Elimination, metals and conversions

Elimination. Heat a haloalkane with alcoholic KOH (or sodium ethoxide), and instead of substitution, the base removes an H from the next carbon (the β-carbon) while the halide leaves. The result is an alkene: this is β-elimination (dehydrohalogenation).

When there is more than one kind of β-hydrogen, Saytzeff's rule says the main product is the more substituted alkene, the one with fewer H atoms on the double-bond carbons.

The same reagent can do two jobs, so remember:

• Aqueous KOH: substitution, giving an alcohol.
• Alcoholic KOH: elimination, giving an alkene.

Tertiary halides, being crowded, tend to eliminate.

Grignard reagents. A haloalkane reacts with magnesium in dry ether to give RMgX, a Grignard reagent. Its carbon behaves like R⁻, so it is very reactive. It must be kept away from water: even a trace converts it into the hydrocarbon R–H (and D₂O gives R–D). Its main use is to make alcohols and acids from carbonyl compounds and CO₂ (chapters 7 and 8).

Joining carbon chains with sodium.

• Wurtz reaction: two haloalkanes with sodium in dry ether join their alkyl groups: 2R–X + 2Na → R–R + 2NaX. It doubles the chain.
• Fittig reaction: two aryl halides with sodium give a biaryl, Ar–Ar.
• Wurtz-Fittig reaction: an aryl halide and an alkyl halide with sodium give an alkylbenzene, Ar–R.

Adding one carbon. Substitution with KCN gives an alkyl cyanide, R–CN, one carbon longer; hydrolysing it gives an acid, R–COOH.

To plan a conversion, work backwards from the product: what had to be there one step earlier?