Primary and secondary amines — common mistakes

To maximise the yield of the primary amine, use a large excess of ammonia in a sealed tube under heat and pressure (using ethanol as a solvent). The high concentration of ammonia makes it statistically more likely that the halogenoalkane will collide with an ammonia molecule rather than the amine product.

It's a balance of two factors in aqueous solution. Firstly, secondary amines have two electron-donating alkyl groups, making the lone pair more available than in primary amines (the +I effect). Secondly, the resulting ammonium ion (R₂NH₂⁺) is well-stabilised by solvation (hydrogen bonding with water). Tertiary amines have three alkyl groups (strongest +I effect), but their corresponding ammonium ion (R₃NH⁺) is sterically hindered, leading to less effective solvation, which slightly reduces their basicity compared to secondary amines.

A quaternary ammonium salt has four alkyl or aryl groups bonded to the nitrogen atom, giving the nitrogen a permanent positive charge (e.g., [R₄N]⁺X⁻). Unlike amines, it has no lone pair of electrons on the nitrogen and therefore cannot act as a base or a nucleophile. These salts are often used as surfactants and disinfectants.

No, tertiary amines cannot form amides with acyl chlorides because they do not have a hydrogen atom attached to the nitrogen. The nucleophilic addition-elimination mechanism requires the elimination of a small molecule, which is HCl in the case of primary/secondary amines. Without an N-H bond, this elimination cannot occur to form a stable neutral amide product.