Question

Arrange the following in the decreasing order of the ${pK}_b$ values:

$i)C_2{H}_5{NH}_2$

$ii)C_6{H}_5{NHCH}_3$

$iii\left)\right(C_2{H}_5{)}_{2}NH$

$iv)C_6{H}_5{NH}_2$

• A. $i>ii>iii>iv$
• B. $iii>i>ii>iv$
• C. $i>iii>ii>iv$
• D. $ii>i>iii>iv$

Answer 1

Answer: (B)

$iii>i>ii>iv$

$+I$ effect of alkyl group increases electron density on $-N$ of amine and increases basic strength. Thus $(C_2{H}_5{)}_{2}NH$ is the strongest base. In aniline, the lone pair of electrons of $N$ is in delocalization with ring which makes it less available and least basic. Methyl substitution increases basic strength due to its inductive effect. Therefore the order of base strength or $p{K}_b$ is $iii>i>ii>iv$