Explain the basic character of amines. Compare the basic strengths of aliphatic amines, ammonia and aniline, and arrange the methylamines in order of basic strength in water, giving reasons.

1. 1 · State why amines are basic. The lone pair on N accepts a proton: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻. Strength is measured by Kb; the smaller the pKb, the stronger the base.
2. 2 · Aliphatic amines vs ammonia. Alkyl groups push electrons to N (inductive, +I effect), so aliphatic amines (pKb about 3 to 4) are stronger than NH₃ (4.75).
3. 3 · Order in water. +I effect, solvation of the ion by hydrogen bonding, and steric crowding together give (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.
4. 4 · Aniline. The lone pair is delocalised into the ring by resonance, so aniline (pKb 9.38) is weaker than NH₃.
5. 5 · Draw the resonance. Draw aniline with the lone pair on N and curved arrows into the ring; show the negative charge at the ortho and para carbons.
6. 6 · Link to real life. Lemon juice on fish turns smelly amines into odourless salts.
7. 7 · Close in one line. So the more available the lone pair, the stronger the amine as a base.

Scoring tip: Name all three effects (inductive, solvation, steric) when you explain the order in water; naming only the inductive effect loses marks.