Properties Sn Reactions

6.6 Physical Properties

Pure alkyl halides are colourless — the iodides turn yellow to red on standing because light decomposes them and liberates I₂. They carry a distinct sweetish smell. Because the C–X dipole is only moderate and no hydrogen bonding to water is possible, simple haloalkanes are virtually insoluble in water but mix freely with organic solvents such as benzene, ether and CCl₄.

Boiling points rise along the series R–F < R–Cl < R–Br < R–I because larger halogens increase London dispersion forces; within a given halogen, BP increases with molecular mass and decreases with branching (a branched chain is more spherical, has less surface area for dispersion contacts). Densities follow R–I > R–Br > R–Cl — heavier halogens pack more mass into a similar volume.

6.7 Chemical Reactions of Haloalkanes

6.7.1 Nucleophilic Substitution — the Big Picture

The δ⁺ carbon of the C–X bond is electrophilic. A nucleophile — any species with a lone pair or negative charge — can attack it, form a new σ-bond to C and expel X⁻ as a leaving group. Depending on which nucleophile you choose, you can turn R–X into an alcohol, ether, nitrile, amine, thiol or ester:

Table 6.1: Nucleophiles and the products they give with alkyl halides. CN⁻ and NO₂⁻ are ambident — they attack through either end, and the identity of the metal (Na vs Ag) controls which end.

6.7.2 The Two Mechanisms — SN2 and SN1

Kinetic studies show that nucleophilic substitution follows two distinct mechanistic pathways, labelled SN2 (bimolecular) and SN1 (unimolecular). Which pathway operates depends on the substrate, nucleophile, solvent and leaving group.

(a) SN2 — Bimolecular, One Step

The nucleophile attacks the δ⁺ carbon from the side opposite the leaving group (the "back-side"). Bond-making and bond-breaking happen simultaneously through a single five-coordinate transition state. Because both the substrate and the nucleophile appear in the rate-determining step:

• Stereochemistry: complete inversion of configuration at the attacked carbon.
• Substrate order: 1° > 2° > 3° (steric crowding at the carbon blocks back-side attack). Methyl > 1° > 2° ≫ 3°.
• Favoured by: strong nucleophile (HO⁻, CN⁻, RO⁻), polar aprotic solvent (DMSO, DMF, acetone), good leaving group.

(b) SN1 — Unimolecular, Two Steps

For tertiary substrates, steric crowding forbids back-side attack. Instead, the C–X bond heterolyses first to give a flat, sp²-hybridised carbocation plus X⁻ (slow step). In a fast second step, the nucleophile adds to either face of the carbocation:

• Stereochemistry: racemisation (roughly 50:50 R/S product).
• Substrate order: 3° > 2° > 1° — because carbocation stability goes 3° > 2° > 1°.
• Favoured by: polar protic solvent (H₂O, ROH — solvates the carbocation), weak nucleophile, good leaving group.

6.7.3 Head-to-Head Comparison

6.8 Elimination Reactions (E2) — β-Elimination

Instead of substituting the leaving group, a base can abstract a proton from the β-carbon; the electrons of the C–H bond push into the C–C bond forming a new π-bond, while X⁻ leaves. The outcome is an alkene plus HX, and the overall name is β-elimination or dehydrohalogenation.

Saytzeff's Rule

When a haloalkane can give two different alkenes on β-elimination, the major product is the more substituted (more highly alkylated) alkene, because greater substitution raises the stability of the π-system through hyperconjugation.

Substitution vs elimination competition: a strong, bulky base (e.g., tert-butoxide) in alcoholic solvent at high temperature favours elimination; a strong, non-bulky nucleophile in aprotic solvent at low temperature favours substitution. 3° halides favour SN1/E1 or E2; 1° halides favour SN2 with only a little E2.

Worked Examples