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Organic Chemistry

Topic 5 of 9

Carboxylic acids: recognise -COOH

A functional group gives a family its characteristic chemistry.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Carboxylic acids: recognise -COOH

A functional group gives a family its characteristic chemistry.

Carboxylic acids contain the -COOH group, also written -CO2H. This includes a C=O bond and an O-H bond on the same carbon. Ethanoic acid is CH3COOH; it can form when ethanol is oxidised by oxygen in air or by warm acidified potassium manganate(VII). The carbon skeleton is retained in this conversion.

Pure only
Pure: C1-C4 acid structures
Name or connectivityCondensed formula
Methanoic acidH-C(=O)-OH
Ethanoic acidCH3-C(=O)-OH
Propanoic acidCH3-CH2-C(=O)-OH
Butanoic acidCH3-CH2-CH2-C(=O)-OH
Branched C4 acid(CH3)2CH-C(=O)-OH
Pure only

Worked example

Draw the whole carboxyl group

Draw propanoic acid and account for all three carbons.

  1. Draw a three-carbon chain. The end carbon is part of -COOH; do not add a fourth carbon for this group.
  2. Attach one oxygen by a double bond and a second oxygen by a single bond to that end carbon. Attach H to the singly bonded oxygen.
  3. Complete the remaining C-H bonds: the chain is CH3-CH2-C(=O)-OH. The carboxyl carbon already has four bonds and takes no H.
Answer

C3H6O2. In methanoic acid the carboxyl carbon instead bonds to H because there is no carbon chain attached.

Pure only

Displayed methanoic acid

Complete displayed structure with 1 carbon, 2 hydrogen and 2 oxygen atoms: CH2O2. Every bond and hydrogen is shown.

CH2O2. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed ethanoic acid

Complete displayed structure with 2 carbon, 4 hydrogen and 2 oxygen atoms: C2H4O2. Every bond and hydrogen is shown.

C2H4O2. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed propanoic acid

Complete displayed structure with 3 carbon, 6 hydrogen and 2 oxygen atoms: C3H6O2. Every bond and hydrogen is shown.

C3H6O2. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
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Displayed butanoic acid

Complete displayed structure with 4 carbon, 8 hydrogen and 2 oxygen atoms: C4H8O2. Every bond and hydrogen is shown.

C4H8O2. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
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Displayed branched C4 carboxylic acid

Complete displayed structure with 4 carbon, 8 hydrogen and 2 oxygen atoms: C4H8O2. Every bond and hydrogen is shown.

C4H8O2. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Pure: carboxylic acids are weak acids, partly ionising in water. They still react with suitable metals, bases and carbonates. Weak means partly ionised, not incapable of reaction. The salts of ethanoic acid contain CH3COO-, the ethanoate ion.

Pure only
Pure: characteristic ethanoic-acid reactions
PartnerBalanced reaction
Sodium hydroxideCH3COOH(aq) + NaOH(aq) -> CH3COONa(aq) + H2O(l)
Sodium carbonate2CH3COOH(aq) + Na2CO3(aq) -> 2CH3COONa(aq) + H2O(l) + CO2(g)
Magnesium2CH3COOH(aq) + Mg(s) -> (CH3COO)2Mg(aq) + H2(g)
Check your understandingWhat distinguishes the functional group in ethanol from the one in ethanoic acid?Think it through, then reveal the answer
Ethanol contains an alcohol -OH attached to a carbon chain. Ethanoic acid contains -C(=O)-OH: its -OH is part of the carboxyl group. Recognise the whole group, not just the letters OH.