Lesson eLearning

A sample of a monosaccharide gives a brick‑red precipitate in Benedict test at neutral pH. Which of the following is the most likely structure of the sugar?

easy MCQ 3 marks Class 12 · Chemistry · Biomolecules
  1. An aldehyde with five carbons
  2. An aldehyde with six carbons
  3. A ketone with six carbons
  4. A ketone with five carbons
Answer
An aldehyde with six carbons.

The correct answer is An aldehyde with six carbons. — Select the six‑carbon aldehyde as the correct structure.

Step-by-step solution

  1. Identify that a positive Benedict's test indicates a reducing sugar that can reduce the copper(II) ions.
  2. Recognise that only aldehyde groups (not ketones) can reduce Cu²⁺ at neutral pH.
  3. Compare the options: an aldehyde with six carbons is a reducing sugar; the other options either lack an aldehyde or have fewer carbons.
  4. Select the six‑carbon aldehyde as the correct structure.
Common mistake: Assuming ketones can also reduce Benedict's reagent at neutral pH

Concept tested: Identifying reducing sugars at neutral pH

Published · How this solution was prepared and checked

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