Hydrocarbons: Alkanes, Alkenes & Alkynes

Hydrocarbons — the simplest organic compounds containing only C and H — are classified into alkanes (CnH₂n+2, saturated), alkenes (CnH₂n, one C=C), and alkynes (CnH₂n-2, one C-C triple bond). Understanding their conformations, addition reactions, and selectivity rules is essential for NEET.

Alkane Conformations: Ethane (CC) has two extreme conformations viewed via Newman projections: staggered (dihedral angle 60 degrees, most stable, zero torsional strain) and eclipsed (0 degrees, least stable, maximum torsional strain due to electron-electron repulsion). Butane (CCCC) has four key conformations in decreasing stability: anti (180 degrees, most stable) > gauche (60 degrees) > eclipsed (120 degrees) > fully eclipsed (0 degrees, least stable — methyl groups eclipsed).

Free Radical Halogenation proceeds in three steps: (i) Initiation — X₂ →(UV/heat) 2X. (radical); (ii) Propagation — R-H + X. → R. + HX, then R. + X₂ → R-X + X.; (iii) Termination — R. + R. → R-R. Hydrogen abstraction reactivity follows 3 degree H > 2 degree H > 1 degree H, reflecting the stability of the resulting radical intermediate. Among halogens, reactivity follows F₂ > Cl₂ > Br₂ > I₂, but Br₂ is the most selective (preferentially abstracts 3 degree H over 1 degree H by a large factor).

Alkene Electrophilic Addition involves the electron-rich $\pi$ bond attacking an electrophile to form a carbocation intermediate, followed by nucleophilic capture. Markovnikov's rule states that H adds to the carbon with more H atoms (equivalently, the electrophile adds to give the more stable carbocation). For propene (CC=C) + HBr: Markovnikov product is 2-bromopropane (CC(Br)C). Anti-Markovnikov addition (Kharasch effect) occurs ONLY with HBr + organic peroxide — the reaction follows a free radical mechanism where Br. adds first to the less substituted carbon. This produces 1-bromopropane (CCCBr). This works ONLY with HBr: HCl radicals are too unreactive, and HI radicals are too reactive for a sustained chain.

Ozonolysis cleaves C=C bonds: alkene + O₃ → ozonide →(Zn/H₂O, reductive workup) aldehydes and/or ketones. This is used to locate the position of the double bond by identifying the carbonyl fragments. For example, 2-butene (CC=CC) gives 2 molecules of acetaldehyde (CC=O).

Alkyne Acidic Character: Terminal alkynes (RC-CH) have acidic hydrogen because the sp-hybridized C-H bond has 50% s-character — electrons are held closer to the carbon nucleus, making the H easier to release as H+. Acidity order: terminal alkyne (sp) > alkene (sp2) > alkane (sp3). Terminal alkynes react with NaNH₂ to form sodium acetylide (RC-C-Na+). Selective reduction of alkynes: Lindlar's catalyst (Pd/CaCO₃ poisoned with Pb(OAc)₂ and quinoline) gives the cis-alkene (syn-addition of H₂ on the catalyst surface), while Na in liquid NH₃ (Birch reduction) gives the trans-alkene (anti-addition via radical anion mechanism).

The key testable concept is that anti-Markovnikov addition works ONLY with HBr + peroxide (not HCl or HI), and that Lindlar's catalyst gives cis-alkene while Na/liq. NH₃ gives trans-alkene.