Condensation Reaction
Summary: A condensation reaction is a reaction in which two molecules join together with the elimination of a small molecule, usually water (H₂O) or hydrogen chloride (HCl). Esterification and condensation polymerisation are the key examples at IGCSE. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-23
A condensation reaction is a chemical reaction in which two molecules combine to form a larger molecule, with the simultaneous elimination of a small molecule — typically water (H₂O) but occasionally hydrogen chloride (HCl).
Key Examples at IGCSE
1. Esterification
Carboxylic acid + Alcohol → Ester + H₂O
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
The —OH from the acid and —H from the alcohol combine to form water.
2. Condensation Polymerisation
Polyesters: Dicarboxylic acid + Diol → Polyester + H₂O
Polyamides: Dicarboxylic acid + Diamine → Polyamide (e.g. nylon) + H₂O
Proteins: Amino acids → Protein (polypeptide) + H₂O
3. Peptide Bond Formation
Two amino acids join → dipeptide + H₂O
The —OH from one amino acid’s —COOH group and —H from the other’s —NH₂ group form water, leaving a peptide bond (—CONH—).
How to Recognise a Condensation Reaction
- Two molecules → one larger molecule (they join together)
- A small molecule is eliminated (H₂O, HCl)
- Often involves —OH combining with —H to form water
- Mass of product < sum of masses of reactants (byproduct lost)
Condensation vs Addition
| Feature | Condensation | Addition |
|---|---|---|
| Products formed | Large molecule + small byproduct | Single product only |
| Byproduct? | Yes (H₂O, HCl) | No |
| Substrate requirement | Two functional groups | C=C double bond |
| All atoms retained? | No — some lost as byproduct | Yes — all atoms in product |
| Examples | Esterification, nylon, proteins | Bromine + ethene, hydration of ethene |
Key Facts
- Condensation = two molecules join + small molecule eliminated
- Small molecule is usually water (H₂O)
- Occur when —OH and —H combine to form water
- Key examples: esterification, condensation polymerisation, peptide bond formation
- Different from addition — which produces only ONE product with no byproduct
- Mass of products < mass of reactants
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.