Question

The correct order of decreasing acidic strength is

• A. $X>Z>Y$
• B. $Z>X>Y$
• C. $X>Y>Z$
• D. $Z<X<Y$

Answer 1

Answer: (A)

$K_a$ value of carboxylic group is more than $K_a$ of $\stackrel{+}{N}{H}_3$ in amino acid and (Z) will have comparatively more $K_a$ than $—\stackrel{+}{N}{H}_3(Y)$ due to -$I$ effect of carboxylic group. We know that acidic strength is directly proportional to $K_a$
Hence, correct order of acidic strength is :
$\underset{(X)}{-\stackrel{+}{C}{O}_{2}H}>\underset{(Z)}{-\stackrel{+}{N}{H}_3}>\underset{(Y)}{\stackrel{+}{N}{H}_3}$