nitriles

Meaning: A nitrile has the functional group R–C≡N. The carbon and nitrogen are sp hybridized, so the R–C≡N unit is linear. The C≡N bond is polar, so the carbon is electrophilic, much like a carbonyl carbon, and nucleophiles can add to it. Nitriles form when cyanide ion (CN⁻) displaces a halide from a primary or secondary alkyl halide by SN2, or when a primary amide is dehydrated. Heating with aqueous acid or base hydrolyzes a nitrile to a carboxylic acid, through an amide intermediate. Reduction with LiAlH₄ gives a primary amine.

On the MCAT: Expect multistep synthesis questions such as alkyl halide → nitrile → carboxylic acid or amine, where you must notice that the carbon chain grows by one carbon. Cyanohydrins, made from an aldehyde or ketone plus HCN, also contain a nitrile. In IR spectroscopy a nitrile gives a sharp peak near 2250 cm⁻¹, in the region of triple bonds. In naming, the nitrile carbon counts in the parent chain, so CH₃CN is ethanenitrile (acetonitrile).

Common confusion: Students treat the nitrile nitrogen like an amine nitrogen. Its lone pair sits in an sp orbital, which has high s character, so a nitrile is only very weakly basic. Also, cyanide is a fairly strong base, so tertiary halides tend to give elimination instead of the nitrile.