How many steroisomers of butane`-2,3-`diol,
`CH_(3)CH(OH)CH(OH)CH_(3)`, are possible.

Options

A. Two
B. Four
C. Six
D. Three

Answer

D

Explanation

`CH_(3)-overset(**)underset(OH)underset(|)(CH)-overset(**)underset(OH)underset(|)(CH)-CH_(3)`
The molecule of butane `-2,3-`diol contains two chiral carbonds `(C-2` and `C-3)`.Therefore, we might expect that four steroismers `(2^(2)=4)` are possible as shown below:

`I` and `II` are enantiomers as these are mirror images thta are not superposable or interconvertible. An allowed `180^(@)` rotation (clockwise or counterclockwise) of `III` in the plane of the paper gives `IV`. Thus `III` and `IV` are identical as they are superposable mirror images. So there are only three stereoisomers. The steroisomer `III` or `IV` is called the meso form. It is avhiral and optically inactive.