CBSE Class 12 Chemistry: Marking Scheme Solution

Question: Give reasons for any 3 of the following observations.

(a) Penta-acetate of glucose does not react with hydroxylamine ($NH_2OH$).

• Step 1 (Governing Principle): [0.5 Mark] The reaction of glucose with $NH_2OH$ depends on the presence of a free aldehydic group ($-CHO$) to form an oxime.
• Step 2 (Structural Analysis): [0.5 Mark] In glucose penta-acetate, all five $-OH$ groups are acetylated. However, the absence of reaction indicates that the open-chain form of glucose is not present in the penta-acetate derivative.
• Step 3 (Final Conclusion): [0.0 Mark] The penta-acetate of glucose does not contain a free $-CHO$ group because the cyclic hemiacetal structure is locked, preventing the formation of the open-chain aldehyde required for the reaction with $NH_2OH$.

💡 EL10 Concept Insight: Think of the aldehyde group as a "door" that needs to be open for the hydroxylamine to enter; in penta-acetate, that door is permanently locked shut by the chemical modification.

(b) Amino acids behave like salts.

• Step 1 (Governing Principle): [0.5 Mark] Amino acids contain both an acidic group ($-COOH$) and a basic group ($-NH_2$) within the same molecule.
• Step 2 (Mechanism): [0.5 Mark] In aqueous solution, the carboxyl group loses a proton ($H^+$) and the amino group accepts it, forming a dipolar ion known as a zwitterion: $$H_2N-CH(R)-COOH \rightleftharpoons H_3N^+-CH(R)-COO^-$$
• Step 3 (Final Conclusion): [0.0 Mark] Due to the presence of strong electrostatic attractions between the oppositely charged ends of the zwitterion, amino acids exhibit high melting points and solubility characteristics similar to ionic salts.

💡 EL10 Concept Insight: An amino acid is like a person holding their own hand—because it has both a positive and negative charge inside, it sticks to itself and other molecules just like salt does.

(c) Water soluble vitamins must be taken regularly in diet.

• Step 1 (Governing Principle): [0.5 Mark] Vitamins B and C are water-soluble and cannot be stored in the body to a significant extent.
• Step 2 (Physiological Process): [0.5 Mark] Because they are soluble in water, any excess intake is readily excreted from the body through urine.
• Step 3 (Final Conclusion): [0.0 Mark] Since the body cannot store them for long-term use, they must be supplied regularly through the diet to prevent deficiency diseases.

💡 EL10 Concept Insight: Think of your body like a leaky bucket for water-soluble vitamins; since they wash out with your urine, you need to keep "refilling the bucket" every day.

(d) The two strands in DNA are complimentary to each other.

• Step 1 (Governing Principle): [0.5 Mark] The structure of DNA is governed by specific base-pairing rules (Chargaff's Rule).
• Step 2 (Mechanism): [0.5 Mark] Adenine ($A$) always pairs with Thymine ($T$) via two hydrogen bonds, and Guanine ($G$) always pairs with Cytosine ($C$) via three hydrogen bonds.
• Step 3 (Final Conclusion): [0