Question
Arrange the following amines in order of increasing basicity in aqueous solution:
, , , , (aniline)
Explain why is more basic than in water, but less basic in the gas phase.
Solution — Step by Step
Three competing effects determine amine basicity:
- Inductive effect (+I) — alkyl groups donate electron density to N, stabilising the conjugate acid. More alkyl groups, more basicity.
- Steric effect — bulky groups around N hinder protonation.
- Solvation effect (in water) — the conjugate acid is stabilised by H-bonding with water. More N-H bonds means more H-bonds means more stabilisation.
In aniline, the lone pair on N is delocalised into the benzene ring, reducing its availability for protonation. So aniline is the weakest base in this list.
The empirical order in water is:
Note that tops the list — secondary beats tertiary. This is because the tertiary cation has only one N-H bond available for H-bonding with water, while the secondary cation has two.
Without solvent, only the inductive effect matters. The order becomes:
i.e., basicity increases monotonically with the number of alkyl groups, as expected from +I effects alone.
Final: aqueous order has on top; gas-phase order has on top.
Why This Works
The “inverted” order in water is one of the most-asked NEET conceptual questions. The key is that water’s role is non-trivial — it favours species with more N-H bonds for H-bonding. This is why secondary amines often appear surprisingly more basic than tertiary in aqueous solution.
In non-aqueous solvents (like THF or gas phase), the pure inductive trend reasserts itself: more alkyl groups = more basic.
Alternative Method
Compare values:
- : 4.74
- : 3.36
- : 3.27
- : 4.22
- : 9.4
Lower means stronger base. Read off the order directly.
Common Mistake
Students assume is the most basic because it has the most alkyl groups. In water, this is wrong — solvation flips the order. Always specify “in aqueous solution” or “in gas phase” before answering.