Question

Pick out the incorrect statement(s) from the following.

1.  Glucose exists in two different crystalline forms, \( \alpha \)-D-glucose and \( \beta \)-D-glucose.
2.  \( \alpha \)-D-glucose and \( \beta \)-D-glucose are anomers.
3.  \( \alpha \)-D-glucose and \( \beta \)-D-glucose are enantiomers.
4.  Cellulose is a straight chain polysaccharide made of only \( \beta \)-D-glucose units.
5.  Starch is a mixture of amylase and amylopectin, both contain unbranched chain of \(\alpha\)-D-glucose units.

• A. 1 and 2 only
• B. 2 and 3 only
• C. 3 and 4 only
• D. 3 and 5 only

Hint:

When working with sugars, remember that anomers differ at only one stereocenter (the anomeric carbon), while enantiomers differ at all chiral centers.

Answer 1

The Correct Option is D

Let's analyze each statement to identify the incorrect ones:

1. Glucose exists in two different crystalline forms, \( \alpha \)-D-glucose and \( \beta \)-D-glucose.

This statement is correct. Glucose has two anomeric forms: \( \alpha \) and \( \beta \).

2. \( \alpha \)-D-glucose and \( \beta \)-D-glucose are anomers.

This statement is correct. Anomers are isomers of monosaccharides that differ in configuration only at the hemiacetal/acetal carbon, also known as the anomeric carbon.

3. \( \alpha \)-D-glucose and \( \beta \)-D-glucose are enantiomers.

This statement is incorrect. Enantiomers are mirror images of each other and cannot be turned into one another by rotation. \( \alpha \) and \( \beta \) forms are not mirror images; they differ only in the configuration at the anomeric carbon.

4. Cellulose is a straight chain polysaccharide made of only \( \beta \)-D-glucose units.

This statement is correct. Cellulose is composed of \( \beta \)-D-glucose units linked by \( \beta \)-1,4-glycosidic bonds.

5. Starch is a mixture of amylase and amylopectin, both contain unbranched chain of \( \alpha \)-D-glucose units.

This statement is incorrect. Starch contains both amylose and amylopectin, but amylopectin is branched, not unbranched. Only amylose is an unbranched chain of \( \alpha \)-D-glucose units.

Option	Reason
3 and 5 only	Both statements 3 and 5 are incorrect based on the above analysis.

Answer 2

The statements in the options are as follows:
1. Glucose exists in two crystalline forms, \( \alpha \)-D-glucose and \( \beta \)-D-glucose. This statement is correct.
2. \( \alpha \)-D-glucose and \( \beta \)-D-glucose are anomers. This is also correct. Anomers are cyclic forms of sugars that differ only in the configuration of the hydroxyl group on the anomeric carbon.
3. \( \alpha \)-D-glucose and \( \beta \)-D-glucose are not enantiomers; they are diastereomers (since they differ in configuration at one stereocenter, not in all). Therefore, this statement is incorrect.
4. Cellulose is a straight chain polysaccharide made of only \( \beta \)-D-glucose units. This is correct. 5. Starch is a mixture of amylose and amylopectin, both of which contain unbranched chains of \( \alpha \)-D-glucose. This is correct.
Thus, the incorrect statements are 3 and 5, so the answer is (D).