Hydrocarbons: NEET notes

1. Classification and the alkane family

NCERT §9.1, §9.2

• A hydrocarbon contains only two elements, carbon and hydrogen. LPG, CNG and LNG, and petrol, diesel and kerosene (from fractional distillation of petroleum), are all mixtures of hydrocarbons used as fuels.
• Hydrocarbons also feed industry: polythene, polypropene and polystyrene are made from them, higher members serve as paint solvents, and many dyes and drugs start from them.
• By the kind of carbon–carbon bond present they fall into three groups: saturated (only single bonds; open-chain alkanes and ring-shaped cycloalkanes), unsaturated (at least one double or triple bond) and aromatic (a special class of ring compounds).
• Alkanes are saturated open-chain hydrocarbons with general formula CₙH₂ₙ₊₂. Methane, CH₄, heads the series; it occurs in coal mines and marshy places.
• Replacing one H of methane by –CH₃ gives ethane, C₂H₆; repeating the step gives C₃H₈, C₄H₁₀ and so on, each member differing by CH₂.
• The old name paraffins (Latin parum, little; affinis, affinity) reflects how unreactive alkanes are towards acids, bases and most reagents under ordinary conditions.
• Methane is tetrahedral by VSEPR theory, with every H–C–H angle 109.5°. In alkanes the C–C bond is 154 pm and the C–H bond 112 pm; both are σ bonds from sp³ orbitals of carbon (with 1s of H for C–H).

2. Naming alkanes and chain isomers

NCERT §9.2.1

• Methane, ethane and propane each have a single structure. From butane onwards the carbons can be joined in more than one way.
• C₄H₁₀ exists as butane (n-butane, b.p. 273 K) and 2-methylpropane (isobutane, b.p. 261 K). C₅H₁₂ exists as pentane (309 K), 2-methylbutane (isopentane, 301 K) and 2,2-dimethylpropane (neopentane, 282.5 K).
• Isomers that differ only in the carbon skeleton, straight versus branched, are called chain isomers, a kind of structural isomerism. C₆H₁₄ has five, C₇H₁₆ nine, and C₁₀H₂₂ as many as 75.
• The five hexanes are n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.
• A carbon bonded to one other carbon (or none, as in methane) is primary (1°); to two, secondary (2°); to three, tertiary (3°); to four, quaternary or neo (4°). End carbons of a chain are always primary.
• An alkyl group (general formula CₙH₂ₙ₊₁) is an alkane minus one hydrogen, such as –CH₃ or –C₂H₅.
• IUPAC rules: choose the longest chain, number it so substituents get the lowest locants, and list substituents alphabetically. 'sec' and 'tert' are ignored when alphabetising, but isopropyl is treated as a single word.
• To draw a structure from a name, write the parent chain, number it, attach the substituents, then add hydrogens until each carbon has four bonds. For example, '2-ethylpentane' is really 3-methylhexane, because its longest chain has six carbons.

3. Preparation of alkanes

NCERT §9.2.2

• Petroleum and natural gas are the main natural sources of alkanes; the laboratory methods below are used when a particular alkane is wanted.
• Hydrogenation: H₂ adds to alkenes and alkynes over finely divided Pt, Pd or Ni, which adsorb H₂ and weaken the H–H bond. Pt and Pd work at room temperature; Ni needs higher temperature and pressure. Propyne takes up 2H₂ to give propane.
• Reduction of alkyl halides (not fluorides) with zinc and dilute HCl replaces the halogen by hydrogen: CH₃Cl gives CH₄ and C₂H₅Cl gives C₂H₆.
• Wurtz reaction: an alkyl halide with sodium in dry ether joins two alkyl groups, 2CH₃Br + 2Na → C₂H₆ + 2NaBr. It suits higher alkanes with an even number of carbons; two different halides give a mixture.
• Decarboxylation: heating the sodium salt of a carboxylic acid with soda lime (NaOH + CaO) removes CO₂ and gives an alkane with one carbon fewer. Sodium ethanoate gives methane; sodium butanoate gives propane.
• Kolbe's electrolytic method: electrolysis of an aqueous sodium or potassium carboxylate gives an alkane at the anode, where two alkyl free radicals join; H₂ forms at the cathode. Sodium acetate gives ethane.
• Kolbe's method always doubles the alkyl group, so it gives even-carbon alkanes only; methane cannot be made this way.

4. Physical properties of alkanes

NCERT §9.2.3

• C–C and C–H bonds are covalent and carbon and hydrogen differ little in electronegativity, so alkanes are nearly non-polar and are held together only by weak van der Waals forces.
• At 298 K, C₁ to C₄ are gases, C₅ to C₁₇ are liquids and C₁₈ onwards are solids. Alkanes are colourless and odourless.
• Being non-polar they dissolve in non-polar solvents, not in water: 'like dissolves like'. This is why petrol removes grease (a mixture of higher alkanes) in dry cleaning.
• Boiling point rises steadily with molecular mass because a larger molecule has more surface area and stronger van der Waals attraction: methane 111.0 K, ethane 184.4 K, propane 230.9 K, hexane 341.9 K, decane 447.1 K.
• Among isomers, branching lowers the boiling point: pentane 309.1 K, 2-methylbutane 300.9 K, 2,2-dimethylpropane 282.5 K. More branching makes the molecule more nearly spherical, so it touches its neighbours over a smaller area.
• Melting points do not follow the same simple pattern: 2,2-dimethylpropane melts at 256.4 K, far above pentane at 143.3 K.

5. Chemical reactions of alkanes

NCERT §9.2.3

• Halogenation replaces H by halogen at 573–773 K or in diffused sunlight or UV light. Methane with chlorine gives CH₃Cl, CH₂Cl₂, CHCl₃ and finally CCl₄, releasing HCl at each step. Lower alkanes do not undergo nitration or sulphonation.
• Halogen reactivity is F₂ > Cl₂ > Br₂ > I₂, and hydrogens are replaced in the order 3° > 2° > 1°. Fluorination is too violent to control; iodination is slow and reversible, so it is done with an oxidant such as HIO₃ or HNO₃ that destroys the HI formed.
• Halogenation runs by a free-radical chain. Initiation: light or heat splits Cl–Cl (the weakest bond present) homolytically. Propagation: Cl• takes H from CH₄ giving •CH₃ and HCl; •CH₃ takes Cl from Cl₂ giving CH₃Cl and a fresh Cl•. Termination: any two radicals combine.
• Two methyl radicals joining in a termination step explain why some ethane appears when methane is chlorinated.
• Complete combustion gives CO₂ and water with much heat: for CH₄, ΔcH⁻ = −890 kJ mol⁻¹; for C₄H₁₀, −2875.84 kJ mol⁻¹. Too little air gives carbon black, used for inks and black pigments.
• Controlled oxidation: 2CH₄ + O₂ → 2CH₃OH (Cu, 523 K, 100 atm); CH₄ + O₂ → HCHO + H₂O (Mo₂O₃, heat); ethane gives ethanoic acid with manganese acetate. KMnO₄ turns the tertiary H of (CH₃)₃CH into an OH, giving 2-methylpropan-2-ol.
• Isomerisation: n-alkanes heated with anhydrous AlCl₃ and HCl gas become branched; n-hexane gives mainly 2-methylpentane and 3-methylpentane.
• Aromatization (reforming): n-alkanes with six or more carbons, at 773 K and 10–20 atm over oxides of V, Mo or Cr on alumina, lose hydrogen and close into rings; hexane gives benzene and heptane gives toluene.
• Methane and steam over nickel at 1273 K give CO + 3H₂, an industrial source of hydrogen.
• Pyrolysis (cracking) breaks higher alkanes into smaller alkanes and alkenes by heat, through free radicals. Dodecane at 973 K over Pt, Pd or Ni gives heptane, pentene and other products; oil gas is made this way.

6. Conformations of ethane

NCERT §9.2.4

• Electron density in a C–C σ bond is symmetrical about the bond axis, so turning one carbon relative to the other does not break the bond. The resulting interconvertible arrangements are called conformations, conformers or rotamers.
• Rotation is not perfectly free. A small barrier of 1–20 kJ mol⁻¹, due to repulsion between bonds on neighbouring carbons, opposes it; this repulsion is called torsional strain.
• In ethane, the staggered form keeps the H atoms on the two carbons as far apart as possible, the eclipsed form puts them as close as possible, and anything in between is a skew form. Bond lengths and angles are the same in all of them.
• The angle of rotation about the C–C bond is the dihedral (torsional) angle. Torsional strain is least in the staggered form and greatest in the eclipsed form, so staggered is the more stable, preferred conformation.
• The energy gap between the eclipsed and staggered forms of ethane is about 12.5 kJ mol⁻¹. Collisions at ordinary temperature supply this easily, so rotation is almost free in practice and the conformers of ethane cannot be separated.
• Sawhorse projection: the C–C bond is drawn as a longer slanted line, front carbon at the lower end and rear carbon at the upper end, each carrying three bonds 120° apart.
• Newman projection: looking straight down the C–C bond, the front carbon is a point with three bonds at 120°, and the rear carbon is a circle with three shorter bonds at 120°.

7. Alkenes: double bond, names and isomers

NCERT §9.3.1–§9.3.3

• Alkenes contain at least one C=C and have general formula CₙH₂ₙ. They are also called olefins (oil-forming), because ethene gives an oily liquid with chlorine.
• The double bond is one σ bond (about 397 kJ mol⁻¹) from head-on overlap of sp² orbitals and one weaker π bond (about 284 kJ mol⁻¹) from sideways overlap of 2p orbitals. The whole C=C (681 kJ mol⁻¹) is stronger than the C–C of ethane (348 kJ mol⁻¹) and shorter (134 pm against 154 pm).
• The loosely held π electrons make alkenes targets for electrophiles (electron-seeking reagents), so alkenes are less stable than alkanes and readily add reagents across the double bond.
• IUPAC naming: take the longest chain containing the double bond, number from the end nearer to it, and replace -ane by -ene. Ethene (common name ethylene) is the first stable member; CH₂ (methene) is too short-lived.
• Structural isomers of C₄H₈: but-1-ene and but-2-ene are position isomers; each is a chain isomer of 2-methylprop-1-ene. C₅H₁₀ has five alkene structural isomers.
• Rotation about C=C is restricted, so when each doubly bonded carbon carries two different groups, two geometrical isomers exist: cis (identical groups on the same side) and trans (on opposite sides). Types XYC=CXY, XYC=CXZ and XYC=CZW all show it.
• cis and trans forms differ in melting point, boiling point, dipole moment and solubility. cis-But-2-ene has μ = 0.33 D; in trans-but-2-ene the two C–CH₃ dipoles cancel, so μ is zero. Among solids, the trans isomer usually melts higher.
• A compound with two identical groups on one doubly bonded carbon, such as CH₂=CBr₂ or (CH₃)₂C=CH–C₂H₅, shows no cis-trans isomerism.

8. Preparation of alkenes

NCERT §9.3.4

• Partial reduction of alkynes: H₂ over Lindlar's catalyst (palladised charcoal partly poisoned with sulphur compounds or quinoline) gives cis alkenes; sodium in liquid ammonia gives trans alkenes.
• Ethyne and propyne add one H₂ over Pd/C to give ethene and propene. Propene shows no geometrical isomerism because one of its doubly bonded carbons carries two H atoms.
• Dehydrohalogenation: an alkyl halide heated with alcoholic KOH loses HX to give an alkene. H leaves from the β carbon (next to the carbon bearing X), so this is a β-elimination.
• The rate of dehydrohalogenation depends on the halogen (iodine > bromine > chlorine) and the alkyl group (tertiary > secondary > primary).
• Dehalogenation: a vicinal dihalide (halogens on adjacent carbons) with zinc loses ZnX₂: CH₂Br–CH₂Br + Zn → CH₂=CH₂ + ZnBr₂.
• Acidic dehydration: an alcohol heated with concentrated H₂SO₄ loses water to give an alkene; the –OH removes an H from the β carbon, so this too is a β-elimination.

9. Properties and reactions of alkenes

NCERT §9.3.5

• Alkenes C₂ to C₄ are gases, then fourteen members are liquids, and higher ones are solid. Ethene has a faintly sweet smell; other alkenes have none. None of them dissolves in water, but all dissolve fairly well in non-polar solvents such as benzene and petroleum ether. Each added CH₂ raises the boiling point by 20–30 K.
• Bromine or chlorine adds to give a vicinal dihalide (through a cyclic halonium ion); iodine does not add under normal conditions. Loss of the reddish orange colour of bromine in CCl₄ is a test for unsaturation.
• Hydrogen halides add to give alkyl halides, with reactivity HI > HBr > HCl. A symmetrical alkene gives one product: ethene + HBr gives bromoethane.
• Markovnikov rule (1869): when an unsymmetrical reagent adds to an unsymmetrical alkene, its negative part joins the double-bond carbon that carries fewer hydrogens. Propene + HBr gives mainly 2-bromopropane.
• Mechanism: H⁺ attacks first, forming the more stable carbocation (secondary rather than primary), which forms faster; Br⁻ then bonds to that positive carbon.
• Peroxide (Kharash) effect: with benzoyl peroxide, HBr adds to propene against Markovnikov's rule, giving 1-bromopropane, by a free-radical chain through the more stable secondary radical. Hex-1-ene gives 2-bromohexane without peroxide and 1-bromohexane with it.
• Only HBr shows the peroxide effect. H–Cl (430.5 kJ mol⁻¹) is too strong for the radical to break, and I• radicals join to form I₂ rather than add, even though H–I (296.8 kJ mol⁻¹) is weaker than H–Br (363.7 kJ mol⁻¹).
• Cold concentrated H₂SO₄ adds by Markovnikov's rule to give alkyl hydrogen sulphates; water with a few drops of concentrated H₂SO₄ adds by the same rule to give alcohols.
• Cold dilute aqueous KMnO₄ (Baeyer's reagent) turns alkenes into vicinal glycols, and its loss of colour is a test for unsaturation. Acidic KMnO₄ or K₂Cr₂O₇ cleaves them to ketones and/or acids; but-2-ene gives two molecules of ethanoic acid.
• Ozonolysis adds O₃ to form an ozonide, which Zn–H₂O splits into smaller carbonyl compounds; the fragments reveal where the double bond was. Polymerisation at high temperature and pressure with a catalyst turns ethene into polythene and propene into polypropene.

10. Alkynes: triple bond, preparation and reactions

NCERT §9.4

• Alkynes contain at least one C≡C and have general formula CₙH₂ₙ₋₂. Ethyne (acetylene) is the first stable member; burnt with oxygen as the oxyacetylene flame, it is used for welding.
• IUPAC names replace -ane by -yne, and the locant is the first triply bonded carbon. But-1-yne and but-2-yne are position isomers. C₅H₈ has three alkynes (pent-1-yne, pent-2-yne, 3-methylbut-1-yne) and C₆H₁₀ has seven.
• Each carbon of ethyne is sp hybridised: it forms one C–C σ bond and two C–H σ bonds, while its two unhybridised p orbitals, at right angles to each other, overlap sideways with those of the partner carbon to make two π bonds. H–C–C is 180°, so ethyne is linear, and the π cloud is cylindrically symmetrical about the axis.
• C≡C (823 kJ mol⁻¹, 120 pm) is stronger and shorter than C=C (681 kJ mol⁻¹, 133 pm) and C–C (348 kJ mol⁻¹, 154 pm).
• Industrial ethyne: CaCO₃ → CaO + CO₂; CaO + 3C → CaC₂ + CO; CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. Alternatively, a vicinal dihalide treated with alcoholic KOH gives an alkenyl halide, which sodamide converts to the alkyne.
• The first three alkynes are gases, the next eight liquids, the rest solids. They are colourless, weakly polar, lighter than water and immiscible with it; ethyne has a characteristic odour.
• Hydrogen on a triply bonded carbon is acidic. Na or NaNH₂ gives sodium ethynide and H₂; alkenes and alkanes do not react, so this distinguishes them. The sp carbon, with 50% s character, is the most electronegative and holds the C–H electrons tightly. Acidity order: HC≡CH > H₂C=CH₂ > CH₃–CH₃, and HC≡CH > CH₃–C≡CH >> CH₃–C≡C–CH₃.
• Alkynes add two molecules of H₂, halogen or HX. HX addition follows Markovnikov's rule through a vinylic cation and ends in a gem dihalide: ethyne + 2HBr gives 1,1-dibromoethane. Bromine in CCl₄ is decolourised here too.
• With HgSO₄ and dilute H₂SO₄ at 333 K, one molecule of water adds to give a carbonyl compound: ethyne gives ethanal and propyne gives propanone.
• Linear polymerisation gives polyacetylene, which conducts electricity under special conditions and can serve as a light battery electrode. Passing ethyne through a red-hot iron tube at 873 K joins three molecules into benzene.

11. Benzene: structure and aromaticity

NCERT §9.5.1–§9.5.4

• Aromatic hydrocarbons (arenes) are named for the pleasant smell of many of them. Those containing a benzene ring are benzenoid; aromatic rings without one are non-benzenoid.
• All six H atoms of benzene are equivalent, so it has only one monosubstituted derivative, but three disubstituted ones: ortho (1,2 or 1,6), meta (1,3 or 1,5) and para (1,4). The dimethylbenzenes are o-, m- and p-xylene.
• Faraday isolated benzene in 1825. Its formula C₆H₆ suggested heavy unsaturation, and its triozonide pointed to three double bonds, yet it proved unusually stable.
• Kekulé (1865) proposed a six-carbon ring with alternating single and double bonds. His model predicted two different 1,2-dibromobenzenes, but only one exists, so he suggested the double bonds oscillate between two positions.
• Resonance explains it: benzene is a hybrid of the two Kekulé structures, drawn as a hexagon with a circle for six delocalised π electrons.
• Every carbon is sp²: sp²–sp² overlap makes six C–C σ bonds and sp²–1s overlap six C–H σ bonds, all in one plane. Each carbon's leftover p orbital overlaps equally with both neighbours, giving π clouds above and below the ring.
• X-ray diffraction shows a planar molecule with all six C–C bonds 139 pm, between C–C (154 pm) and C=C (133 pm). With no true double bond, benzene resists addition, and delocalisation makes it more stable than hypothetical cyclohexatriene.
• Aromaticity needs (i) a planar ring, (ii) complete delocalisation of π electrons and (iii) (4n + 2) π electrons, n = 0, 1, 2 … (Hückel rule). Benzene has 6 π electrons (n = 1).
• Benzene is obtained commercially from coal tar. In the laboratory it comes from cyclic polymerisation of ethyne, from sodium benzoate heated with soda lime, or from phenol vapour passed over heated zinc dust.

12. Electrophilic substitution and directive influence

NCERT §9.5.5, §9.5.6, §9.6

• Arenes are non-polar, colourless liquids or solids with a typical smell. They do not mix with water but mix readily with organic solvents, and they burn with a sooty flame. Naphthalene balls repel moths.
• Their typical reactions are electrophilic substitutions. Nitration uses conc. HNO₃ + conc. H₂SO₄; halogenation uses a Lewis acid (anhydrous FeCl₃, FeBr₃ or AlCl₃); sulphonation uses oleum; Friedel-Crafts alkylation uses R–X with anhydrous AlCl₃; Friedel-Crafts acylation uses an acyl halide or anhydride with AlCl₃. Excess Cl₂ with AlCl₃ goes all the way to C₆Cl₆.
• The mechanism has three steps: the electrophile is generated; it attacks the ring to form an arenium ion (σ-complex) with one sp³ carbon, which is resonance-stabilised but no longer aromatic; then a proton is lost, removed by [AlCl₄]⁻ or [HSO₄]⁻, and aromaticity returns.
• In nitration the electrophile is NO₂⁺ (nitronium ion), formed when H₂SO₄ gives a proton to HNO₃; here sulphuric acid acts as the acid and nitric acid as the base. AlCl₃ generates Cl⁺, R⁺ or RC⁺O (acylium ion).
• Under severe conditions benzene adds too: H₂ over Ni at high temperature and pressure gives cyclohexane, and three Cl₂ under UV light give benzene hexachloride, C₆H₆Cl₆ (gammaxane). Combustion: C₆H₆ + 15/2 O₂ → 6CO₂ + 3H₂O.
• In a monosubstituted benzene the group already present, not the incoming one, decides where the next group goes. This is the directive influence.
• Ortho and para directors that activate the ring: –OH, –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅. In phenol, resonance raises electron density at o- and p-positions more than the –I effect lowers it.
• Halogens are o- and p-directing but deactivating: their strong –I effect lowers electron density on the whole ring, while resonance keeps o- and p-positions richer than meta.
• Meta directors deactivate the ring: –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H. In nitrobenzene, resonance and the –I effect pull electron density mostly from o- and p-positions, so the meta position is attacked.
• Benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and carcinogenic. They form when tobacco, coal or petroleum burns incompletely and, inside the body, can damage DNA.

Must-know facts

1. General formulas: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂.
2. Chain isomer counts: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9, C₁₀H₂₂ 75.
3. Bond lengths: C–C 154 pm, C=C 134 pm (133 pm in the alkyne and benzene sections), C≡C 120 pm, benzene C–C 139 pm, alkane C–H 112 pm.
4. Bond enthalpies: C–C 348, C=C 681, C≡C 823 kJ mol⁻¹; in C=C the σ part is about 397 and the π part about 284 kJ mol⁻¹.
5. Wurtz reaction (Na, dry ether) and Kolbe electrolysis both give even-carbon alkanes; Kolbe cannot make methane.
6. Decarboxylation with soda lime gives an alkane with one carbon fewer than the acid.
7. Branching lowers boiling point: pentane 309.1 K > 2-methylbutane 300.9 K > 2,2-dimethylpropane 282.5 K.
8. Halogenation of alkanes: F₂ > Cl₂ > Br₂ > I₂; hydrogens replaced 3° > 2° > 1°; free-radical chain of initiation, propagation, termination.
9. Ethane appears during chlorination of methane because two •CH₃ radicals combine.
10. Ethane: staggered is more stable than eclipsed by about 12.5 kJ mol⁻¹; general barrier 1–20 kJ mol⁻¹.
11. cis-But-2-ene μ = 0.33 D; trans-but-2-ene μ = 0. Solid trans isomers usually melt higher.
12. Lindlar's catalyst gives cis alkenes; Na in liquid NH₃ gives trans alkenes.
13. Dehydrohalogenation rate: I > Br > Cl and 3° > 2° > 1°.
14. HX addition reactivity: HI > HBr > HCl. Markovnikov rule goes through the more stable carbocation.
15. Peroxide effect works with HBr only; propene gives 1-bromopropane.
16. Tests for unsaturation: decolourisation of Br₂ in CCl₄ and of Baeyer's reagent (cold, dilute, aqueous KMnO₄).
17. Alkyne hydration (HgSO₄, dil. H₂SO₄, 333 K): ethyne gives ethanal, propyne gives propanone.
18. Acidity: HC≡CH > H₂C=CH₂ > CH₃–CH₃, because sp carbon has 50% s character.
19. Three ethyne molecules give benzene in a red-hot iron tube at 873 K.
20. Aromatic: planar, fully delocalised, (4n + 2) π electrons.
21. Nitronium ion NO₂⁺ is the electrophile in nitration; H₂SO₄ is the acid and HNO₃ the base.
22. –OH, –NH₂, –OCH₃, –CH₃ are o/p and activating; halogens are o/p but deactivating; –NO₂, –CN, –CHO, –COOH, –SO₃H are meta and deactivating.
23. Benzene hexachloride (gammaxane) C₆H₆Cl₆ forms by addition of 3Cl₂ under UV light, not by substitution.

Common traps

Formulas

Key terms

Hydrocarbon

A compound made of carbon and hydrogen only.

Chain isomers

Isomers that differ only in the carbon skeleton, straight or branched.

Wurtz reaction

Coupling of two alkyl halide molecules by sodium in dry ether to form a higher alkane.

Decarboxylation

Loss of CO₂ from a carboxylate, as when its sodium salt is heated with soda lime.

Free radical

A species with an unpaired electron, formed by homolytic bond breaking.

Conformation

An arrangement reached by rotation about a single bond, without breaking any bond.

Torsional strain

Repulsion between bonds on adjacent carbons that makes eclipsed forms less stable.

Geometrical isomers

cis and trans forms that exist because rotation about C=C is restricted.

β-Elimination

Loss of a small molecule in which H leaves from the carbon next to the one bearing the leaving group.

Markovnikov rule

The negative part of an unsymmetrical reagent joins the double-bond carbon with fewer hydrogens.

Peroxide effect

Anti-Markovnikov addition of HBr in the presence of a peroxide, by a free-radical route.

Ozonolysis

Cleavage of a double bond by ozone and then Zn–H₂O into carbonyl compounds.

Aromaticity

Special stability of a planar, fully delocalised ring with 4n + 2 π electrons.

Arenium ion

The carbocation (σ-complex) formed when an electrophile attacks a benzene ring.

Directive influence

Control by the group already on a benzene ring over where a new group enters.

Carcinogen

A substance able to cause cancer.