# Question #e08bf

Sep 22, 2015

Cold, alkaline permanganate oxidation produces vicinal diols.

#### Explanation:

The mechanism involves a cyclic manganate ester, so the addition is stereospecific.

(a) Propene

$\text{CH"_3"CH=CH"_2 stackrelcolor (blue)("alk. MnO"_4^-)(→) "CH"_3stackrelcolor (blue)(✳)("C")"HOHCH"_2"OH}$

Note that the starred carbon atom is chiral, so the product is a racemic mixture of ($R$)- and ($S$)-propane-1,2-diol.

(b) cis-But-2-ene

${\text{CH"_3"CH=CHCH"_3 stackrelcolor (blue)("alk. MnO"_4^-)(→) "CH"_3stackrelcolor (red)(✳)("C")"HOH"stackrelcolor (red)(✳)("C")"HOHCH}}_{3}$

Here we have two chiral carbons.

If you replace the wedged $\text{H}$ atom in the diagram above with a ${\text{CH}}_{3}$ group, you have the mechanism for the reaction with cis-but-2-ene.

Note that the two ${\text{CH}}_{3}$ groups are both "in front", so the molecule has an internal plane of symmetry.

The product is meso-butane-2,3-diol.

(c) trans-But-2-ene

${\text{CH"_3"CH=CHCH"_3 stackrelcolor (blue)("alk. MnO"_4^-)(→) "CH"_3stackrelcolor (red)(✳)("C")"HOH"stackrelcolor (red)(✳)("C")"HOHCH}}_{3}$

(the same equation)

But here the second ${\text{CH}}_{3}$ group replaces the $\text{H}$ atom on the dashed bond.

Both $\text{C}$ atoms are chiral, but there is no longer a mirror plane.

The product is a racemic mixture of ($2 R , 3 R$)- and ($2 S , 3 S$)-butane-1,2-diol.