Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They serve as the fundamental framework for all other organic compounds and represent the primary constituents of petroleum, natural gas, and fossil fuels.

Classification of Hydrocarbons

Hydrocarbons are systematically classified based on their structural architecture and the nature of carbon-carbon bonds:

Saturated Hydrocarbons: Alkanes

Alkanes are also known as paraffins (Latin: parum affinis = little affinity) because they are relatively unreactive under normal conditions due to the presence of strong, inert C-C and C-H σ-bonds. All carbon atoms in alkanes are sp³ hybridized with a tetrahedral geometry and a bond angle of 109.5°.

Conformations of Alkanes

Alkanes exhibit conformational isomerism due to the free rotation about the carbon-carbon single (σ) bond. Taking ethane (CH₃-CH₃) as a prime example, two extreme spatial arrangements exist:

• Eclipsed Conformation: Hydrogen atoms attached to both carbon atoms are directly opposite each other, maximizing steric hindrance and torsional strain. This is the least stable, highest-energy conformation.
• Staggered Conformation: Hydrogen atoms are arranged as far apart as possible, minimizing torsional strain. This is the most stable, lowest-energy conformation.

Key Chemical Reactions of Alkanes

• Free Radical Substitution: Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet (UV) light or high temperatures.
  CH₄ + Cl₂ hν→ CH₃Cl + HCl
• Combustion: Complete oxidation of alkanes in excess oxygen produces carbon dioxide, water, and a large amount of heat, making them excellent fuels.
  CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (Δ H = -890 kJ/mol)
• Controlled Oxidation: Alkanes yield different products depending on the catalyst used:
  • Methane with a Copper tube at 523 K yields Methanol (CH₃OH).
  • Methane with Molybdenum oxide (MoO₃) yields Methanal (HCHO).
• Isomerization: Heating n-alkanes with anhydrous AlCl₃ and HCl gas converts them into branched-chain isomers, which burn more efficiently in internal combustion engines.

Unsaturated Hydrocarbons: Alkenes and Alkynes

Unsaturated hydrocarbons contain at least one multiple carbon-carbon bond, making them significantly more reactive than alkanes due to the accessible electron density in their π-bonds.

Alkenes (Olefins)

Alkenes contain at least one carbon-carbon double bond (>C = C<). The double-bonded carbons are sp² hybridized, featuring a planar geometry with bond angles of approximately 120°. They are called olefins (oil-forming) because their lower members react with halogens to form oily liquids.

Core Reactions of Alkenes

• Electrophilic Addition Reactions: The primary reaction pathway for alkenes where the π-bond breaks to accommodate new atoms.
  • Hydrogenation: Addition of H₂ in the presence of Raney Nickel, Platinum, or Palladium catalysts to form alkanes.
  • Halogenation: Addition of Br₂ in CCl₄. The fading of the reddish-brown color of bromine is the standard test for unsaturation.
• Regioselectivity Rules:
  • Markovnikov’s Rule: When an unsymmetrical reagent (like HBr) adds to an unsymmetrical alkene, the negative part of the addendum (Br^-) attaches to the carbon atom possessing the lesser number of hydrogen atoms.
  • Anti-Markovnikov’s Rule (Peroxide Effect): In the presence of organic peroxides, the addition of HBr (and only HBr) to an unsymmetrical alkene yields the opposite product, where the bromine atom attaches to the carbon with more hydrogen atoms.
• Ozonolysis: Alkenes react with ozone (O₃) followed by reduction with Zinc and water (Zn/H₂O) to cleave the carbon-carbon double bond, yielding aldehydes and/or ketones. This reaction is vital for locating the position of double bonds in unknown structures.

Alkynes

Alkynes contain at least one carbon-carbon triple bond (-C≡ C-). The triple-bonded carbon atoms are sp hybridized, presenting a linear geometry with a bond angle of 180°.

Core Reactions and Properties of Alkynes

• Acidity of Terminal Alkynes: Hydrogen atoms attached directly to sp hybridized carbons show weak acidic character due to the high s-character (50%), which makes the carbon highly electronegative and capable of stabilizing a negative charge. Terminal alkynes (like ethyne) react with sodium metal or sodamide (NaNH₂) to liberate hydrogen gas.
• Cyclic Polymerization: Passing ethyne (acetylene) through a red-hot iron tube at 873 K results in cyclic polymerization, synthesizing Benzene (C₆Hex).

Aromatic Hydrocarbons: Arenes

Aromatic hydrocarbons, or arenes, are cyclic, planar unsaturated compounds that exhibit a unique stability termed aromatic stabilization energy. They do not readily undergo addition reactions despite their high unsaturation.

Criteria for Aromaticity (Hückel’s Rule)

For a compound to be classified as aromatic, it must satisfy all four of the following criteria:

1. The molecule must be cyclic.
2. The molecule must be completely planar (all atoms in the ring must be sp² or sp hybridized).
3. There must be complete delocalization of π-electrons across the entire ring system.
4. The ring system must contain a total of (4n + 2) π-electrons, where n is an integer (n = 0, 1, 2, 3 …).

Chemical Reactivity of Benzene

Due to the resonance stabilization of its π-cloud, benzene resists addition reactions and instead undergoes Electrophilic Aromatic Substitution (EAS), preserving its aromatic ring structure.

• Nitration: Benzene reacts with a nitrating mixture (concentrated HNO₃ + concentrated H₂SO₄) to introduce a nitro group (-NO₂), producing Nitrobenzene.
• Halogenation: Reaction with Cl₂ or Br₂ in the presence of a Lewis acid catalyst (anhydrous FeCl₃ or AlCl₃) to produce Halobenzenes.
• Friedel-Crafts Alkylation: Alkylation of the benzene ring using an alkyl halide in the presence of anhydrous AlCl₃ to yield alkylbenzenes (e.g., producing Toluene from benzene and CH₃Cl).
• Friedel-Crafts Acylation: Introduction of an acyl group (-COCH₃) using an acyl halide and anhydrous AlCl₃ to produce aromatic ketones (e.g., Acetophenone).

UPSC Prelims Fact File: Industrial Applications and Applied Chemistry

Petroleum, Refining, and Fuel Quality Metrics

Crude petroleum consists primarily of complex mixtures of alkanes, cycloalkanes, and aromatic hydrocarbons.

• Fractional Distillation: Processes crude oil into various commercially viable fractions based on boiling point gradients (e.g., LPG, Petrol, Kerosene, Diesel, Heavy Fuel Oils).
• Octane Number: Measures the antiknock properties of gasoline (petrol) inside internal combustion engines. Highly branched alkanes and aromatic hydrocarbons have higher octane ratings. Iso-octane is arbitrarily assigned an octane rating of 100, while n-heptane is assigned 0.
• Cetane Number: Measures the ignition delay period of diesel fuel. Contrary to petrol, straight-chain alkanes (like n-cetane, assigned 100) are preferred for high cetane ratings, whereas highly branched or aromatic structures lower the cetane value.

Environmental Impact and Atmospheric Chemistry

• Polycyclic Aromatic Hydrocarbons (PAHs): Hydrocarbons containing multiple fused benzene rings (e.g., Benzo[a]pyrene). Formed during incomplete combustion of organic matter like coal, petroleum, tobacco, and wood. They are highly carcinogenic and mutagenic pollutants monitored strictly under environmental safety standards.
• Photochemical Smog: Unsaturated hydrocarbons emitted from automobile exhausts react with nitrogen oxides (NO_x) in the presence of sunlight to form toxic secondary pollutants like Ozone (O₃) and Peroxyacetyl Nitrate (PAN).
• Carbide Ripening: Calcium carbide (CaC₂) reacts with environmental moisture to release Ethyne (Acetylene gas), which is widely utilized for the artificial ripening of fruits. However, industrial-grade calcium carbide often contains arsenic and phosphorus impurities, making its use for food ripening illegal under food safety regulations.