Chapter 18, Problem 18.63SP

(a)

Interpretation Introduction

Interpretation:

The two structures of the sugar fructose are given. The cyclic structure predominates in aqueous solution. The number of carbon atoms present in the cyclic structure and the functional group at $\text{C2}$ in the cyclic form is to be identified.

Concept introduction:

Carbohydrates and the sugars mainly exist as cyclic acetals and hemiacetals. The formation of acetals is similar to imine formation and has two steps. First is the acid catalyzed addition of the alcohol to the carbonyl group. Second is substitution of the protonated hemiacetals.

(b)

Interpretation Introduction

Interpretation:

The two structures of the sugar fructose are given. The cyclic structure predominates in aqueous solution. A mechanism for the cyclization of the given sugar structure is to be predicted.

Concept introduction:

The steps followed by the cyclization are stated below:

• The first step is the protonation of carbonyl group.
• The second step is cyclization by the attack of lone pair of oxygen on the carbonyl carbon.
• The third step is the deprotonation of cyclised fructose.