# Hydrocarbons

> Hydrocarbons for JEE Chemistry: alkane halogenation, Markovnikov and anti-Markovnikov additions, ozonolysis and electrophilic substitution.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/hydrocarbons
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Hydrocarbons", PrepElephant, https://prepelephant.com/topics/jee/chemistry/hydrocarbons

## 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.