Hydrocarbons: Alkanes, Alkenes, Alkynes & Aromatic Compounds

1. Classification, Isomerism and Nomenclature

• Classification by saturation: alkanes (C_nH_2n+2, all C–C single bonds, saturated) vs alkenes (C_nH_2n, one C=C, DU = 1) vs alkynes (C_nH_2n−2, one C≡C, DU = 2) vs aromatic hydrocarbons (benzene ring, DU = 4 per ring: 1 for the ring + 3 for the three formal double bonds).
• Hybridization fixes geometry directly: alkane carbon is sp³ (tetrahedral, 109.5°); alkene carbon is sp² (planar, ~120°, with the 121.2° vs 117.2° split in ethene arising because double-bond–single-bond repulsion exceeds single-bond–single-bond repulsion); alkyne carbon is sp (linear, 180°).
• Bond parameters scale together: as hybridization moves sp³ → sp² → sp, %s-character rises (25% → 33.3% → 50%), bond length falls (1.54 Å → 1.34 Å → 1.20 Å), and bond energy rises (415 → 615 → 835 kJ/mol) — more s-character pulls bonding electrons closer to the nucleus, shortening and strengthening the bond.
• Isomerism specific to this unit: geometrical (cis-trans/E-Z) isomerism needs a C=C (or ring) with two different groups on each restricted carbon — alkynes CANNOT show this at all, since the sp carbon's strict 180° linearity leaves no second spatial arrangement possible around the triple bond.
• Nomenclature essentials: longest chain through the multiple bond gets lowest locant to the multiple bond itself; a ring is named as a prefix (cyclo-) or, for benzene derivatives, substituent positions are numbered to give the lowest locant set, with common names (toluene, xylene, cumene, styrene) still expected at JEE level alongside IUPAC names.

2. Alkanes: Conformational Analysis

• Newman projection: view directly down a C–C bond; the front carbon's three bonds are drawn from a central point, the back carbon's three bonds from behind a circle. Sawhorse projection: a 3-D side-on view along the same axis, showing both carbons offset diagonally — same information, different drawing convention, and JEE questions can mix the two freely.
• Ethane's torsional barrier is PURELY torsional: since all three hydrogens on each carbon are identical, there is no steric difference between any of the three staggered (or three eclipsed) positions — the barrier (~12.5 kJ/mol) comes entirely from eclipsing-bond electron repulsion, not from bulky groups being close together.
• n-Butane adds a steric contribution on top: because the two methyl groups are bulky and distinguishable from the four hydrogens, n-butane's three staggered conformers are no longer equal energy (anti, with methyls 180° apart, is lowest; gauche, with methyls 60° apart, is higher due to steric clash) and its two eclipsed conformers also differ (methyl-over-methyl eclipsing is far worse than methyl-over-H).
• Population at room temperature: the staggered conformers are energy minima and are far more heavily populated than the eclipsed conformers (energy maxima / rotational transition states) at any instant — conformers interconvert freely by bond rotation and are never isolable as separate compounds.

3. Alkanes: Methods of Preparation

• Catalytic hydrogenation of alkenes/alkynes: H₂ over Ni/Pt/Pd at 25°C; heterogeneous (two-phase, finely divided metal) hydrogenation is a strict SYN addition — cis-alkene + syn addition gives a meso product, trans-alkene + syn addition gives a racemic pair, when the resulting alkane has two new stereocentres.
• Wurtz reaction: 2 R–X + 2 Na (dry ether) → R–R + 2 NaX, coupling via either an ionic (R⁻ carbanion attacking R–X by S_N2) or free-radical pathway. Limited to 1°/2° halides (3° gives E2 elimination instead); mixing two different alkyl halides gives an inseparable mixture of three alkanes (R–R, R–R′, R′–R′); methane CANNOT be made this way (needs at least two fragments to couple); moisture destroys the organosodium intermediate, giving R–H instead of R–R.
• Reduction of alkyl halides: metal/acid (Zn/HCl, Zn–Cu couple, etc.) or metal hydride — LiAlH₄ reduces 1°/2° R–X by S_N2 but ELIMINATES 3° R–X (strong base → E2) instead of reducing it; NaBH₄ is poor/unreactive with 1° R–X but reduces 2°/3°.
• Organometallic routes: Grignard reagents (RMgX) react with ANY active-hydrogen compound (H₂O, ROH, NH₃, terminal alkynes, RCOOH, RSH) to give R–H, which is why Grignard reactions must be run under strictly anhydrous conditions. Corey-House synthesis (R–X → R–Li → R₂CuLi, then + R′–X) is the preferred route to unsymmetrical alkanes: Gilman's reagent tolerates –NO₂, –CN, and >C=O groups that would destroy a Grignard reagent, and works best when R′–X is 1°.
• From carboxylic acids: decarboxylation with soda lime (R–COONa + NaOH → R–H + Na₂CO₃, CaO/Δ) loses one carbon; Kolbe's electrolysis (2 RCOOK + 2 H₂O → R–R at the anode) gives an alkane with DOUBLE the carbons of one acid fragment via a radical coupling mechanism (RCOO^• → R^• + CO₂, then R^• + R^• → R–R).
• Reduction of carbonyl compounds: Clemmensen (Zn–Hg/conc. HCl) and Wolff-Kishner (NH₂NH₂/KOH) both convert >C=O to >CH₂, but Clemmensen fails on acid-sensitive groups (–OH, –NH₂, C=C) while Wolff-Kishner fails on base-sensitive groups (–X, esters) — the choice between them is dictated entirely by what ELSE is on the molecule.

4. Alkanes: Halogenation, Selectivity and Other Reactions

• Free-radical chain mechanism (R–H + X₂, UV/heat): Initiation (X₂ → 2 X^•, endothermic homolysis) → Propagation (X^• + R–H → R^• + HX, the rate-determining H-abstraction step, then R^• + X₂ → R–X + X^•) → Termination (any two radicals combine). Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂ (F₂ is explosive even in the dark; I₂ is endothermic/reversible and needs an oxidiser like HIO₃ to destroy the HI byproduct and pull the equilibrium forward).
• Selectivity vs reactivity: product yield depends on BOTH the probability factor (number of equivalent H atoms) and the inherent C–H reactivity (3° > 2° > 1° > CH₄, tracking radical stability). Chlorination's narrow rate spread (1 : 3.8 : 5.0) makes it low-selectivity — the statistical count of H atoms competes heavily with intrinsic reactivity, giving messy product mixtures. Bromination's huge rate spread (1 : 82 : 1600) makes it highly selective for the 3° position, often >99% yield there even when 3° hydrogens are a small minority of the total.
• Why Cl₂ → 2 Cl^• initiates (not CH₄ → CH₃^• + H^•): the Cl–Cl bond dissociation energy (~58 kcal/mol) is far lower than a C–H bond in methane (~104 kcal/mol), so homolysing the halogen is thermally the much easier first step.
• Other reactions: sulphonation (C_≥6 alkanes + oleum/SO₃, 400°C → alkanesulphonic acid); isomerization (straight-chain → branched, anhyd. AlCl₃/HCl, 300°C); aromatization/reforming (C_6–8 alkanes dehydrogenate + cyclize over Cr₂O₃/Al₂O₃, 600°C, high pressure → benzene/alkylbenzenes); pyrolysis/cracking (thermal C–C and C–H bond breaking, 400–500°C, no air → smaller alkanes + alkenes + H₂).
• Octane number: more branched alkanes have a LOWER heat of combustion (more stable, thermodynamically) and a HIGHER octane number (better anti-knock performance) — iso-octane (2,2,4-trimethylpentane) is fixed at 100, n-heptane at 0.

5. Alkenes: Structure, Stability and Preparation

• Stability order (substitution rule): tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted, driven by hyperconjugation (more alpha-H on the substituents) and homoconjugative resonance. Among disubstituted alkenes specifically, trans > cis (less steric strain between the two substituents) — but isobutene (CH₂=C(CH₃)₂, 6 alpha-H concentrated on ONE carbon) is slightly MORE stable than trans-2-butene, a tested exception to plain substitution-counting.
• Heat of hydrogenation (ΔH_hyd) measures stability INVERSELY: ΔH_hyd ∝ 1/stability for isomeric alkenes — a LOWER heat of hydrogenation means the alkene started at a LOWER (more stable) energy level before being hydrogenated down to the same final alkane.
• Preparation of alkenes: partial hydrogenation of alkynes via Lindlar's catalyst (H₂, Pd/CaCO₃ + Pb(OAc)₂/quinoline, syn addition → cis-alkene) or Birch reduction (Na/Li in liquid NH₃, anti addition via radical-anion → trans-alkene; FAILS for terminal alkynes since the acidic H reacts with Na to form a sodium salt instead); dehalogenation of vicinal dihalides (Zn or NaI); dehydrohalogenation of alkyl halides (E2, alc. KOH, Saytzeff's rule unless a bulky base gives Hofmann product); dehydration of alcohols (conc. H₂SO₄/160°C or H₃PO₄, E1 via a carbocation — watch for rearrangement).

6. Alkenes: Electrophilic Addition Mechanisms

• General EAR mechanism: Step 1 (RDS) — the alkene's loosely-held pi electrons attack an electrophile, generating a carbocation or a cyclic bridged ion; Step 2 (fast) — a nucleophile attacks that intermediate to complete addition. Electron-releasing groups on the alkene (+M, +I) speed up Step 1.
• Halogenation (X₂): goes through a cyclic halonium ion, NOT a free carbocation — so there is no rearrangement, and addition is strictly ANTI (cis-alkene + Br₂ → dl-pair; trans-alkene + Br₂ → meso compound, when applicable).
• Markovnikov addition of HX (no peroxide): H^⊕ adds to the carbon that already has more hydrogens, generating the MORE stable carbocation (this is the actual mechanistic reason for "Markovnikov's rule," not just a memorised pattern) — the resulting carbocation is prone to 1,2-hydride/methyl shifts toward an even more stable cation before X^⊖ attacks.
• Kharasch (peroxide) effect: exclusively for HBr + peroxide (R–O–O–R) + light. RO^• abstracts H from HBr to generate Br^•, which then adds FIRST to the terminal carbon (forming the more stable secondary/tertiary radical), giving the anti-Markovnikov product. HF and HCl bonds are too strong for the second propagation step; HI's weak bond lets iodine radicals recombine to I₂ before they can add — so only HBr shows this effect.
• Hydration, three ways (see the comparison table in the diagram): acid-catalyzed hydration goes through a free carbocation (Markovnikov, non-stereoselective, CAN rearrange); oxymercuration-demercuration goes through a bridged mercurinium ion (Markovnikov, anti-addition, no rearrangement); hydroboration-oxidation goes through a concerted four-centred transition state (anti-Markovnikov, syn-addition, no rearrangement).

7. Alkynes: Acidity, Preparation and Reactions

• Acidic character of terminal alkynes: the sp carbon's 50% s-character pulls C–H bonding electron density closer to carbon, stabilising the conjugate base (acetylide ion, R–C≡C^⊖) — acidity order H–OH > H–OR > H–C≡CR > H–NH₂ > H–CH=CH₂ > H–CH₂CH₃. Terminal alkynes react with strong bases (NaNH₂ or Na metal) to form acetylide salts, but are NOT acidic enough to react with weaker bases like NaOH.
• Preparation: double dehydrohalogenation of vicinal/geminal dihalides (alc. KOH, then NaNH₂/Δ for the second, harder elimination — vinyl halide is unreactive toward the first base alone due to resonance stabilisation of the C–X bond); hydrolysis of carbides (CaC₂ + 2 H₂O → ethyne; Mg₂C₃ + 4 H₂O → propyne).
• Addition reactions: HX/X₂ — 1 mole gives a vinyl halide, 2 moles give a geminal dihalide; HOX — 1 mole gives an α-haloketone (via an enol intermediate), 2 moles give an α,α-dihaloketone. Mercuric-ion catalyzed hydration (HgSO₄/dil. H₂SO₄) follows Markovnikov addition via an enol that tautomerises to a methyl ketone — EXCEPT ethyne itself, which gives acetaldehyde since there's no alkyl group to make "methyl ketone" apply.
• Hydroboration-oxidation of alkynes: terminal alkyne → aldehyde (R–CH₂–CHO, anti-Markovnikov); internal alkyne → ketone (R–CH₂–CO–R′).
• Cyclic polymerization: 3 molecules of ethyne through a red-hot iron tube at 873 K → benzene (trimerization); 4 molecules with Ni(CN)₂ → 1,3,5,7-cyclooctatetraene (tetramerization).

8. Conjugated Dienes: 1,2- vs 1,4-Addition

• 1,3-Butadiene (CH₂=CH–CH=CH₂) undergoes electrophilic addition through a delocalised ALLYLIC carbocation that has partial positive charge at BOTH C2 and C4, so nucleophilic attack can occur at either position.
• Low temperature (−80°C): the 1,2-addition product dominates — the Kinetic Control Product (KCP), formed faster because the nucleophile attacks the nearer, more accessible charge centre directly.
• High temperature (40°C): the 1,4-addition product dominates — the Thermodynamic Control Product (TCP), more stable because it retains a MORE substituted internal double bond, and is favoured once there's enough thermal energy to equilibrate back through the allylic cation and let the more stable product accumulate.
• The general principle (kinetic product forms faster at low temperature; thermodynamic product dominates at high temperature or given enough time to equilibrate) recurs across organic chemistry wherever a reaction can revert through a common intermediate — conjugated diene addition is simply its cleanest textbook example.

9. Aromatic Hydrocarbons: Structure and Aromaticity

• Benzene is a resonance hybrid, not an equilibrium between two Kekulé structures: neither individual Kekulé structure (alternating single/double bonds) is the real molecule — all six C–C bonds are experimentally identical at 1.39 Å, exactly intermediate between a pure single bond (1.54 Å) and a pure double bond (1.34 Å), because the six pi electrons are fully delocalised around the ring.
• Hückel's rule: a ring is aromatic if it is (i) cyclic, (ii) planar, (iii) fully conjugated (every ring atom has a p orbital), and (iv) has exactly (4n+2) pi electrons in that delocalised system (n = 0, 1, 2…). Benzene has n = 1 → 6 pi electrons.
• Resonance/delocalisation energy: the extra stability benzene has compared to a hypothetical "cyclohexatriene" with three isolated double bonds, experimentally measured as roughly 36 kcal/mol via heat-of-hydrogenation comparison — this large stabilisation is exactly why benzene strongly prefers SUBSTITUTION (which preserves the aromatic sextet) over ADDITION (which would destroy it), unlike an ordinary alkene.
• Nomenclature: mono-substituted benzenes often keep common names (toluene = methylbenzene, cumene = isopropylbenzene, styrene = vinylbenzene); di-substituted patterns are ortho (1,2-), meta (1,3-), or para (1,4-); tri- and higher-substituted rings are numbered for the lowest overall locant set.

10. Electrophilic Aromatic Substitution and Directing Effects

• General EAS mechanism: the ring's pi system attacks an electrophile (E^⊕) to form a resonance-stabilised arenium ion (sigma complex, positive charge delocalised onto the ortho/para-related ring carbons but NEVER the ipso carbon itself), then a base removes the proton from the sp³ carbon to restore full aromaticity.
• Nitration: conc. HNO₃ + conc. H₂SO₄ generates the nitronium ion (NO₂^⊕) electrophile, which attacks the ring.
• Friedel-Crafts alkylation/acylation: R–X or R–COCl with anhydrous AlCl₃ (Lewis acid catalyst generates R^⊕ or R–CO^⊕). FAILS entirely on strongly deactivated rings (nitrobenzene, and other rings bearing –NO₂, –CN, –SO₃H, or an existing carbonyl/carboxyl group) because the ring is too electron-poor to attack the electrophile; also fails on aniline-type rings (the Lewis acid complexes with the basic –NH₂ lone pair instead of activating the catalyst).
• Directing-group logic: activating groups (+R dominant, like –OH/–NH₂/–OR, or +I/hyperconjugation alone, like alkyl groups) donate electron density, speeding up substitution and directing to ortho/para (where that extra density concentrates in the arenium-ion resonance structures). Deactivating groups (–R dominant, like –NO₂/–CN/–COOH/–SO₃H) withdraw electron density, slowing substitution and directing to meta (the one position where the positive arenium charge is NOT placed directly adjacent to the electron-withdrawing group in any resonance structure).
• Halogens are the sole deactivating-yet-o/p-directing exception: strong –I effect withdraws electron density overall (deactivating, slower than benzene itself), but a lone pair on the halogen still donates by resonance (+R) specifically into the ortho/para positions, directing there even though the net reaction is slower.

11. Laboratory Identification Tests

• Baeyer's reagent (cold dil. alk. KMnO₄): detects C=C or C≡C unsaturation — pink colour disappears as a vicinal diol forms.
• Bromine water / Br₂ in CCl₄: detects C=C or C≡C unsaturation — reddish-brown colour decolorizes via addition (benzene does NOT give this test under ordinary conditions, see Section 9).
• Tollens' reagent (AgNO₃ + NH₄OH) and ammoniacal cuprous chloride: detect TERMINAL alkynes specifically (not internal alkynes, not alkenes) — white silver acetylide (R–C≡C–Ag↓) or red copper acetylide (R–C≡C–Cu↓) precipitates, since only a terminal alkyne has the acidic C–H needed to form the acetylide salt.

12. How JEE Frames These Questions

• "Identify the product" sequences: work the sequence one arrow at a time, naming the specific intermediate (carbocation vs bridged ion vs radical vs arenium ion) at each step before deciding the regiochemistry/stereochemistry of that step — most wrong answers come from skipping straight to a remembered "rule" without checking which mechanism actually applies here.
• Reactivity-ordering of substituted benzenes (nitration, halogenation, sulphonation): classify each substituent as activating or deactivating first, then rank purely by that electronic effect — a strongly activating group (anisole-type) reacts fastest, a strongly deactivating group (nitrobenzene-type) reacts slowest, regardless of what the electrophile actually is.
• Assertion-reason / two-statement questions on the peroxide effect: always check whether the halogen acid is specifically HBr — if the question substitutes HCl, HF, or HI "with peroxide," the anti-Markovnikov claim is false no matter how the rest of the statement reads.
• Percentage-composition / mass-percent numericals built on a reaction sequence: identify the final product's structure fully (via the reaction sequence) before computing the percentage — the arithmetic itself is routine, the structure identification is where marks are actually lost.
• "Separation method" questions following a reaction that gives two products: match the method to what actually differs between the two products — different boiling points → (fractional) distillation; volatile vs non-volatile → steam distillation; solid sublimes vs doesn't → sublimation.

13. Quick Sheet and Checklist

Before the exam, check you can:

• Explain why alkynes show zero geometrical isomerism, directly from sp hybridization and 180° linearity.
• Distinguish Newman and Sawhorse projections, and explain why ethane's conformers are equal-energy within each type while n-butane's are not.
• State which organometallic route (Grignard vs Corey-House) survives a nitro, cyano, or carbonyl group elsewhere on the molecule, and why.
• Derive (not just recall) why more substituted alkenes have a LOWER heat of hydrogenation despite being MORE stable.
• Name the exact conditions under which the peroxide effect operates (HBr only) and explain mechanistically why HCl/HF/HI don't show it.
• Compare the three hydration methods by intermediate, regiochemistry, stereochemistry, and rearrangement risk.
• State the acidity order of terminal alkynes relative to water, alcohols, and ammonia, and explain it from s-character.
• Predict 1,2- vs 1,4-addition on a conjugated diene from the reaction temperature alone.
• Explain why benzene gives substitution rather than addition, in terms of resonance/delocalisation energy.
• Classify any ring substituent as activating/deactivating and o/p- or m-directing, including the halogen exception.