Carboxylic Acids

Summary: Carboxylic acids contain the -COOH functional group. Ethanoic acid (CH3COOH) is a weak acid found in vinegar. They react with alcohols to form esters (esterification). Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16

What are Carboxylic Acids?

Carboxylic acids are a homologous series with the general formula:

CnH2n+1COOH (or R-COOH, where R = alkyl group)

The functional group is the carboxyl group (-COOH):

  O
  ||
  C
 / \
O   OH

The -COOH group consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. It is the combination of these two groups that gives carboxylic acids their acidic properties.

Key structural point: Only the H atom in the -COOH group is acidic (can be donated as H+). Any H atoms attached to carbon atoms in the alkyl chain (e.g., the three H atoms in CH3-) are NOT acidic.

The First Three Carboxylic Acids

Note that n in the general formula CnH2n+1COOH refers to the number of carbon atoms in the alkyl chain, not including the -COOH carbon. Methanoic acid has n=0, ethanoic acid has n=1, propanoic acid has n=2. The total number of carbons is n+1.

Drawing displayed formulas:

• Methanoic acid (HCOOH): One carbon atom with a C=O double bond and a C-OH single bond, plus one H attached to the carbon: H-COOH
• Ethanoic acid (CH3COOH): A methyl group (CH3-) attached to a -COOH group: CH3-COOH
• Propanoic acid (C2H5COOH): An ethyl group (CH3CH2-) attached to a -COOH group: CH3CH2-COOH

Properties of Carboxylic Acids

Weak Acids

Carboxylic acids are weak acids. This means they only partially dissociate (ionise) in water:

CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq)
Ethanoic acid        Ethanoate ion

The equilibrium lies well to the left — most of the acid remains as undissociated CH3COOH molecules. Unlike strong acids (HCl, H2SO4, HNO3), carboxylic acids do not fully dissociate.

Consequences of being a weak acid:

• Higher pH than a strong acid of the same concentration (less H+ in solution)
• Slower reaction with metals, bases, and carbonates
• Lower electrical conductivity than a strong acid of the same concentration

“Weak” refers to the degree of dissociation, NOT the concentration. Concentrated ethanoic acid is still a weak acid because it is only partially dissociated. Dilute hydrochloric acid is still a strong acid because it is fully dissociated.

Typical Acid Reactions

Carboxylic acids undergo the typical reactions of acids, but more slowly than strong acids:

With reactive metals (above hydrogen in the reactivity series):

2CH3COOH (aq) + Mg (s) --> (CH3COO)2Mg (aq) + H2 (g)
Ethanoic acid + Magnesium --> Magnesium ethanoate + Hydrogen

With bases (neutralisation):

CH3COOH (aq) + NaOH (aq) --> CH3COONa (aq) + H2O (l)
Ethanoic acid + Sodium hydroxide --> Sodium ethanoate + Water

With carbonates:

2CH3COOH (aq) + Na2CO3 (s) --> 2CH3COONa (aq) + H2O (l) + CO2 (g)
Ethanoic acid + Sodium carbonate --> Sodium ethanoate + Water + Carbon dioxide

Observation: effervescence (fizzing) as CO2 gas is produced. This is the test for an acid using a carbonate.

Naming salts of carboxylic acids:

• The salt is named: [metal] [alkanoate]
• Methanoic acid ⇒ methanoates, ethanoic acid ⇒ ethanoates, propanoic acid ⇒ propanoates

Formation of Ethanoic Acid

Ethanoic acid can be produced by oxidation of ethanol:

C2H5OH + 2[O] --> CH3COOH + H2O
Ethanol             Ethanoic acid

Method 1: Using oxidising agents (laboratory method)

• Warm ethanol with acidified potassium manganate(VII) (KMnO4/H2SO4): purple ⇒ colourless
• OR warm ethanol with acidified potassium dichromate(VI) (K2Cr2O7/H2SO4): orange ⇒ green

Method 2: Biological oxidation (natural process)

• Wine left exposed to air turns sour: ethanol in wine is oxidised by oxygen and bacteria to ethanoic acid
• This is how vinegar is produced naturally (vinegar is ~5% ethanoic acid solution)

Esterification (Reaction with Alcohols)

Carboxylic acids react with alcohols to form esters. This is called esterification:

Carboxylic acid + Alcohol ⇌ Ester + Water

Example: Ethanoic acid + Ethanol:

CH3COOH (l) + C2H5OH (l) ⇌ CH3COOC2H5 (l) + H2O (l)
Ethanoic acid   Ethanol        Ethyl ethanoate      Water

Conditions:

• Catalyst: Concentrated sulfuric acid (H2SO4) — a few drops
• Warm gently (do not boil — alcohols are flammable)
• The reaction is reversible (equilibrium sign: ⇌)

Role of concentrated H2SO4:

• Acts as a catalyst (speeds up the reaction)
• Also acts as a dehydrating agent — removes water produced, shifting equilibrium to the right (increasing ester yield)

Observations during esterification:

• The mixture often has a characteristic fruity/fragrant smell (the ester)

Naming Esters

Esters are named in two parts:

[Alcohol part (alkyl)] + [Acid part (-anoate)]

Rules:

1. The first part comes from the alcohol: e.g., methanol ⇒ methyl, ethanol ⇒ ethyl, propanol ⇒ propyl
2. The second part comes from the carboxylic acid: e.g., methanoic acid ⇒ methanoate, ethanoic acid ⇒ ethanoate

Examples:

Drawing the ester link: The ester functional group is -COO-:

  O
  ||
  C - O - R'
  |
  R

where R is from the acid part and R’ is from the alcohol part.

To draw an ester, join the acid part and alcohol part together, losing water (H from acid’s -OH, OH from alcohol’s -OH). The acid part provides the C=O; the alcohol part provides the O-R’ link.

Uses of Esters

Vinegar

• Vinegar is a dilute solution of ethanoic acid (typically ~5% CH3COOH in water)
• Produced by oxidation of ethanol (from fermented wine, cider, or malt)
• Used as a food flavouring and preservative
• Ethanoic acid gives vinegar its characteristic sour taste and pungent smell

Key Points

• General formula: CnH2n+1COOH
• Functional group: carboxyl (-COOH)
• First three: methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (C2H5COOH)
• Weak acids — partially dissociate in water: CH3COOH ⇌ CH3COO- + H+
• With metals: acid + metal ⇒ salt + H2
• With bases: acid + base ⇒ salt + H2O
• With carbonates: acid + carbonate ⇒ salt + H2O + CO2
• Oxidation of ethanol: C2H5OH + 2[O] ⇒ CH3COOH + H2O
• Esterification: carboxylic acid + alcohol ⇌ ester + H2O (conc. H2SO4 catalyst, warm)
• Ester naming: [alkyl from alcohol] + [alkanoate from acid], e.g., ethanol + ethanoic acid ⇒ ethyl ethanoate
• Vinegar = dilute ethanoic acid (~5%)

Key Concepts from Past Papers

• Fractional distillation depends on boiling point
• Esterification requires concentrated H2SO4 catalyst and warming

Keywords from Past Papers

methane, ethene, point, bonds, carbon, fractions, energy, hydrocarbons, boiling, idea, structure, water, alcohol, addition, poly

Related Notes

• Organic Chemistry Fundamentals
• Alkanes
• Alkenes
• Alcohols

Sources

• OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Carboxylic Acids)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Carboxylic Acids], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Carboxylic Acids), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 25: Carboxylic Acids], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
• 0620/32 Feb/March 2018: Q66(a)(i) (1m)
• 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
• 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
• 0620/32 Feb/March 2021: Q66(c)(i) (1m)
• 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
• 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
• 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
• 0620/33 May/June 2017: Q44(a)(ii) (1m)
• 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)
• 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)

Common Misconceptions