Haloalkanes and Haloarenes: SN1, SN2 and the Chloroform Bottle

In the paper

Unit 3, Organic chemistry, carries 17 of the 50 chemistry marks; with eleven chapters sharing them, this one averages a little over one and a half (MEC publishes weights by unit, not by chapter). It is the chapter that supplies reagents to every other one — the Grignard reagent, the Williamson ether, the nitrile that becomes an acid or an amine all start here.

MEC scope: Nomenclature, isomerism and classification; Preparation and properties of monohaloalkanes; Concept of SN1 and SN2 mechanisms; Preparation and chemical properties of chloroform and chlorobenzene.

Three ways it is asked:

• Recall: which reagent turns an alcohol into a chloroalkane most cleanly; what chloroform is stored with; the product of an alkyl halide with AgCN.
• Understanding: why R–I is more reactive than R–Cl although iodine is the least electronegative; why chlorobenzene resists nucleophiles; why SN2 inverts configuration.
• Application: predict the effect on rate of doubling the nucleophile concentration; decide which mechanism a named substrate follows; name the alkene from dehydrohalogenation by Saytzeff's rule.