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Practical Organic Chemistry question

2020 · 9 Jan · Shift 2 · Q4
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Practical Organic Chemistry question

2020 · 9 Jan · Shift 2 · Q4

JEE MainChemistryPractical Organic ChemistryMCQ+4 / −1
A, B and C are three biomolecules. The results of the tests performed on them are given below:  Molisch’s  Test  Barfoed  Test  Biuret  Test  A  Positive  Negative  Negative  B  Positive  Positive  Negative  C  Negative  Negative  Positive \begin{array}{|l|l|l|l|} \hline & \begin{array}{l} \text { Molisch's } \\ \text { Test } \end{array} & \begin{array}{l} \text { Barfoed } \\ \text { Test } \end{array} & \begin{array}{l} \text { Biuret } \\ \text { Test } \end{array} \\ \hline \text { A } & \text { Positive } & \text { Negative } & \text { Negative } \\ \hline \text { B } & \text { Positive } & \text { Positive } & \text { Negative } \\ \hline \text { C } & \text { Negative } & \text { Negative } & \text { Positive } \\ \hline \end{array} A  B  C ​ Molisch’s  Test ​ Positive  Positive  Negative ​ Barfoed  Test ​ Negative  Positive  Negative ​ Biuret  Test ​ Negative  Negative  Positive ​​ A, B and C are respectively :
  1. A
    A = Lactose, B = Glucose, C = Albumin
  2. B
    A = Lactose, B = Fructose, C = Alanine
  3. C
    A = Lactose, B = Glucose, C = Alanine
  4. D
    A = Glucose, B = Fructose, C = Albumin
View written solutionFree

Correct answer: A

  1. Interpret each qualitative test

    • Molisch's test: Positive for carbohydrates.
    • Barfoed's test: Positive for monosaccharides; generally negative for disaccharides under test conditions.
    • Biuret test: Positive for proteins/peptides having two or more peptide bonds.

  2. Analyze biomolecule A

    Given for A:

    • Molisch's test: Positive
    • Barfoed's test: Negative
    • Biuret test: Negative

    Therefore, A is:

    • a carbohydrate (Molisch positive)
    • not a monosaccharide (Barfoed negative)
    • not a protein (Biuret negative)

    So A is most likely a disaccharide, such as lactose.

  3. Analyze biomolecule B

    Given for B:

    • Molisch's test: Positive
    • Barfoed's test: Positive
    • Biuret test: Negative

    Therefore, B is:

    • a carbohydrate
    • a monosaccharide
    • not a protein

    So B can be glucose or fructose.

  4. Analyze biomolecule C

    Given for C:

    • Molisch's test: Negative
    • Barfoed's test: Negative
    • Biuret test: Positive

    Therefore, C is:

    • not a carbohydrate
    • a protein/peptide

    Albumin is a protein, so it gives Biuret positive. Alanine is an amino acid, not a protein, and does not give Biuret test positive.

    Hence, C = Albumin.

  5. Check options systematically

    • Option A: A=A=A= Lactose, B=B=B= Glucose, C=C=C= Albumin

      • Lactose: carbohydrate, disaccharide ⇒\Rightarrow⇒ Molisch +++, Barfoed −-−
      • Glucose: carbohydrate, monosaccharide ⇒\Rightarrow⇒ Molisch +++, Barfoed +++
      • Albumin: protein ⇒\Rightarrow⇒ Biuret +++
      • All match.

    • Option B: A=A=A= Lactose, B=B=B= Fructose, C=C=C= Alanine

      • C=C=C= Alanine would give Biuret negative, not positive.
      • Incorrect.

    • Option C: A=A=A= Lactose, B=B=B= Glucose, C=C=C= Alanine

      • Again, alanine does not give Biuret positive.
      • Incorrect.

    • Option D: A=A=A= Glucose, B=B=B= Fructose, C=C=C= Albumin

      • A=A=A= Glucose is a monosaccharide, so Barfoed test should be positive, but A has Barfoed negative.
      • Incorrect.

  6. Final conclusion

    Therefore, A=Lactose,B=Glucose,C=AlbuminA = \text{Lactose}, \quad B = \text{Glucose}, \quad C = \text{Albumin}A=Lactose,B=Glucose,C=Albumin

    So the correct option is A.

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