Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen. They are classified into three main families based on the type of C–C bonding: alkanes (only single bonds, general formula Cₙ$H_{2}$ₙ₊{2}, saturated), alkenes (containing at least one C=C double bond, general formula Cₙ$H_{2}$ₙ), and alkynes (containing at least one C≡C triple bond, general formula Cₙ$H_{2}$ₙ₋{2}). Each family has distinct hybridization: $sp^{3}$ for alkanes, $sp^{2}$ at the double bond for alkenes, and sp at the triple bond for alkynes.

Alkane Conformations. A key concept for NEET is the three-dimensional arrangement of atoms in alkanes. Because C–C single bonds allow free rotation, alkanes exist in multiple conformations. The Newman projection technique views a molecule along a C–C bond axis: the front carbon appears as a dot and the back as a circle. For ethane (CC), two extremes exist: the staggered conformation (60° dihedral angle between adjacent H atoms, most stable, minimum torsional strain) and the eclipsed conformation (0° dihedral, least stable, maximum torsional strain from electron–electron repulsion between overlapping bonding orbitals). Butane (CCCC) has four key conformations viewed along the C2–C3 bond. In decreasing order of stability: anti (180° dihedral, 0 kJ/mol reference, methyl groups maximally separated), gauche (60°, ~3.8 kJ/mol above anti, mild methyl–methyl van der Waals interaction), eclipsed (120°, ~16 kJ/mol above anti, methyl eclipsing H), and fully eclipsed (0°, ~19 kJ/mol above anti, both methyl groups directly eclipsing each other — maximum steric and torsional strain). The mnemonic "A Gentle Evening Falls" encodes Anti > Gauche > Eclipsed > Fully eclipsed.

Free Radical Halogenation. Alkanes undergo halogenation with $X_{2}$ under UV light or heat via a free radical chain mechanism with three distinct stages. Initiation: UV or heat causes homolytic cleavage of $X_{2}$ into two X• radicals. Propagation (two steps): (i) R–H + X• → R• + HX (H abstraction, generating an alkyl radical); (ii) R• + $X_{2}$ → R–X + X• (halogen transfer, giving the product alkyl halide and regenerating X• to continue the chain). Termination: any two radicals combine (R• + R•, R• + X•, or X• + X•) to destroy radicals and end the chain. H-abstraction selectivity follows 3° H > 2° H > 1° H, reflecting the stability order of the resulting alkyl radicals ($sp^{2}$-hybridized, planar centers stabilized by hyperconjugation). Among halogens, reactivity is $F_{2}$ > $Cl_{2}$ > $Br_{2}$ > $I_{2}$, but selectivity is the inverse. $Br_{2}$ is most selective — Br• has a higher activation energy for H abstraction, making it sensitive to C–H bond strength differences and causing it to preferentially abstract the weakest (3°) bonds by a factor of ~1600:1 compared to 1° H per hydrogen atom.

Alkene Electrophilic Addition — Markovnikov's Rule. The electron-rich C=C π bond of alkenes attacks electrophiles. For HX addition (ionic mechanism, no peroxide), $H^{+}$ adds first to form a carbocation intermediate. Markovnikov's rule: H adds to the carbon bearing more H atoms, generating the more stable (more substituted) carbocation. For propene (CC=C) + HBr (no peroxide): $H^{+}$ adds to C-1 (more H atoms), producing a secondary carbocation at C-2, followed by $Br^{-}$ attack to give 2-bromopropane (CC(Br)C). The rule's underlying logic is always carbocation stability: more substituted carbocation (3° > 2° > 1°) = lower activation energy = favored pathway.

Anti-Markovnikov Addition (Kharasch Effect). When HBr is added to an alkene in the presence of an organic peroxide (ROOR), the reaction follows a free radical mechanism and gives the opposite (anti-Markovnikov) regiochemistry. The peroxide homolytically generates Br• radicals. Br• adds first to the less substituted carbon (placing the radical at the more substituted, more stable position), then H• (from HBr) adds to give the product. For propene + HBr + ROOR: product is 1-bromopropane (CCCBr). This anti-Markovnikov addition works ONLY with HBr. HCl fails because the Cl• addition step is endothermic (chain cannot sustain). HI fails because I• causes premature chain termination. This HX specificity is the single most tested point in NEET for this topic.

Ozonolysis. Alkenes react with ozone ($O_{3}$) to form an unstable ozonide intermediate, which is then decomposed. With reductive workup (Zn/$H_{2}$O), the C=C bond is cleaved and each doubly-bonded carbon becomes a carbonyl carbon: if that carbon carried at least one H, it becomes an aldehyde (–CHO); if it carried no H, it becomes a ketone (–CO–). For 2-butene (CC=CC), ozonolysis gives two molecules of acetaldehyde (ethanal, CC=O). Ozonolysis is used analytically to determine the position of C=C bonds by identifying the carbonyl fragments and reconstructing the original alkene structure.

Alkyne Acidic Character. Terminal alkynes (RC≡CH) have acidic C–H bonds due to the sp hybridization at the C bearing H. The sp hybrid orbital has 50% s-character, meaning electrons are held close to the nucleus, making the carbon highly electronegative and the C–H bond polar with H easily released as $H^{+}$. Acidity order: terminal alkyne (sp, 50% s, pKa ~25) > terminal alkene ($sp^{2}$, 33.3% s, pKa ~44) > alkane ($sp^{3}$, 25% s, pKa ~50). Terminal alkynes react with NaN$H_{2}$ (conjugate acid $NH_{3}$, pKa ~38, which is stronger base than needed) to form sodium acetylide (RC≡C^{-}$$Na^{+}) — a useful carbon nucleophile.

Selective Alkyne Reduction. Internal alkynes can be selectively reduced to either geometric isomer of the alkene. Lindlar's catalyst (Pd on CaC$O_{3}$, poisoned with Pb(OAc)_{2} and quinoline) + $H_{2}$ delivers $H_{2}$ via syn-addition (both H atoms from the same face of the solid catalyst surface), giving the cis-alkene (Z-configuration). Na in liquid $NH_{3}$ (Birch-type dissolving metal reduction) proceeds by sequential electron transfer from Na to give a radical anion and then a vinyl anion. The vinyl anion prefers the anti geometry (trans-alkene, E-configuration) to minimize steric interactions. Complete hydrogenation with $H_{2}$/Pd–C (undeactivated) reduces the alkyne all the way to the alkane.

The overarching theme of OC-02 is that mechanism determines regiochemistry and stereochemistry. Ionic mechanisms follow carbocation stability (Markovnikov); radical mechanisms follow radical stability (anti-Markovnikov); surface mechanisms give syn-addition (Lindlar's); dissolving metal mechanisms give anti-addition (Na/$NH_{3}$). Mastering these mechanistic principles enables the student to predict any product without memorizing individual reactions.