Animated Solution for Chemistry - Biomolecules: The number of chiral carbons present in sucrose is ……… .

To find the number of chiral carbons in sucrose, we first need to visualize its molecular structure. Sucrose is a common disaccharide, meaning it is composed of two simpler monosaccharide units joined together. Specifically, it consists of an $\alpha$-D-glucose unit and a $\beta$-D-fructose unit.

These two rings are connected by a glycosidic linkage. In sucrose, this linkage forms between the $C_1$ anomeric carbon of the glucose ring and the $C_2$ anomeric carbon of the fructose ring. This specific connection is known as an $\alpha (1\to 2)\beta$ glycosidic bond.

Chirality

Before we start counting, let's quickly recall what makes a carbon atom chiral. A chiral carbon (or stereocenter) is a carbon atom that is bonded to four completely different atoms or groups of atoms. If any two groups attached to the carbon are identical, it is achiral.

Our task is to systematically examine every carbon atom in both the glucose and fructose rings to see if they meet this criterion.

Let's focus on the $\alpha$-D-glucose ring first, which is a six-membered pyranose ring.

Starting at the anomeric carbon ($C_1$), we see it is bonded to a hydrogen atom, the glycosidic oxygen, the ring oxygen, and the $C_2$ carbon. Since all four paths/groups are different, $C_1$ is chiral.

Moving around the ring, we examine $C_2$, $C_3$, $C_4$, and $C_5$. Each of these carbons is bonded to a hydrogen atom, a hydroxyl group ($-OH$), and two different segments of the carbon ring. Therefore, $C_2,C_3,C_4,$ and $C_5$ are all chiral.

What about $C_6$? The $C_6$ carbon is part of a $-C{H}_{2}OH$ group. Because it is bonded to two identical hydrogen atoms, it is achiral.

So, the glucose unit contributes exactly $5$ chiral carbons.

Now, let's examine the $\beta$-D-fructose ring, which is a five-membered furanose ring.

We start at its anomeric carbon, which is $C_2$. It is bonded to a $-C{H}_{2}OH$ group, the glycosidic oxygen, the ring oxygen, and the $C_3$ carbon. All four groups are distinct, making $C_2$ chiral.

Continuing around the furanose ring, we look at $C_3$, $C_4$, and $C_5$. Just like in the glucose ring, each of these carbons is bonded to four different groups (including the different paths around the ring). Thus, $C_3,C_4,$ and $C_5$ are chiral.

Fructose has two $-C{H}_{2}OH$ groups (at $C_1$ and $C_6$). Since both of these carbons are bonded to two identical hydrogen atoms, they are achiral.

So, the fructose unit contributes exactly $4$ chiral carbons.

We have successfully identified all the stereocenters in the molecule. Now, we simply add them together:

Therefore, a single molecule of sucrose contains exactly $9$ chiral carbons.