CCEA A Level Chemistry 2.5 Halogenoalkanes Exam Questions

About CCEA A Level Chemistry 2.5 Halogenoalkanes

Section 2.5 of Unit AS 2 covers halogenoalkanes, which are versatile intermediates in synthesis. You need boiling point trends, classification as primary, secondary or tertiary, and how to prepare a liquid halogenoalkane from an alcohol in the laboratory. Reactions include nucleophilic substitution with aqueous alkali, ammonia and potassium cyanide, and elimination with ethanolic potassium hydroxide. You must define nucleophile, draw the nucleophilic substitution mechanism for primary and tertiary halogenoalkanes, and explain the relative rates of hydrolysis using carbon-halogen bond enthalpy. CFCs and ozone depletion complete the section. The mechanisms link forward to rates of reaction (SN1 and SN2) in A2 1.

What you need to know

• Classification and properties Primary, secondary and tertiary depend on how many carbons are bonded to the C-X carbon. Boiling point rises from fluoro to iodo compounds and with chain length, and falls with branching.
• Preparation A halogenoalkane such as 2-chloro-2-methylpropane is made by shaking the alcohol with concentrated HCl in a separating funnel, washing with sodium hydrogencarbonate solution and water, drying with anhydrous sodium sulfate and distilling.
• Nucleophilic substitution A nucleophile is an electron pair donor. With aqueous NaOH, an alcohol forms; with ethanolic ammonia (heated under pressure), an amine forms; with ethanolic KCN, a nitrile forms (adding one carbon).
• Mechanisms Primary: OH- attacks the delta+ carbon as the C-X bond breaks in one step. Tertiary: the C-X bond breaks first to form a carbocation, which is then attacked by OH-.
• Hydrolysis rates Iodoalkanes hydrolyse fastest because the C-I bond has the lowest bond enthalpy, despite C-Cl being more polar. Follow rates with aqueous silver nitrate in ethanol: AgI (yellow) precipitates first.
• Elimination and CFCs Hot ethanolic KOH eliminates HX to form an alkene; unsymmetrical halogenoalkanes can give more than one alkene. CFCs release chlorine radicals in UV light in the stratosphere, which catalyse ozone breakdown.

Common mistakes in 2.5 Halogenoalkanes

• Saying C-Cl hydrolyses fastest because it is most polar. Rate is controlled by bond enthalpy, so C-I is fastest.
• Using aqueous KOH for elimination. Elimination needs hot ethanolic KOH; aqueous conditions favour substitution.
• Starting the curly arrow from the negative charge on OH- rather than a lone pair. Start the arrow at a lone pair on oxygen.
• Forgetting that KCN extends the carbon chain. The nitrile has one more carbon than the halogenoalkane.
• Saying CFCs deplete ozone because they are toxic. Chlorine radicals released by UV act as catalysts for ozone breakdown.

Try three 2.5 Halogenoalkanes exam-style questions