Class 11 Chemistry Hydrocarbons Notes

Class 11 Chemistry Hydrocarbons Notes

9.1 Classification of Hydrocarbons

  • Alkanes (Saturated hydrocarbons): Only single bonds (C–C).
  • Alkenes (Unsaturated hydrocarbons): Contain double bonds (C=C).
  • Alkynes (Unsaturated hydrocarbons): Contain triple bonds (C≡C).
  • Aromatic hydrocarbons: Contain benzene ring or conjugated cyclic systems.

9.2 Alkanes

  • General formula: CnH2n+2C_nH_{2n+2}Cn​H2n+2​
  • Bonding: sp³ hybridized carbons, tetrahedral shape.
  • Nomenclature: IUPAC system, longest chain, substituents.
  • Reactions:
    • Combustion: CnH2n+2+O2→CO2+H2OC_nH_{2n+2} + O_2 \rightarrow CO_2 + H_2OCn​H2n+2​+O2​→CO2​+H2​O
    • Substitution (halogenation): CH4+Cl2→hvCH3Cl+HClCH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HClCH4​+Cl2​hv​CH3​Cl+HCl
  • Isomerism: Chain and position isomers.

9.3 Alkenes

  • General formula: CnH2nC_nH_{2n}Cn​H2n​
  • Bonding: sp² hybridized, planar geometry, 120° bond angle.
  • Nomenclature: Longest chain containing C=C, locants for double bond.
  • Reactions:
    • Addition reactions (H₂, X₂, HX, H₂O)
    • Polymerization
  • Isomerism:
    • Structural (position, chain)
    • Geometrical (cis-trans)

9.4 Alkynes

  • General formula: CnH2n−2C_nH_{2n-2}Cn​H2n−2​
  • Bonding: sp hybridized, linear, 180° bond angle.
  • Nomenclature: Longest chain containing C≡C, number the triple bond.
  • Reactions:
    • Addition reactions (similar to alkenes)
    • Combustion
    • Acidity of terminal alkynes

9.5 Aromatic Hydrocarbons

  • Contain benzene rings (C₆H₆) with delocalized π-electrons.
  • Properties: Planar, resonance stabilized, undergo electrophilic substitution.
  • Examples: Benzene, Toluene, Xylene.
  • Reactions:
    • Nitration
    • Halogenation
    • Sulfonation
    • Friedel-Crafts alkylation/acylation

9.6 Carcinogenicity and Toxicity

  • Some hydrocarbons, especially aromatic compounds, are carcinogenic or toxic.
  • Example: Benzene is toxic and can cause leukemia.
  • Polycyclic aromatic hydrocarbons (PAHs) from combustion of fuels can be carcinogenic.
  • Proper handling and precautions are essential in lab and industrial use.

Key Points to Remember

  • Hydrocarbons are classified into saturated, unsaturated, and aromatic.
  • Alkanes: Single bonds; Alkenes: Double bonds; Alkynes: Triple bonds.
  • Aromatic hydrocarbons are stable due to resonance.
  • Some hydrocarbons are toxic or carcinogenic, requiring safety measures.