Q. 13.58 - An Introduction to General, Organic, and Biological Chemistry

Question

Raffinose, found in Australian manna and in cottonseed meal, has the following Haworth structure: 

$(13.4,13.5,13.6)$

a. Is raffinose a mono-, di-, or trisaccharide?

b. What monosaccharides are present in raffinose?

c. Is raffinose a reducing sugar?

Step-by-Step Solution

Answer

(Part a) Raffinose is a polysaccharide.

(Part b) Aldohexoses are galactose and glucose, and ketopentose is fructose.

Step 1: Given information (Part a)

The haworth structure is 

We have to find raffinose is a polysaccharide.

Step 2 : Explanation for (a)

If we look at the structure of raffinose (Figure in Step l), we can see that this sugar has two glycoside bonds (a bond that is formed when the hydroxyl group of one monosaccharide reacts with the hydroxyl group of another monosaccharide).

If it has two glycosidic bonds, it means that three sugars are connected, which means that raffinose is a polysaccharide.

Step 3 : Given information (b)

The haworth structure is 

We have to find that monosaccharides are present in raffinose 

Step 4 : Explanation for (b)

If we look closely at the structure of raffinose, we can see that there are two six-carbon sugars, which are aldohexose, and one five-carbon sugar, which is ketopentose. Aldohexoses are galactose and glucose, and ketopentose is fructose.

Step 5 : Given information for (c)

The haworth structure is 

We have to find that raffinose is a reducing sugar 

Step 6 : Explanation for (b)

Reducing sugars are carbohydrates with an aldehyde group that can reduce the ${\mathrm{Cu}}^{2+}$ in Benedict's reagent.

This means that every sugar that has an aldehyde group in an open-chain form is a reducing sugar.

To know which sugar has an aldehyde group in an open-chain form, we have to look for the hemiacetal group (a hemiacetal is a carbon connected to two oxygen atoms, where one oxygen is alcohol $\left(OH\right)$ and the other is an ether $\left(OR\right))$.

In raffinose, there are no hemiacetal groups, that can be opened into the aldehyde group, so raffinose is a nonreducing sugar.