Q39 / 160 · WBJEE 2015 · Chemistry
Within the list shown below, the correct pair of structures of alanine in $\mathrm{pH}$ ranges $2-4$ and $9-11$ is
I. $\mathrm{H}_{3} \mathrm{N}^{+}-\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2} \mathrm{H}$
II. $\mathrm{H}_{2} \mathrm{N}-\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2}^{-}$
III. $\mathrm{H}_{3} \mathrm{N}^{+}-\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2}^{-}$
$\mathrm{IV} . \mathrm{H}_{2} \mathrm{N}-\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2} \mathrm{H}$
- Correct answerAI and II
- BI and III
- CII and III
- DIII and IV
Solution
Isoelectric point for neutral amino acids ranges from $\mathrm{pH} 5.5$ to 6.3 . Acidic amino acids have isoelectric point around 3 while basic amino acids have ranges from pH 7.6 to 10.8 . At isoelectric point, amino acids exist as Zwitter ions. At low pH, it exists as cation and at high $\mathrm{pH}$, it exists as anion. Hence, alanine (pH range $2-4$ ) will exist as cation while alanine (pH range $9-11$ ) will exist as anion.
Acidic medium:
$$
\mathrm{H}_{3} \mathrm{N}^{+} -\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2} \mathrm{H}
$$
Basic medium:
$$
\mathrm{H}_{2} \mathrm{N}-\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2}^{-}
$$
Hence, the correct answer is option: 'A'.