Hydrocarbons — NEET Notes

Classification of hydrocarbons

Every hydrocarbon belongs to one of four families. The difference is not in what they are made of — every one of them is just carbon and hydrogen — but in how the carbons bond to one another. Alkanes have only single C–C bonds and a saturated skeleton. Alkenes contain at least one C=C double bond. Alkynes carry a C≡C triple bond. Arenes have a special closed-loop conjugated π system — the benzene ring being the archetype. The general formulas fall out of these structural facts and are worth memorising cold because nearly every problem starts by asking you to identify the family.

The classification is not academic. It dictates reactivity. Saturated alkanes have no electron-rich site for electrophiles to attack; they undergo only radical chemistry. Alkenes and alkynes have exposed π electrons and so undergo electrophilic addition. Benzene also has a π system, but the loss of aromaticity that addition would cause is so energetically expensive that benzene chooses substitution instead — preserving the aromatic stability.

Alkanes — preparation

NCERT lists three named alkane syntheses that NEET returns to repeatedly: Wurtz reaction, Frankland (via Grignard reagents in NCERT's NIOS treatment), and Kolbe electrolysis. Together with the catalytic reduction of alkyl halides and unsaturated hydrocarbons, these are everything you need.

The Wurtz reaction couples two molecules of an alkyl halide using sodium metal in dry ether to yield a symmetrical alkane with twice as many carbons. The reaction is excellent for symmetrical alkanes — ethane from bromomethane, n-butane from bromoethane — but is hopeless for unsymmetrical alkanes because mixing two different halides gives all three possible cross-products. NEET 2020 (Q.140) tested this directly: n-heptane cannot be made in good yield because seven is odd, forcing an unsymmetrical coupling.

Frankland's method uses a Grignard reagent (R–MgX) — alkyl halide and magnesium in dry ether — and quenches it with an active hydrogen source (water, alcohol, or acid). The R group acquires an H to become R–H. Frankland is the textbook way to make a specific small alkane from a halide of the same carbon count.

Kolbe electrolysis takes a sodium or potassium salt of a carboxylic acid and electrolyses its concentrated aqueous solution. At the anode the carboxylate loses an electron to form an acyloxy radical, which spits out CO₂ and leaves an alkyl radical; two alkyl radicals couple to give a symmetrical alkane. Sodium acetate → ethane is the textbook example. Kolbe is again restricted to symmetrical alkanes for the same reason as Wurtz, and a fourth route — heating the sodium salt of a carboxylic acid with soda lime (NaOH + CaO, 3:1) — gives an alkane with one carbon less than the parent acid. NEET 2021 (Q.90) tested that the "missing reagent" in the soda-lime decarboxylation is CaO.

Conformations of ethane — staggered vs eclipsed

The C–C single bond in ethane permits free rotation. As the back CH₃ group rotates relative to the front, the molecule passes through infinitely many arrangements — but only two have names. In the staggered conformation, the six C–H bonds are as far apart as geometry allows, with a dihedral angle of 60° between front and back hydrogens. In the eclipsed conformation, the dihedral angle is 0° — every front C–H is directly behind a back C–H, generating torsional strain. The staggered conformer is the more stable of the two by about 12 kJ mol⁻¹.

Three NEET papers in three years have tested this exact pair. NEET 2021 asked for the dihedral angle of the least stable conformer (answer: 0°). NEET 2016 asked which conformer is more stable and why (answer: staggered, no torsional strain). NEET 2017 asked whether bond angle or bond length change between the two (answer: neither — only the dihedral angle changes).

Alkane reactions — halogenation, combustion, cracking, isomerisation, aromatisation

Saturated and electron-poor, alkanes will not react with polar reagents under ordinary conditions. They react with halogens only via a free-radical chain initiated by UV light or heat. The mechanism has the three classical phases — initiation (Cl₂ → 2 Cl•), propagation (Cl• + CH₄ → •CH₃ + HCl; •CH₃ + Cl₂ → CH₃Cl + Cl•), and termination (radical coupling). The reaction does not stop at monosubstitution; methane chlorinated in excess Cl₂ gives a mixture of CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄. The order of halogen reactivity is F₂ > Cl₂ > Br₂ > I₂.

Combustion is the defining reaction of alkanes economically — every gram of petrol that burns in an engine is alkane oxidation. CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O. The reactions are extraordinarily exothermic — methane releases 890 kJ mol⁻¹ — which is why alkanes dominate the world's fuel supply.

Three further industrial reactions appear in NCERT and on NEET. Pyrolysis (cracking) heats long-chain alkanes to about 873 K in the absence of air; bonds break, and the molecule fragments into shorter alkenes and alkanes — the basis of petroleum cracking. Isomerisation in the presence of AlCl₃ + HCl converts straight-chain alkanes to their branched isomers (n-butane → isobutane); this raises the octane rating of petrol. Aromatisation — heating an alkane with six or more carbons over a Pt–Al₂O₃ catalyst at high temperature — dehydrogenates and cyclises it to an arene (n-hexane → benzene), the industrial route to benzene from naphtha.

Alkenes — preparation & geometrical isomerism

The C=C double bond consists of one σ bond (axial overlap) and one π bond (lateral overlap of unhybridised p orbitals). The π bond is weaker and exposed above and below the σ-bond plane — making it the reactive site for every characteristic alkene reaction. Each alkene carbon is sp²-hybridised, so the geometry around the double bond is planar and the bond angle is about 120°. Crucially, the π bond locks the C=C against rotation: there is no free rotation across a double bond.

The standard alkene preparations are dehydrohalogenation of alkyl halides with alcoholic KOH (CH₃CH₂Br → ethene), dehydration of alcohols using concentrated H₂SO₄ at ~433 K or Al₂O₃ at 623–633 K (ethanol → ethene), and partial hydrogenation of alkynes with Lindlar's catalyst. When more than one alkene can form, the major product follows Saytzeff's rule: the more substituted alkene predominates, because it is more stable.

The locked geometry of the C=C bond creates a new isomerism: cis–trans (geometrical) isomerism. If each carbon of the double bond carries two different substituents, the molecule has two distinct configurations. In the cis isomer, the higher-priority groups sit on the same side of the double bond; in the trans isomer they sit on opposite sides. The two are different compounds with different physical properties — different boiling points, different melting points, different dipole moments. NCERT lists cis-but-2-ene (m.p. 134 K) versus trans-but-2-ene (m.p. 167 K) as the canonical example.

Markovnikov & the anti-Markovnikov (Kharasch) peroxide effect

The single most-tested reaction of alkenes is the addition of a hydrogen halide. When HBr (or HCl, or HI) adds across the C=C of an unsymmetrical alkene — one in which the two carbons of the double bond carry different numbers of hydrogens — the regiochemistry is governed by Markovnikov's rule: hydrogen goes to the carbon already richer in hydrogens; halogen goes to the carbon that has fewer. Mechanistically, this is the path that produces the more stable carbocation intermediate (tertiary > secondary > primary). Propene + HBr gives 2-bromopropane as the major product.

When the reagent is HBr specifically and an organic peroxide such as benzoyl peroxide is present, the regiochemistry flips. This is the Kharasch peroxide effect, or anti-Markovnikov addition. The peroxide initiates a chain mechanism that proceeds through bromine radicals rather than protons, and Br• adds to the alkene at the less substituted carbon to give the more stable secondary radical intermediate. Propene + HBr / peroxide gives 1-bromopropane as the major product. The peroxide effect is observed only with HBr — HCl bonds are too strong to break homolytically under these conditions, and HI bonds are too weak to give a productive chain.

Ozonolysis, hydration and polymerisation

Beyond hydrogen halide addition, alkenes undergo four further reactions that NEET keeps testing. Hydrogenation with Ni/Pt/Pd reduces the double bond to a single bond, converting ethene to ethane and forming the basis of vegetable-oil hardening into vanaspati ghee. Halogen addition (Br₂ in CCl₄) gives a 1,2-dihaloalkane; the disappearance of the reddish-brown bromine colour is the classical test for unsaturation. Acid-catalysed hydration in dilute H₂SO₄ adds water across the double bond, giving an alcohol — Markovnikov-selective.

Ozonolysis is the diagnostic reaction. Ozone adds across the C=C to form an unstable molozonide which rearranges to an ozonide; reductive workup with Zn / H₂O cleaves the ozonide so that each carbon of the original double bond becomes a separate carbonyl. The reaction breaks the C=C completely and lets you locate the original double bond by identifying the fragments. But-1-ene gives propanal + methanal; but-2-ene gives two molecules of ethanal. NEET 2022 (Q.98) gave the fragments (formaldehyde + 2-methylpropanal) and asked for the parent alkene (answer: 3-methylbut-1-ene); NEET 2020 (Q.159) ran the same question with methanal as one fragment.

Polymerisation at high temperature and pressure converts ethene into polyethene — the chain repeating thousands of times. Chloroprene (2-chloro-1,3-butadiene) polymerises to neoprene, a fact NEET 2023 (Q.66) tested directly. Other alkenes give polypropene, polystyrene, polyvinyl chloride — the entire commercial plastics industry rests on this single reaction.

Alkynes — acidity of the terminal C–H

Alkynes have a C≡C triple bond — one σ bond plus two π bonds. The two carbons are sp-hybridised, the geometry is linear (180°), and the molecule is held more rigidly than an alkene. Preparation in the laboratory is straightforward: heating calcium carbide with water gives ethyne (CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂), and double dehydrohalogenation of a vicinal or geminal dihalide with alcoholic KOH gives a terminal alkyne. Higher alkynes are built by alkylating sodium acetylide (the conjugate base of ethyne) with a primary alkyl halide.

The reactions of alkynes parallel those of alkenes — hydrogenation, halogen addition, hydrohalogenation, polymerisation — but with two differences. First, alkynes add reagents twice, because there are two π bonds to consume. Second, terminal alkynes have an acidic C–H bond that alkenes and alkanes do not.

The acidity is a direct consequence of hybridisation. An sp orbital has 50% s character, an sp² orbital 33%, an sp³ orbital 25%. The greater the s character, the closer the bonding electrons sit to the carbon nucleus, the more electronegative the carbon, and the more easily the C–H proton departs. The numerical effect is dramatic.

This acidity is the basis of two signature tests. Pass ethyne through ammoniacal AgNO₃ and a white precipitate of silver acetylide forms. Through ammoniacal Cu₂Cl₂ and a red precipitate of cuprous acetylide forms. Neither alkenes nor non-terminal alkynes give these precipitates — only terminal alkynes have an H acidic enough to be replaced by Ag or Cu. NEET 2016 (Q.33) used the alkylation of sodium acetylide twice to build 3-hexyne, testing precisely this property.

One distinct alkyne reaction worth memorising is the hydration. Adding water to ethyne in the presence of dilute H₂SO₄ and HgSO₄ produces an unstable enol (vinyl alcohol, CH₂=CHOH) which immediately tautomerises to acetaldehyde (CH₃CHO). For higher alkynes the same reaction gives a ketone — Markovnikov-selective — because water adds to put OH on the more substituted carbon. NEET 2017 (Q.30) tested exactly this mechanism for propyne → acetone.

Ozonolysis of an alkyne cleaves the triple bond but, unlike the alkene case, the products are 1,2-dicarbonyl compounds (glyoxals) when reductively worked up. Combustion of ethyne in pure oxygen yields the famous oxyacetylene flame at about 2800 °C, used for welding and cutting steel.

Benzene structure, resonance and aromaticity

Benzene has the molecular formula C₆H₆ — the formula of a triene — yet behaves nothing like one. It does not decolourise bromine water, does not respond to Baeyer's test, and refuses to add HX or H₂O. Instead, it undergoes substitution: a hydrogen is replaced by an electrophile while the ring stays intact. The explanation rests on three structural facts that Kekulé's 1865 alternating single–double-bond picture could not capture.

First, every C–C bond in benzene is the same length — 139 pm — intermediate between a single bond (154 pm) and a double bond (134 pm). Second, the ring is a perfect regular hexagon, every C–C–C angle exactly 120°, every carbon sp²-hybridised with its remaining p orbital perpendicular to the plane. Third, the six p orbitals overlap sideways all the way around the ring to form a single delocalised π system — a doughnut of electron density above and below the molecular plane. Benzene is a resonance hybrid; the two Kekulé structures are merely the boundary contributors.

Hückel's rule generalises benzene's aromaticity. A molecule is aromatic if and only if it is (a) cyclic, (b) planar, (c) fully conjugated, with every ring atom sp² and a p orbital in the π system, and (d) contains 4n+2 π electrons where n is a non-negative integer. For n = 0 the count is 2 (the cyclopropenyl cation); for n = 1 it is 6 (benzene, the cyclopentadienyl anion, pyridine, the tropylium cation); for n = 2 it is 10 (naphthalene, [10]annulene). Planar conjugated rings with 4n π electrons (cyclobutadiene, the cyclopentadienyl cation) are antiaromatic — unstable. Any system that cannot stay planar or conjugated is simply non-aromatic.

NEET 2023 (Q.95) gave seven cyclic species and asked how many obey Hückel's rule — the answer was four. NEET 2022 (Q.52) asked which one of four cyclic species is not aromatic. Both questions reward the student who can count π electrons and check planarity quickly.

Benzene's industrial preparation routes mirror the alkane-to-arene transition described earlier. The destructive distillation of coal gives benzene-rich light oil. Cyclotrimerisation of ethyne over hot iron gives benzene (3 HC≡CH → C₆H₆). Decarboxylation of sodium benzoate with soda lime gives benzene. Catalytic dehydrogenation (aromatisation) of n-hexane over Pt–Al₂O₃ at high temperature gives benzene.

Electrophilic aromatic substitution

The signature reactivity of benzene is electrophilic aromatic substitution (EAS): a hydrogen on the ring is replaced by an electrophile while the aromatic π system is preserved. Every EAS reaction — halogenation, nitration, sulphonation, Friedel–Crafts alkylation, Friedel–Crafts acylation — runs by the same three-step mechanism.

The five EAS reactions on the NEET syllabus differ only in the electrophile generated in step 1.

Halogenation: benzene + Cl₂ (or Br₂) over FeCl₃ (or FeBr₃) at room temperature gives chlorobenzene (or bromobenzene). The Lewis acid polarises the halogen. Iodination is slow because the HI by-product reduces iodobenzene back to benzene; HNO₃ or HIO₃ is added to scavenge HI.

Nitration: benzene + concentrated HNO₃ + concentrated H₂SO₄ at about 323–333 K gives nitrobenzene. H₂SO₄ protonates HNO₃ and a water molecule departs, generating the nitronium ion NO₂⁺ — the active electrophile. NEET 2016 (Q.35) used this: adding KHSO₄ to the nitrating mixture suppresses NO₂⁺ formation by common-ion effect, slowing the reaction.

Sulphonation: benzene + oleum (fuming H₂SO₄, which contains free SO₃) gives benzenesulphonic acid. The electrophile is SO₃ itself or its protonated form. Sulphonation is reversible — heating the sulphonic acid with dilute H₂SO₄ regenerates benzene — a property organic chemists exploit to "park" a sulphonate group at a position they want to block.

Friedel–Crafts alkylation: benzene + an alkyl halide with anhydrous AlCl₃ gives an alkylbenzene. AlCl₃ abstracts the halide from R–Cl to generate the carbocation R⁺. Benzene + CH₃Cl gives toluene. The reaction has limitations: it fails on rings deactivated by strong meta directors, and the alkyl-cation intermediate can rearrange.

Friedel–Crafts acylation: benzene + an acyl chloride RCOCl with AlCl₃ gives an aryl ketone (acetyl chloride + benzene → acetophenone). The electrophile is the acylium ion R–C≡O⁺, which is resonance-stabilised and does not rearrange.

Directive influence — ortho/para directors vs meta directors

The first substituent on a benzene ring controls where the second one will go. Electron-donating groups (alkyl, –OH, –OR, –NH₂) push electron density into the ring through induction or resonance, activate the ring towards further EAS, and steer the next electrophile to the ortho and para positions — exactly the positions where their lone pairs can stabilise the arenium intermediate. Electron-withdrawing groups (–NO₂, –SO₃H, –COOH, –COR, –CN, –CHO) drain electron density out of the ring, deactivate the ring, and steer the next electrophile to the meta position — the only position where their destabilising influence on the arenium intermediate is least pronounced.

Halogens are a special case: they are deactivating (because of strong induction) but still ortho/para directors (because their lone pairs can still donate by resonance).

NEET 2018 (Q.74) used directive influence to track a four-step transformation. C₇H₈ is toluene; Cl₂/Δ on the methyl side chain gives benzyl chloride or trichloromethyl benzene depending on conditions; Br₂/Fe is ring bromination of the side-chain-substituted toluene; –CCl₃ is a meta director, so the bromine goes meta; Zn/HCl reduces the –CCl₃ back to –CH₃; the final product is m-bromotoluene. Tracking the director at each step is the only way to solve it.

Carcinogenicity & toxicity of polynuclear aromatics

Benzene itself is a known human carcinogen — chronic exposure causes leukaemia, which is why it has been removed from petrol additives and laboratory solvents wherever possible. The danger grows with ring count. Polynuclear (polycyclic) aromatic hydrocarbons — molecules built from two, three, or more fused benzene rings (naphthalene, anthracene, phenanthrene, pyrene, benzo[a]pyrene) — are produced whenever organic matter burns incompletely. Cigarette smoke, diesel exhaust, charred meat, coal-tar pitch and forest-fire smoke all contain them.

Of these, benzo[a]pyrene is the most studied and the most dangerous. It is metabolised in the liver to an epoxide that binds covalently to DNA bases, causing mutations that lead to skin and lung cancers. NCERT identifies it specifically by name as a carcinogen "found in cigarette smoke and in the exhaust from automobiles." The wider lesson — that polynuclear aromatics from combustion are systemically harmful — is the public-health rationale behind clean-air standards, smoke-free workplaces, and the regulation of coal-tar exposure.