Question

Define the following terms:

(a) Anomers

(b) Invert sugar

(c) Glycosidic linkage

Hint:

Anomers differ at C-1 only. Invert sugar is the glucose and fructose mixture from sucrose. A glycosidic linkage is the C-O-C bridge between two sugar units.

Answer 1

Step 1: Part (a), anomers:
In the ring form of glucose, carbon 1 becomes a new chiral centre. It is called the anomeric carbon.
Two cyclic forms are possible, and they differ only in the position of the OH group on C-1.
Such a pair of cyclic stereoisomers is called anomers.
Example: \( \alpha\text{-D-glucose} \) and \( \beta\text{-D-glucose} \).

Step 2: Part (b), invert sugar:
Sucrose is dextrorotatory. When it is hydrolysed by dilute acid or the enzyme invertase, it gives equal amounts of D-glucose and D-fructose.
\[ \mathrm{C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6} \]
Glucose is dextrorotatory (+52.5 degrees) and fructose is strongly laevorotatory (-92.4 degrees). The mixture is therefore laevorotatory.
Because the sign of rotation changes from dextro to laevo, the mixture is called invert sugar.

Step 3: Part (c), glycosidic linkage:
Two monosaccharide units join when the OH group of the anomeric carbon of one unit reacts with an OH group of the other unit, and a water molecule is lost.
This leaves an oxide bridge \( \mathrm{C-O-C} \) between the two units.
This bridge is called a glycosidic linkage. Examples: the C1-C4 link in maltose and the C1-C2 link in sucrose.

Final Answer:
Anomers differ only at C-1. Invert sugar is the equimolar glucose and fructose mixture from sucrose. A glycosidic linkage is the C-O-C link between two monosaccharides.
\[ \boxed{\text{Anomers; invert sugar; glycosidic linkage (defined above)}} \]