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(f) Carboxylic acids

Syllabus
2024
Topic
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Level
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Learning objectives

434C Carboxylic acid functional groupKnow that carboxylic acids contain the –COOH functional group.435C Carboxylic acid structures and namesUnderstand how to draw structural and displayed formulae for unbranched-chain carboxylic acids with up to four carbon atoms in the molecule, and name each compound436C Carboxylic acid reactionsDescribe the reactions of aqueous solutions of carboxylic acids with metals and metal carbonates437C Vinegar and ethanoic acidKnow that vinegar is an aqueous solution containing ethanoic acid

Recognise the carboxylic acid functional group

Carboxylic acids contain the carboxyl functional group −COOH\ce{-COOH}.

Inside this group, the same carbon atom is double-bonded to one oxygen and single-bonded to an −O−H\ce{-O-H} group: −C(=O)OH\ce{-C(=O)OH}. In ethanoic acid, CHX3COOH\ce{CH3COOH}, the final carbon is part of the functional group.

Do not circle only −OH\ce{-OH} or only C=O\ce{C=O}: both belong to the one −COOH\ce{-COOH} group. An ester has −COO−\ce{-COO-} between two carbon groups and no O−H\ce{O-H} in that linkage.

Draw and name carboxylic acids up to four carbons

The required unbranched carboxylic acids end in “anoic acid”; the carbon in −COOH\ce{-COOH} counts as part of the chain.

Total carbon atoms Name Structural formula
1 methanoic acid HCOOH\ce{HCOOH}
2 ethanoic acid CHX3COOH\ce{CH3COOH}
3 propanoic acid CHX3CHX2COOH\ce{CH3CH2COOH}
4 butanoic acid CHX3CHX2CHX2COOH\ce{CH3CH2CH2COOH}

For a displayed formula, draw the unbranched carbon chain, make the final carbon double-bonded to O and single-bonded to O-H, then add hydrogens so every carbon has four bonds, oxygen two and hydrogen one.

Do not add an extra carbon after naming the alkyl-looking part: CHX3COOH\ce{CH3COOH} has two carbons and is ethanoic acid. A displayed formula must show both the C=O\ce{C=O} and O−H\ce{O-H} bonds.

Predict reactions of carboxylic acids with metals and carbonates

Aqueous carboxylic acids react like other acids: with a suitable metal they form a carboxylate salt and hydrogen; with a metal carbonate they form a carboxylate salt, carbon dioxide and water.

Reactant added Products Gas observed
metal carboxylate salt + hydrogen HX2\ce{H2}; effervescence as the metal gets smaller
metal carbonate carboxylate salt + carbon dioxide + water COX2\ce{CO2}; effervescence

\ce{2CH3COOH(aq) + 2Na(s) -> 2CH3COONa(aq) + H2(g)}

\ce{2CH3COOH(aq) + K2CO3(aq) -> 2CH3COOK(aq) + CO2(g) + H2O(l)}

The acid name changes from “anoic acid” to “anoate” in the salt: ethanoic acid forms an ethanoate, such as sodium ethanoate or potassium ethanoate.

Fizzing alone does not identify the gas. A metal produces hydrogen, whereas a carbonate produces carbon dioxide and water; do not omit the carboxylate salt in either equation.

Identify what vinegar contains

Vinegar is an aqueous solution containing ethanoic acid.

Aqueous means dissolved in water, so vinegar contains water with ethanoic acid molecules in solution. Ethanoic acid gives vinegar its acidic behaviour, including reactions with suitable metals and carbonates.

Vinegar is not pure ethanoic acid and the words are not interchangeable. “Vinegar” names the aqueous mixture; “ethanoic acid” names the acid compound it contains.