Question

cis-But`-2-`ene on reaction with cold dilute `KMnO_(4)` solution yields
A. `(+)-`butane`-2,3`-diol
B. `(-)`butane-`2,3`-diol
C. `(+-)`butane`-2,3-`diol
D. meso-butane`-2,3-`diol

Answer 1

Correct Answer - D
The mechanisms for the formation of glycols by permaganate ion (from `KMnO_(4))` and osmium tetraoxide oxidations first involve the formation of cyclic intermediates. Then in serval steps cleavage at the oxygen metal bond takes place (at the dashed lines in the following reactions) ultimately producing the glycol and `MnO_(2)` or osmium metal.

The course of these reactions is syn hydroxylation:

`OH^(-)` is used to hydrolyze the Mn complex while `H_(2)O_(2)` oxidately cleaves the `Os` complex. A similr metal complex is formed in both the case. The `Mn` complex has -ve charge delocalized to the lone oxygen atoms while there is no charge on the isolable osmate complex. In the breakdown of the cyclic complexes, the `C-O` bonds are not broken, their geometry is not changed and the syn addition is preserved.