Q.Two successive alkanes differ from
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Homologous Series
You are walking up a staircase. Each step is a little higher than the one before, but the shape of the step — the rise, the tread, the material — is exactly the same. If you know how to climb the first step, you know how to climb the hundredth. That is the intuition behind a homologous series.
In organic chemistry, a homologous series is a family of organic compounds that share the same general formula and the same functional group, and whose successive members differ by a constant unit: a −CH2− group. The word "homologous" comes from Greek — homos (same) and logos (proportion) — meaning "having the same ratio or pattern."
A homologous series is defined by three fixed rules:
- All members have the same general formula (e.g., CnH2n+2 for alkanes).
- Each member differs from the next by exactly one −CH2− unit (i.e., a mass difference of 14 u).
- All members contain the same functional group (if any), so their chemical properties are similar.
Let’s see it in action with the simplest series: the alkanes.
- Methane: CH4 (n=1)
- Ethane: C2H6 (n=2)
- Propane: C3H8 (n=3)
- Butane: C4H10 (n=4)
Notice the pattern: each step adds one carbon and two hydrogens — exactly a −CH2− group. The general formula is CnH2n+2. If you know the formula for one member, you can write the formula for the next by adding CH2.
Now, what does this mean for properties? Because the functional group is the same, the chemical behaviour of all members is similar. All alkanes burn in oxygen to give CO2 and water; all react with halogens in the same way (substitution). But physical properties change gradually — this is called gradation. As the chain gets longer, boiling points rise, melting points rise, and the compounds become less volatile and more viscous. Methane is a gas at room temperature; octane (C8H18) is a liquid; waxes with very long chains are solids.
A common mistake is to think that "same general formula" means all members have identical chemical properties. They do not — the type of reaction is the same, but the rate and conditions can change. For example, larger alkanes ignite less easily than smaller ones. The similarity is in the kind of chemistry, not the exact numbers.
Other important homologous series you will meet:
| Series | General Formula | Functional Group | First Member |
|---|---|---|---|
| Alkenes | CnH2n | C=C double bond | Ethene (C2H4) |
| Alkynes | CnH2n−2 | C≡C triple bond | Ethyne (C2H2) |
Alkanes form a homologous series with general formula CnH2n+2, where each successive member differs from the previous one …
Successive alkanes in the homologous series differ by a CH2 unit (14 mass units).
Alkanes follow the general formula CnH2n+2. Going from one member to the next (e.g. methane CH4 → ethane C2H6 → propane C3H8), each step adds exactly one carbon and two hydrogens, i.e. one CH2 unit. This constant difference in c …
- CBSE 2026Set ANNUAL1 markQ.Give the name of first stable member of alkyne series.
›Reveal solutionSolution
The first member of the alkyne series is ethyne (acetylene), HC≡CH, with molecular formula C2H2.
Alkynes are hydrocarbons containing at least one carbon-carbon triple bond, with the general formula CnH2n-2. The simplest possible alkyne would require two carbon atoms joined by a triple bond, i.e., ethyne (HC≡CH, C2H2), commonly known by its trivial name acetylene. (A single-carbon 'methyne' analogue does not exist as a stable compound, since a triple bond requires at least two carbon atoms.) Ethyne is …
- CBSE 2026Set ANNUAL1 markMCQQ.The functional group of acetone is(a) − OH(b) − COOH(c) >C=O(d) − CHO (−C(=O)H)
›Reveal solutionSolution
Acetone's functional group is the carbonyl group >C=O.
Acetone, CH3–CO–CH3, is the simplest ketone. Its characteristic functional group is the carbonyl group, >C=O, bonded to two carbon atoms. …
- CBSE 2026Set ANNUAL1 markMCQQ.The functional group present in carboxylic acids is(a) − CHO(b) − CO −(c) − COOH(d) − COOR
›Reveal solutionSolution
Carboxylic acids contain the −COOH (carboxyl) group.
The carboxyl group −COOH (a combination of carbonyl >C=O and hydroxyl −OH on the same carbon) is the functional group of carboxylic acids, e.g …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is the functional group of secondary amine ?(a) − NH2(b) − NH −(c) − N< (−N−)(d) None of these
›Reveal solutionSolution
Secondary amine functional group = −NH− (R2NH).
Amines are classified by how many carbon groups are on nitrogen: primary R−NH2 (−NH2), secondary R2NH (−NH−), tertiary R3N (−N<). In a secondary amine, nitrogen i …
- CBSE 2026Set ANNUAL1 markMCQQ.The drug Azidothymidine (AZT) is used in the treatment of :(1) Cancer(2) AIDS(3) Bronchitis(4) Ulcer
›Reveal solutionSolution
AZT (Azidothymidine) is used in the treatment of AIDS — option (2).
Azidothymidine (AZT), also called zidovudine, is a nucleoside-analogue antiretroviral drug. It blocks the enzyme reverse transcriptase, which HIV needs to copy its RNA into DNA, thereby slowing …
- CBSE 2024Set ANNUAL1 markMCQQ.Two successive alkanes differ from(a) CH2(b) CH(c) CH3(d) C2H4
›Reveal solutionSolution
Successive alkanes in the homologous series differ by a CH2 unit (14 mass units).
Alkanes follow the general formula CnH2n+2. Going from one member to the next (e.g. methane CH4 → ethane C2H6 → propane C3H8), each step adds exactly one carbon and two hydrogens, i.e. one CH2 unit. This constant difference in c …
- CBSE 2020Set ANN1 markQ.The class of organic compound differ by a ⟩CH2 group between adjacent members are called _____.
›Reveal solutionSolution
A series of organic compounds in which each member differs from the next by a constant -CH2- unit is called a homologous series.
A homologous series is a family of organic compounds having:
- The same general formula (e.g., alkanes: CnH2n+2)
- The same functional group and similar chemical properties
- Adjacent members differing in molecular formula by a -CH2- (14 mass units) unit
- A gradual change in physical properties (boiling point, melting point) with increasing molecular mass …
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