Haloalkanes, Haloarenes, Alcohols, Phenols and Ethers
This domain tests substitution mechanisms, the relative reactivity of haloalkanes and haloarenes, the preparation and reactions of alcohols and phenols, and the cleavage and formation of ethers.
Haloalkanes
- SN2: one step, backside attack, inversion of configuration, rate ∝ [RX][Nu⁻]; favoured by CH₃ > 1° > 2° > 3° and polar aprotic solvents.
- SN1: two steps through a carbocation, rate ∝ [RX]; favoured by 3° > 2° > 1° and polar protic solvents.
- Leaving-group ability: I⁻ > Br⁻ > Cl⁻ > F⁻, because the C–I bond is weakest.
- Reagents: aqueous KOH gives an alcohol, alcoholic KOH an alkene, KCN a nitrile, AgCN an isocyanide, Mg/dry ether a Grignard reagent.
Haloarenes
In chlorobenzene the C–Cl bond has partial double-bond character (resonance) and the carbon is sp², so SN1 and SN2 are extremely slow; substitution needs electron-withdrawing groups at ortho/para positions or extreme conditions (Dow process, 623 K, 300 atm). Electrophilic substitution is normal and ortho/para directing.
Alcohols, phenols, ethers
Alcohols are weaker acids than water, oxidise by degree of substitution (1° → aldehyde → acid, 2° → ketone, 3° resistant), and dehydrate with conc. H₂SO₄. Lucas test: 3° gives immediate turbidity, 2° in about five minutes, 1° no cloudiness at room temperature. Phenol is acidic enough for NaOH but not for NaHCO₃, and its ring is strongly activated: Reimer–Tiemann gives salicylaldehyde, Kolbe gives salicylic acid. Ethers are made by Williamson synthesis (alkoxide + 1° halide) and cleaved by HI.
Worked example
Which reacts faster with OH⁻ in SN2, 1-bromobutane or 2-bromobutane? In SN2 the nucleophile attacks the carbon bearing the halogen, so steric crowding controls the rate. 2-Bromobutane is secondary and more hindered, so 1-bromobutane reacts faster, even though the secondary carbocation would be more stable - a factor that matters only in SN1.
Traps that cost marks
- Quoting carbocation stability to predict SN2 rates.
- Treating chlorobenzene as a normal alkyl halide.
- Assuming phenol dissolves in NaHCO₃ with effervescence (only carboxylic acids do).
- Forgetting that HI cleavage of an ether gives the alkyl iodide and the alcohol.