2.1 Nomenclature and Isomerism

Nomenclature follows the IUPAC system. Methane, ethane, and propane have only one structure. Higher alkanes can have more than one structure. For example, C₄H₁₀ has two structures (butane and 2-methylpropane). These are Chain Isomers. Carbon atoms are classified as Primary (1°), Secondary (2°), Tertiary (3°), and Quaternary (4°).

2.2 Preparation

Primarily obtained from petroleum and natural gas. They can also be prepared by:

• From Unsaturated Hydrocarbons (Hydrogenation): Addition of dihydrogen gas to alkenes and alkynes.
• From Alkyl Halides: Reduction with zinc and dilute HCl, or the Wurtz Reaction with sodium metal.
• From Carboxylic Acids: Decarboxylation with soda lime or Kolbe’s Electrolytic Method.

2.3 Properties

Physical: Non-polar, weak van der Waals forces, insoluble in water. Boiling point increases with molecular mass; branching decreases boiling point. Chemical: Generally inert but undergo substitution (e.g., Halogenation via free radical mechanism), combustion, controlled oxidation, isomerisation, aromatization, reaction with steam, and pyrolysis (cracking).

2.4 Conformations

Different spatial arrangements from rotation around a C-C single bond. The energy barrier is called torsional strain. For ethane, the staggered conformation (hydrogens far apart) is the most stable, and the eclipsed conformation (hydrogens close) is the least stable.

3.1 - 3.3: Structure, Nomenclature, and Isomerism

The C=C double bond is shorter (134 pm) than a C-C single bond. The π bond makes alkenes susceptible to attack by electrophiles. Alkenes exhibit both structural isomerism and geometrical (cis-trans) isomerism due to restricted rotation around the double bond. The cis isomer has identical groups on the same side; the trans isomer has them on opposite sides.

3.4 Preparation

Prepared by:

• From Alkynes: Partial reduction using Lindlar’s catalyst (for cis-alkenes) or sodium in liquid ammonia (for trans-alkenes).
• From Alkyl Halides (Dehydrohalogenation): Heating with alcoholic potash (KOH).
• From Vicinal Dihalides (Dehalogenation): Reacting with zinc metal.
• From Alcohols by Acidic Dehydration: Heating with concentrated H₂SO₄.

3.5 Properties

Rich in π electrons, they undergo addition reactions. Key reactions include: addition of dihydrogen, halogens (decolorization of bromine water is a test for unsaturation), hydrogen halides (following Markovnikov's rule, or Anti-Markovnikov/peroxide effect with HBr/peroxide), sulphuric acid, and water. They also undergo oxidation (Baeyer’s reagent, ozonolysis) and polymerisation.

4.1 - 4.3: Structure, Nomenclature, Preparation

The H-C-C bond angle is 180°, making ethyne a linear molecule. The C≡C bond is shorter (120 pm) and stronger than double or single bonds. They are prepared industrially from calcium carbide or from vicinal dihalides via dehydrohalogenation.

4.4 Properties

Acidic Character: Hydrogen atoms on a triply bonded carbon are acidic due to the high electronegativity of the sp hybridised carbon. This allows them to react with strong bases like NaNH₂. Addition Reactions: They add two molecules of H₂, halogens, or HX. Addition of water (with HgSO₄/H₂SO₄) forms carbonyl compounds. Polymerisation: Ethyne undergoes linear polymerisation to form polyacetylene or cyclic polymerisation in a red hot iron tube to form benzene.

5.1 & 5.2: Nomenclature and Structure of Benzene

Disubstituted benzenes have ortho (1,2), meta (1,3), and para (1,4) isomers. Benzene's structure is a resonance hybrid of Kekulé structures. All C-C bond lengths are identical (139 pm) due to the six π electrons being delocalised in a ring above and below the plane of the sp² hybridised carbon atoms.

5.3 Aromaticity (Hückel's Rule)

A compound is aromatic if it is planar, has complete delocalisation of π electrons in a ring, and possesses (4n + 2) π electrons, where n is an integer (0, 1, 2, ...).

5.4 & 5.5: Preparation and Properties

Benzene is prepared by cyclic polymerisation of ethyne, decarboxylation of aromatic acids, or reduction of phenol. It is characterized by electrophilic substitution reactions (Nitration, Halogenation, Sulphonation, Friedel-Crafts Alkylation/Acylation). Addition reactions (e.g., hydrogenation) occur only under vigorous conditions.

5.6 Directive Influence of a Functional Group

Substituents direct incoming groups. Ortho and Para Directing Groups (activating groups like –OH, –NH₂, –CH₃, and halogens) direct to the o- and p-positions. Meta Directing Groups (deactivating groups like –NO₂, –CN, –COOH) direct to the m-position by withdrawing electron density from the ring.