon \, dioxide$

Example

$ethanoic \space acid + magnesium \space carbonate \rightarrow magnesium\space ethanoate + water + carbon \space dioxide\\2CH_3COOH (aq) + MgCO_3(s) \rightarrow (CH_3COO^-)_2Mg^{2+}(aq) +\,H_2O(l) +\,CO_2(g)$

Reaction with metals

$carboxylic \space acids\, + metal \rightarrow salt \, +\,hydrogen$

Example

$ethanoic \space acid + magnesium \rightarrow magnesium\space ethanoate + hydrogen\\2CH_3COOH(aq) + Mg (s) \rightarrow (CH_3COO^-)_2Mg^{2+} (aq) +\,H_2(g)$

Reaction with bases

$Alkali: carboxylic \space acids\, + alkali \rightarrow salt \, +\,water \\Metal \,oxides: carboxylic \space acids\, + metal\space oxide \rightarrow salt \, +\,water$

Example

$ethanoic \space acid + magnesium \space hydroxide \rightarrow magnesium\space ethanoate + water\\2CH_3COOH (aq) + Mg(OH)_2 (aq) \rightarrow (CH_3COO^-)_2Mg^{2+} (aq) +\,2H_2O(l)$

$ethanoic \space acid + magnesium \space oxide \rightarrow magnesium\space ethanoate + water\\ 2CH_3COOH (aq) + MgO (aq) \rightarrow (CH_3COO^-)_2Mg^{2+} (aq) +\,H_2O(l)$