1.

Which of the following does not exist as a zwitter ion?

Answer»

GLYCINE
Alanine
Sulphanilic acid
p-Aminobenzoic acid

Solution :In p-aminobenzoic acid (PABA), the lone PAIR of electrons on the `NH_(2)` group is donated towards the benzene ring. As a result, acidic character ofCOOH group and basic character of NH group decreases. Therefore, the weakly acidic COOH group cannot, transfer a `H^(+)` ion to the weakly basic -`-NH_(2)` group. As a result, PABA does not exist as a zwitter ion.
In contrast, in sulphanilic acid, `SO_(3)H` group being a much STRONGER acidic group than COOH group donates a proton to weakly basic `- NH_(2)` group to form a zwitter ion.

In glycine and alanine, no such electron withdrawing benzene ring is present. As a result, -`NH_(2)` group is sufficiently basic and HENCE accepts a proton from --COOH group to form a zwitter ion.
`H_(2) N - CHR - COOH to H_(3)overset(+)(N) - CHR - COO^(-)`


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