AP EAPCET 20 May 2024 Shift 2 Solved Paper

The correct answer is Threonine.A chiral centre is a carbon atom bonded to four different groups.For the options given, we examine the structure of each amino acid:Leucine: $\text{(CH}_3\text{)}_2\text{CH}-\text{CH}_2-\text{CH}(\text{NH}_2)-\text{COOH}$.Only the $\alpha$-carbon is bonded to four different groups, so it has only one chiral centre.Valine: $\text{(CH}_3\text{)}_2\text{CH}-\text{CH}(\text{NH}_2)-\text{COOH}$.Here again only the $\alpha$-carbon is chiral, so it has only one chiral centre.Serine: $\text{HO}-\text{CH}_2-\text{CH}(\text{NH}_2)-\text{COOH}$.The $\beta$-carbon bears two identical hydrogen atoms, so it is not chiral; serine has only one chiral centre.Threonine: $\text{CH}_3-\text{CH}(\text{OH})-\text{CH}(\text{NH}_2)-\text{COOH}$.Both the $\alpha$-carbon (bonded to $\text{NH}_2$, $\text{COOH}$, $\text{H}$ and the side chain) and the $\beta$-carbon (bonded to $\text{OH}$, $\text{H}$, $\text{CH}_3$ and the rest of the molecule) are bonded to four different groups.Therefore threonine possesses two chiral centres and hence exists as four stereoisomers ($\text{L}$-threonine, $\text{D}$-threonine, $\text{L}$-allothreonine and $\text{D}$-allothreonine).Among the given options, only threonine has two chiral centres.