Which among the following is the strongest Bronsted base?

Solution

Chemistry ma’am · 3 steps

Look at the lone pair on N: is it pushed out by alkyl groups, crowded by them, or pulled into a double bond?

1. What decides it. A Bronsted base accepts H⁺ on the nitrogen lone pair. It is strongest when alkyl groups push electron density onto N (+I effect), nothing crowds the lone pair, and the R₃NH⁺ ion formed is easy to stabilise by solvation.

   Option	Amine	State of the lone pair
   (a)	triethylamine, 3°	three +I groups, but the freely swinging ethyl chains crowd N and block solvation of Et₃NH⁺
   (b)	diethylamine, 2°	only two +I groups, so less electron push onto N
   (c)	N-methyl cyclic enamine	N sits on a C=C carbon, so the lone pair is delocalised into the double bond
   (d)	bicyclic 3° amine (1-azabicyclo[2.2.1]heptane)	three +I groups, but they are tied back into rings, so the lone pair is fully exposed

2. Why the cage wins. In (d) the three carbon arms on N are locked into the ring frame and point away from the lone pair. You get the full +I effect of a tertiary amine without the steric crowding of (a), so H⁺ approaches easily and the cation formed is well solvated.
3. So. The bicyclic amine (d) is the strongest Bronsted base, and the enamine (c) is the weakest.

The idea to keep. Tying alkyl groups back into a ring keeps their +I push but removes their crowding, so caged amines beat open-chain ones. A lone pair conjugated with C=C is always a poor base.

Answer from the NTA final answer key. Solution by PrepWiser.