Choose and explain: a. Strongest base to H (gas phase): NH3, CH3NH2, (CH3)2NH, (CH3)3N
b. Strongest base to BMe3: pyridine,
2-methylpyridine, 4-methylpyridine

a) (CH3)3N

b) 4-methylpyridine

Explanation:

Let us bear in mind that the basicity of amines depend on;

1) the availability of the lone pair

2) the stability of the conjugate acid of the amine.

In the gaseous phase, the basicity of the amine strictly depends on the availability of the lone pair. The tertiary amine is better able to accept a proton in the gaseous phase since it is surrounded by three methyl groups having an electron pushing effect thereby reinforcing the lone pair on the nitrogen. This order is reversed in solution due to solvation.

Here again, the electron donation to the nitrogen bearing the lone pair is important. The 4-methylpyridine is more basic than 2-methylpyridine to BMe3 due to steric hindrance that hinders the bonding of 2-methylpyridine to BMe3.

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