Aromatic Compounds Containing Nitrogen - Result Question 2
####2. The increasing order of the $p K _b$ of the following compound is
(A)
(B)
(D)
(a) (A) $<$ (C) $<$ (D) $<$ (B)
(2019 Main, 12 April I)
(b) (C) $<$ (A) $<$ (D) $<$ (B)
(c) (B) $<$ (D) $<$ (A) $<$ (C)
(d) (B) $<$ (D) $<$ (C) $<$ (A)
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Answer:
Correct Answer: 2. (c)
Solution:
- Key Idea $p K _b$ value is defined as the minus lagarithm of $K _b$ smallar the value of $p K _b$ stronger is the base and vice-versa.
In the given options, $+R$ effect is shown by $-CH _3$ and $-OCH _3$ group $\left(-OCH _3>-CH _3\right)$. These group increases the electron density at $o$ and $p$-positions. Groups such as $-F$ and $-NO _2$ shows $-R$-effect $\left(-NO _2>-F\right)$. These group decreases the electron density at $o$ and $p$-positions.
Increase in electron density at $p$-position makes the unshared electron pair of ’ $N$ ’ more available and decrease in electron density at $p$-position makes the unshared electron pair of ’ $N$ ’ less available. Compound containing $-OCH _3$ group act as strongest base and hence possess lowest value of $p K _b$. So, the correct increasing order of $p K _b$ in the given compound is
(C)
(A)
(D)
(B)
