Chapter 9, Problem 9.77P

Interpretation Introduction

(a)

Interpretation:

The glycosidic linkage and acetal carbon in the lactose are to be identified.

Concept introduction:

An acetal is characterized by a C atom bonded to two alkyl (or H) groups and two alkoxy (RO–) groups. The compound in which the OH from a sugar molecule has been replaced by OR is called glycoside. The $\text{C-O-C}$ bond that connects the sugars is called glycosidic linkage.

Interpretation Introduction

(b)

Interpretation:

The type of glycosidic linkage that lactose has is to be determined.

Concept introduction:

If the alkoxy (RO-) group on the acetal carbon is in axial position, it is designated as $\text{α}$, and if it is in equatorial position, it is designated as $\text{β}$. If the ring on the other side of glycosidic linkage is bonded to C4, it is notified as 1, 4’, where the prime (’) indicates that C4 and C1 are on different sugar units.

Interpretation Introduction

(c)

Interpretation:

The complete, detailed mechanism and the products of the acid catalyzed hydrolysis of lactose are to be drawn.

Concept introduction:

The acid catalyzed hydrolysis of lactose gives two monosaccharides, which are linked by glycoside linkage. In Step 1, protonation of electron rich O atom takes place. The bond of acetal carbon and electron poor O of the glycosidic linkage breaks in heterolysis step. In Step 3, the newly formed carbocation is attacked by water as a nucleophile. This can take place on either side of the plane of the carbocation C. Finally, in Step 4, deprotonation removes the positive charge from O and forms $\text{D-Galactose}$.