MHT CET · Chemistry · Biomolecules

In which of the following carbohydrate, molecular mass increases by 84 u after complete acetylation?

A Aldotriose
B Aldotetrose
C Ketotetrose
D Ketopentose

Verified Solution

Answer & Solution

Correct Answer

(A) Aldotriose

Step-by-step Solution

Detailed explanation

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.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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