Question

A certain compound gives negative test with ninhydrin and positive test with Benedict’s solution. The compound is
A.A protein
B.A monosaccharide
C.A lipid
D.An amino acid

Answer 1

Hint:Ninhydrin test is the test which is used to detect the ammonia or primary and secondary amines or carboxylic acid group present in protein and peptide.
Benedict’s solution test is the test which is used for the detection of reducing sugar like glucose. It is used at the place of Fehling’s solution.

Complete step by step answer:
First we will discuss about the chemical tests which is given in the question,
-Ninhydrin test is a chemical test which is used to check if a given analyst contains amine or alpha amino acid. The test used for both qualitative and quantitative analysis.
When amino group of amino acid react with ninhydrin, ninhydrin undergoes oxidative deamination reaction, resulting the Hydrindantin is form with liberation of $CO{}_{2},NH{}_{3}$and aldehydes.

Ninhydrin + amino acid → Hydrindantin + $2H{}_{2}O$
Hydrindantin + Ninhydrin + $NH{}_{3}\to $ Ruhemann purple
Some free amino acids, proteins and lipids give the positive purple colour result of ninhydrin test except prolin or hydroxyproline (give negative test).
-Benedict’s solution tests the test which is used to detect the presence of carbohydrates or reducing sugar (monosaccharide and disaccharide) and the solution is made by sodium carbonate, sodium citrate and copper sulfate pentahydrate.
Approximately 1ml sample placed in tube and add a few drops of Benedict’s solution. The solution is heated in a water bath for 3-5 minutes and observe the colour of solution. The brick red or orange colour indicates the presence of carbohydrates or reducing sugar.

$C{{u}^{2+}}+Aldehyde\to Carboxylic Acid+cuprous oxide$ (Red color compound)
-When the reducing sugar is absent in the sample so that Benedict's solution gives the negative test. For example: sucrose.
So according to this theoretical concept, the correct answer is (B) which is monosaccharide.

Note:
Ninhydrin is commonly used to detect the latent fingerprints present at the crime scene because the terminal amine of lysine residue in peptides and proteins sloughs off in fingerprints reacts with ninhydrin. Benedict’s test is used to detect the glucose in urine.