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3.2.3—Homologous series

Syllabus
First assessment 2025
Objective
3.2.3
Level
SL

Homologous Series

Members of a homologous series share a functional-group pattern and general formula. Successive members differ by CH₂.

Recognize the series by its functional group and general formula, including alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, acids, ethers, amines, amides, esters, and halogenoalkanes.

Homologous series Recognition pattern / common acyclic general formula
Alkane only C-C single bonds; CXnHX2n+2\ce{C_nH_{2n+2}}
Alkene / alkyne C=C: CXnHX2n\ce{C_nH_{2n}}; C≡C: CXnHX2n−2\ce{C_nH_{2n-2}}
Halogenoalkane C-X; CXnHX2n+1X\ce{C_nH_{2n+1}X}
Alcohol / ether C-OH: CXnHX2n+1OH\ce{C_nH_{2n+1}OH}; C-O-C: CXnHX2n+2O\ce{C_nH_{2n+2}O}
Aldehyde / ketone terminal -CHO or internal C=O; CXnHX2nO\ce{C_nH_{2n}O}
Carboxylic acid / ester -COOH or -COO-; CXnHX2nOX2\ce{C_nH_{2n}O2}
Primary amine / amide -NH2: CXnHX2n+3N\ce{C_nH_{2n+3}N}; -CONH2: CXnHX2n+1NO\ce{C_nH_{2n+1}NO}

Moving from one member to the next adds CH₂, so molar mass and dispersion forces change gradually while the shared functional group gives similar reaction patterns. Use both the functional group and general formula: formula alone can overlap with another structural family.

Recognising Homologous Series

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

State the general formula for the homologous series of alkenes.

Organic Analysis Summary

Retrieve the route: translate formulae, identify functional groups and series, name and classify isomers, then use mass, IR, and NMR evidence together to determine structure.

Check connectivity, functional-group evidence, formula/mass constraint, shifts and integration, splitting neighbours, and agreement across every technique.