Lesson 13 of 13 · 16 min
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Must-know facts
23 facts
- 1General formulas: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂.
- 2Chain isomer counts: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9, C₁₀H₂₂ 75.
- 3Bond 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.
- 4Bond 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.
- 5Wurtz reaction (Na, dry ether) and Kolbe electrolysis both give even-carbon alkanes; Kolbe cannot make methane.
- 6Decarboxylation with soda lime gives an alkane with one carbon fewer than the acid.
- 7Branching lowers boiling point: pentane 309.1 K > 2-methylbutane 300.9 K > 2,2-dimethylpropane 282.5 K.
- 8Halogenation of alkanes: F₂ > Cl₂ > Br₂ > I₂; hydrogens replaced 3° > 2° > 1°; free-radical chain of initiation, propagation, termination.
- 9Ethane 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
Where marks are lost
Using Wurtz or Kolbe to make an odd-carbon alkane or methane.
Expecting the most branched isomer to boil highest.
Calling ethane's conformers separable isomers.
Saying bond angles or lengths change between staggered and eclipsed forms.
Applying the peroxide effect to HCl or HI.
Saying propene or but-1-ene show cis-trans isomerism.
Treating halogens as activating because they direct ortho and para.
Calling benzene hexachloride a substitution product.
Expecting ethyne + water to give an alcohol.
Making every H in an alkyne acidic.
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.