Halogenoalkanes (AS Level) flashcards

Topic 21: Halogenoalkanes

Why do nucleophiles attack halogenoalkanes?

The halogen is more electronegative than carbon, so the C–X bond is polar: C is δ+ and attracts the lone pair of a nucleophile.

Nucleophilic substitution: reagents and conditions for OH⁻, CN⁻ and NH₃

OH⁻: NaOH or KOH(aq), warm → alcohol. CN⁻: KCN in aqueous ethanol, reflux → nitrile (one more carbon). NH₃: excess NH₃ in ethanol, heated in a sealed container → primary amine.

Order of reactivity of halogenoalkanes, and the reason

Iodo > bromo > chloro > fluoro. The C–X bond enthalpy decreases from C–F to C–I, and the weaker bond breaks faster. Polarity predicts the opposite order.

Why is excess ammonia used to make a primary amine?

The amine product has a lone pair and can itself act as a nucleophile; excess NH₃ reduces further substitution.

Substitution vs elimination with KOH: conditions that favour each

Substitution (alcohol): aqueous, warm; OH⁻ acts as a nucleophile. Elimination (alkene): ethanol, hot; OH⁻ acts as a base.

Name the product of 1-bromopropane with KCN

Butanenitrile, CH₃CH₂CH₂CN (the C of C≡N is counted).

How do chlorine radicals destroy ozone? Give the equations.

UV breaks C–Cl in CFCs, releasing Cl•. Cl• + O₃ → ClO• + O₂ ClO• + O₃ → 2O₂ + Cl• Cl• is regenerated, so it acts as a catalyst.

Hydrolysis with aqueous silver nitrate: which halogenoalkane gives a precipitate first?

The iodoalkane (yellow AgI), then bromo (cream AgBr), then chloro (white AgCl), following C–X bond enthalpy.