Organic Chemistry-1 BCH1T02

Syllabus topics (official RTMNU)

(A) Structure and Bonding

• Hybridization in case of Methane, Ethane, Ethylene and acetylene, Bond lengths, bond angles and bond energies.
• Inductive effect, Electromeric effect.
• Resonance effect.
• Hyperconjugation definition, examples and application of these effects.
• Hydrogen bonding in organic compounds (with reference to alcohol, phenols, amines, acids) and consequences.

(B) Mechanism of Organic Reactions

• Homolytic and heterolytic bond breaking examples and factors favouring the bond fission.
• Electrophiles and nucleophiles definition and example both neutral and charged.
• Types of organic reactions addition, substitution, elimination, rearrangement.
• Energy consideration.
• Reactive intermediates carbocations, carbanions, free radicals, carbenes, formation, geometry, stability and reactions given by these intermediates.

(A) Structure and bonding

Hybridisation in methane, ethane, ethylene and acetylene

• In ethylene the unhybridised 2p orbitals of the two C atoms overlap sideways to form one π bond above and below the molecular plane.
• In acetylene two unhybridised p orbitals (pᵧ, p_z) on each C give two π bonds at right angles; the electron cloud is cylindrical.
• More s character → shorter, stronger bond and more electronegative carbon (sp > sp² > sp³). This is why acetylene is acidic.

Electronic effects

Applications

• Acid strength: ClCH₂COOH > CH₃COOH (−I of Cl stabilises carboxylate); HCOOH > CH₃COOH (+I of CH₃ destabilises).
• Basicity of amines: in gas phase (CH₃)₂NH > CH₃NH₂ > NH₃ due to +I; aniline is weaker than NH₃ because the lone pair is delocalised into the ring (+R).
• Stability of carbocations and alkenes by hyperconjugation: 3° > 2° > 1° > CH₃⁺; more substituted alkenes are more stable (more α-H). Number of hyperconjugative structures = number of α-H atoms.
• Resonance explains the equal C–O bond lengths of carboxylate ion and the acidity of phenol.

Hydrogen bonding in organic compounds

• Alcohols: intermolecular H-bonds (R–O–H····O(H)R), so b.p. is much higher than ethers of same mass (ethanol 78 °C vs dimethyl ether −24 °C); lower alcohols are miscible with water.
• Phenols: high b.p.; o-nitrophenol forms intramolecular H-bond, so it is steam-volatile and less soluble than p-nitrophenol.
• Amines: 1° and 2° amines form N–H····N bonds (weaker than O–H····O, since N is less electronegative); 3° amines cannot (no N–H), so their b.p. is lower.
• Carboxylic acids: form cyclic dimers by two H-bonds; so b.p. is higher than alcohols of similar mass, and molar mass by cryoscopy in benzene comes out double.

(B) Mechanism of organic reactions

Homolytic and heterolytic fission

• Homolytic: each atom takes one electron of the bond → free radicals. A–B → A• + B•. Favoured by non-polar bonds, gas phase or non-polar solvents, high temperature, light (hν) or peroxides. Example: Cl₂ → 2Cl• (in light).
• Heterolytic: one atom takes both electrons → ions. A–B → A⁺ + :B⁻. Favoured by polar bonds, polar solvents, and difference in electronegativity. Example: (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻.

Electrophiles and nucleophiles

Types of organic reactions

• Addition: two molecules combine; π bond broken. CH₂=CH₂ + Br₂ → BrCH₂–CH₂Br.
• Substitution: one atom/group replaced by another. CH₃Br + OH⁻ → CH₃OH + Br⁻.
• Elimination: atoms removed from adjacent carbons to form a multiple bond. CH₃CH₂Br + alc. KOH → CH₂=CH₂ + KBr + H₂O.
• Rearrangement: atoms in a molecule reorganise. NH₄CNO → (heat) NH₂CONH₂; pinacol → pinacolone.

Energy consideration: a reaction proceeds through a transition state of higher energy. Energy of activation (Ea) = energy of transition state − energy of reactants. ΔH = energy of products − energy of reactants (negative for exothermic). A reactive intermediate sits in a valley between two transition states on the energy profile.

Reactive intermediates

Likely exam questions

Long answer

1. Explain inductive effect, electromeric effect, resonance and hyperconjugation with examples and applications.
2. What are reactive intermediates? Discuss the formation, structure and stability of carbocations, carbanions, free radicals and carbenes.

Short answer

1. Distinguish between homolytic and heterolytic fission with examples.
2. Define electrophile and nucleophile. Give two neutral and two charged examples of each.
3. Explain the effect of hydrogen bonding on the boiling points of alcohols and carboxylic acids.

Syllabus topics (official RTMNU)

(A) Stereochemistry of Organic Compounds

• Concept of isomerism.
• Types of isomerism with suitable examples.
• Optical isomerism-elements of symmetry, molecular chirality, enantiomers, stereogenic centre(lactic acid as example).
• Optical activity, properties of enantiomers, chiral and achiral molecules with two stereogenic centres (Tartaric acid) diastereomers, mesocompound.
• Resolution of enantiomers biological and chemical methods.
• Inversion retention and racemisation.
• Asymmetric synthesis.
• Relative and absolute configuration, sequence rules, D and L and R and S system of nomenclature.

(B) Geometrical isomerism

• E and Z system of nomenclature, geometric isomerism in alkenes with examples, maleic acid and fumaric acid, 2-butene.
• Conformational isomerism : Conformational analysis of ethane, n-butane and substituted n-butane, conformations of Cyclohexane, axial and equatorial bonds.
• Newman’s projection and sawhorse formulae.
• Difference between configuration and conformation.

(A) Stereochemistry of organic compounds

Isomerism: compounds with the same molecular formula but different structures or arrangements are isomers.

• Structural isomerism: chain (n-butane, isobutane), position (1-propanol, 2-propanol), functional (ethanol, dimethyl ether), metamerism (diethyl ether, methyl propyl ether), tautomerism (keto–enol of acetoacetic ester).
• Stereoisomerism: same connectivity, different spatial arrangement – optical, geometrical and conformational.

Optical isomerism

• Optical activity: ability to rotate plane-polarised light. Dextrorotatory (d or +) rotates to the right; laevorotatory (l or −) to the left. Measured with a polarimeter. Specific rotation [α] = α/(l × c) (l in dm, c in g mL⁻¹).
• Elements of symmetry: plane of symmetry (σ), centre of symmetry (i), alternating axis (Sₙ). A molecule having any of these is achiral.
• Chirality: a molecule that is not superimposable on its mirror image is chiral. A carbon attached to four different groups is a stereogenic (chiral) centre.
• Lactic acid CH₃–CH(OH)–COOH has one chiral C; it exists as (+) and (−) forms, which are mirror images (enantiomers). An equimolar mixture is the racemic (±) form, optically inactive by external compensation.
• Number of optical isomers = 2ⁿ (n = number of different chiral centres).

Two stereogenic centres: tartaric acid

HOOC–CH(OH)–CH(OH)–COOH has two similar chiral centres, so only 3 stereoisomers (not 4): (+)-tartaric acid, (−)-tartaric acid (enantiomers), and meso-tartaric acid. The meso form has a plane of symmetry, so it is optically inactive by internal compensation (rotation by one half cancelled by the other). Meso and (+) forms are diastereomers.

Resolution of a racemic mixture

• Mechanical: Pasteur picked out mirror-image crystals of sodium ammonium tartrate.
• Biochemical: a micro-organism (e.g. Penicillium glaucum) eats one enantiomer and leaves the other.
• Chemical: react racemic acid with a pure chiral base (e.g. brucine) → two diastereomeric salts with different solubilities; separate by fractional crystallisation, then decompose with acid.

Inversion, retention, racemisation; asymmetric synthesis

• Inversion: configuration is turned inside out (umbrella-like), as in SN2 (Walden inversion).
• Retention: configuration stays the same (no bond to the chiral C broken).
• Racemisation: an optically active compound becomes a racemic mixture, e.g. via a planar carbocation in SN1.
• Asymmetric synthesis: making an optically active compound from an achiral one using a chiral reagent, catalyst or enzyme, giving one enantiomer in excess. Example: reduction of pyruvic acid to (−)-lactic acid by enzymes.

Configuration: relative and absolute; D/L and R/S

• Relative (D/L): compared with glyceraldehyde. In the Fischer projection, if –OH (or the key group) on the lowest chiral C is on the right → D; on the left → L. D/L has no relation with (+)/(−).
• Absolute (R/S) – Cahn–Ingold–Prelog sequence rules:
  1. Higher atomic number of the atom directly attached = higher priority (I > Br > Cl > OH > NH₂ > CH₃ > H).
  2. If tied, compare the next atoms outward.
  3. Double/triple bonds are counted as duplicated/triplicated atoms (–CHO counts as C with O, O, H).
• View with the lowest-priority group pointing away. 1 → 2 → 3 clockwise = R; anticlockwise = S.
• Example: in lactic acid priorities are –OH > –COOH > –CH₃ > –H; (+)-lactic acid is S.

(B) Geometrical isomerism

Arises due to restricted rotation about C=C (or in rings), when each doubly bonded carbon carries two different groups.

• cis/trans: similar groups on the same side = cis; opposite = trans. cis-2-butene (b.p. 4 °C, polar) and trans-2-butene (b.p. 1 °C, more stable, higher m.p.).
• Maleic acid (cis-butenedioic acid) and fumaric acid (trans). Maleic acid: m.p. 130 °C, more soluble, stronger first acid, forms anhydride on heating (groups close). Fumaric acid: m.p. ≈ 287 °C, less soluble, no anhydride.
• E/Z system: assign CIP priority to the two groups on each carbon. Higher-priority groups on the same side = Z (zusammen); opposite = E (entgegen). Used when all four groups differ, e.g. 1-bromo-1-chloropropene.

Conformational isomerism

Different arrangements produced by rotation about a C–C single bond are conformations. They interconvert easily and cannot be separated.

• Newman projection: look along the C–C bond; front C is a point, back C a circle. Sawhorse: the C–C bond drawn diagonally with groups shown in perspective.
• Ethane: staggered (most stable) and eclipsed (least stable); energy difference ≈ 12.5 kJ mol⁻¹ (torsional strain). Energy curve vs dihedral angle shows minima at 60°, 180°, 300° and maxima at 0°, 120°, 240°.
• n-Butane (around C2–C3): stability order anti (180°) > gauche (60°) > partially eclipsed (120°) > fully eclipsed (0°). Anti is ≈ 3.8 kJ mol⁻¹ lower than gauche; fully eclipsed ≈ 19 kJ mol⁻¹ above anti.
• Substituted n-butane (e.g. 1,2-dichloroethane, ethylene glycol): for ethylene glycol the gauche form is more stable because of intramolecular H-bonding; for 1,2-dichloroethane, anti is preferred (dipole repulsion).
• Cyclohexane: chair (most stable, strain-free, all bonds staggered), twist-boat, boat (flagpole H interactions, eclipsing) and half-chair (least stable). Chair is ≈ 29 kJ mol⁻¹ more stable than boat.
• Axial and equatorial bonds: in the chair, 6 axial bonds are parallel to the ring axis (3 up, 3 down) and 6 equatorial bonds point outward. Ring flipping interconverts them. A substituent prefers the equatorial position (avoids 1,3-diaxial interactions); in methylcyclohexane ≈ 95 % is equatorial.

Likely exam questions

Long answer

1. Explain optical isomerism in lactic acid and tartaric acid. What is a meso compound? Describe chemical and biochemical methods of resolution.
2. Discuss the conformational analysis of n-butane with an energy profile diagram. Explain the chair and boat conformations of cyclohexane with axial and equatorial bonds.

Short answer

1. State the sequence rules and assign R/S configuration with one example.
2. Explain geometrical isomerism in maleic and fumaric acids.
3. Distinguish between configuration and conformation.

Syllabus topics (official RTMNU)

(A) Alkanes

• IUPAC nomenclature of branched and unbranched alkanes.
• Alkyl group, Isomerism in alkanes.
• Methods of formation (Ethane)– Wurtz reaction, Kolbe reaction, decarboxylation of carboxylic acid.
• Physical properties and chemical reactions of alkanes-halogenation, nitration, sulphonation, isomerization, cyclization, aromatization, pyrolysis and cracking oxidation, L.P.G., Octane number.
• Mechanism of free radical halogenation of alkane.

(B) Cycloalkanes

• Nomenclature, methods of formation of cyclohexane from dihalides, benzene and cyclohexanone, chemical reactions of cyclohexane oxidation, aromatization, chlorination, Baeyer’s strain theory and its limitations.
• Ring strain in small rings cyclopropane and cyclobutane.
• Theory of strainless rings.

(C) Alkenes

• Nomenclature of alkenes, methods of formation- dehydrogenation, dehydrohalogenation of alkyl halides, dehydration of alcohol dehalogenation of dihalides.
• Mechanism of dehydration of alcohol and dehydrohalogenation of Alkyl halides.
• Saytzeff rule, Hofmann elimination reaction.
• Chemical reactions of alkenes- hydroboration, oxidation KMnO4, HIO4, Epoxidation, Ozonolysis, Hydroxylation, Polymerization Substitution in allylic and vinylic position of alkenes.
• Industrial applications of ethylene and propylene.
• Markownikoff Rule and peroxide effect.
• Ionic Mechanism of addition of Br2 to ethene and HBr to propene.
• Free radical mechanism of addition of HBr to propene.
• Stereochemistry of bromine and KMnO4 addition to alkene.

(A) Alkanes

IUPAC nomenclature and alkyl groups

• General formula CₙH₂ₙ₊₂. Select the longest carbon chain; number from the end nearer the first branch; name substituents with locants in alphabetical order; use di-, tri- for repeated groups.
• Example: (CH₃)₂CH–CH₂–CH(CH₃)–CH₃ = 2,4-dimethylpentane.
• Alkyl group: alkane minus one H (CₙH₂ₙ₊₁–): methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl.
• Isomerism: chain isomerism begins with butane (2 isomers); pentane has 3 (n-pentane, isopentane, neopentane).

Methods of formation (ethane)

• Wurtz reaction: 2CH₃Br + 2Na → (dry ether) CH₃–CH₃ + 2NaBr.
• Kolbe's electrolysis: electrolysis of aqueous sodium acetate: 2CH₃COONa + 2H₂O → CH₃–CH₃ (anode) + 2CO₂ + 2NaOH + H₂ (cathode).
• Decarboxylation: heating sodium propanoate with soda lime: CH₃CH₂COONa + NaOH → (CaO, heat) CH₃–CH₃ + Na₂CO₃.

Physical properties

Non-polar, insoluble in water, soluble in organic solvents. C₁–C₄ gases, C₅–C₁₇ liquids, higher solids. B.p. increases with molar mass; branching lowers b.p. (less surface area): n-pentane 36 °C > isopentane 28 °C > neopentane 9.5 °C.

Chemical reactions

• Halogenation: CH₄ + Cl₂ → (hν) CH₃Cl + HCl → CH₂Cl₂ → CHCl₃ → CCl₄.
• Nitration (vapour phase, 400 °C): CH₄ + HNO₃ → CH₃NO₂ + H₂O.
• Sulphonation (higher alkanes, fuming H₂SO₄): R–H + H₂SO₄ → R–SO₃H + H₂O; easiest at 3° H.
• Isomerisation: n-butane → (AlCl₃/HCl) isobutane.
• Cyclisation and aromatisation: n-hexane → (Cr₂O₃/Al₂O₃, 773 K) benzene + 4H₂ (via cyclohexane).
• Pyrolysis/cracking: heating in absence of air breaks large molecules into smaller alkanes and alkenes: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
• Oxidation/combustion: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ mol⁻¹. Controlled oxidation gives alcohols, aldehydes, acids.

Mechanism of free radical halogenation (CH₄ + Cl₂)

1. Chain initiation: Cl₂ → (hν) 2Cl•
2. Chain propagation: Cl• + CH₄ → CH₃• + HCl; CH₃• + Cl₂ → CH₃Cl + Cl• (Cl• regenerated, chain continues)
3. Chain termination: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl; CH₃• + CH₃• → C₂H₆ (ethane traces prove the radical path)

Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂; of H: 3° > 2° > 1° (radical stability).

(B) Cycloalkanes

• General formula CₙH₂ₙ. Named with prefix cyclo-: cyclopropane, cyclobutane, methylcyclohexane.
• Formation of cyclohexane: (i) from 1,6-dibromohexane with Zn or Na (intramolecular Wurtz): Br(CH₂)₆Br + 2Na → C₆H₁₂ + 2NaBr; (ii) from benzene: C₆H₆ + 3H₂ → (Ni, 200 °C) C₆H₁₂; (iii) from cyclohexanone by Clemmensen reduction (Zn-Hg/HCl) or Wolff–Kishner reduction → cyclohexane.
• Reactions of cyclohexane: oxidation (HNO₃ or air/catalyst) → cyclohexanol/cyclohexanone, then adipic acid; aromatisation (Pt or Pd, heat) → benzene + 3H₂; chlorination (hν) → chlorocyclohexane (free radical substitution).

Baeyer's strain theory (1885)

• Assumed rings are planar. Deviation of ring angle from 109°28′ causes angle strain. Deviation d = ½(109°28′ − ring angle).
• Cyclopropane 60° → d = 24°44′; cyclobutane 90° → 9°44′; cyclopentane 108° → 0°44′; cyclohexane 120° → −5°16′.
• Predicted cyclopentane most stable and cyclohexane and larger rings increasingly strained.
• Limitations: cyclohexane and larger rings are actually strain-free (heats of combustion per CH₂ ≈ 658 kJ mol⁻¹, same as open chains); rings are not planar.

Ring strain in small rings: cyclopropane uses bent ("banana") bonds with poor overlap and has eclipsed H's (angle + torsional strain), so it opens easily: with H₂/Ni at 80 °C gives propane; adds Br₂ and HBr (1-bromopropane). Cyclobutane is slightly puckered and needs 200 °C for hydrogenation.

Sachse–Mohr theory of strainless rings: rings of six or more carbons are puckered (non-planar, e.g. chair cyclohexane), keeping the tetrahedral angle 109°28′, so they are strain-free.

(C) Alkenes

General formula CₙH₂ₙ; IUPAC suffix -ene with lowest locant for C=C (CH₃CH=CHCH₃ = but-2-ene).

Methods of formation

• Dehydrogenation of alkanes (Cr₂O₃, 500 °C): CH₃CH₃ → CH₂=CH₂ + H₂.
• Dehydrohalogenation: CH₃CH₂Br + alc. KOH → CH₂=CH₂ + KBr + H₂O.
• Dehydration of alcohols: C₂H₅OH → (conc. H₂SO₄, 443 K) CH₂=CH₂ + H₂O.
• Dehalogenation of vicinal dihalides: BrCH₂CH₂Br + Zn → CH₂=CH₂ + ZnBr₂.

Mechanisms

1. Dehydration of alcohol (E1): (i) –OH is protonated by H⁺ → R–OH₂⁺; (ii) water leaves → carbocation (slow, rate-determining); (iii) loss of β-H⁺ → alkene.
2. Dehydrohalogenation (E2): base (OH⁻/RO⁻) removes β-H while C–X breaks and C=C forms in one step through a transition state; H and X must be anti-periplanar. Rate = k[RX][base].

• Saytzeff (Zaitsev) rule: the more substituted alkene is the major product. 2-bromobutane + alc. KOH → but-2-ene (major, ≈ 80 %) + but-1-ene.
• Hofmann elimination: quaternary ammonium hydroxides (or bulky bases) give the less substituted alkene: [CH₃CH₂CH(CH₃)N(CH₃)₃]⁺OH⁻ → (heat) but-1-ene (major) + N(CH₃)₃ + H₂O.

Reactions of alkenes

• Hydroboration–oxidation: 3RCH=CH₂ + BH₃ → (RCH₂CH₂)₃B → (H₂O₂/OH⁻) 3RCH₂CH₂OH (anti-Markovnikov, syn addition).
• Oxidation by cold dilute alkaline KMnO₄ (Baeyer's reagent): CH₂=CH₂ + H₂O + [O] → HOCH₂CH₂OH (ethylene glycol; syn-hydroxylation; purple colour disappears – test for unsaturation). Hot acidic KMnO₄ cleaves the C=C to acids/ketones.
• HIO₄: cleaves the glycol formed after hydroxylation into two carbonyl compounds.
• Epoxidation: alkene + peracid (RCO₃H) → epoxide (oxirane); acid hydrolysis gives anti-diol.
• Ozonolysis: alkene + O₃ → ozonide → (Zn/H₂O) two carbonyl compounds. CH₃CH=CHCH₃ → 2CH₃CHO. Locates the position of C=C.
• Polymerisation: nCH₂=CH₂ → (high pressure, O₂ or Ziegler–Natta catalyst) (–CH₂–CH₂–)ₙ polyethylene.
• Allylic substitution: CH₃CH=CH₂ + Cl₂ → (500 °C) ClCH₂CH=CH₂ (allyl chloride); NBS gives allylic bromination. Vinylic H is very hard to substitute.

Industrial uses: ethylene – polythene, ethylene glycol (antifreeze), ethanol, ripening of fruit; propylene – polypropylene, isopropanol, acetone, cumene.

Markovnikov's rule and peroxide effect

Markovnikov's rule: in addition of HX to an unsymmetrical alkene, H goes to the carbon having more H atoms. CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane).

1. Ionic mechanism (HBr + propene): H⁺ adds to the CH₂ end → more stable 2° carbocation CH₃C⁺HCH₃ (slow); Br⁻ attacks it → 2-bromopropane.
2. Br₂ + ethene: Br₂ is polarised by the π cloud; Br⁺ adds forming a cyclic bromonium ion; Br⁻ attacks from the opposite side → 1,2-dibromoethane (anti addition).
3. Peroxide (Kharasch) effect – free radical mechanism: in presence of peroxide, HBr adds anti-Markovnikov. Initiation: R–O–O–R → 2RO•; RO• + HBr → ROH + Br•. Propagation: Br• + CH₃CH=CH₂ → CH₃C•H–CH₂Br (2° radical, more stable); CH₃C•HCH₂Br + HBr → CH₃CH₂CH₂Br + Br•. Termination: radicals combine. Only HBr shows this (HCl bond too strong, HI gives I• which is too unreactive).

Stereochemistry: Br₂ adds anti (via bromonium ion): cis-2-butene gives racemic 2,3-dibromobutane, trans-2-butene gives meso. Cold KMnO₄ adds syn (via cyclic manganate ester): cis-2-butene gives meso-2,3-butanediol, trans gives racemic.

Likely exam questions

Long answer

1. Give the mechanism of free radical chlorination of methane. Describe Wurtz reaction, Kolbe's electrolysis and decarboxylation for preparing ethane.
2. State Markovnikov's rule. Explain the ionic mechanism of addition of HBr to propene and the free radical mechanism of the peroxide effect.

Short answer

1. Explain Baeyer's strain theory and give its limitations.
2. What is ozonolysis? Give its importance with an example.
3. State Saytzeff's rule and explain Hofmann elimination with an example.

Syllabus topics (official RTMNU)

(A) Dienes

• Nomenclature and classification of dienes Methods of formation of 1, 3 - butadiene. 1,2 and 1,4 addition reactions of substituted 1,3-butadiene, Diels-Alder reaction.

(B) Alkynes

• Nomenclature, structure and bonding in Alkynes.
• Methods of formation of acetylene from - calcium carbide, dehydrohalogenation of dihalides Chemical reaction - hydroboration, oxidation metal ammonia reduction and polymerization.
• Oxyacetylene flame.
• Acidity of alkynes.

(C) Aromatic Compounds and Aromaticity

• Nomenclature of Benzene derivatives, structure of benzene, Molecular formula and Kekule structure.
• Resonance structure, MO picture, Huckel rule, aromatic ions (cyclopentadienyl anion and cycloheptatrienyl cation).
• Aromatic electrophillic substitution mechanism with energy profile diagram (eg. nitration and sulphonation).

(A) Dienes

Hydrocarbons with two C=C bonds (CₙH₂ₙ₋₂). Named with -diene and locants: CH₂=CH–CH=CH₂ = buta-1,3-diene.

Methods of formation of 1,3-butadiene

• Dehydrogenation of butane or butenes: CH₃CH₂CH₂CH₃ → (Cr₂O₃/Al₂O₃, 600 °C) CH₂=CH–CH=CH₂ + 2H₂.
• Dehydration of butane-1,3-diol or butane-1,4-diol (acid/heat): HOCH₂CH₂CH₂CH₂OH → CH₂=CH–CH=CH₂ + 2H₂O.
• From ethanol (Lebedev process, MgO/SiO₂): 2C₂H₅OH → CH₂=CH–CH=CH₂ + 2H₂O + H₂.
• Dehydrohalogenation of 1,3- or 1,4-dibromobutane with alcoholic KOH.

1,2- and 1,4-addition

HBr + 1,3-butadiene gives two products through one allylic carbocation:

1. H⁺ adds to C1 → allylic carbocation CH₃–C⁺H–CH=CH₂ ↔ CH₃–CH=CH–C⁺H₂ (resonance stabilised).
2. Br⁻ attacks C2 → 1,2-adduct 3-bromobut-1-ene; attacks C4 → 1,4-adduct 1-bromobut-2-ene.

At low temperature (−80 °C) the 1,2-product is major (kinetic control, forms faster). At 40 °C the 1,4-product is major (thermodynamic control, more stable, more substituted alkene). The same pattern holds for Br₂ and for substituted dienes such as isoprene.

Diels–Alder reaction

A [4 + 2] cycloaddition between a conjugated diene (in s-cis form) and a dienophile (alkene with electron-withdrawing group) to give a cyclohexene ring in one concerted step.

CH₂=CH–CH=CH₂ + CH₂=CH–CHO → (heat) cyclohex-3-ene-1-carbaldehyde. Butadiene + maleic anhydride → cis-1,2,3,6-tetrahydrophthalic anhydride. Features: stereospecific (syn addition), no intermediate, endo product favoured; used to make six-membered rings.

(B) Alkynes

• General formula CₙH₂ₙ₋₂; suffix -yne (CH₃C≡CH = propyne).
• Structure: each C≡C carbon is sp hybridised; molecule linear (180°); one σ (sp–sp) and two π bonds; C≡C 120 pm, C–H 106 pm.

Formation of acetylene

• From calcium carbide: CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂ (CaC₂ made from CaO + 3C → CaC₂ + CO in an electric furnace).
• Dehydrohalogenation of vicinal dihalides: BrCH₂–CH₂Br + 2KOH(alc) → HC≡CH + 2KBr + 2H₂O (second step needs NaNH₂ or strong heating).

Reactions

• Hydroboration: alkyne + BH₃ (or disiamylborane), then H₂O₂/OH⁻ → enol → aldehyde (terminal alkynes give aldehydes; anti-Markovnikov). Protonolysis of the vinylborane with CH₃COOH gives cis-alkene.
• Oxidation: hot alkaline KMnO₄ cleaves the triple bond: R–C≡C–R′ → RCOOH + R′COOH; acetylene gives oxalic acid / CO₂. Ozonolysis also gives acids.
• Metal–ammonia reduction (Birch-type): R–C≡C–R + Na/liquid NH₃ → trans-alkene. (H₂ with Lindlar catalyst gives cis-alkene.)
• Polymerisation: 3HC≡CH → (red-hot Fe tube, 873 K) benzene (cyclic trimerisation); 2HC≡CH → (Cu₂Cl₂/NH₄Cl) vinylacetylene CH₂=CH–C≡CH; 4HC≡CH → cyclooctatetraene (Ni(CN)₂).

Acidity of alkynes

H on a triple-bonded carbon (≡C–H) is weakly acidic (pKa ≈ 25) because the sp carbon has 50 % s character, holds the electron pair close to the nucleus, and stabilises the acetylide ion. Order: HC≡CH > CH₂=CH₂ (pKa 44) > CH₃–CH₃ (pKa 50). Reactions: HC≡CH + 2Na → NaC≡CNa + H₂; HC≡CH + NaNH₂ → HC≡CNa + NH₃; with ammoniacal AgNO₃ gives white silver acetylide, with ammoniacal Cu₂Cl₂ gives red copper acetylide – tests for terminal alkynes.

(C) Aromatic compounds and aromaticity

Nomenclature of benzene derivatives

Monosubstituted: chlorobenzene, nitrobenzene, toluene (methylbenzene), phenol, aniline, benzoic acid. Disubstituted: ortho (1,2-), meta (1,3-), para (1,4-), e.g. 1,4-dimethylbenzene = p-xylene; 2-nitrophenol = o-nitrophenol.

Structure of benzene

• Molecular formula C₆H₆ (from analysis and vapour density); highly unsaturated, yet it undergoes substitution, not addition.
• Kekulé structure (1865): six-membered ring with alternate single and double bonds, oscillating between two forms. It could not explain only one ortho-disubstituted product, the low reactivity, or equal bond lengths.
• Resonance: benzene is a hybrid of two Kekulé structures (plus Dewar structures). All C–C bonds are 139 pm (between 154 and 134 pm). Resonance energy ≈ 150 kJ mol⁻¹ (from heats of hydrogenation: 3 × 120 = 360 expected, 208 observed).
• MO picture: each C is sp², ring planar (120°). Six unhybridised p orbitals overlap sideways to form a continuous π cloud above and below the ring; 6 π electrons are delocalised. Six π MOs: three bonding (filled) and three antibonding (empty).

Hückel's rule

A compound is aromatic if it is cyclic, planar, fully conjugated (each ring atom has a p orbital) and has (4n + 2) π electrons (n = 0, 1, 2…; i.e. 2, 6, 10…). With 4n π electrons a planar ring is anti-aromatic.

• Cyclopentadienyl anion C₅H₅⁻: removing H⁺ from cyclopentadiene (pKa ≈ 16, unusually acidic) gives a planar ring with 6 π electrons → aromatic.
• Cycloheptatrienyl (tropylium) cation C₇H₇⁺: removal of H⁻ from cycloheptatriene gives a planar ring with 6 π electrons → aromatic and stable (tropylium bromide is ionic).
• Cyclopentadienyl cation (4 π) and cyclobutadiene (4 π) are anti-aromatic; cyclooctatetraene (8 π) is non-planar (tub-shaped) and non-aromatic.

Aromatic electrophilic substitution – general mechanism

1. Generation of the electrophile (E⁺).
2. E⁺ attacks the π cloud → arenium ion (σ-complex, carbocation), resonance stabilised but non-aromatic. This step is slow (rate-determining).
3. Loss of H⁺ from the sp³ carbon restores aromaticity (fast).

Energy profile: two humps (two transition states) with the arenium ion in the valley between; the first hump is higher (slow step). Products are lower in energy than reactants.

• Nitration: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻. NO₂⁺ (nitronium ion) attacks benzene → σ-complex → nitrobenzene + H⁺ (conc. HNO₃ + conc. H₂SO₄, 50–60 °C).
• Sulphonation: 2H₂SO₄ ⇌ SO₃ + H₃O⁺ + HSO₄⁻; SO₃ (neutral electrophile) attacks benzene → benzenesulphonic acid (fuming H₂SO₄). Sulphonation is reversible – heating with dilute acid/steam removes the –SO₃H group.

Likely exam questions

Long answer

1. Discuss the structure of benzene on the basis of Kekulé structure, resonance and molecular orbital picture. State Hückel's rule with examples of aromatic ions.
2. Give the mechanism of aromatic electrophilic substitution with an energy profile diagram, taking nitration and sulphonation as examples.

Short answer

1. Explain 1,2- and 1,4-addition of HBr to 1,3-butadiene.
2. What is the Diels–Alder reaction? Give one example.
3. Why is acetylene acidic? Give two reactions showing its acidic nature.