Hydrocarbons — Class 11 MCQs with Answers

Hydrocarbons — important questions & MCQs with answers (Class 11 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   General formula of alkenes:

   ACₙH₂ₙ₊₂BCₙH₂ₙCCₙH₂ₙ₋₂DCₙHₙ
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   Answer: CₙH₂ₙ

   Explanation: One C=C bond removes 2 H from the alkane formula: CₙH₂ₙ₊₂ → CₙH₂ₙ, e.g. propene C₃H₆. Adding the extra +2 gives the alkane formula instead, a common mix-up.

2. Q2 of 6Easy

   Molecular formula of benzene:

   AC₂H₆BC₆H₁₂CCH₄DC₆H₆
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   Answer: C₆H₆

   Explanation: Benzene's ring has 6 C and 6 H — degree of unsaturation = (2×6+2−6)/2 = 4, matching 1 ring + 3 C=C. Mistaking it for cyclohexane (C₆H₁₂) skips the unsaturation entirely.

3. Q3 of 6Medium

   Alkynes have ___ bonds between carbons.

   ASingleBDoubleCTripleDNone
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   Answer: Triple

   Explanation: The −yne suffix specifically denotes a C≡C triple bond (2π + 1σ), giving sp hybridised carbons, unlike −ane (single) or −ene (double). Picking 'double' confuses alkynes with alkenes.

4. Q4 of 6Medium

   Hückel's rule for aromaticity: (4n+2) π electrons. For benzene, n =

   A1B0C2D3
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   Answer: 1

   Explanation: Benzene has 6 π electrons; solving 4n+2 = 6 gives n = 1. Dividing 6 by 4 directly (instead of solving the equation) is the mistake that gives a non-integer or wrong n.

5. Q5 of 6Hard

   Markovnikov's rule applies to addition of HX to:

   AEstersBAlkanesCAlkenes (asymmetric)DAmines
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   Answer: Alkenes (asymmetric)

   Explanation: Markovnikov's rule governs HX addition to unsymmetrical (asymmetric) alkenes: H adds to the carbon already bearing more H, via the more stable carbocation. Symmetric alkenes give only one product.

6. Q6 of 6Hard

   Benzene typically undergoes:

   AElectrophilic substitutionBNucleophilic additionCFree radical additionDNone
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   Answer: Electrophilic substitution

   Explanation: Benzene reacts by electrophilic substitution, replacing one ring H while keeping all 3 π bonds and the resonance stabilisation intact. Addition, as with alkenes, would destroy that delocalisation.

Hydrocarbons notes: key concepts (Class 11 Chemistry)

Hydrocarbons split into saturated alkanes, unsaturated alkenes and alkynes, and aromatic rings. This chapter covers their preparation and physical trends plus the defining reactions: free-radical substitution in alkanes, electrophilic addition and ozonolysis in alkenes/alkynes, and electrophilic substitution in arenes.

Alkanes

Saturated hydrocarbons CₙH₂ₙ₊₂ with only σ bonds; fairly inert but undergo free-radical halogenation and combustion, and show chain isomerism.

Preparation of alkanes

Wurtz reaction couples two alkyl halides with Na; decarboxylation of sodium carboxylates and catalytic hydrogenation of alkenes also give alkanes.

Conformations of ethane

Rotation about C–C gives staggered (most stable, least torsional strain) and eclipsed (least stable) forms; the barrier is only about 12.5 kJ/mol.

Alkenes

Contain a C=C double bond (CₙH₂ₙ, one sp² π bond); being electron-rich they undergo electrophilic addition and show cis–trans isomerism.

Markovnikov's rule

In HX addition to an unsymmetrical alkene, H adds to the carbon bearing more hydrogens, giving the more stable, more-substituted carbocation.

Peroxide (anti-Markovnikov) effect

With HBr and peroxides the addition reverses via a free-radical chain; only HBr shows it — not HCl or HI — for bond-energy reasons.

Ozonolysis

O₃ cleaves a C=C to a cyclic ozonide, which on reductive work-up (Zn/H₂O) gives carbonyl compounds; it pinpoints the double-bond position.

Alkynes

Contain a C≡C triple bond (CₙH₂ₙ₋₂, two π bonds); a terminal alkyne's acidic sp C–H reacts with Na and with ammoniacal AgNO₃ or Cu₂Cl₂.

Preparation of alkenes/alkynes

Alkenes come from dehydrohalogenation or dehydration (Saytzeff product); acetylene from CaC₂ and water or from a vicinal dihalide by double elimination.

Aromaticity & Hückel rule

A planar, cyclic, fully conjugated ring with (4n+2) π electrons is aromatic and unusually stable; benzene has 6 π electrons (n = 1).

Electrophilic substitution in arenes

Benzene keeps its aromatic sextet by substituting, not adding: nitration, sulphonation, halogenation and Friedel–Crafts alkylation/acylation.

Directive effects

o/p-directors (−OH, −NH₂, −OR, alkyl, halogens) activate the ring except halogens; −NO₂, −COOH and −CN are meta-directing deactivators.

Physical-property trends

Boiling point rises with chain length and falls with branching; hydrocarbons are non-polar, insoluble in water and less dense than water.

Combustion & flame test

Complete combustion gives CO₂ + H₂O and heat; a sooty, luminous flame from incomplete combustion signals high-carbon aromatic compounds.