Aliphatic Compounds Containing Nitrogen - Result Question 16
17. Considering the basic strength of amines in aqueous solution, which one has the smallest $p K _b$ value?
(2014 Main)
(a) $\left(CH _3\right) _2 NH$
(b) $CH _3 NH _2$
(c) $\left(CH _3\right) _3 N$
(d) $C _6 H _5 NH _2$
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Answer:
Correct Answer: 17. (a)
Solution:
This problem can be solved by using the concept of effect of steric hindrance, hydration and H-bonding in basic strength of amines. Order of basic strength of aliphatic amine in aqueous solution is as follows (order of $K _b$ )
$ \left(CH _3\right) _2 \ddot{N} H>CH _3 \ddot{N} H _2>\left(CH _3\right) _3 \ddot{N}>C _6 H _5 \ddot{N} H _2 $
As we know, $p K _b=-\log K _b$
So, $\left(CH _3\right) _2 \ddot{N} H$ will have smallest $p K _b$ value. In case of phenyl amine, $N$ is attached to $s p^{2}$-hybridised carbon, hence it has highest $p K _b$ and least basic strength.
BETA
