Chapters

Everything NCERT Class 11 Unit 9 (Hydrocarbons) says that can be asked, plus what JEE Main adds beyond the book (marked Beyond NCERT). Conformations and cis–trans isomerism are covered fully in Isomerism, aromaticity in General Organic Chemistry, and the reactions of benzene in more depth in Aromatic Compounds.

Formula sheet

Every formula in the chapter on one screen, for the last pass. Details and examples are in the sections below.

Quantity	Formula	Watch
Alkane, alkene, alkyne	CₙH₂ₙ₊₂, CₙH₂ₙ, CₙH₂ₙ₋₂	Cycloalkane is CₙH₂ₙ too
Alkyl group	CₙH₂ₙ₊₁
Degree of unsaturation	(2C + 2 − H − X + N)/2	Rings + π bonds
Alkane combustion	CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → n CO₂ + (n + 1) H₂O	8 mol O₂ → n = 5
Any hydrocarbon	CₓHᵧ + (x + y/4) O₂ → x CO₂ + y/2 H₂O	Gas volumes = moles
Moles of H₂ taken up	= number of π bonds	A ring takes none
Ozonolysis	each C=C → two C=O	Join the C=O carbons back to get the alkene
Bond enthalpy (kJ mol⁻¹)	C–C 348, C=C 681, C≡C 823	π of C=C ≈ 284; σ ≈ 397
Bond length (pm)	C–C 154, C=C 134, C≡C 120, benzene 139	C–H 112
Halogen reactivity	F₂ > Cl₂ > Br₂ > I₂	H replaced: 3° > 2° > 1°
HX reactivity	HI > HBr > HCl	Peroxide effect only with HBr
Acidity	HC≡CH > CH₂=CH₂ > CH₃CH₃	HC≡CH > CH₃C≡CH ≫ CH₃C≡CCH₃

0 · What JEE Main asks from this chapter

143 questions in JEE Main 2019–2026, about 1 per paper; 25 are numerical answers. Grouped by type (counts are approximate):

Type	Share	Typical question
Addition to alkenes: Markovnikov, peroxide, hydration, Br₂, rearrangement	~31%	(CH₃)₃C–CH=CH₂ + Hg(OAc)₂, H₂O; NaBH₄ → (CH₃)₃C–CH(OH)CH₃ (2022)
Alkanes: preparation, halogenation, oxidation, isomerisation	~24%	Excess isobutane + Br₂, light, 125 °C → (CH₃)₃CBr (2021)
Alkynes: acidity, hydration, Lindlar, Na/NH₃, preparation	~17%	Which is the Lindlar catalyst (2021)
Ozonolysis and KMnO₄: find the alkene	~17%	C₆H₈ takes 2 H₂, ozonolysis gives 2 OHC–CH₂–CHO → cyclohexa-1,4-diene (2023)
Benzene in this unit (polymerisation, BHC, aromatization)	~6%	Benzene + Cl₂ in sunlight → C₆H₆Cl₆: 6 H (2022)
Bonding, stability, physical properties	~5%	"The C≡C bond of ethyne is weaker than C=C": incorrect (2024)

Most questions are one reagent, one product. Write the intermediate (carbocation or radical) before you pick the product.

1 · Classification and alkanes

Hydrocarbons: compounds of C and H only. LPG, CNG and LNG are hydrocarbon fuels; petrol, diesel and kerosene come from the fractional distillation of petroleum; coal gas from the destructive distillation of coal.

• Saturated: C–C and C–H single bonds only: open chains are alkanes, rings are cycloalkanes.
• Unsaturated: at least one C=C or C≡C: alkenes, alkynes.
• Aromatic: a special class of cyclic compounds (benzene and its relatives).

Alkanes were once called paraffins (Latin parum affinis, little affinity): inert to acids, bases and most reagents. Methane is tetrahedral (Fig. 9.1), all H–C–H angles 109.5°; C–C 154 pm, C–H 112 pm; σ bonds from sp³ orbitals.

Isomers and carbon types

• Methane, ethane and propane have one structure each. C₄H₁₀ has 2 isomers (butane, b.p. 273 K; 2-methylpropane, 261 K); C₅H₁₂ has 3 (pentane 309 K, 2-methylbutane 301 K, 2,2-dimethylpropane 282.5 K); C₆H₁₄ 5; C₇H₁₆ 9; C₁₀H₂₂ 75. These are chain isomers.
• A carbon bonded to one other C (or none, as in methane) is primary (1°); to two, secondary (2°); three, tertiary (3°); four, quaternary (4°). Terminal carbons are always 1°. The H's on them are 1°, 2°, 3° H.

Example (NCERT Problem 9.1) · C₆H₁₄: hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,2-dimethylbutane.

Example (NCERT Problem 9.2) · The eight C₅H₁₁– groups give eight alcohols: pentan-1-ol, pentan-2-ol, pentan-3-ol, 3-methylbutan-1-ol, 2-methylbutan-1-ol, 2-methylbutan-2-ol, 2,2-dimethylpropan-1-ol, 3-methylbutan-2-ol.

Example (NCERT Problems 9.3, 9.5) · (CH₃)₃CCH₂C(CH₃)₃ is 2,2,4,4-tetramethylpentane; (CH₃)₂C(C₂H₅)₂ is 3,3-dimethylpentane; tetra-tert-butylmethane is 3,3-di-tert-butyl-2,2,4,4-tetramethylpentane. "2-Ethylpentane" is really 3-methylhexane (the longest chain has six carbons).

Example (2025, 2026) · 2,2,3,3-Tetramethylpentane has just one CH₂, so two 2° H. The C₅H₁₂ (72 g mol⁻¹) with three 1° carbons is 2-methylbutane.

Preparation of alkanes

1. Hydrogenation of alkenes and alkynes with finely divided Pt, Pd or Ni (the metal adsorbs H₂ and activates the H–H bond). Pt and Pd work at room temperature; Ni needs higher T and P. CH₂=CH₂ + H₂ → CH₃CH₃; propyne + 2H₂ → propane.
2. Reduction of alkyl halides (not fluorides) with Zn and dil. HCl: CH₃Cl + H₂ → CH₄ + HCl.
3. Wurtz reaction: alkyl halide + Na in dry ether → higher alkane with twice the carbons: 2CH₃Br + 2Na → CH₃CH₃ + 2NaBr. Gives alkanes with an even number of C; two different halides give a mixture of three alkanes.
4. Decarboxylation: sodium salt of an acid + soda lime (NaOH + CaO), heated → alkane with one carbon fewer: CH₃COONa + NaOH → CH₄ + Na₂CO₃. Propane needs sodium butanoate (NCERT Problem 9.6).
5. Kolbe's electrolysis: aqueous sodium or potassium salt electrolysed; the alkane (even C) forms at the anode, H₂ at the cathode. Methane cannot be made this way.

At the anode: CH₃COO⁻ loses an electron → CH₃COO• → •CH₃ + CO₂; two •CH₃ join to give ethane.

Example (2024) · Electrolysis of CH₃COONa + C₂H₅COONa gives ethane, butane and propane (CH₃–C₂H₅): 3 alkanes.

Example (2026) · Methane comes from sodium ethanoate + soda lime, or CH₃MgBr + water, but not from Kolbe's electrolysis (that gives ethane), and not by hydrogenating any unsaturated hydrocarbon (the smallest has two carbons).

Physical properties

• Almost non-polar (C–C and C–H covalent, little electronegativity difference): only weak van der Waals forces.
• At 298 K: C₁–C₄ gases, C₅–C₁₇ liquids, C₁₈ and above solids. Colourless, odourless, insoluble in water (like dissolves like); grease (higher alkanes) dissolves in petrol.
• b.p. rises steadily with molecular mass (larger surface, stronger forces). Branching lowers b.p.: a more spherical molecule has less surface contact. Pentane 309.1 K > 2-methylbutane 300.9 K > 2,2-dimethylpropane 282.5 K.
• But neopentane has the highest m.p. of the three (256.4 K vs 143.3 K for pentane): a symmetrical molecule packs well in the crystal (2025).

Chemical properties of alkanes

1. Halogenation (free radical substitution) at 573–773 K or in diffused sunlight or UV light. Lower alkanes don't undergo nitration or sulphonation.

• Rate with halogens: F₂ > Cl₂ > Br₂ > I₂. Fluorination is too violent to control; iodination is very slow and reversible, so it is done with an oxidising agent (HIO₃ or HNO₃) that destroys the HI: HIO₃ + 5HI → 3I₂ + 3H₂O (2021).
• H replaced: 3° > 2° > 1°.
• Mechanism:
  1. Initiation: light or heat breaks Cl–Cl (weaker than C–C and C–H) into 2 Cl•.
  2. Propagation: Cl• + CH₄ → •CH₃ + HCl; •CH₃ + Cl₂ → CH₃Cl + Cl•. Further steps (Cl• + CH₃Cl → •CH₂Cl + HCl, and so on) give the polychlorinated products.
  3. Termination: Cl• + Cl• → Cl₂; •CH₃ + •CH₃ → CH₃CH₃; •CH₃ + Cl• → CH₃Cl. The second step explains the ethane by-product.

Selectivity · Beyond NCERT

• Relative rate per H, chlorination at 25 °C: 1° : 2° : 3° ≈ 1 : 3.8 : 5. Bromination is far more selective, about 1 : 82 : 1600, so Br₂ takes the 3° H almost only: excess isobutane + Br₂, light → (CH₃)₃CBr (2021).
• Allylic and benzylic H are replaced most easily (the radical is resonance-stabilised): but-1-ene + Br₂, light → the γ (allylic) H goes (2019); cyclohexene + Cl₂, hν → 3-chlorocyclohexene, but Cl₂ in CCl₄ → 1,2-dichlorocyclohexane (2024); 3-ethylbenzonitrile + Br₂, UV → 3-(1-bromoethyl)benzonitrile (2021).

Example (NCERT, 2024) · Monochlorination of 2-methylbutane gives four constitutional products and 6 with stereoisomers (worked in Isomerism).

2. Combustion: complete oxidation to CO₂ and H₂O with much heat; hence fuels. Incomplete combustion gives carbon black (ink, printer ink, black pigments, fillers).

Example (2026) · An alkane needing 8 mol O₂: (3n + 1)/2 = 8 gives n = 5, C₅H₁₂, so C + H = 17. If it gives only one monochloro product it is neopentane, with 4 primary carbons.

3. Controlled oxidation (regulated O₂, high pressure, catalyst):

Reaction	Conditions	Product
2CH₄ + O₂	Cu, 523 K, 100 atm	2CH₃OH (methanol)
CH₄ + O₂	Mo₂O₃	HCHO + H₂O (methanal)
2CH₃CH₃ + 3O₂	(CH₃COO)₂Mn	2CH₃COOH + 2H₂O
(CH₃)₃CH	KMnO₄	(CH₃)₃COH

Ordinary alkanes resist oxidation; only an alkane with a tertiary H is oxidised by KMnO₄, to a 3° alcohol. 2-Methylbutane → 2-methylbutan-2-ol is true; "n-alkanes are easily oxidised by KMnO₄" is false (2021).

4. Isomerisation: n-alkanes with anhydrous AlCl₃ + HCl give branched isomers: n-hexane → 2-methylpentane + 3-methylpentane (major).

5. Aromatization (reforming): n-alkanes with six or more C, at 773 K and 10–20 atm over oxides of V, Mo or Cr on alumina, are dehydrogenated and cyclised: n-hexane → benzene; n-heptane → toluene. (2026: the compound that isomerises to 2,4-dimethylpentane and aromatizes to toluene is heptane.)

6. With steam: CH₄ + H₂O → CO + 3H₂ at 1273 K over Ni: the industrial route to H₂.

7. Pyrolysis (cracking): higher alkanes break into smaller alkanes and alkenes on strong heating; a free radical reaction. Dodecane at 973 K over Pt, Pd or Ni → heptane + pentene + others. Oil gas and petrol gas are made this way.

Conformations

Rotation about the C–C σ bond gives conformations (rotamers), hindered by a 1–20 kJ mol⁻¹ barrier (torsional strain). Ethane: staggered most stable, eclipsed least, others skew; the gap is about 12.5 kJ mol⁻¹, so the conformers can't be isolated. Drawn as sawhorse or Newman projections. The dihedral angle in the staggered form is 60°; eclipsed is not the most stable (2024). Full detail in Isomerism.

2 · Alkenes

Alkenes (olefins, "oil forming"): at least one C=C, general formula CₙH₂ₙ. Ethene with chlorine gives an oily liquid, hence the name. Methene (CH₂) is too short-lived; ethene is the first stable member.

The double bond: one strong σ bond (sp²–sp² head-on, about 397 kJ mol⁻¹) and one weak π bond (sideways 2p–2p, about 284 kJ mol⁻¹). C=C is 134 pm. The loosely held π electrons make alkenes less stable than alkanes and open to electrophiles, yet the C=C bond as a whole (681 kJ mol⁻¹) is stronger than C–C (348 kJ mol⁻¹) (2022).

Naming: the longest chain containing the C=C, numbered from the end nearer the double bond; "-ene" replaces "-ane". Propene; but-1-ene; but-2-ene; buta-1,3-diene; 2-methylprop-1-ene; 3-methylbut-1-ene.

Example (NCERT Problems 9.7, 9.8) · (CH₃)₂CH–CH=CH–CH₂–CH=CH–CH(C₂H₅)–CH₃ is 2,8-dimethyldeca-3,6-diene (33 σ, 2 π). CH₂=CH–CH=CH–CH=CH–CH=CH₂ is octa-1,3,5,7-tetraene (17 σ, 4 π). CH₂=C(CH₂CH₂CH₃)₂ is 2-propylpent-1-ene (23 σ, 1 π).

Isomerism: C₄H₈ alkenes: but-1-ene, but-2-ene (position isomers) and 2-methylpropene (chain isomer of both). C₅H₁₀: pent-1-ene, pent-2-ene, 2-methylbut-2-ene, 3-methylbut-1-ene, 2-methylbut-1-ene. Alkenes also show geometrical (cis–trans) isomerism from restricted rotation: cis-but-2-ene μ = 0.33 D, trans ≈ 0; cis has the higher b.p., trans the higher m.p. Full detail in Isomerism.

Preparation of alkenes

1. From alkynes (partial reduction): H₂ with Lindlar's catalyst (palladised charcoal partly deactivated with sulphur compounds or quinoline) gives the cis alkene; Na in liquid NH₃ gives the trans alkene. Pd/C with H₂: ethyne → ethene, propyne → propene.
2. From alkyl halides (dehydrohalogenation): heat with alcoholic KOH; HX is lost, H from the β-carbon (β-elimination). Rate: I > Br > Cl; 3° > 2° > 1°.
3. From vicinal dihalides (dehalogenation): with Zn, ZnX₂ is lost: BrCH₂CH₂Br + Zn → CH₂=CH₂ + ZnBr₂.
4. From alcohols (acidic dehydration): heat with conc. H₂SO₄; water is lost (also β-elimination). Ethanol gives ethene (2022).

Which alkene forms · Beyond NCERT

• Saytzeff rule: elimination gives mainly the more substituted (more stable) alkene. 2-Bromobutane + alc. KOH → but-2-ene (major).
• A bulky base (KOᵗBu) takes the easier, less hindered H: the less substituted (Hofmann) alkene.
• Dehydration goes through a carbocation, so it can rearrange first.

Example (2023) · trans-Ph–CH=CH–CH₃ → cis: Br₂ (dibromide), alc. KOH and NaNH₂ (alkyne), then H₂ with Lindlar's catalyst (cis).

Properties of alkenes

Physical: like alkanes. C₂–C₄ gases, the next fourteen liquids, the rest solids. Ethene is a colourless gas with a faint sweet smell; the rest are colourless and odourless, insoluble in water, soluble in benzene and petroleum ether. Each –CH₂– adds about 20–30 K to the b.p.; straight chains boil higher than branched isomers.

Chemical: addition across the π bond, mostly electrophilic; some free radical additions; oxidation, ozonolysis and polymerisation.

1. Hydrogen: with finely divided Ni, Pd or Pt → alkane.

2. Halogens: Br₂ or Cl₂ add to give vicinal dihalides; iodine doesn't add normally. The reddish orange colour of Br₂ in CCl₄ is discharged: the test for unsaturation. Electrophilic addition through a cyclic halonium ion.

3. Hydrogen halides: reactivity HI > HBr > HCl; electrophilic addition.

• Symmetrical alkenes give one product: CH₂=CH₂ + HBr → CH₃CH₂Br.
• Markovnikov's rule (1869): the negative part of the adding molecule goes to the carbon with fewer H atoms. Propene + HBr → 2-bromopropane (major).
• Why: H⁺ adds first. It can give a 1° carbocation (CH₃CH₂CH₂⁺) or a 2° one (CH₃CH⁺CH₃). The 2° is more stable, forms faster, and Br⁻ adds to it (2021).

4. Peroxide effect (Kharash effect, anti-Markovnikov): with a peroxide such as benzoyl peroxide, HBr adds the other way: propene → 1-bromopropane. Observed by Kharash and Mayo, 1933.

• Mechanism (free radical): the peroxide gives C₆H₅• radicals; C₆H₅• + HBr → C₆H₆ + Br•; Br• adds first, to give the more stable 2° radical CH₃ĊHCH₂Br; this takes H from HBr, giving 1-bromopropane and a new Br•.
• Only HBr: the H–Cl bond (430.5 kJ mol⁻¹) is too strong for the radical to break; H–I (296.8) breaks, but I• radicals join to I₂ instead of adding. H–Br is 363.7.

Example (NCERT Problem 9.12) · Hex-1-ene + HBr gives 2-bromohexane; with peroxide, 1-bromohexane.

5. Sulphuric acid: cold conc. H₂SO₄ adds by Markovnikov's rule → alkyl hydrogen sulphate: CH₃CH=CH₂ → CH₃CH(OSO₃H)CH₃.

6. Water: a few drops of conc. H₂SO₄ catalyse Markovnikov addition → alcohol: (CH₃)₂C=CH₂ + H₂O → (CH₃)₃COH.

7. Oxidation:

• Cold, dilute, aqueous KMnO₄ (Baeyer's reagent) → vicinal glycol (diol); the pink colour fades: also a test for unsaturation. Ethene → ethane-1,2-diol.
• Acidic KMnO₄ or K₂Cr₂O₇ → ketones and/or acids. (CH₃)₂C=CHCH₃ → (CH₃)₂CO + CH₃COOH; but-2-ene → 2CH₃COOH.

8. Ozonolysis: O₃ adds to give an ozonide, which Zn–H₂O cleaves to aldehydes and ketones. Each C=C is cut and each end becomes C=O: this locates the double bond.

9. Polymerisation: at high temperature and pressure with a catalyst, many alkene molecules (monomers) join to a polymer. n CH₂=CH₂ → (–CH₂–CH₂–)ₙ, polythene (bags, sheets, squeeze bottles, toys, pipes); n CH₃CH=CH₂ → (–CH(CH₃)–CH₂–)ₙ, polypropene (milk crates, buckets, moulded articles).

More additions · Beyond NCERT

Reagent	Product, rule	Rearranges?
H₂O, H⁺	alcohol, Markovnikov	yes (carbocation)
Hg(OAc)₂, H₂O; then NaBH₄ (oxymercuration–demercuration)	alcohol, Markovnikov	no
BH₃ (B₂H₆) in THF; then H₂O₂, OH⁻ (hydroboration–oxidation)	alcohol, anti-Markovnikov, syn addition	no
Br₂ in CCl₄	vicinal dibromide, anti addition	no
Br₂ water (or Cl₂ water)	halohydrin: OH on the more substituted C	no
Br₂ in CH₃OH	bromo-ether: OCH₃ on the more substituted C	no
hot KMnO₄ or KMnO₄/H⁺	=CH₂ → CO₂; =CHR → RCOOH; =CR₂ → R₂CO
O₃; then H₂O or H₂O₂ (oxidative)	aldehydes become acids

• Rearrangement: when H⁺ (or HX) gives a carbocation next to a more substituted carbon, a 1,2-H or 1,2-CH₃ shift (or ring expansion) happens first. 3-Methylbut-1-ene + HCl → 2-chloro-2-methylbutane.
• Groups that destroy the cation: F₃C–CH=CH₂ + HCl gives the anti-Markovnikov product, because −I CF₃ destabilises the cation next to it (2019).
• Br₂ anti addition: trans-but-2-ene → meso-2,3-dibromobutane; cis-but-2-ene → the racemic pair (2021).
• Halohydrin: water attacks the bromonium ion at the more substituted carbon: propene + Br₂ water → 1-bromopropan-2-ol (2021); Cl₂/H₂O → CH₃CH(OH)CH₂Cl (2019).
• Conjugated dienes add 1,2 or 1,4; with heat, the 1,4 product (more substituted C=C) dominates: isoprene + HBr → (CH₃)₂C=CH–CH₂Br (2021).
• Alkene stability rises with substitution (more α-H): tetrasubstituted > tri > di > mono; trans > cis. A more stable alkene has a lower heat of hydrogenation and combustion; so does a conjugated diene compared with an isolated one (2020).

Example (2022) · (CH₃)₃C–CH=CH₂ + Hg(OAc)₂, H₂O, then NaBH₄ → (CH₃)₃C–CH(OH)–CH₃: Markovnikov, no methyl shift. With BH₃ then H₂O₂/OH⁻ → (CH₃)₃C–CH₂–CH₂OH (anti-Markovnikov).

Example (2024) · 3-Methylhex-2-ene + HBr with peroxide: Br• adds at C2, leaving the 3° radical at C3, so the product is CH₃CHBrCH(CH₃)CH₂CH₂CH₃. C2 and C3 are both chiral: 4 stereoisomers.

Working back from ozonolysis

1. Count the C=C from the H₂ taken up (rings take none) or the O₃ used.
2. Write the C=O products with their oxygens facing each other.
3. Remove the oxygens and join those carbons by C=C. Two ends of one dicarbonyl molecule joined back mean a ring.

Example (NCERT exercise 9.5–9.7) · Ethanal + pentan-3-one come from 3-ethylpent-2-ene, CH₃CH=C(C₂H₅)₂. Propanal + pentan-3-one come from CH₃CH₂CH=C(C₂H₅)₂, 3-ethylhex-3-ene. An alkene with 3 C–C σ, 8 C–H σ and one π that gives 2 mol of an aldehyde of 44 u (CH₃CHO) is but-2-ene.

Example (2023) · One O₃ per mole, giving ethanal and propanone: (CH₃)₂C=CHCH₃, C₅H₁₀, 70 g mol⁻¹.

Example (2023) · C₆H₈ takes 2 H₂ (two C=C, so one ring); ozonolysis gives 2 mol OHC–CH₂–CHO → cyclohexa-1,4-diene.

Example (2022) · 5.0 g pent-1-ene (70 g mol⁻¹) = 0.0714 mol, takes 0.0714 mol Br₂ = 11.43 g → 1143 × 10⁻² g.

Example (2021, 2022) · C₄H₈ that gives acetone plus CO₂ (effervescence) with KMnO₄/H⁺, and acetone on ozonolysis: 2-methylpropene.

3 · Alkynes

Alkynes: at least one C≡C, general formula CₙH₂ₙ₋₂. Ethyne (acetylene) is the first; the oxyacetylene flame is used in welding.

Structure	Common name	IUPAC name
HC≡CH (C₂H₂)	acetylene	ethyne
CH₃C≡CH (C₃H₄)	methylacetylene	propyne
CH₃CH₂C≡CH (C₄H₆)	ethylacetylene	but-1-yne
CH₃C≡CCH₃ (C₄H₆)	dimethylacetylene	but-2-yne

Isomers of C₅H₈ alkynes: pent-1-yne, pent-2-yne (position), 3-methylbut-1-yne (chain). C₆H₁₀ (the 5th member): hex-1-yne, hex-2-yne, hex-3-yne, 3-methylpent-1-yne, 4-methylpent-1-yne, 4-methylpent-2-yne, 3,3-dimethylbut-1-yne: 7 (NCERT Problem 9.13).

The triple bond: each C is sp; one C–C σ (sp–sp), two C–H σ (sp–1s), and two π bonds from two pairs of perpendicular 2p orbitals. H–C–C angle 180°, a linear molecule; the π cloud is cylindrical about the axis. C≡C: 823 kJ mol⁻¹, 120 pm, stronger and shorter than C=C and C–C (2024).

Preparation of alkynes

1. From calcium carbide (industrial):

2. From vicinal dihalides: alcoholic KOH removes one HX → an alkenyl (vinylic) halide; sodamide (NaNH₂) removes the second → alkyne. CH₃CH₂CHBrCH₂Br → but-1-yne (2019).

Properties of alkynes

Physical: first three gases, next eight liquids, higher ones solids. Colourless; ethyne has a characteristic odour, the others none. Weakly polar, lighter than water, immiscible with it, soluble in ether, CCl₄, benzene. m.p., b.p. and density rise with molar mass.

A. Acidic character. Na and NaNH₂ (strong bases) react with ethyne to give sodium ethynide (acetylide) and H₂; ethene and ethane don't. The C–H carbon is sp (50% s character), the most electronegative, so it holds the bond pair tightly and lets H go as H⁺. Only H on a triply bonded carbon is acidic.

• Acidity: HC≡CH > CH₂=CH₂ > CH₃–CH₃; HC≡CH > CH₃–C≡CH ≫ CH₃–C≡C–CH₃ (no acidic H; doesn't react with Na, 2021). These reactions tell terminal alkynes from alkenes and alkanes.
• 1 mol propyne with excess Na gives ½ mol H₂ (one acidic H) (2025).

Acetylides · Beyond NCERT

• Terminal alkynes give a white precipitate with ammoniacal AgNO₃ (Tollens') and a red precipitate with ammoniacal Cu₂Cl₂; internal alkynes don't.
• A sodium acetylide + a 1° alkyl halide → a longer alkyne: CH₃C≡C⁻Na⁺ + CH₃CH₂CH₂Br → CH₃C≡CCH₂CH₂CH₃ (2024). A 3° halide gives elimination instead.

B. Addition. Two molecules can add. Most additions are electrophilic; the intermediate is a vinylic cation, and unsymmetrical alkynes follow Markovnikov's rule.

• H₂: Pt/Pd/Ni → alkene → alkane (HC≡CH → CH₂=CH₂ → CH₃CH₃).
• Halogens: Br₂ in CCl₄ adds twice (→ 1,1,2,2-tetrabromo), and its reddish orange colour is discharged.
• Hydrogen halides: two HX add to give a gem dihalide (both X on one carbon): HC≡CH + HBr → CH₂=CHBr → CH₃CHBr₂ (1,1-dibromoethane).
• Water: alkynes don't react with water alone, but with HgSO₄ and dil. H₂SO₄ at 333 K one H₂O adds → enol → carbonyl compound. Ethyne → ethanal; propyne → propanone; every other alkyne gives a ketone.

Example (2019) · CH₃C≡CH + DCl (1 equiv), then DI: both add Markovnikov, so both halogens end on C2 → CH₃C(Cl)(I)CHD₂.

Example (2020) · O₂N–C₆H₄–C≡C–C₆H₄–OCH₃ + Hg²⁺/H⁺: the vinylic cation forms next to the methoxyphenyl ring (+R), so the C=O ends there: 1-(4-methoxyphenyl)-2-(4-nitrophenyl)ethan-1-one.

C. Polymerisation.

• Linear: ethyne → polyacetylene (polyethyne), –(CH=CH–CH=CH)ₙ–, which conducts electricity under special conditions; thin films serve as battery electrodes (lighter and cheaper than metals).
• Cyclic: ethyne through a red-hot iron tube at 873 K → benzene: the route from aliphatic to aromatic compounds. Propyne gives mesitylene (1,3,5-trimethylbenzene) (2021).

Example (NCERT Problem 9.14) · Ethanoic acid → benzene: CH₃COOH → CH₃COONa → (soda lime) CH₄ → (Cl₂, hν) CH₃Cl → (Na, dry ether) C₂H₆ → (Cl₂) C₂H₅Cl → (alc. KOH) CH₂=CH₂ → (Br₂) BrCH₂CH₂Br → (alc. KOH, then NaNH₂) HC≡CH → (red-hot Fe tube) benzene.

Example (2023, 2026) · But-2-yne + H₂: Lindlar (or Pd/C, one mole) gives cis-but-2-ene, Na/liq. NH₃ gives trans: stereoisomers, cis with the higher dipole moment. Na/NH₃ then cold dil. KMnO₄ gives butane-2,3-diol, 2 O atoms (2024).

4 · Aromatic hydrocarbons

Arenes, from Greek aroma (pleasant smell). Most contain a benzene ring, which is highly unsaturated yet keeps its unsaturation in most reactions. Benzenoid (benzene ring) and non-benzenoid (another unsaturated ring). Examples: benzene, toluene, naphthalene, biphenyl.

Isomerism: all six H's of benzene are equivalent, so there is one monosubstituted product. Two substituents give three position isomers: ortho (1,2 or 1,6), meta (1,3 or 1,5), para (1,4). The xylenes: 1,2-, 1,3- and 1,4-dimethylbenzene.

Structure of benzene

• Faraday isolated it (1825). C₆H₆ looks highly unsaturated, yet benzene is stable, forms a triozonide (three double bonds) and gives only one monosubstituted product (all six C and H equivalent).
• Kekulé (1865): a ring of six carbons with alternate single and double bonds. It predicts two 1,2-dibromobenzenes, but only one exists; Kekulé suggested the double bonds oscillate. It still couldn't explain the unusual stability and the preference for substitution over addition.
• Resonance: benzene is a hybrid of the two Kekulé structures, drawn as a hexagon with a circle (six delocalised electrons).
• Orbitals: each C is sp²; six C–C σ bonds and six C–H σ bonds lie in one plane; each C keeps one p orbital perpendicular to the ring. They overlap equally all round, giving two doughnut-shaped π clouds above and below the ring. The delocalised cloud is held more strongly than localised π bonds, so benzene is more stable than hypothetical cyclohexatriene.
• X-ray: planar; all six C–C bonds 139 pm, between C–C 154 pm and C=C 133 pm. No pure double bond, so benzene is reluctant to add.

Aromaticity (Hückel rule): (i) planarity, (ii) complete delocalisation of the π electrons in the ring, (iii) (4n + 2) π electrons, n = 0, 1, 2… Details and the antiaromatic and non-aromatic cases are in General Organic Chemistry.

Preparation of benzene

Commercially from coal tar. In the laboratory:

1. Cyclic polymerisation of ethyne (red-hot iron tube, 873 K).
2. Decarboxylation: sodium benzoate + soda lime → benzene + Na₂CO₃.
3. Reduction of phenol: phenol vapour over heated zinc dust → benzene + ZnO.

Properties of arenes

Physical: non-polar; colourless liquids or solids with a characteristic smell (naphthalene balls, a moth repellent); immiscible with water, miscible with organic solvents; burn with a sooty flame.

Electrophilic substitution is their characteristic reaction:

Reaction	Reagent → product	Electrophile
Nitration	conc. HNO₃ + conc. H₂SO₄ (nitrating mixture), heat → nitrobenzene	NO₂⁺
Halogenation	Cl₂ or Br₂ + anhydrous FeCl₃, FeBr₃ or AlCl₃ → chlorobenzene	Cl⁺
Sulphonation	fuming H₂SO₄ (oleum), heat → benzenesulphonic acid	SO₃
Friedel–Crafts alkylation	R–X + anhydrous AlCl₃ → alkylbenzene	R⁺
Friedel–Crafts acylation	RCOCl or (RCO)₂O + anhydrous AlCl₃ → acylbenzene (ketone)	RC⁺=O (acylium)

• Excess Cl₂ with AlCl₃ goes all the way to hexachlorobenzene, C₆Cl₆.
• Benzene + 1-chloropropane + AlCl₃ gives isopropylbenzene, not n-propylbenzene: the 1° cation rearranges to the 2° one.

Mechanism (S_E, three steps):

1. Generate the electrophile. AlCl₃, a Lewis acid, takes Cl⁻ from Cl₂, RCl or RCOCl to free Cl⁺, R⁺ or RCO⁺. For nitration, H₂SO₄ protonates HNO₃, which loses water to give NO₂⁺: here sulphuric acid acts as the acid and nitric acid as the base.
2. The electrophile attacks the ring: a carbocation, the arenium ion (σ-complex), with one sp³ carbon. It is resonance-stabilised but has lost its aromaticity (delocalisation stops at the sp³ carbon).
3. Loss of H⁺ from the sp³ carbon, taken by [AlCl₄]⁻ or [HSO₄]⁻, restores the aromatic ring.

Addition (only under vigorous conditions):

• H₂ with Ni at high temperature and pressure → cyclohexane.
• Cl₂ under UV light: three molecules add → benzene hexachloride (BHC), C₆H₆Cl₆, gammaxane. It keeps all six H (2022).

Combustion: sooty flame.

Example (2026) · 80 mL of a hydrocarbon with 264 mL O₂; after cooling, 224 mL of gas remain. Ethyne fits: C₂H₂ + 5/2 O₂ → 2CO₂ + H₂O(l). 80 mL uses 200 mL O₂ and gives 160 mL CO₂; with 64 mL O₂ left, 160 + 64 = 224 mL. So C₂H₂.

Directive influence of substituents

A group already on the ring decides where the next one goes; the entering group doesn't matter.

• Ortho–para directors, activating: –OH (its lone pair raises the electron density at o and p by resonance; the −I effect lowers it slightly, but resonance wins), –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅.
• Halogens: o/p-directing but deactivating. Strong −I lowers the electron density of the whole ring, so substitution is harder; but resonance (+R) leaves o and p richer than m.
• Meta directors, deactivating: –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H. Nitro lowers the density by −I and −R, most at o and p, so the electrophile goes to the relatively richer meta position.

Example (NCERT exercise 9.13) · Benzene → p-nitrobromobenzene: brominate first (Br is o/p-directing), then nitrate. → m-nitrochlorobenzene: nitrate first (NO₂ is meta-directing), then chlorinate. → acetophenone: CH₃COCl, anhydrous AlCl₃.

5 · Carcinogenicity and toxicity

Benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and carcinogenic. They form on incomplete combustion of tobacco, coal and petroleum, enter the body, and through biochemical reactions damage DNA and cause cancer. Examples: 1,2-benzanthracene, 3-methylcholanthrene, 1,2-benzpyrene (benzo[a]pyrene), 1,2,5,6-dibenzanthracene.

6 · Traps

Trap	What goes wrong	Do this
Peroxide effect	Using it with HCl or HI	Only HBr
Markovnikov reasoning	Placing Br by counting H's alone	Draw both cations; check for a 1,2-shift
Hydration of alkenes	Assuming no rearrangement	H₃O⁺ rearranges; oxymercuration and hydroboration don't
Lindlar vs Na/NH₃	Swapping cis and trans	Lindlar cis; Na/liq. NH₃ trans
Alkyne + H₂O	Ethyne → acetone	Ethyne → ethanal; others → ketones
Kolbe's electrolysis	Making methane	Only even-carbon alkanes (ethane up)
Wurtz with two halides	Expecting one product	Three alkanes
Decarboxylation	Keeping the carbon count	One carbon fewer
Branching	Lower b.p. means lower m.p.	Neopentane: lowest b.p. of the pentanes, highest m.p.
KMnO₄ strength	Cold dilute vs hot acidic	Cold dil.: diol; hot or acidic: cleavage
Ozonolysis work-up	Ignoring the second reagent	Zn/H₂O: aldehydes; H₂O or H₂O₂: acids
Acidic H	Counting all H of propyne	Only ≡C–H; but-2-yne has none
Bond strengths	"C≡C is weaker"	C≡C 823 > C=C 681 > C–C 348
Friedel–Crafts with 1-chloropropane	n-propylbenzene	Isopropylbenzene (rearrangement)
BHC	Counting H lost	Addition: C₆H₆Cl₆ keeps 6 H

7 · Numbers worth knowing

• Bond enthalpies (kJ mol⁻¹): C–C 348, C=C 681 (σ 397 + π 284), C≡C 823; H–Cl 430.5, H–Br 363.7, H–I 296.8.
• Bond lengths (pm): C–C 154, C=C 134 (Unit 9 also gives 133), C≡C 120, benzene 139; C–H 112.
• ΔcH: CH₄ −890 kJ mol⁻¹; C₄H₁₀ −2875.84 kJ mol⁻¹.
• Ethane rotation barrier 12.5 kJ mol⁻¹; cis-but-2-ene 0.33 D.
• Pentanes, b.p. (K): 309.1, 300.9, 282.5; m.p. pentane 143.3, neopentane 256.4.
• Conditions: halogenation 573–773 K; aromatization 773 K, 10–20 atm; steam reforming 1273 K; cracking dodecane 973 K; alkyne hydration 333 K; ethyne → benzene 873 K.
• Isomer counts: C₆H₁₄ 5, C₇H₁₆ 9, C₁₀H₂₂ 75; C₆H₁₀ alkynes 7.

8 · For the boards only

JEE Main doesn't ask this, but the CBSE paper can.

• LPG is used as a domestic fuel with the least pollution; kerosene pollutes a little; petrol and CNG vehicles pollute less. Higher hydrocarbons are solvents for paints and starting materials for dyes and drugs.
• Models: toothpicks and plasticine balls for alkanes; spring models for alkenes, alkynes and arenes. Two cardboards joined by two nails show restricted rotation.
• Markovnikov framed his rule in 1869; Kharash and Mayo saw the peroxide effect in 1933 at the University of Chicago.
• Kekulé (1829–1896) described dreaming of a snake seizing its own tail; polybenzenoid "kekulenes" were named at his centenary.
• Excessive use of polythene and polypropene is an environmental concern.