Hydrocarbons

In short

Hydrocarbons are compounds of carbon and hydrogen only, divided into saturated alkanes, unsaturated alkenes and alkynes, and aromatic arenes. The topic is the backbone of organic chemistry because nearly every functional group is introduced onto a hydrocarbon skeleton, and because the reagents taught here (electrophiles, free radicals, catalysts) recur in every later chapter. It carries heavy weight in B.Sc. and M.Sc. entrance papers, CSIR-UGC NET, GATE Chemistry (CY), IIT-JAM and the written rounds of Reliance Industries, IOCL, BPCL, HPCL, ONGC, GAIL, Indian Oil R&D, Deepak Nitrite, Aarti Industries, SRF, Pidilite and the big pharma intermediate makers. On the job it underlies refinery and petrochemical streams, solvent selection, polymer feedstocks and the toxicology of benzene and PAHs. Expect questions that ask you to predict the major product and justify it by stability of the intermediate.

Explained

Classification, structure and bonding

• Aliphatic: open-chain or non-aromatic rings. Saturated = alkanes (CnH2n+2, only sigma bonds). Unsaturated = alkenes (CnH2n, one C=C) and alkynes (CnH2n−2, one C≡C).
• Alicyclic: cycloalkanes CnH2n; aromatic: benzene and its derivatives, plus fused arenes (naphthalene C10H8, anthracene, phenanthrene).
• Hybridisation and geometry: sp³ tetrahedral 109.5°, C–C 154 pm; sp² trigonal planar 120°, C=C 134 pm; sp linear 180°, C≡C 120 pm.
• Bond strength and s-character: more s-character shortens the bond, raises the electronegativity of that carbon and raises acidity of attached H. Hence C–H acidity order: HC≡CH (pKa ≈ 25) > CH2=CH2 (≈ 44) > CH3–CH3 (≈ 50).
• Degree of unsaturation (double-bond equivalents) for CcHhNnXx: DoU = (2c + 2 + n − h − x)/2. Each ring or double bond counts 1; a triple bond counts 2; a benzene ring counts 4.
• Physical trends: all hydrocarbons are non-polar (alkenes very slightly polar), insoluble in water, soluble in benzene, ether, CCl4. Boiling point rises with chain length (~20–30 K per CH2 in the lower members) and falls with branching (less surface contact). Melting point of alkanes shows the odd-even alternation — even-numbered chains pack better.

Alkanes: preparation and free-radical chemistry

Preparations

1. Hydrogenation of alkenes/alkynes: H2 / Ni (Sabatier–Senderens, 250–300 °C) or Pt, Pd at room temperature.
2. Wurtz reaction: 2 R–X + 2 Na in dry ether → R–R. Good only for symmetrical alkanes; two different halides give a mixture of three products. Fails for tertiary halides (elimination) and for CH3X it gives ethane.
3. Corey–House: R2CuLi + R′X → R–R′ — the unsymmetrical alternative to Wurtz.
4. Reduction of haloalkanes: Zn + dilute HCl, or Zn–Cu couple in ethanol, or LiAlH4.
5. Kolbe electrolysis: electrolysis of aqueous sodium/potassium carboxylate; anodic decarboxylation gives R–R (acetate → ethane).
6. Decarboxylation (soda-lime): RCOONa + NaOH/CaO, heat → RH + Na2CO3 — one carbon shorter.
7. Hydrolysis of Grignard reagent: RMgX + H2O → RH + Mg(OH)X.

Reactions

• Halogenation by free-radical substitution — chain mechanism: (i) initiation Cl2 → 2Cl· (hv or 250–400 °C); (ii) propagation Cl· + CH4 → CH3· + HCl, then CH3· + Cl2 → CH3Cl + Cl·; (iii) termination radical couplings.
• Reactivity of halogens: F2 (explosive, uncontrolled) > Cl2 > Br2 > I2 (reversible, needs an oxidant like HIO3). Selectivity is the reverse: Br2 is far more selective than Cl2 (late, more endothermic H-abstraction → more product-like transition state).
• Radical stability 3° > 2° > 1° > methyl > vinyl, from hyperconjugation and inductive release. Relative H-abstraction rates per H by Cl· ≈ 5.0 : 3.8 : 1 (3° : 2° : 1°); by Br· ≈ 1600 : 82 : 1.
• Allylic and benzylic bromination: NBS (N-bromosuccinimide) in CCl4 with a trace of peroxide gives a low steady Br2 concentration — substitution at the allylic position instead of addition to the double bond.
• Combustion: CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O, strongly exothermic — the basis of LPG, CNG and petrol as fuels.
• Controlled oxidation: CH4 + O2 (Cu, 200 atm) → CH3OH; (Mo2O3) → HCHO; alkane with a 3° H + KMnO4 → 3° alcohol.
• Isomerisation: n-butane + anhydrous AlCl3/HCl, 300 °C → isobutane. Aromatisation / reforming: n-hexane over Cr2O3–Al2O3 at 500–600 °C, 10–20 atm → benzene.
• Pyrolysis / cracking: C–C and C–H homolysis above 500 °C giving shorter alkanes, alkenes and H2 — thermal, catalytic (zeolite, FCC) and hydrocracking versions.
• Conformations of ethane and butane: staggered (anti) most stable; ethane rotational barrier ≈ 12.5 kJ/mol; n-butane: anti < gauche (+3.8) < eclipsed gauche/skew (+16) < fully eclipsed syn (+19–25 kJ/mol).

Alkenes: preparation, addition and oxidation

Preparations (all eliminations; Saytzeff rule — the more substituted, more stable alkene is major; bulky base like t-BuOK gives the Hofmann, less substituted product)

• Dehydrohalogenation: R–X + alcoholic KOH, heat (E2, anti-periplanar). Ease 3° > 2° > 1°.
• Dehydration of alcohols: conc. H2SO4 (170 °C) or Al2O3 (350 °C); E1 via carbocation, so rearrangement possible. Ease 3° > 2° > 1°.
• Dehalogenation of vicinal dihalide: Zn dust in methanol.
• Partial reduction of alkynes: H2/Pd-BaSO4 quinoline (Lindlar) → cis; Na or Li in liquid NH3 (Birch) → trans.
• Kolbe electrolysis of a dicarboxylate salt, and pyrolytic eliminations (Hofmann, Chugaev, Cope).

Addition reactions (electrophilic, through the most stable carbocation)

• Hydrogenation H2/Ni, Pt, Pd — syn addition; heat of hydrogenation measures alkene stability (more substituted = lower ΔH).
• Halogen addition: Br2/CCl4 — anti addition via the cyclic bromonium ion; decolourisation of bromine water is the classic unsaturation test.
• HX addition — Markovnikov rule: H goes to the carbon with more hydrogens, X to the carbon that can carry the more stable positive charge. Reactivity HI > HBr > HCl > HF.
• Peroxide (Kharasch) effect: HBr + peroxide → anti-Markovnikov product by a free-radical path (Br· adds to give the more stable radical). Works only for HBr, not HCl or HI.
• Hydration: dilute H2SO4 → Markovnikov alcohol (rearrangement possible). Hydroboration–oxidation (B2H6 then H2O2/NaOH) → anti-Markovnikov alcohol, syn addition, no rearrangement. Oxymercuration–demercuration → Markovnikov alcohol without rearrangement.
• Hypohalous acid HOX → vicinal halohydrin (OH on the more substituted carbon).
• Hydroxylation: cold dilute alkaline KMnO4 (Baeyer's reagent, decolourised, brown MnO2 forms) or OsO4/NMO → syn diol; peracid then H3O+ → anti diol.
• Ozonolysis: O3 then Zn/H2O (reductive) gives aldehydes/ketones; oxidative work-up (H2O2) converts the aldehyde fragments to acids. Used to locate the position of the double bond.
• Hot concentrated KMnO4 cleaves C=C to acids/ketones; CO2 from a terminal =CH2.
• Polymerisation: ethene → polyethene (Ziegler–Natta Ti/Al catalyst, LLDPE/HDPE), propene → polypropene, styrene → polystyrene.
• Diels–Alder [4+2]: conjugated diene + dienophile → cyclohexene, suprafacial-suprafacial, cis-principle retained.

Alkynes: acidity and characteristic reactions

• Preparation: CaC2 + 2H2O → HC≡CH + Ca(OH)2 (industrially 1,2-dibromoethane + 2KOH-alc, or vicinal tetrahalide + Zn); alkylation of sodium acetylide with 1° R–X extends the chain.
• Terminal alkyne acidity: HC≡C–H loses H+ to Na/NaNH2 in liquid NH3 giving an acetylide. Tests: ammoniacal AgNO3 → white Ag acetylide; ammoniacal Cu2Cl2 → red Cu acetylide. Non-terminal alkynes give no precipitate — this is how 1-butyne is told from 2-butyne.
• Addition of HX, X2: two successive Markovnikov additions; the alkyne is *less* reactive than the alkene to electrophiles (sp carbons hold electrons more tightly and the vinyl cation is unstable).
• Hydration: dilute H2SO4/HgSO4 → enol → tautomerises to the carbonyl; acetylene gives acetaldehyde, all other terminal alkynes give methyl ketones.
• Hydroboration–oxidation of a terminal alkyne gives the aldehyde (anti-Markovnikov enol).
• Reduction: H2/Ni (full), Lindlar (cis-alkene), Na/liq. NH3 (trans-alkene).
• Polymerisation: 3 HC≡CH over red-hot iron tube (873 K) → benzene; 2 HC≡CH + Cu2Cl2/NH4Cl → vinylacetylene (precursor of chloroprene/neoprene).
• Acetylene burns in O2 at about 3,300 °C — oxy-acetylene welding.

Arenes: aromaticity and electrophilic substitution

• Hückel's rule: planar, cyclic, fully conjugated and (4n + 2) π electrons → aromatic. 4n π → antiaromatic (cyclobutadiene); non-planar or non-conjugated → non-aromatic (cyclooctatetraene is tub-shaped). Aromatic ions: cyclopentadienyl anion (6 π), tropylium cation (6 π).
• Benzene structure: all C–C 139 pm (between 154 and 134), resonance energy ≈ 150 kJ/mol; measured heat of hydrogenation 208 kJ/mol against ~360 kJ/mol predicted for "cyclohexatriene". MO picture: six p orbitals → three bonding MOs filled.
• Preparation: decarboxylation of sodium benzoate with soda-lime; phenol + Zn dust; cyclohexane dehydrogenation; acetylene trimerisation; catalytic reforming of naphtha.
• Mechanism of electrophilic aromatic substitution (EAS): E+ attacks the π cloud → arenium ion (sigma complex, Wheland intermediate), a resonance-stabilised carbocation → loss of H+ restores aromaticity. Substitution, not addition, preserves the resonance energy.
• The five reactions and their electrophiles: nitration HNO3/H2SO4 (NO2+); sulphonation fuming H2SO4 (SO3 or HSO3+, reversible); halogenation X2/anhydrous AlCl3 or FeCl3 (X+); Friedel–Crafts alkylation R–X/AlCl3 (R+); Friedel–Crafts acylation RCOCl/AlCl3 (acylium RCO+).
• Friedel–Crafts limitations: carbocation rearrangement, polyalkylation (product more reactive than benzene), and total failure on strongly deactivated rings (nitrobenzene, aniline — the Lewis base N complexes the AlCl3). Acylation avoids rearrangement and polysubstitution; Clemmensen/Wolff–Kishner reduction of the ketone then gives the straight-chain alkylbenzene.
• Directive effects: o/p-directing activators –NH2, –OH, –OR, –NHCOR, –R, –Ph (+R/+I); o/p-directing deactivator –X (–I deactivates, +R directs); m-directing deactivators –NO2, –CN, –SO3H, –CHO, –COR, –COOH, –CF3, –NR3+ (–R/–I). Reason: the activator stabilises the arenium ion when attack is ortho/para; a deactivator destabilises it most at those positions, leaving meta as the least bad.
• Side-chain reactions: toluene + alkaline KMnO4 → benzoic acid (the ring survives); Cl2 in sunlight (no catalyst) → benzyl chloride by a benzylic radical route; ring versus side chain is set by the conditions.
• Addition under force: H2/Ni 200 °C → cyclohexane; Cl2 in UV → benzene hexachloride (lindane, now banned under the Stockholm Convention and the Insecticides Act regime in India).
• Carcinogenicity: benzene and polynuclear aromatics (benzo[a]pyrene from tobacco smoke, coal tar, incomplete combustion) are confirmed human carcinogens; benzene is restricted as a solvent and is a regulated workplace exposure and ambient air parameter (NAAQS annual limit 5 µg/m³, CPCB).

Petroleum, LPG and the Indian context

• Petroleum fractions by boiling range: gaseous (<20 °C, LPG), petroleum ether 20–60, light naphtha 60–100, heavy naphtha/petrol 40–200, kerosene 175–275, diesel/gas oil 250–400, lubricating oil and wax, then residue (bitumen, petroleum coke).
• Cracking: thermal (free-radical), catalytic FCC (zeolite, carbocation route, gives branched high-octane petrol), steam cracking of naphtha/ethane for ethene and propene — the start of the whole petrochemical chain.
• Reforming converts straight-chain naphtha to aromatics and branched alkanes to raise octane; isomerisation, alkylation and MTBE/ethanol blending also raise octane.
• Knocking and ratings: octane number against iso-octane = 100 and n-heptane = 0; branching and aromaticity raise it. Cetane number uses n-hexadecane = 100, α-methylnaphthalene = 0 — opposite requirement.
• Indian practice: BIS specifications govern motor gasoline and diesel (IS 2796 for motor gasoline, IS 1460 for diesel — editions are revised, so quote the current one); BS-VI fuel from April 2020 caps sulphur at 10 ppm and benzene in petrol at 1% v/v; E20 ethanol-blended petrol is now the market norm and CNG (mainly methane) is sold through city gas networks. LPG is a propane–butane mix with ethyl mercaptan added as odourant.
• Natural gas is >80% methane; coal bed methane and gas hydrates are exploration targets. Hydrocarbon exploration and production are regulated by the DGH under MoPNG; the PNGRB regulates pipelines and city gas distribution.

Key points

• Alkane - CnH2n+2, sp³, only sigma bonds, paraffin (low affinity), reacts by free-radical substitution.
• Alkene - CnH2n, sp², a planar π bond that is a nucleophile, reacts by electrophilic addition.
• Alkyne - CnH2n−2, sp, linear; terminal C–H is weakly acidic (pKa ≈ 25) so it forms metal acetylides.
• Markovnikov rule - the electrophile's H adds so as to generate the more stable (more substituted) carbocation.
• Peroxide effect - HBr + ROOR gives the anti-Markovnikov product by a radical chain; only HBr.
• Hydroboration–oxidation - B2H6/H2O2-NaOH gives the anti-Markovnikov alcohol, syn, with no rearrangement.
• Saytzeff vs Hofmann - small base gives the more substituted alkene, bulky base (t-BuOK) the less substituted.
• Baeyer's test - cold dilute alkaline KMnO4 decolourised = unsaturation; gives a syn diol.
• Ozonolysis - locates C=C; reductive work-up keeps aldehydes, oxidative work-up makes acids.
• Lindlar vs Birch - Pd-BaSO4/quinoline gives cis-alkene; Na/liq. NH3 gives trans-alkene.
• Acetylide test - ammoniacal AgNO3 (white) or Cu2Cl2 (red) precipitate proves a terminal alkyne.
• Alkyne hydration - HgSO4/H2SO4 gives an enol that tautomerises to a methyl ketone (acetaldehyde from acetylene only).
• Hückel's rule - planar cyclic conjugated with (4n+2) π electrons = aromatic; 4n π = antiaromatic.
• Arenium ion - the resonance-stabilised cation intermediate of every electrophilic aromatic substitution.
• Directive effect - activators (OH, NH2, R) are o/p; deactivators (NO2, COOH, SO3H) are meta; halogens deactivate yet direct o/p.
• Friedel–Crafts - alkylation suffers rearrangement and polysubstitution; acylation does not, and fails on deactivated rings.
• Selectivity vs reactivity - Cl2 is more reactive but far less selective than Br2 in alkane halogenation.
• Octane vs cetane - petrol wants branched/aromatic (high octane), diesel wants long straight chains (high cetane).

Formulas and examples

Example 1 — molecular formula from DoU. A hydrocarbon C8H8 decolourises Br2 water and, on vigorous oxidation, gives benzoic acid.

• DoU = (2×8 + 2 − 8)/2 = (16 + 2 − 8)/2 = 5.
• A benzene ring uses 4, leaving 1 π bond outside the ring.
• Oxidation to benzoic acid (C7H6O2) means one carbon chain attached to the ring. With C8 and one extra double bond: styrene, C6H5–CH=CH2. Answer: styrene.

Example 2 — ozonolysis to identify an alkene. An alkene C6H12 on ozonolysis with Zn/H2O gives only propanal (CH3CH2CHO), 2 mol per mol of alkene.

• Two identical fragments ⇒ the C=C is symmetrical.
• Propanal fragment CH3CH2CH= on each side ⇒ CH3CH2CH=CHCH2CH3.
• Answer: hex-3-ene (C6H12, DoU = (12 + 2 − 12)/2 = 1, consistent). Its cis and trans forms give the same ozonolysis result, so ozonolysis fixes position, not geometry.

Example 3 — combustion stoichiometry of LPG. A 14.2 kg domestic cylinder, taken as pure butane C4H10 (M = 58 g/mol), is fully burnt.

• Moles of butane = 14,200/58 = 244.8 mol.
• C4H10 + 13/2 O2 → 4 CO2 + 5 H2O.
• O2 needed = 244.8 × 6.5 = 1,591 mol = 1,591 × 32 = 50,920 g ≈ 50.9 kg.
• Air required (O2 is 21% by mole) = 1,591/0.21 = 7,576 mol ≈ 7,576 × 22.4/1,000 = 169.7 m³ at STP.
• CO2 produced = 244.8 × 4 = 979 mol = 979 × 44 = 43,080 g ≈ 43.1 kg — about 3 kg of CO2 per kg of LPG.

Example 4 — product prediction with and without peroxide. 2-methylpropene (isobutylene) + HBr.

• No peroxide (ionic): H+ adds to =CH2 giving the 3° carbocation (CH3)3C+ (more stable than the 1° alternative) → Br− attacks → 2-bromo-2-methylpropane (Markovnikov).
• With peroxide (radical): Br· adds to =CH2 giving the 3° radical (CH3)2C·–CH2Br (more stable than the 1° alternative) → abstracts H from HBr → 1-bromo-2-methylpropane (anti-Markovnikov).
• Same reagents, opposite products: the rule is always "whichever intermediate — cation or radical — is more stable".

Example 5 — selectivity in alkane bromination. Isobutane (CH3)3CH has 9 primary H and 1 tertiary H. Relative rate per H for Br· is 1600 (3°) : 1 (1°).

• Tertiary contribution = 1 × 1600 = 1600; primary contribution = 9 × 1 = 9.
• Fraction of 2-bromo-2-methylpropane = 1600/1609 = 99.4%; 1-bromo isomer = 9/1609 = 0.6%.
• With Cl· (5.0 : 1): 3° = 1 × 5.0 = 5.0, 1° = 9 × 1 = 9 → tertiary = 5.0/14.0 = 36%, primary = 64%. Chlorination is unselective; bromination is clean.

Where it is used

• Refining and petrochemicals: Reliance Jamnagar, IOCL Panipat/Paradip, BPCL Kochi, HPCL Visakh, ONGC/GAIL gas processing — FCC, reformer and steam-cracker operation, naphtha and LPG blending, octane and cetane control.
• Polymers and plastics: ethene, propene, butadiene and styrene from crackers feed HDPE/LLDPE, polypropene, SBR and polystyrene at Reliance, Haldia Petrochemicals, GAIL Pata, OPaL Dahej.
• Bulk and fine chemicals: benzene–toluene–xylene (BTX) chains to phenol, caprolactam, nitrobenzene, aniline, phthalic/terephthalic acid — Aarti Industries, Deepak Nitrite, Atul, SRF, Gujarat Alkalies.
• Pharma and agrochemical intermediates: Friedel–Crafts acylation, nitration and side-chain oxidation of toluene/xylene are everyday unit reactions in Hyderabad and Ankleshwar plants.
• Fuels and energy roles: refinery process chemist, QC chemist on IS fuel specifications, CNG/LPG quality and odourisation, LNG terminals, E20 ethanol blending.
• Environment and safety: benzene and PAH monitoring under CPCB NAAQS, VOC emission control, flammability and MSDS work, hydrocarbon spill assessment.

Interview questions

• Q: Why is benzene less reactive to addition than an alkene? A: Addition destroys the ring's aromatic delocalisation (resonance energy about 150 kJ/mol), so benzene prefers substitution, which restores aromaticity after loss of H+.
• Q: State Markovnikov's rule in its modern form. A: The electrophile adds so that the reaction passes through the more stable carbocation; the nucleophile then attacks that carbon.
• Q: Why does the peroxide effect work for HBr but not HCl or HI? A: Only HBr has both a favourable H-abstraction step and a favourable Br· addition step; H–Cl is too strong to abstract and I· addition is endothermic, so neither chain propagates.
• Q: How would you distinguish 1-butyne from 2-butyne? A: 1-butyne is terminal and gives a white precipitate with ammoniacal AgNO3 (red with ammoniacal Cu2Cl2); 2-butyne gives none.
• Q: Why is bromination of alkanes more selective than chlorination? A: The Br· hydrogen-abstraction step is endothermic and has a late, product-like transition state, so differences in radical stability show up fully; Cl· abstraction is exothermic and nearly indiscriminate.
• Q: Halogens deactivate the benzene ring yet direct ortho/para — explain. A: The −I effect withdraws electron density and slows the reaction, but lone-pair donation (+R) stabilises the arenium ion only for ortho and para attack.
• Q: Why is acylation preferred to alkylation in Friedel–Crafts? A: The acylium ion does not rearrange, and the ketone product is deactivated, so polysubstitution does not occur; the ketone can then be reduced to the alkyl chain.
• Q: Why is a terminal alkyne acidic while an alkane is not? A: The carbanion's lone pair sits in an sp orbital with 50% s-character, close to the nucleus and well stabilised; an sp³ carbanion has only 25%.
• Q: What product does propyne give on HgSO4/H2SO4 hydration, and why not an aldehyde? A: Acetone — water adds by Markovnikov rule to give the enol CH3C(OH)=CH2, which tautomerises to the methyl ketone. Only acetylene itself gives an aldehyde.
• Q: Is cyclooctatetraene aromatic? A: No. It has 8 π electrons (4n) and adopts a non-planar tub shape, so it is non-aromatic, not antiaromatic.
• Q: Why does branching lower the boiling point but raise the octane number? A: Branched molecules are more spherical with weaker van der Waals contact (lower b.p.), and their C–H bonds resist the pre-flame radical chemistry that causes knocking (higher octane).
• Q: How does Wurtz synthesis fail for an unsymmetrical alkane? A: Two different halides give three coupled products (R–R, R′–R′, R–R′) that are hard to separate; use the Corey–House reaction instead.

Common mistakes

• Applying Markovnikov's rule mechanically to the "carbon with fewer H" without checking which carbocation is actually more stable (resonance can beat substitution).
• Forgetting that acid-catalysed hydration and dehydration go through carbocations, so hydride and methyl shifts can move the functional group.
• Writing benzene as cyclohexatriene with alternating long and short bonds, and treating it as a simple alkene towards Br2/CCl4 or Baeyer's reagent.
• Confusing ring and side-chain conditions for toluene: Cl2/AlCl3 in the dark halogenates the ring; Cl2 in sunlight attacks the benzylic position.
• Giving an aldehyde for the HgSO4 hydration of a substituted terminal alkyne, or forgetting tautomerisation of the enol altogether.
• Mixing up Lindlar (cis) with Na/liquid NH3 (trans), and using Lindlar expecting a fully reduced alkane.
• Adding the arenium ion's positive charge count wrongly — only three resonance structures exist, and the key test is whether the substituent sits on a positive carbon.
• Balancing combustion equations without the (n+1) water or using 22.4 L/mol at the wrong temperature and pressure.

Related topics

Organic nomenclature and isomerism · Organic reaction mechanisms · Stereochemistry · Alcohols, phenols and ethers · Aldehydes and ketones · Polymers and petrochemical technology