Question

Number of compounds from the following which will not produce orange red precipitate with Benedict solution is ____________
Glucose, maltose, sucrose, ribose, 2-deoxyribose, amylose, lactose

Answer 1

Correct Answer: 3

Benedict’s solution is used to test for the presence of reducing sugars. Reducing sugars are carbohydrates that have a free aldehyde or ketone group capable of reducing \( \text{Cu}^{2+} \) ions in Benedict’s solution to \( \text{Cu}^+ \), resulting in the formation of an orange-red precipitate of copper(I) oxide (\( \text{Cu}_2\text{O} \)).

Analysis of the Given Compounds:

• Glucose: Reducing sugar (✓) — Has a free aldehyde group.
• Maltose: Reducing sugar (✓) — Contains a free reducing end from glucose units.
• Sucrose: Non-reducing sugar (×) — The glucose and fructose units are linked by their anomeric carbons, leaving no free aldehyde or ketone group.
• Ribose: Reducing sugar (✓) — Has a free aldehyde group.
• 2-Deoxyribose: Reducing sugar (✓) — Has a free aldehyde group.
• Amylose: Non-reducing sugar (×) — Does not have a free aldehyde group accessible for reduction.
• Lactose: Reducing sugar (✓) — Contains a free reducing end from glucose units.

Compounds That Do Not React:

The two compounds that do not give a positive Benedict’s test are:

• Sucrose: Due to the linkage of glucose and fructose at their anomeric carbons, it lacks a free aldehyde or ketone group.
• Amylose: Does not have a free aldehyde group to react with Benedict’s reagent.

Final Answer: 

Two compounds (sucrose and amylose) will not produce an orange-red precipitate with Benedict’s solution.