Lesson 13 of 13 · 16 min
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Must-know facts
23 facts
- 01General formulas: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂.
- 02Chain isomer counts: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9, C₁₀H₂₂ 75.
- 03Bond 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.
- 04Bond 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.
- 05Wurtz reaction (Na, dry ether) and Kolbe electrolysis both give even-carbon alkanes; Kolbe cannot make methane.
- 06Decarboxylation with soda lime gives an alkane with one carbon fewer than the acid.
- 07Branching lowers boiling point: pentane 309.1 K > 2-methylbutane 300.9 K > 2,2-dimethylpropane 282.5 K.
- 08Halogenation of alkanes: F₂ > Cl₂ > Br₂ > I₂; hydrogens replaced 3° > 2° > 1°; free-radical chain of initiation, propagation, termination.
- 09Ethane appears during chlorination of methane because two •CH₃ radicals combine.
- 10Ethane: staggered is more stable than eclipsed by about 12.5 kJ mol⁻¹; general barrier 1–20 kJ mol⁻¹.
- 11cis-But-2-ene μ = 0.33 D; trans-but-2-ene μ = 0. Solid trans isomers usually melt higher.
- 12Lindlar's catalyst gives cis alkenes; Na in liquid NH₃ gives trans alkenes.
- 13Dehydrohalogenation rate: I > Br > Cl and 3° > 2° > 1°.
- 14HX addition reactivity: HI > HBr > HCl. Markovnikov rule goes through the more stable carbocation.
- 15Peroxide effect works with HBr only; propene gives 1-bromopropane.
- 16Tests for unsaturation: decolourisation of Br₂ in CCl₄ and of Baeyer's reagent (cold, dilute, aqueous KMnO₄).
- 17Alkyne hydration (HgSO₄, dil. H₂SO₄, 333 K): ethyne gives ethanal, propyne gives propanone.
- 18Acidity: HC≡CH > H₂C=CH₂ > CH₃–CH₃, because sp carbon has 50% s character.
- 19Three ethyne molecules give benzene in a red-hot iron tube at 873 K.
- 20Aromatic: planar, fully delocalised, (4n + 2) π electrons.
- 21Nitronium 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.
- 23Benzene hexachloride (gammaxane) C₆H₆Cl₆ forms by addition of 3Cl₂ under UV light, not by substitution.
Common traps
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Formulas
6 to know
Alkane series
CₙH₂ₙ₊₂
Alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂; alkyl groups CₙH₂ₙ₊₁.
Combustion of an alkane
CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → nCO₂ + (n + 1)H₂O
n = 4 gives 13/2 O₂ for butane.
Combustion of any hydrocarbon
CₓHᵧ + (x + y/4) O₂ → xCO₂ + (y/2) H₂O
Benzene: x = 6, y = 6 gives 15/2 O₂.
Methane combustion
CH₄ + 2O₂ → CO₂ + 2H₂O, ΔcH⁻ = −890 kJ mol⁻¹
Butane: ΔcH⁻ = −2875.84 kJ mol⁻¹.
Hückel rule
π electrons = 4n + 2 (n = 0, 1, 2 …)
Benzene: 6 π electrons, n = 1.
Ethyne from carbide
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂
CaC₂ from CaO + 3C → CaC₂ + CO.
Key terms
16 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.