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Hydrocarbons

NCERT Class 11 Chemistry Chapter 9

Hydrocarbons for Class 11 Chemistry, JEE and NEET: alkanes, alkenes, alkynes and benzene, with conformations, reaction mechanisms and directive influence.

6 topics Updated 8 Nov 2026

For: Class 11, JEE and NEET, unless a topic says otherwise

In the syllabus: JEE Class 11 syllabus, NEET Class 11 syllabus

With your textbook

NCERT Chemistry Part II (Class 11), Chapter 9 “Hydrocarbons”, reprint 2026-27.

Open the official NCERT chapter (PDF, 3.3 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Classification of hydrocarbons

    Saturated, unsaturated and aromatic, general formulas, fuels and degree of unsaturation.

    NCERT Section 9.1 11 min read 4 solved examples 8 practice questions

  2. Alkanes

    Isomers, Wurtz, Kolbe and decarboxylation, free radical halogenation, combustion and the conformations of ethane.

    NCERT Section 9.2 32 min read 7 solved examples 20 practice questions

  3. Alkenes

    The C=C bond, cis-trans isomerism, preparation, Markovnikov’s rule, the peroxide effect and ozonolysis.

    NCERT Section 9.3 26 min read 7 solved examples 24 practice questions

  4. Alkynes

    Naming, the triple bond, preparation, acidic character of terminal alkynes, addition reactions and polymerisation.

    NCERT Section 9.4 22 min read 6 solved examples 18 practice questions

  5. Aromatic hydrocarbons

    Benzene’s structure, Hückel’s rule, electrophilic substitution and the directive influence of groups.

    NCERT Section 9.5 25 min read 6 solved examples 17 practice questions

  6. Carcinogenicity and toxicity of hydrocarbons

    Benzene and polynuclear aromatic hydrocarbons, their sources and how they damage DNA.

    Class 11 only NCERT Section 9.6 9 min read 3 solved examples 8 practice questions

Key formulas in this chapter

Classification of hydrocarbons

  • Alkanes CnH2n+2; cycloalkanes and alkenes CnH2n; alkynes CnH2n−2
  • Degree of unsaturation of CxHy = (2x + 2 − y)/2 = rings + π bonds

Alkanes

  • Alkanes CnH2n+2; alkyl groups CnH2n+1
  • CnH2n+2 + ((3n + 1)/2) O2 → nCO2 + (n + 1)H2O
  • Wurtz: 2RX + 2Na → R–R + 2NaX; decarboxylation: RCOONa + NaOH → RH + Na2CO3
  • Halogenation rate F2 > Cl2 > Br2 > I2; hydrogens replaced 3° > 2° > 1°
  • Ethane: staggered more stable than eclipsed by about 12.5 kJ mol−1

Alkenes

  • Alkenes CnH2n (one C=C); C=C = one σ bond + one π bond
  • Open chain: σ bonds = number of atoms − 1; π bonds = number of double bonds
  • Markovnikov: CH3CH=CH2 + HBr → CH3CHBrCH3; with peroxide → CH3CH2CH2Br
  • Reactivity HI > HBr > HCl; dehydrohalogenation rate I > Br > Cl and 3° > 2° > 1°
  • Ozonolysis: R2C=CHR′ → R2C=O + R′CHO

Alkynes

  • Alkynes CnH2n−2; C≡C = one σ bond + two π bonds
  • CaC2 + 2H2O → Ca(OH)2 + C2H2
  • HC≡CH + NaNH2 → HC≡CNa + NH3; only H on a triply bonded carbon is acidic
  • RC≡CH + 2HBr → RCBr2CH3 (gem dihalide, Markovnikov twice)
  • 3C2H2 → C6H6 (red hot iron tube, 873 K)

Aromatic hydrocarbons

  • Hückel’s rule: aromatic if planar, fully delocalised and with (4n + 2) π electrons (n = 0, 1, 2, …)
  • CxHy + (x + y/4) O2 → xCO2 + (y/2) H2O
  • Benzene C–C 139 pm, between C–C 154 pm and C=C 133 pm

Carcinogenicity and toxicity

  • Complete combustion: CH4 + 2O2 → CO2 + 2H2O; incomplete: CH4 + O2 → C + 2H2O
  • Degree of unsaturation of CxHy = (2x + 2 − y)/2 = rings + π bonds
  • 1,2-Benzanthracene C18H12 (4 rings); 1,2-benzpyrene C20H12 (5 rings)

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