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).