Hydrocarbons

Concept overview

Hydrocarbons — compounds of only carbon and hydrogen — form the backbone of organic chemistry. Four families cover the JEE syllabus: alkanes (single bonds only, sp³, saturated), alkenes (at least one C=C, sp²), alkynes (at least one C≡C, sp), and aromatic hydrocarbons (a planar ring with delocalised π electrons obeying Hückel's rule: 4n+2 π electrons for n = 0, 1, 2 …).

Alkanes are relatively unreactive at room temperature; the main JEE-relevant reactions are free-radical halogenation (Cl₂ or Br₂ under UV light) and combustion. Alkenes and alkynes are the workhorses of hydrocarbon reactivity — they undergo electrophilic addition of HX, H₂O, X₂, and hydroboration-oxidation. Markovnikov's rule predicts regioselectivity for HX addition (H to the carbon with more H; X to the more substituted carbon), while peroxide-mediated HBr addition gives anti-Markovnikov products via a radical mechanism.

Aromatic hydrocarbons — benzene and its derivatives — undergo electrophilic aromatic substitution (EAS) rather than addition, because addition would destroy the aromatic stability (~150 kJ/mol). The five classic EAS reactions are halogenation, nitration, sulfonation, Friedel-Crafts alkylation and Friedel-Crafts acylation. Directive effects of substituents on the ring — activating vs deactivating, ortho/para-directing vs meta-directing — determine where the incoming electrophile attaches. Master these directive rules and every EAS problem becomes a two-step lookup.

Key reactions & principles

• Markovnikov's rule: In HX addition to C=C, H adds to the carbon with more H; X to the more substituted carbon
• Anti-Markovnikov (HBr + peroxide): Reversed regioselectivity via free-radical mechanism
• Hydroboration-oxidation: Anti-Markovnikov OH addition to alkene
• Ozonolysis: R₂C=CR'₂ + O₃ / Zn/H₂O → 2 carbonyls (cleaves C=C)
• Hückel's rule: Aromaticity requires 4n+2 π electrons in a planar cyclic system
• EAS activator/deactivator: –OH, –NH₂, alkyl activate; –NO₂, –CN, –SO₃H deactivate
• Directive effect: Activators are ortho/para directors; deactivators are meta directors (except halogens, o/p)

Common mistakes to avoid

• Using Markovnikov's rule in the presence of peroxides — peroxides flip HBr to anti-Markovnikov (the 'peroxide effect').
• Assuming all halogens are deactivating meta-directors — halogens are deactivating but ortho/para-directing (they withdraw inductively but donate by resonance).
• Forgetting that Friedel-Crafts reactions fail on strongly deactivated rings — you cannot nitrate then Friedel-Crafts alkylate.
• Writing addition products for a benzene ring instead of substitution — benzene resists addition to preserve aromaticity.

JEE Main relevance

JEE Main asks 2 hydrocarbon problems every paper — one regio-selectivity or mechanism (Markovnikov, hydroboration, ozonolysis) and one EAS directive-effect problem. All the archetypes are pattern-recognition.

JEE Advanced relevance

Advanced hydrocarbon problems combine two mechanisms in one reaction — for instance, an alkene undergoing halogenation followed by base-promoted elimination, giving a specific alkyne or diene. Retrosynthesis problems that ask you to work backwards from an aromatic product to identify the substitution sequence are also common at the Advanced level.

Previous-year questions (PYQ)

The Hydrocarbons chapter is among the most predictable in the PYQ bank. Practise EAS directive-effect problems in one focused session — the same three or four substituent combinations recycle.