Class 12 Chemistry Chapter 16 – Hydrocarbons Notes

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Chapter 16: Hydrocarbons

In Class 12 Chemistry, Chapter 16 focuses on hydrocarbons, which are the simplest organic compounds composed entirely of carbon and hydrogen. This chapter covers the classification, structure, nomenclature, properties, and reactions of hydrocarbons.

Classification of Hydrocarbons

  1. Aliphatic Hydrocarbons
  • These are hydrocarbons with open-chain structures, which can be either straight or branched.
  • Saturated Hydrocarbons (Alkanes): Contain only single bonds between carbon atoms. They have the general formula ( C_nH_{2n+2} ). Example: methane (CH₄), ethane (C₂H₆).
  • Unsaturated Hydrocarbons:
    • Alkenes: Contain at least one carbon-carbon double bond. General formula ( C_nH_{2n} ). Example: ethene (C₂H₄), propene (C₃H₆).
    • Alkynes: Contain at least one carbon-carbon triple bond. General formula ( C_nH_{2n-2} ). Example: ethyne (C₂H₂), propyne (C₃H₄).
  1. Aromatic Hydrocarbons
  • These hydrocarbons contain one or more benzene rings, which are stable ring structures with alternating double and single bonds.
  • Example: benzene (C₆H₆), toluene (C₇H₈), naphthalene (C₁₀H₈).

Nomenclature of Hydrocarbons

  1. IUPAC Naming Rules
  • Alkanes: Named by identifying the longest continuous chain of carbon atoms, and adding the suffix “-ane”. Substituents are named as prefixes.
  • Alkenes and Alkynes: Similar to alkanes, but the suffixes “-ene” and “-yne” are used for alkenes and alkynes, respectively. The position of the double or triple bond is indicated by the lowest numbered carbon atom involved in the bond.
  • Aromatic Compounds: Named based on the parent aromatic structure, with substituents named as prefixes.
  1. Common Names
  • Some hydrocarbons have common names, especially those that are widely used or have historical significance.

Isomerism in Hydrocarbons

  1. Structural Isomerism
  • Chain Isomerism: Different arrangements of the carbon chain.
  • Position Isomerism: Different positions of a functional group or multiple bonds within the same carbon skeleton.
  • Functional Group Isomerism: Same molecular formula but different functional groups.
  1. Geometric (Cis-Trans) Isomerism
  • Found in alkenes, where different groups attached to the double-bonded carbons can be on the same side (cis) or opposite sides (trans).

Properties of Hydrocarbons

  1. Physical Properties
  • Alkanes: Generally non-polar, low reactivity, low melting and boiling points that increase with molecular size. Soluble in non-polar solvents.
  • Alkenes and Alkynes: Similar properties to alkanes but are more reactive due to the presence of multiple bonds. They have lower boiling points than alkanes of similar molecular weight.
  • Aromatic Hydrocarbons: Stable, non-polar, with characteristic aromatic odor. Generally have higher melting and boiling points compared to aliphatic hydrocarbons.
  1. Chemical Properties
  • Alkanes: Undergo combustion and substitution reactions. Relatively unreactive due to strong C-H and C-C bonds.
  • Alkenes: Undergo addition reactions, including hydrogenation, halogenation, and hydrohalogenation. The double bond acts as a site of reactivity.
  • Alkynes: Similar to alkenes but more reactive due to the triple bond. Can undergo addition reactions and act as acids (terminal alkynes).
  • Aromatic Hydrocarbons: Undergo substitution reactions, such as halogenation, nitration, and sulfonation, due to the stability of the aromatic ring.

Reactions of Hydrocarbons

  1. Combustion
  • Complete combustion of hydrocarbons produces carbon dioxide and water, while incomplete combustion can produce carbon monoxide and soot.
  1. Halogenation
  • Alkanes react with halogens in the presence of UV light or heat to form haloalkanes.
  1. Hydrogenation
  • Alkenes and alkynes can be hydrogenated to alkanes using a catalyst like platinum, palladium, or nickel.
  1. Electrophilic Addition
  • Alkenes react with halogens, hydrogen halides, and water in the presence of acids to form various products.
  1. Polymerization
  • Alkenes can undergo polymerization to form polymers, such as polyethylene from ethene.

Aromaticity and Benzene

  1. Structure of Benzene
  • Benzene has a hexagonal ring structure with alternating double bonds, known as resonance.
  1. Aromatic Stability
  • Benzene and other aromatic compounds are stabilized by delocalized π-electrons over the ring structure.
  1. Reactions of Benzene
  • Benzene undergoes electrophilic substitution reactions, such as nitration, sulfonation, and Friedel-Crafts alkylation/acylation.

Hydrocarbons are fundamental organic compounds with a wide range of applications in everyday life, including fuels, solvents, and as raw materials for various chemicals and polymers. Understanding their properties, reactions, and nomenclature is crucial for further studies in organic chemistry and industrial applications.

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