Halogen compounds

📚The chemistry

Making halogenoalkanes

• From alkanes — free-radical substitution with Cl₂ or Br₂ in UV light (topic 14). Gives a mixture.
• From alkenes — electrophilic addition of HX(g) at room temperature, or of X₂ (which gives a dihalogenoalkane).
• From alcohols — substitution of the OH group by a halogen. Several reagents do it:
  • HX(g), or HCl made in situ from KCl with concentrated H₂SO₄ or concentrated H₃PO₄: ROH + HCl → RCl + H₂O;
  • PCl₃ and heat: 3ROH + PCl₃ → 3RCl + H₃PO₃;
  • PCl₅ at room temperature: ROH + PCl₅ → RCl + POCl₃ + HCl (steamy fumes — a test for an OH group);
  • SOCl₂: ROH + SOCl₂ → RCl + SO₂ + HCl. Both by-products are gases, so the product is easy to purify.

Primary, secondary and tertiary

Classify by the carbon that carries the halogen: count the alkyl groups attached to that carbon.

(Halogenomethanes, CH₃X, have none and behave like primary ones.)

Nucleophilic substitution

Halogens are more electronegative than carbon, so the C–X bond is polar: C^δ+–X^δ−. A nucleophile is attracted to the δ+ carbon, forms a new bond to it using its lone pair, and the halogen leaves as a halide ion. Four reactions are named:

• NaOH(aq), heat (under reflux) → alcohol. This is also called hydrolysis. \[ \mathrm{CH_3CH_2Br + OH^{-} \rightarrow CH_3CH_2OH + Br^{-}} \]
• KCN in ethanol, heat → nitrile. The chain gets one carbon longer, which makes this reaction valuable in synthesis. \[ \mathrm{CH_3CH_2Br + CN^{-} \rightarrow CH_3CH_2CN + Br^{-}} \] The product is propanenitrile: the C of the CN group counts in the name.
• NH₃ in ethanol, heated under pressure (in a sealed tube) → amine. Excess ammonia is used, because the amine formed is itself a nucleophile and would react further. \[ \mathrm{CH_3CH_2Br + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br} \]
• Aqueous silver nitrate in ethanol — water acts as the nucleophile, slowly releasing the halide ion, which then precipitates with Ag⁺. The colour of the precipitate identifies the halogen: AgCl white, AgBr cream, AgI yellow (topic 11). For bromoethane: \[ \mathrm{CH_3CH_2Br + H_2O \rightarrow CH_3CH_2OH + H^{+} + Br^{-}} \] \[ \mathrm{Ag^{+}(aq) + Br^{-}(aq) \rightarrow AgBr(s)} \] Ethanol is the solvent because halogenoalkanes do not mix with water.

Elimination

Change the solvent to ethanol and hydroxide ions act as a base instead of a nucleophile: they remove H⁺ from the carbon next to the C–X, and HX is lost, forming a C=C double bond.

Substitution and elimination always compete. Aqueous conditions favour substitution (alcohol); ethanolic conditions and higher temperature favour elimination (alkene). Tertiary halogenoalkanes eliminate more readily than primary ones. With an unsymmetrical secondary or tertiary halogenoalkane, elimination can give more than one alkene.

The two mechanisms

S_N2 — primary halogenoalkanes

Substitution, Nucleophilic, bimolecular — one step, involving two species.

1. The OH⁻ approaches the δ+ carbon from the side opposite the halogen. Curly arrow from a lone pair on the O of OH⁻ to the carbon; curly arrow from the C–Br bond to the Br.
2. For an instant there is a transition state in which the carbon is partly bonded to both OH and Br (drawn with dashed bonds and an overall negative charge in square brackets); the other three groups are flattened into a plane.
3. The C–Br bond breaks as the C–O bond forms: product alcohol + Br⁻. The molecule is turned inside out, like an umbrella in the wind.

The rate depends on both the halogenoalkane and the hydroxide concentration.

S_N1 — tertiary halogenoalkanes

Substitution, Nucleophilic, unimolecular — two steps.

1. Slow step: the C–Br bond breaks heterolytically on its own (curly arrow from the C–Br bond to Br), forming a tertiary carbocation, (CH₃)₃C⁺, and Br⁻.
2. Fast step: OH⁻ attacks the planar carbocation (curly arrow from its lone pair to C⁺), forming (CH₃)₃COH.

The rate depends only on the halogenoalkane, because only it is involved in the slow step.

Why the class decides the mechanism

• Tertiary → S_N1. Three alkyl groups are electron-donating (positive inductive effect): they stabilise the carbocation, so it forms readily. They are also bulky, so they physically block a nucleophile approaching from behind (steric hindrance), which rules out S_N2.
• Primary → S_N2. A primary carbocation would have only one alkyl group to stabilise it, so it is too unstable to form; but the carbon is uncrowded, so the nucleophile can reach it from behind.
• Secondary → a mixture of both, depending on the structure and the conditions.

Reactivity and the C–X bond

From the data section, C–Cl 340, C–Br 280, C–I 240 kJ mol⁻¹. Reactivity increases from chloroalkanes to iodoalkanes. You might expect the opposite, because the C–Cl bond is the most polar and so the carbon the most δ+. But experiment shows that iodoalkanes react fastest, so bond strength, not polarity, controls the rate: the weaker the C–X bond, the lower the activation energy for breaking it.

The silver nitrate experiment shows this directly. Warm equal amounts of 1-chloro-, 1-bromo- and 1-iodobutane with aqueous ethanolic AgNO₃ in a water bath: the yellow AgI precipitate appears first, the cream AgBr next, and the white AgCl much later.