Question 4

The artificial sweetener X described is Sucralose.

1. Monosaccharide Units: It consists of a 4-chloro-4-deoxy-$\alpha$-D-galactopyranose unit and a 1,6-dichloro-1,6-dideoxy-$\beta$-D-fructofuranose unit.

2. Stereochemistry at C4 of Galactose: In D-galactose (as seen in the given Fischer projection, the -OH group at C4 is on the left), which corresponds to the 'up' (axial) position in the 3D chair/Haworth projection. Therefore, for the 4-chloro-4-deoxy-$\alpha$-D-galactose unit, the Chlorine atom at C4 must also be in the 'up' (axial) position.

3. Analyzing the Options (Left Ring):

   • In Option (A), the Cl atom at C4 of the left pyranose ring is pointing up (axial). This correctly represents the galacto configuration.

   • In Option (B), the Cl atom at C4 is pointing down (equatorial). This represents a gluco configuration, which contradicts the given IUPAC name.

4. Glycosidic Linkage & Fructose Unit: The units are joined by an $\alpha(1 \to 2)$ glycosidic bond (linkage between C1 of the galactose pyranose ring and C2 of the fructose furanose ring). The fructose unit has chlorine atoms at C1 and C6 (forming $-CH_2Cl$ groups). Option (A) correctly depicts both the $\alpha(1 \to 2)$ linkage and the chlorinated fructose unit.

Thus, Option (A) is the correct structure.