Question #e08bf

1 Answer
Sep 22, 2015

Answer:

Cold, alkaline permanganate oxidation produces vicinal diols.

Explanation:

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

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(a) Propene

#"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

#"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 #"H"# atom in the diagram above with a #"CH"_3# group, you have the mechanism for the reaction with cis-but-2-ene.

Note that the two #"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

#"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 #"CH"_3# group replaces the #"H"# atom on the dashed bond.

Both #"C"# atoms are chiral, but there is no longer a mirror plane.

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