Hydroxylationthe first formed carbonium ion (XIX) can add to the double bond of
a second molecule to form a second carbonium ion (XX). This in its
turn can add on to the double bond of a third molecule to yield (XXI)
or, alternatively, lose a proton to yield the alkene (XXII). Such
successive additions can lead to unwanted by-products in, for
example, the simple addition of hydrogen halides, but they may be
specifically promoted to yield polymers by the presence of Lewis
acids, e.g. A1C1 3 , SnCl 4 , BF 3 , as catalysts. Many polymerisations of
olefines are radical-induced however (p. 247).
m
(iii) Hydroxylation
Investigation of the action of osmium tetroxide on alkenes has led to
the isolation of cyclic osmic esters (XXIII) which undergo ready
hydrolysis to yield the 1,2-diol:/ \ Os0 4 / \ H 2 0 /^ vr" ~ Vr"
O. O ^ - HO OH
A X. X^
Os _ HO OIL-
6
° AO o
(XXIII) (XXIV)As the hydrolysis results in the splitting of the osmium-oxygen and
not the oxygen-carbon bonds in (XXIII), no inversion of configura
tion can take place at the carbon atoms and the glycol produced
must, like the cyclic osmic ester itself, be cis, i.e. this is a stereospecific
cis addition. The expense and toxicity of osmium tetroxide preclude
its large scale use but it can be employed in catalytic amounts in the
presence of hydrogen peroxide which reoxidises osmic acid (XXIV)
to the tetroxide.
The cis glycol is also obtained with permanganate, the classical
reagent for the hydroxylation of double bonds, and though no cyclic
permanganic esters have been isolated it is not unreasonable to
suppose that the reaction follows a similar course. This is supported
by the fact that use of lsO labelled Mn0 4 e results in both oxygen
F 145