question_answer

The increasing order of pKa of the following ammo acids in aqueous solution is: Gly Asp Lys Arg [JEE Main Online Paper (Held On 09-Jan-2019 Morning]

A) \[Asp<Gly<Lys<Arg\]

B) \[Arg<Lys<Gly<Asp\]

C) \[Gly<Asp<Arg<Lys\]

D) \[Asp<Gly<Arg<Lys\]

Correct Answer: A

Solution :

Acidic strength \[\propto \,\,\frac{1}{pKa}\,\,\propto \,\,EWG\] \[pKa\,\,=\,\,-\,\log \,\,Ka\]   Acidic strength in solution \[\Rightarrow \text{ }ASP\text{ }>\text{ }Gly\text{ }>Lys\text{ }>\text{ }Arg\] pKa order should be \[\Rightarrow \text{ }Asp\text{ }<\text{ }Gly\text{ }<\text{ }Lys\text{ }<Arg\] [Though data suggest this is order of Isoelectric pH value]