Chemistry · Ch 8 — Organic Chemistry – Some Basic Principles and Techniques

Homologous Series

8.4.2

Homologous Series

The Concept of a Homologous Series

Organic compounds are vast in number, but they are not a random collection. They fall into families. A homologous series is a family of organic compounds that share the same functional group and whose members differ from one another by a fixed unit — a methylene ( $-CH_2-$ ) group. Each member of such a series is called a homologue.

The key idea is that the entire series can be described by a single general molecular formula. For example, all alkanes fit $C_nH_{2n+2}$ , all unbranched alkenes (with one double bond) fit $C_nH_{2n}$ , and all primary alcohols (alkanols) fit $C_nH_{2n+1}OH$ or $C_nH_{2n+2}O$ .

Important

The successive members of any homologous series differ in molecular formula by exactly one $-CH_2-$ unit. This means their molecular masses differ by 14 u (12 for carbon + 2 for the two hydrogens).

Common Homologous Series

The textbook lists several important series. Each is defined by its characteristic functional group and general formula.

Series Name	Functional Group	General Formula	Example (first member)
Alkanes	None (single C–C bonds)	$C_nH_{2n+2}$	$CH_4$ (methane)
Alkenes	Carbon-carbon double bond ( $C=C$ )	$C_nH_{2n}$	$C_2H_4$ (ethene)
Alkynes	Carbon-carbon triple bond ( $C\equiv C$ )	$C_nH_{2n-2}$	$C_2H_2$ (ethyne)
Haloalkanes	Halogen atom ( $-X$ , where $X = F, Cl, Br, I$ )	$C_nH_{2n+1}X$	$CH_3Cl$ (chloromethane)
Alkanols (Alcohols)	Hydroxyl group ( $-OH$ )	$C_nH_{2n+1}OH$	$CH_3OH$ (methanol)
Alkanals (Aldehydes)	Aldehyde group ( $-CHO$ )	$C_nH_{2n+1}CHO$	$HCHO$ (methanal)
Alkanones (Ketones)	Carbonyl group ( $>C=O$ )	$C_nH_{2n}O$	$CH_3COCH_3$ (propanone)
Alkanoic Acids (Carboxylic Acids)	Carboxyl group ( $-COOH$ )	$C_nH_{2n+1}COOH$	$HCOOH$ (methanoic acid)
Amines	Amino group ( $-NH_2$ )	$C_nH_{2n+1}NH_2$	$CH_3NH_2$ (methanamine)

Note

The general formula for a series applies strictly to compounds that are monofunctional (containing only one functional group) and acyclic (no rings). For example, the formula $C_nH_{2n}$ for alkenes assumes one double bond and no rings. A compound with two double bonds would have a different general formula ( $C_nH_{2n-2}$ ).

Polyfunctional Compounds

A single organic molecule can carry more than one functional group. These groups may be identical (e.g., a diol with two $-OH$ groups) or different (e.g., an amino acid with both $-NH_2$ and $-COOH$ groups). Such compounds are called polyfunctional compounds. They do not belong to a single simple homologous series but are instead described by the combination of functional groups they possess. …

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