Oxidation of alkenes·
Alkenes
are relatively electron rich (p-bond)
and can be oxidized to compounds such as diols, aldehydes, ketones and
caboxylic acids.Ê The reactions of
alkenes with oxidizing agents such as OsO4, KMnO4, O3Ê and HIO4 are summarized
here.
(a)
hydroxylation
(cis-diol products)·
KMnO4 is a beautiful deep purple.Ê The color disappears and is replaced by an
ugly brown ppt.
HOMnO2
(Mn V) is an unstable species which disproportionates to Mn (IV) = MnO2
and an Mn (VI) species which can enter into further alkene oxidation.
(b) hydroxylation (trans-diol products)·
What is the mechanism of the last step?Ê It is similar to the reaction of an halonium cation with water to form a halohydrin.
ÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊ
(c) alkene cleavage·
ÊÊÊ (1) with KMnO4·
ÊÊÊÊÊÊÊÊÊÊ (2)
ozonolysis·
Decomposition of the ozonide with Zn + acid (reducing
conditions) prevent oxidation of the aldehyde to a carboxylic acid.
ÊÊÊÊÊÊÊÊÊÊ
ÊÊÊÊÊÊÊÊÊÊ (3) diol cleavage·
HIO4
will cleave diols with axial and equatorial OHâs, but not if both OHâs are
axial.Ê The trans-diol shown below does not react.
