Chapter 1: Alkane Structure and Conformations

Alkanes (Cₙ$H_{2}$ₙ₊_{2}) are saturated hydrocarbons with all $sp^{3}$-hybridized carbons. The most important concept for NEET is conformational analysis via Newman projections. Ethane (CC) can adopt staggered (60° dihedral, most stable) or eclipsed (0° dihedral, least stable) conformations. The energy difference (~12 kJ/mol) is due to torsional strain — electronic repulsion between eclipsing C–H bonding orbitals.

Butane (CCCC) has four key conformations along the C2–C3 bond. The stability order (most to least): Anti (180°, 0 kJ/mol) > Gauche (60°, ~3.8 kJ/mol) > Eclipsed (120°, ~16 kJ/mol) > Fully eclipsed (0°, ~19 kJ/mol). The "fully eclipsed" conformation is least stable because both methyl groups directly eclipse each other, creating both torsional and steric strain. In contrast, the gauche conformation has only mild van der Waals methyl–methyl interaction (~3.8 kJ/mol) and is NOT the least stable.

Chapter 2: Free Radical Halogenation of Alkanes

Alkanes react with halogens ($X_{2}$) under UV light or heat via a three-step free radical chain: (1) Initiation — $X_{2}$ homolysis to 2X•; (2) Propagation — H abstraction (R–H + X• → R• + HX) and halide transfer (R• + $X_{2}$ → R–X + X•); (3) Termination — radical combination reactions. Selectivity for H abstraction follows 3° > 2° > 1°, reflecting radical stability. $Br_{2}$ is highly selective (~1600:1 for 3° vs 1° H); $Cl_{2}$ is moderately selective; $F_{2}$ is non-selective and explosively reactive.

Chapter 3: Addition Reactions of Alkenes

Alkenes undergo electrophilic addition to their π bond. The key reactions are:

Markovnikov addition (HX, no peroxide): H adds to the more-H carbon; more stable carbocation intermediate forms; $X^{-}$ then adds. Propene + HBr → 2-bromopropane.

Anti-Markovnikov addition (HBr + ROOR only): Radical mechanism; Br• adds first to the less-substituted carbon; generates more stable radical; H added from HBr. Propene + HBr + ROOR → 1-bromopropane. CRITICAL: only HBr works, not HCl or HI.

Ozonolysis ($O_{3}$/Zn/$H_{2}$O): Cleaves C=C completely; each C=C carbon becomes a C=O; –CH= → –CHO (aldehyde); –CR= → –CO– (ketone). Used to identify double bond position.

Other reactions: $H_{2}$/Pd gives syn-addition (alkane); $Br_{2}$/$CCl_{4}$ gives anti-addition via bromonium ion (vicinal dihalide); $H_{2}$O/H_{2}$$SO_{4} gives Markovnikov alcohol.

Chapter 4: Alkynes — Acidity and Selective Reduction

Terminal alkynes are uniquely acidic among hydrocarbons due to their 50% s-character sp C–H bond. Acidity order: alkyne > alkene > alkane (pKa ~25, ~44, ~50 respectively). NaN$H_{2}$ deprotonates terminal alkynes to give sodium acetylides (RC≡C^{-}$$Na^{+}) — valuable synthetic nucleophiles.

Selective reduction: Lindlar's catalyst (Pd/CaC$O_{3}$ + Pb(OAc)_{2} + quinoline) + $H_{2}$ → cis-alkene via syn-addition. Na/liq. $NH_{3}$ → trans-alkene via anti-addition (radical anion mechanism). Excess $H_{2}$/Pd–C → complete reduction to alkane.