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Homologous Series and Its Characteristics

Chemistry · General Principles Of Organic Chemistry · NEET

A homologous series is a family of organic compounds that share the same functional group and the same general formula, where each member (a homologue) differs from the next by one -CH2 unit, which is a mass of 14 u. Memory hook: "same group, step by CH2." Because members of one series show a smooth, predictable change in properties, NEET often tests whether two compounds belong to the same series and how their formula or boiling point changes.
Alkane Homologous Series (CnH2n+2)MethaneCH4n = 1EthaneC2H6n = 2PropaneC3H8n = 3+ CH2(14 u)+ CH2(14 u)Each step adds one CH2 unit: same functional family, mass rises by 14 u.Boiling point increases gradually as the chain grows.
The alkane homologous series: methane, ethane and propane share the general formula CnH2n+2, and each successive member differs by one -CH2 unit (a mass difference of 14 u), producing a smooth rise in boiling point.

Your doubts, answered

Do all members of a homologous series have the same molecular formula?

No. They share the same GENERAL formula, not the same molecular formula. For alkanes the general formula is CnH2n+2, so methane (CH4), ethane (C2H6), propane (C3H8) all fit the pattern but each has its own molecular formula. Same general formula, different molecular formulas.

Why do successive members differ by -CH2 and not by -CH3?

A carbon in the middle of the chain needs 2 hydrogens to stay tetravalent, so inserting one more carbon into the chain adds exactly one CH2 group. A CH3 would be a chain end, not an insertion. The mass difference between two neighbours is therefore 12 + 2(1) = 14 u. Remember the number 14 u; NEET uses it.

Is a homologous series the same thing as a functional group?

No. The functional group is the reactive atom or group (like -OH or -COOH). A homologous series is the whole FAMILY of compounds built around one functional group. The functional group defines the series; the series is all the members that carry it (e.g. all alkanols: CH3OH, C2H5OH, C3H7OH).

Can two compounds with the same functional group be in different homologous series?

Yes, if they differ in more than a CH2 step or in the type of carbon skeleton, but within the SAME functional group and general formula they belong to the same series. The key test: same functional group AND same general formula AND neighbours differ by CH2. Alkanols and alkanoic acids both contain oxygen but are different series with different general formulas.

What is the difference between a homologue and an isomer?

Homologues differ in molecular formula by one or more CH2 units and belong to a series (methane and ethane). Isomers have the SAME molecular formula but different structures (butane and 2-methylpropane, both C4H10). Homologues = different formula, same series; isomers = same formula, different structure.

Why do boiling points increase as you go down a homologous series?

Each step adds a CH2 unit, raising molecular mass and increasing the surface area for van der Waals (London) forces. Stronger intermolecular attraction means more energy is needed to boil, so boiling point rises gradually and predictably down the series. This smooth gradation in physical properties is a defining characteristic.

⚠️ The NEET trap
✗ Students think any two compounds with an -OH group are homologues, so they treat ethanol (an alcohol) and phenol as members of the same homologous series.
✓ Homologues must share the same general formula and differ only by CH2 units. Ethanol (C2H5OH, an alkanol) and phenol (C6H5OH, an aromatic) have different general formulas and different skeletons, so they are NOT in the same series even though both carry -OH. Always check general formula plus a clean CH2 gradation, not just the functional group.
🧠 Same functional group does NOT automatically mean same homologous series.

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Frequently asked

What is a homologous series in simple words?

It is a set of organic compounds with the same functional group and the same general formula, where each compound differs from the next by one CH2 group. The individual members are called homologues.

What is the mass difference between two successive homologues?

14 u, because the difference is one -CH2 unit (carbon 12 plus two hydrogens, 12 + 2 = 14). NEET often uses this fact directly.

What are the main characteristics of a homologous series?

All members: (1) have the same functional group, (2) can be written with one general formula, (3) differ from neighbours by CH2 (14 u), (4) are made by similar methods and show similar chemical properties, and (5) show a gradual, regular change in physical properties like boiling point down the series.

Give the general formula for common homologous series asked in NEET.

Alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n-2, alkanols (alcohols) CnH2n+1OH, alkanals (aldehydes) CnH2nO, and alkanoic acids (carboxylic acids) CnH2nO2.

Why is the homologous series concept useful for NEET?

Because properties change in a smooth, predictable way, you can predict the formula, mass, or trend for any member once you know the general formula. This saves memorising each compound and helps in matching, naming, and boiling-point trend questions.

Related concepts:
What is a Functional Group in Organic ChemistryIUPAC Nomenclature of Branched Chain AlkanesClassification of Organic Compounds (Acyclic, Cyclic)Chain Isomerism in Organic CompoundsCatenation: Why Carbon Forms Long Chains
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)General Principles Of Organic Chemistry PYQs