Hydrocarbons
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Direct answer
Hydrocarbons — alkanes, alkenes, alkynes, arenes — carry one mechanism family each. Alkanes react by free-radical substitution (halogenation); alkenes and alkynes by electrophilic addition across the pi bond, guided by Markovnikov's rule and carbocation stability; benzene by electrophilic aromatic substitution that preserves the stable ring, with regiochemistry set by directing effects. Ozonolysis and oxidation cleave the pi bond to expose the carbon skeleton.
What you must remember
- Alkane halogenation runs initiation (Cl2 → 2Cl•), propagation (hydrogen abstraction, then chlorine capture) and termination; bromine is far more selective than chlorine because its abstraction step is more endothermic.
- Markovnikov addition: hydrogen goes to the carbon with more hydrogens; the real driver is carbocation stability (3° > 2° > 1°). Acid-catalysed hydration of propene gives 2-propanol.
- Peroxide (Kharasch) effect: HBr with peroxides adds anti-Markovnikov through a radical chain — only HBr; HCl and HI cannot sustain the chain energetically.
- Ozonolysis (O3, then Zn with H2O) cleaves alkenes to aldehydes and ketones — rebuilding the parent alkene from the fragments is a favourite exercise; cold dilute KMnO4 (Baeyer) gives vicinal diols.
- Terminal alkynes are weakly acidic (pKa about 25): ammoniacal AgNO3 gives a white precipitate, ammoniacal CuCl a red one — tests for the terminal C-H; HgSO4-H2SO4 hydration gives ketones (acetylene alone gives acetaldehyde) via an enol.
- Partial alkyne reduction: Lindlar catalyst gives the cis alkene; sodium in liquid ammonia gives the trans alkene.
- Benzene: nitration (mixed HNO3 + H2SO4, electrophile NO2+), sulphonation, halogenation with a Lewis acid, Friedel-Crafts alkylation and acylation with AlCl3 — acylation avoids polyalkylation and rearrangement.
- Directing: -OH, -NH2, -OR and alkyl activate and direct ortho/para; -NO2, -CN, -CHO, -COR, -COOH, -SO3H deactivate and direct meta; halogens deactivate yet direct ortho/para. Hot KMnO4 oxidises any side chain with an alpha hydrogen to -COOH.
Common confusion
The commonest error is mechanical: predicting addition products by reciting Markovnikov without checking carbocation rearrangements, which can shift the product. The second is overextending the peroxide effect to HCl and HI — only HBr's two propagation steps are both energetically favourable. Students also swap the two alkyne reductions (Lindlar cis, dissolving metal trans) and forget that tert-butylbenzene, lacking an alpha hydrogen, survives hot permanganate.
Exam-focused takeaway
JEE Main tests product prediction at single-concept level: major addition product, ozonolysis fragments, distinguishing tests between alkane, alkene and alkyne. JEE Advanced chains the logic — reconstruct an alkene from ozonolysis data, choose reagents for a stereochemical outcome, count monochlorination products, predict regiochemistry on a disubstituted benzene. Write the intermediate (carbocation or radical) on paper; that is what the examiner tests.
Frequently asked questions
Why does the peroxide effect work only with HBr?
Only HBr's propagation steps are both exothermic; HCl's hydrogen abstraction and HI's iodine addition are not, so neither chain runs.
What does ozonolysis of an alkene tell you?
Each alkene carbon becomes a carbonyl carbon, so the fragments reveal where the double bond sat in the parent.
How do Lindlar and dissolving-metal reductions differ?
Lindlar (poisoned Pd, H2) delivers both hydrogens to one face for the cis alkene; sodium in liquid ammonia gives trans.
Why is bromination more selective than chlorination?
Bromine's hydrogen abstraction is more endothermic with a later transition state, so radical stability governs the rate more strongly.
What makes a substituent ortho/para or meta directing?
Ortho/para directors donate electron density to adjacent positions (resonance from -OH or -NH2, hyperconjugation from alkyl); meta directors withdraw. Halogens are the famous exception — deactivating overall, yet donating at ortho/para.
Why does acetylene give acetaldehyde while other alkynes give ketones?
Markovnikov hydration puts OH on the internal carbon; the enol tautomerises, and only acetylene's enol leaves a hydrogen on the carbonyl carbon.