Question

How many of the following are stereoisomers of $D$-glucose?
i. $D$-fructose
ii. $D$-ribose
iii. $L$-glucose
iv. $D$-galactose

Answer 1

Stereoisomers have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space.
$D$-ribose is not an isomer of $D$-glucose as both have different molecular formulae.
$D$-fructose and $D$-glucose have same molecular formula, but different structural formula. Hence, they are structural isomers and not stereoisomers.
$L$-glucose and $D$-glucose are enantiomers (stereoisomers that are mirror images of each other).
$D$-galactose and $D$-glucose are diastereomers (stereoisomers that are not mirror images).