Animated Solution for Chemistry - Biomolecules: Which one of the following statements is not true?

Biomolecules can sometimes feel like a maze of complex rings and confusing stereochemistry, but once you understand the underlying logic, they become incredibly fascinating. Today, we are going to dissect the structure of lactose, the primary sugar found in milk, and evaluate a series of statements to find the imposter.

Let's visualize the structure of lactose. It is a disaccharide, meaning it is constructed from two simple sugar units (monosaccharides) joined together. On the left side of the molecule, we have a $\beta$-D-galactose ring, and on the right side, we have a D-glucose ring.

Now, how are they connected? Look closely at the bond joining them. It connects carbon-1 ($C_1$) of the galactose unit to carbon-4 ($C_4$) of the glucose unit. Because the hydroxyl ($-OH$) group on carbon-1 of galactose was pointing upwards (in the $\beta$ position) before the bond was formed, this specific connection is called a $\beta$-1,4-glycosidic linkage.

Option (a) claims that lactose contains an $\alpha$-glycosidic linkage. Based on our structural analysis, this is clearly incorrect! The linkage is strictly $\beta$-1,4.

Let's check the other options to be absolutely sure. Is lactose a reducing sugar? For any carbohydrate to be classified as a reducing sugar, it must possess a free anomeric carbon. This means the hemiacetal group must be intact so it can open up into a reactive aldehyde chain in an aqueous solution.

If we look at the glucose unit on the right side of the lactose molecule, its carbon-1 ($C_1$) has a free hydroxyl group! This free hemiacetal group makes lactose a reducing sugar, meaning it will readily reduce $C{u}^{2+}$ to $C{u}^{+}$ and give a positive Fehling's test. So, option (b) is a perfectly true statement.

What about the number of hydroxyl groups? Let's count them together.

In the galactose ring, we have free $-OH$ groups at positions $C_2$, $C_3$, $C_4$, and $C_6$, giving us 4 hydroxyl groups. In the glucose ring, we have free $-OH$ groups at positions $C_1$, $C_2$, $C_3$, and $C_6$, giving us another 4 hydroxyl groups.

That is a total of 8 hydroxyl groups. The chemical formula for lactose is indeed $C_{12}{H}_{22}{O}_{11}$. So, option (c) is also a true statement.

Acid Hydrolysis

Finally, what happens if we boil lactose with dilute acid? The $\beta$-1,4-glycosidic bond breaks in a process called hydrolysis.

We get exactly what we started with! One molecule of D-(+)-galactose and one molecule of D-(+)-glucose. So, option (d) is also true.

Therefore, the only incorrect statement among the choices is option (a). Always remember the specific linkages for common disaccharides: Lactose is $\beta$-1,4, Maltose is $\alpha$-1,4, and Sucrose is $\alpha$-1, $\beta$-2. Keep these distinctions crystal clear in your mind!