Question

The structure of a disaccharide $M$ (molecular mass $342$) is given below. Hydrolysis of the disaccharide gives two monosaccharides $X$ and $Y$. Acetylation using acetic anhydride is one of the methods used in structure determination of carbohydrates. On the basis of the structure of $M$ given below, select the correct option(s).

• A. $M$ is a non-reducing sugar
• B. On hydrolysis, $M$ gives one aldohexose and one aldopentose
• C. If the specific rotation $\left(\alpha \right)$ of $X,Y$ and $M$ are $+{52}^{\circ },-{74}^{\circ }$ and $+{29}^{\circ }$ respectively, the mixture of $X$ and $Y$, obtained upon hydrolysis of $M$ is laevorotatory
• D. $0.171g$ of $M$ requires $0.153g$ of acetic anhydride for complete acetylation

Answer 1

Answer: (A), (C)

If the specific rotation $\left(\alpha \right)$ of $X,Y$ and $M$ are $+{52}^{\circ },-{74}^{\circ }$ and $+{29}^{\circ }$ respectively, the mixture of $X$ and $Y$, obtained upon hydrolysis of $M$ is laevorotatory
$M$ is a non-reducing sugar

M is a non-reducing sugar as it do not have an OH group attached to the anomeric carbon so it cannot reduce other compounds.
On hydrolysis, M gives one aldohexose and one ketopentose.
$0.171g$ ($\frac{0.171}{342}=0.0005$ mol) of $M$ requires $0.0005\times 8\times 102=0.408$ of acetic anhydride for complete acetylation.
If the specific rotation $\left(\alpha \right)$ of $X,Y$ and $M$ are $+{52}^{\circ },-{74}^{\circ }$ and $+{29}^{\circ }$ respectively, the mixture of $X$ and $Y$, obtained upon hydrolysis of $M$ is laevorotatory
This is because the magnitude of specific rotation of $Y$ is more than that of $X$.