Homologous series

In chemistry, a homologous series is a series of compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and showing a gradation in physical properties as a result of increase in molecular size and mass (see relative molecular mass). For example, ethane has a higher boiling point than methane since it has more Van der Waals forces (intermolecular forces) with neighbouring molecules. This is due to the increase in the number of atoms making up the molecule.

Alkanes (paraffins), alkenes (olefins), ethers, and alkynes (acetylenes) form such series in which members differ in mass by 14 atomic mass units. For example, the alkane homologous series begins with methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), and pentane (C₅H₁₂), each member differing from the previous one by a CH₂ group (or 14 atomic mass units). The CH₂ group is called Methylene Group.

Similarly, there is the alcohol homologous series that starts with methanol (CH₄O), ethanol (C₂H₆O), as primary alcohols, isopropanol (C₃H₈O) as a simple secondary alcohol, and a simple tertiary alcohol is tert-butanol (C₄H₁₀O).

Even though the general formula remains constant in a homologous series, specific members of the series may have different structures or entirely different properties, though qualitatively, reactivity usually remains the same. Compounds in each series typically have a similar group of atoms called a functional group. Most chemical properties of organic compounds are due to the presence of functional groups.

Homologous series	General formula	Example	Functional group
Straight Chain Alkanes	CnH2n + 2 (n ≥ 1)	CH4, n = 1
Straight Chain Perfluoroalkanes	CnF2n + 2 (n ≥ 1)	CF4, n = 1
Alkyl	CnH2n + 1 (n ≥ 1)	CH3, n = 1
Alkenes and Cyclic Alkanes	CnH2n (n ≥ 2)	C2H4, n = 2	C = C
Alkynes	CnH2n − 2 (n ≥ 2)	C2H2, n = 2	C ≡ C
Alcohols	CnH(2n + 1)OH (n ≥ 1)	CH3OH, n = 1	- OH
Carboxylic acids	CnH2n+1COOH (n ≥ 0)	CH2O2, n = 0	- COOH
Carbohydrates	Cx(H2O)y (n ≥ 1)	C6H12O6

Where n represents the number of carbon atoms present.

A Homologation reaction is any chemical process which converts one member of a homologous series to the next member.

Inorganic Homologous Series

Homologous series are not unique to organic chemistry. Titanium, vanadium, and molybdenum oxides all form homologous series (e.g. V_nO_2n-1 for 2 < n < 10), as do the silanes, Si_nH_2n+2 (with n up to 8) that are analogous to the alkanes, C_nH_2n+2.