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: \[ \boxed{(2)} \]