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The general formula of alkenes is:
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Answer: Option B — CnH2n
Detailed Solution
Hydrocarbons are compounds made only of carbon and hydrogen. They are divided into groups called homologous series. Members of a series have the same general formula, similar chemical properties and differ from each other by a CH2 unit.
Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). Their general formula is CnH2n, where n is the number of carbon atoms. Since a double bond needs at least two carbon atoms, the first member is ethene (C2H4), also called ethylene. Other members are propene (C3H6) and butene (C4H8). Alkenes end with the suffix "-ene".
Because of the double bond, alkenes are more reactive than alkanes. They undergo addition reactions; for example, they add hydrogen in the presence of a nickel catalyst to form alkanes (this is the basis of the hydrogenation of vegetable oils to make vanaspati ghee). Alkenes decolourise bromine water and Baeyer's reagent (cold, dilute alkaline potassium permanganate), which are common tests for unsaturation. Ethene is used to make polythene and to ripen fruits artificially.
The three main series are:
Alkanes: CnH2n+2, single bonds only, saturated (methane CH4, ethane C2H6). They mainly undergo substitution reactions.
Alkenes: CnH2n, at least one double bond (ethene C2H4).
Alkynes: CnH2n-2, at least one triple bond (ethyne C2H2, also called acetylene, used in oxy-acetylene welding).
Why not the other options?
- ✗CnH2n+2 is the general formula of alkanes.
- ✗CnH2n-2 is the general formula of alkynes.
- ✗CnHn is not a general formula of any of these series, though benzene (C6H6) fits it.
Quick tip
Remember: Alkanes = CnH2n+2 (single bonds); alkenes = CnH2n (double bond, ethene); alkynes = CnH2n-2 (triple bond, ethyne or acetylene).
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Carbon & Organic Chemistry — Preparation
This question tests your understanding of Carbon & Organic Chemistry, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers carbon and its compounds: the allotropes of carbon (diamond, graphite and fullerenes), hydrocarbons, functional groups, the nomenclature of simple organic compounds, alcohols and acids, soaps and detergents, polymers and plastics. Questions commonly ask which allotrope of carbon conducts electricity (graphite), which is the hardest natural substance (diamond), what the main component of natural gas is (methane), and how soaps differ from detergents. Know the monomers of common polymers, such as ethylene for polythene, and which plastics are thermoplastics and which are thermosetting. Practise the carbon and organic chemistry questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.