Question

The incorrect statement from the following with respect to carbohydrates is :

• A. All monosaccharides are reducing sugars.
• B. The monosaccharide units obtained from hydrolysis of oligosaccharides are always the same.
• C. Starch and cellulose are typical examples of polysaccharides, which are very high molecular weight compounds of more than ten monosaccharide units.
• D. Open chain and cyclic structures co-exist at equilibrium that are responsible for certain properties as in the case of D-(+)-glucose.

Hint:

Remember:

• All monosaccharides are reducing sugars.
• Oligosaccharides may contain same or different monosaccharide units.
• Glucose shows mutarotation because cyclic and open-chain forms coexist.

Answer 1

The Correct Option is B

Concept: Carbohydrates are polyhydroxy aldehydes or ketones or compounds that produce them upon hydrolysis. They are classified into:

• Monosaccharides
• Oligosaccharides
• Polysaccharides

Reducing sugars contain a free aldehydic or ketonic group capable of reducing Fehling’s solution or Tollens’ reagent.

Step 1: Checking Statement (1). All monosaccharides possess either:

• Free aldehyde group
• Or ketone group capable of tautomerisation

Therefore, all monosaccharides act as reducing sugars. Examples: \[ \text{Glucose, fructose, galactose} \] Thus, Statement (1) is correct.

Step 2: Checking Statement (2). Oligosaccharides may produce:

• Same monosaccharide units
• Different monosaccharide units

Example:

• Sucrose on hydrolysis gives glucose and fructose.

Since the hydrolysis products are not always identical, the statement saying “always the same” is incorrect. Hence, Statement (2) is incorrect.

Step 3: Checking Statement (3). Polysaccharides contain large numbers of monosaccharide units. Starch and cellulose are classical examples of polysaccharides. They possess: \[ n > 10 \] monosaccharide units and very high molecular masses. Hence, Statement (3) is correct.

Step 4: Checking Statement (4). Glucose exists in equilibrium between:

• Open chain structure
• Cyclic hemiacetal structures

This equilibrium explains phenomena like:

• Mutarotation
• Reducing behaviour

Therefore, Statement (4) is also correct. Thus, the incorrect statement is: \[ \boxed{(2)} \]