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Q.
Arrange in order of increasing acidic strength.
Organic Chemistry – Some Basic Principles and Techniques
Solution:
$p K_a$ value of carboxylic group is less than $p K_a$ of $N\overset{+}{H _3}$ in amino acid and $- N\overset{+}{H _3}( Z )$ will have comparatively less $p K_a$ than $- N\overset{+}{H _3}( Y )$ due to -I effect of carboxylic group. We know that acidic strength in inversely proportional to $p K_a$. Hence, correct order of acidic strength is
$- CO _2 H (X) >- N\overset{+}{H _3}(Z) >- N\overset{+}{H _3}(Y)$
