Carboxylic Acids

Carboxylic acids bring together two familiar ideas: the C=O of a carbonyl and the O–H of an alcohol. The combination creates a functional group — –COOH — that has its own distinctive chemistry. Carboxylic acids are weak acids that react with bases, carbonates and metals just like dilute mineral acids, and they act as the starting point for almost every ester you will meet.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.1 (a)–(c): structure, nomenclature, acidity and reactions of carboxylic acids.

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1. The Carboxyl Group

The carboxyl group, –COOH, can be thought of as a carbonyl (C=O) with a hydroxyl (O–H) attached to the same carbon. Its compact formula hides a great deal of chemistry.

Key Definition — Carboxyl group: The functional group –COOH, in which a carbon is double-bonded to one oxygen and single-bonded to a hydroxyl group.

The two oxygens of –COOH are not equivalent in any one molecule (one is doubly bonded and one singly bonded), but when the acid dissociates to give –COO⁻, the negative charge is delocalised evenly over both oxygens. This resonance stabilisation is the reason carboxylic acids are acidic at all — more on that in Section 3.

graph LR
  A[Carboxyl -COOH] --> B[C=O carbonyl]
  A --> C[O-H hydroxyl]
  A --> D[After loss of H+: -COO- with delocalised charge]

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2. Nomenclature

Carboxylic acids take the suffix -oic acid. The –COOH carbon is always carbon 1, just like the –CHO carbon in aldehydes.

Rules:

1. Find the longest chain including –COOH.
2. Number from the –COOH carbon as C1.
3. Replace the final e of the parent alkane with oic acid.
4. Add locants for substituents.

Examples:

Formula	Name
HCOOH	Methanoic acid (formic acid)
CH₃COOH	Ethanoic acid (acetic acid)
CH₃CH₂COOH	Propanoic acid
CH₃CH₂CH₂COOH	Butanoic acid
CH₃CH(CH₃)COOH	2-methylpropanoic acid
HOOC–COOH	Ethanedioic acid (oxalic acid)
C₆H₅COOH	Benzoic acid

Dicarboxylic acids take the suffix -dioic acid — the final e of the alkane is kept. Note "ethanedioic", not "ethandioic".

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3. Why Carboxylic Acids Are (Weakly) Acidic

Carboxylic acids ionise slightly in water:

$R-COOH + H_2O \rightleftharpoons R-COO^- + H_3O^+$R−COOH+H2​O⇌R−COO−+H3​O+

For ethanoic acid, Kₐ ≈ 1.8 × 10⁻⁵ at 25 °C, giving a pKₐ ≈ 4.76. Compared with a mineral acid (HCl pKₐ ≈ –7) this is very weak — only about 1 in 300 ethanoic acid molecules in a 0.1 mol dm⁻³ solution is ionised.

But it is much more acidic than a typical alcohol (ethanol pKₐ ≈ 16). Why?

3.1 Resonance stabilisation of the carboxylate

When an alcohol loses H⁺, the negative charge is localised on a single O atom. When a carboxylic acid loses H⁺, the negative charge is spread ("delocalised") over both oxygen atoms by resonance:

  R-C(=O)-O(-)   <-->   R-C(-O-)=O

The real ion is a hybrid of these two structures, with two equivalent C–O bonds intermediate in length between single and double. Delocalisation lowers the energy of the anion, so the equilibrium lies further to the right than for an alcohol.

3.2 Effect of electron-donating or electron-withdrawing groups

Groups that withdraw electrons (–Cl, –NO₂, –COOH) stabilise the carboxylate further and make the acid stronger. Groups that donate electrons (–CH₃, –C₂H₅) destabilise the anion and make the acid weaker.

Acid	pKₐ	Why
Cl₃C–COOH (trichloroethanoic)	0.65	Three Cl pull electrons away, stabilising the anion
Cl–CH₂–COOH (chloroethanoic)	2.87	One Cl stabilises the anion
HCOOH (methanoic)	3.75	No alkyl group, baseline strength
CH₃–COOH (ethanoic)	4.76	Methyl group donates electrons, destabilising the anion
CH₃–CH₂–COOH (propanoic)	4.87	Ethyl donates slightly more

You should be able to compare two acids qualitatively and predict which is stronger. OCR will occasionally ask you to explain this in terms of inductive (±I) effects.

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4. Reactions of Carboxylic Acids