1. Alkanes: nomenclature, isomerism and preparation
General formula ; sp³ carbons, tetrahedral, C–C 154 pm. Naming: longest chain, lowest locants for the first branch, substituents alphabetical (2,2,4-trimethylpentane). Chain isomers: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9.
Preparation:
- Hydrogenation of alkenes or alkynes (Ni, Pt or Pd): same carbon number.
- Reduction of alkyl halides: .
- Wurtz: (a mixture of alkyl halides gives a mixture of alkanes).
- Decarboxylation: (one carbon less).
- Kolbe electrolysis: (R–R at the anode).
| Method | Carbons in the alkane |
|---|---|
| Hydrogenation | Same as the alkene |
| Wurtz | Twice those of R in RX |
| Decarboxylation | One less than the acid salt |
| Kolbe | Twice those of R in RCOO⁻ |
Formulas
- Alkane general formula
Key terms
- Chain isomers
Compounds with the same molecular formula but different carbon skeletons.
- Wurtz reaction
Coupling of two alkyl halide molecules by sodium in dry ether to give an alkane with twice the carbons.
- Kolbe electrolysis
Electrolysis of an aqueous sodium or potassium carboxylate to give an alkane R–R at the anode.
2. Alkanes: properties and conformations of ethane
Physical: non-polar; boiling point rises with chain length and falls with branching (pentane 309 K, 2-methylbutane 301 K, 2,2-dimethylpropane 283 K).
Chemical: combustion; controlled oxidation (CH₄ → CH₃OH with Cu, 523 K, 100 atm; ethane → ethanoic acid with Mn(II) ethanoate); isomerisation (AlCl₃/HCl); aromatisation (hexane → benzene, Cr₂O₃/V₂O₅, 773 K); pyrolysis (cracking). Halogenation is a free-radical chain: initiation (Cl₂ → 2Cl•), propagation (Cl• + CH₄ → HCl + CH₃•; CH₃• + Cl₂ → CH₃Cl + Cl•), termination (radical + radical). Rates of H abstraction: 3° > 2° > 1°; F₂ > Cl₂ > Br₂ > I₂.
Conformations of ethane: staggered (dihedral angle 60°, minimum) and eclipsed (0°, maximum). The eclipsed form is about 12.5 kJ mol⁻¹ higher (torsional strain, about 4 kJ mol⁻¹ per eclipsed H–H pair), but rotation is nearly free at room temperature.
Formulas
- Torsional energy of ethane (θ from the eclipsed form)
Key terms
- Free radical
A species with an unpaired electron, formed by homolytic fission.
- Staggered conformation
Ethane conformation with the hydrogens of one carbon midway between those of the other; lowest energy.
- Torsional strain
Extra energy of the eclipsed conformation caused by repulsion between electron clouds of eclipsed bonds.
3. Alkenes: structure, geometric isomerism and preparation
General formula ; sp² carbons, planar, 120°, C=C 134 pm; one σ and one π bond (restricted rotation). Geometric (cis–trans) isomerism needs two different groups on each C of C=C: cis-but-2-ene (bp 277 K, mp 134 K, polar) and trans-but-2-ene (bp 274 K, mp 168 K, non-polar, packs better).
Preparation: partial reduction of alkynes (Lindlar → cis; Na/liquid NH₃ → trans); dehydrohalogenation (alcoholic KOH); dehydration of alcohols (conc. H₂SO₄, 443 K); dehalogenation of vicinal dihalides (Zn).
Key terms
- Geometric isomers
Stereoisomers differing in the arrangement of groups about a double bond (or ring).
- Lindlar catalyst
Palladium on calcium carbonate poisoned with quinoline; stops alkyne hydrogenation at the cis alkene.
4. Alkenes: reactions
Addition: H₂ (Ni) → alkane; X₂ → vicinal dihalide (bromine in CCl₄ decolourised); HX and H₂O (H⁺) follow Markovnikov's rule: H adds to the carbon with more H. The π bond attacks H⁺ to give the more stable carbocation (3° > 2° > 1°), then X⁻ attacks.
Peroxide (Kharasch) effect: with peroxide, HBr only adds anti-Markovnikov through a radical chain (Br• adds to give the more stable radical): but-1-ene → 1-bromobutane.
Ozonolysis (O₃, then Zn/H₂O): C=C is cut and each carbon becomes C=O: =CH₂ → HCHO, =CHR → RCHO, =CR₂ → R₂C=O. Baeyer's test: cold dilute alkaline KMnO₄ is decolourised (brown MnO₂), vicinal diol formed. Polymerisation: n CH₂=CH₂ → polythene.
Key terms
- Markovnikov’s rule
In the addition of HX to an unsymmetrical alkene, hydrogen adds to the carbon that already has more hydrogens.
- Peroxide effect
Anti-Markovnikov addition of HBr in the presence of peroxides.
- Ozonolysis
Cleavage of C=C by ozone followed by reductive work-up, giving aldehydes and ketones.
5. Alkynes
General formula ; sp carbons, linear, C≡C 120 pm. Acidity: sp carbon (50% s-character) is more electronegative, so terminal alkynes are weakly acidic: ethyne > ethene > ethane. ; .
Preparation: ; vicinal dihalide + alcoholic KOH (twice).
Addition: H₂ (Ni → alkane; Lindlar → cis; Na/NH₃ → trans); X₂ (twice); HX (Markovnikov, twice); H₂O/HgSO₄/H₂SO₄ → ethanal from ethyne, propanone from propyne.
Cyclic polymerisation: .
| Hydrocarbon | C–H carbon | pKa (approx.) |
|---|---|---|
| Ethyne | sp | 25 |
| Ethene | sp² | 44 |
| Ethane | sp³ | 50 |
Key terms
- Terminal alkyne
An alkyne with the triple bond at the end of the chain, R–C≡C–H.
- Cyclic polymerisation
Joining of three ethyne molecules into the ring of benzene.
6. Benzene: structure, resonance and aromaticity
Benzene : planar regular hexagon, all C–C 139 pm, bond angles 120°. Resonance hybrid of two Kekulé structures; delocalised π cloud of six electrons above and below the plane. Resonance energy about 150 kJ mol⁻¹ (hydrogenation: −208 observed against −360 expected for three C=C). Benzene prefers substitution to addition.
Aromaticity (Hückel): planar, cyclic, fully conjugated, (4n + 2) π electrons: benzene 6, naphthalene 10, anthracene 14. Cyclobutadiene (4) and cyclooctatetraene (8, tub-shaped) are not aromatic.
Preparation: cyclic polymerisation of ethyne; ; ; aromatisation of hexane.
Formulas
- Hückel rule
Key terms
- Resonance hybrid
The real structure of a molecule that is represented by two or more contributing structures.
- Aromatic compound
A planar, cyclic, fully conjugated compound with (4n + 2) π electrons.
- Resonance energy
The extra stability of a molecule compared with its most stable single contributing structure.
7. Electrophilic substitution, directing effects and toxicity
Mechanism: E⁺ is attacked by the π cloud (slow) to give the arenium ion; loss of H⁺ (fast) restores the ring.
Reactions: nitration (conc. HNO₃/H₂SO₄, 333 K; NO₂⁺); halogenation (X₂, FeX₃); sulfonation (fuming H₂SO₄; SO₃); Friedel–Crafts alkylation (RCl, anhydrous AlCl₃) and acylation (RCOCl, anhydrous AlCl₃, giving an aryl ketone). Friedel–Crafts fails on strongly deactivated rings such as nitrobenzene.
Directing effects: –OH, –NH₂, –OCH₃, –CH₃ activate and direct o/p; –Cl, –Br deactivate but direct o/p; –NO₂, –CN, –CHO, –COOH, –SO₃H deactivate and direct m.
Toxicity: benzene harms bone marrow; polynuclear hydrocarbons such as benzpyrene (formed by incomplete combustion of coal, petroleum and tobacco) are carcinogenic because they damage DNA.
| Group | Rate | Orientation |
|---|---|---|
| –OH, –NH₂, –CH₃ | Activating | o and p |
| –Cl, –Br | Deactivating | o and p |
| –NO₂, –COOH, –CHO | Deactivating | m |
Key terms
- Electrophile
An electron-poor species that attacks regions of high electron density.
- Arenium ion
The resonance-stabilised carbocation intermediate formed when an electrophile attacks benzene.
- Directing group
A substituent that determines the position at which the next group enters the ring.
Exam tips
Common mistakes
- Giving the peroxide effect for HCl or HI (it is for HBr only).
- Writing H adding to the carbon with fewer hydrogens in Markovnikov addition.
- Drawing the Newman projection without the circle for the back carbon, or calling the eclipsed form the more stable one.
- Calling cyclooctatetraene or cyclobutadiene aromatic.
- Writing the product of Friedel–Crafts acylation as an alkylbenzene.
- Forgetting that only terminal alkynes react with sodium.
How marks are usually awarded
- 1-mark items: product, reagent, name or classification.
- Mechanism answers: electrophile or radical generated (1), slow step with intermediate (1), product (1).
- Prediction (2 marks): rule named (1) + product (1).
- Ozonolysis: carbonyl products (1) + alkene structure (1).