91Ó°ÊÓ

• The Fischer projection, a way of visualising three-dimensional structures of molecules on a page.
• Horizontal lines represent bonds that are projecting toward the viewer from the plane of the paper, while vertical lines represent bonds that are projecting away from the spectator.
• In this problem, we have to explain the structural difference (in Fischer projections) of $D-$galactose from $D-$glucose.

• $D-$galactose and $D-$glucose are both aldohexoses (six carbons and an aldehyde group) with a right-hand penultimate hydroxy group.
• We can observe that the sole change in the structure of both sugars is on carbon number four. The stereogenic centre on $D-$carbon galactose's number four differs from that of $D-$gluocse.
• $D-$glucose and $D-$galactose are hence $C_4$ epimers.

Epimers are diastereoisomers with several stereogenic centres but only one stereogenic centre.