Question

Glycine has pKa values of 2.34 and 9.60. Therefore, glycine will exist predominately as a zwitterion in an aqueous solution with the following pH.

A. pH = 2.34 and pH = 9.60
B. pH > 2.34 and pH < 9.60
C. pH < 2.34 and pH > 9.60
D. pH = 2.34 and pH > 9.60

Answer 1

Option (B) pH > 2.34 and pH < 9.60.

Step-by-step explanation:

In a zwitterionic structure, overall negative charge should be 0.

It means, for glycine, the two functional groups e.g.
`-NH_(2)` and -COOH should bear equal and opposite charges in it's zwitterionic structure.

`pK_(a)` of -COOH (in glycine) = 2.34

Therefore, at a pH > 2.34, -COOH group of glycine exists predominantly as
`-COO^(-)`.

`pK_(a)` of
`-NH_(2)` (in glycine) = 9.60

Therefore, at a pH < 9.60,
`-NH_(2)` group of glycine exists predominantly as
`-NH_(3)^(+)`.

Hence we can say that at 2.34 < pH < 9.60, glycine exists predominantly in zwitterionic form due to presence of
`-COO^(-)` and
`-NH_(3)^(+)` in equal amount.