In which of the following carbohydrate, molecular mass increases by 84 u after complete acetylation?
In which of the following carbohydrate, molecular mass increases by 84 u after complete acetylation?
Aldotriose
Aldotetrose
Ketotetrose
Ketopentose
Solution
In acetylation reaction $\mathrm{H}$ atom of an $(-\mathrm{OH})$ group is replaced by an acetyl group $\left(-\mathrm{COCH}_3\right)$. This results increase in molecular mass by $[(12+16+12+3 \times 1)-1]$, that as, $42 \mathrm{u}$.
Increase in molecular mass $=84 \mathrm{u}$
$\therefore \quad$ Number of $-\mathrm{OH}$ groups $=\frac{84 \mathrm{u}}{42 \mathrm{u}}=2$
The carbohydrate must be aldotriose as it contains two alcoholic groups.
