Hydrocarbons

Hydrocarbons

🟢 Lite — Quick Review (1h–1d)

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Hydrocarbons contain only carbon and hydrogen, divided into alkanes (sp³, CₙH₂ₙ₊₂), alkenes (sp², CₙH₂ₙ), alkynes (sp, CₙH₂ₙ₋₂), and aromatics (Hückel’s 4n+2 π-electrons, e.g. benzene C₆H₆). JEE Main tests three recurring skills: IUPAC naming with isomerism, Markovnikov/anti-Markovnikov addition on alkenes, and electrophilic aromatic substitution on benzene.

• Markovnikov addition of HX places H on the carbon already carrying more H’s; the peroxide (Kharasch) effect reverses this only for HBr.
• EAS directing groups: –OH, –NH₂, –CH₃, –X are ortho/para directors; –NO₂, –CHO, –COOH, –SO₃H are meta directors.
• Benzene resists addition; it substitutes via an arenium ion (Wheland intermediate) to preserve aromaticity (~36 kcal/mol resonance energy).

Family	Hybridisation	General formula	Character test
Alkane	sp³	CₙH₂ₙ₊₂	Inert to Br₂/CCl₄
Alkene	sp²	CₙH₂ₙ	Decolourises Br₂/CCl₄
Alkyne	sp	CₙH₂ₙ₋₂	Decolourises Br₂; forms acetylide with NaNH₂
Aromatic	sp² (ring)	e.g. C₆H₆	Substitution, not addition

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification and Nomenclature

IUPAC rules name alkanes with the –ane suffix, alkenes –ene (lowest locant for C=C), alkynes –yne, and arenes as substituted benzenes. Hydrocarbons exhibit chain, position, functional-group, ring-chain, and geometric (cis-trans) isomerism. Optical isomerism appears in hydrocarbons with a chiral carbon (e.g. 3-methylhexane) or restricted rotation creating axial chirality.

Conformations of Alkanes

Ethane shows staggered (dihedral 60°, most stable, ~2.9 kcal/mol lower) and eclipsed (0°, highest energy) forms, visualised by Newman projections. n-Butane adds gauche and anti conformers; the energy barrier to rotation (~3–6 kcal/mol) is small but measurable.

Preparation Methods

Alkanes come from Wurtz coupling (2 R–X + 2 Na → R–R), Kolbe’s electrolysis (2 R–COO⁻ → R–R + 2 CO₂), decarboxylation (soda lime), and catalytic hydrogenation of alkenes. Alkenes form via dehydrohalogenation (alc. KOH, Saytzeff product) and dehydration of alcohols. Alkynes arise from calcium carbide hydrolysis (CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂) and double dehydrohalogenation of tetrahalides.

Reactions of Alkenes and Alkynes

Alkenes undergo electrophilic addition: halogenation (Br₂/CCl₄ decolourisation = Baeyer’s qualitative test), hydrohalogenation (Markovnikov), hydration (H₂O/H⁺, oxymercuration–demercuration, hydroboration–oxidation), and ozonolysis (O₃ then Zn/H₂O → aldehydes/ketones). Alkynes show acidic terminal H (pKa ≈ 25), enabling acetylide formation with NaNH₂; Lindlar’s catalyst (Pd/CaCO₃/PbO) reduces alkyne to cis-alkene, while Na/NH₃(l) gives the trans-alkene (Birch-type reduction).

Aromatic Substitution

Benzene reacts via electrophilic aromatic substitution (EAS): nitration (HNO₃/H₂SO₄), halogenation (X₂/FeX₃), sulfonation (oleum), and Friedel–Crafts alkylation/acylation (AlCl₃). The mechanism proceeds through a Wheland intermediate that rearomatises by losing H⁺.

Substituent	Electronic effect	Directing
–OH, –NH₂, –OR	Strong activator	ortho/para
–CH₃, –C₂H₅	Weak activator	ortho/para
–X (F, Cl, Br)	Deactivator (but o/p director)	ortho/para
–NO₂, –CN, –CHO, –COOH	Strong deactivator	meta

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Carbocation Stability and Rearrangement

The order 3° > 2° > 1° > methyl governs Markovnikov outcomes through hyperconjugation: a tertiary carbocation has nine α-hydrogens donating into the empty p-orbital, compared with three for a primary. During addition of HCl to 3,3-dimethyl-1-butene, the initial 2° cation rearranges to a more stable 3° cation by a methyl shift, yielding 2-chloro-2,3-dimethylbutane instead of the Markovnikov-only prediction.

Ozonolysis as a Structural Tool

Ozonolysis cleaves every C=C into two carbonyl fragments. Reductive workup (Zn/H₂O or Me₂S) preserves aldehydes; oxidative workup (H₂O₂) oxidises any aldehyde fragment to a carboxylic acid. A single experiment can therefore deduce the position of the original double bond from the carbonyl products.

Aromaticity in Non-Benzenoid Rings

Hückel’s rule, π-electrons = 4n + 2, classifies rings independent of the number of atoms. Cyclopropenyl cation (2 π-electrons, n = 0) is aromatic despite having only three carbons; cyclopentadienyl anion (6 π-electrons, n = 1) is aromatic after deprotonation, while the neutral cyclopentadienyl radical (5 π-electrons) is non-aromatic. Cyclobutadiene (4 π-electrons) is anti-aromatic and distorts to a rectangular geometry.

Named Reactions Worth Memorising

• Wurtz–Fittig: aryl halide + alkyl halide + Na → alkylbenzene.
• Fittig: two aryl halides + Na → biaryl.
• Sandmeyer: ArN₂⁺ + CuCl (or CuBr) → ArCl (or ArBr).
• Gattermann–Koch: benzene + CO + HCl/AlCl₃/CuCl → benzaldehyde.
• Birch reduction: benzene + Na/NH₃(l)/EtOH → 1,4-cyclohexadiene.

Practice Prompts

1. Predict the major product when 1-methylcyclohexene reacts with HBr in the presence of benzoyl peroxide, and explain the stereochemistry.
2. Identify the structure of a hydrocarbon C₆H₁₀ that, upon ozonolysis with reductive workup, gives only cyclohexanone. Draw its mechanism through the Wheland intermediate if it undergoes Friedel–Crafts acylation.

Common Traps to Avoid

• Treating benzene like an alkene and writing addition products.
• Forgetting that anti-Markovnikov addition works only for HBr, not HCl or HI, with peroxides.
• Assuming halogens are pure deactivators; they are ortho/para directors through resonance donation.
• Missing stereochemistry: alkene additions must specify syn/anti (e.g. Br₂ adds anti).

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