Ozonolysis(iv) Hydrogenation
The addition of hydrogen to alkenes in the presence of metallic
catalysts, e.g^ Ni, Pt, Pd, etc., is usually a cis addition. This comes
about because reduction takes place when the alkene is adsorbed at
the metallic surface; approach of active hydrogen occurs from one
side of the alkene only, i.e. from the interior of the metal where the
hydrogen is readily adsorbed, probably as reactive free atoms, in
reasonable concentration: metals that are effective hydrogenation
catalysts have the capacity of adsorbing quite large amounts of
hydrogen. The alkene is probably bound to the metal surface by an
interaction involving its IT electrons for, after reduction has taken
place, the reduced product becomes desorbed very readily and so
leaves the catalysts surface free for adsorption of more alkene. For
similar reasons, the partial hydrogenation of a^cetylene would be
expected to lead to a cis alkene. Thus 1,2-cHmethylcyclohexene
(XXVII) yields the cis cyclbhexane derivative (XxVlII) and dimethyl-
acetylene (XXIX), the cis 2-butene (XXX):I V
/ \
/ Me Me
Me—DC—Me ^C^c'
Me Me Me Me
(XXVII) (XXVIII) (XXIX) (XXX)W •- Vr*
Stereospecific cis hydrogenation has been of very great use in con
firming molecular structures by synthetic methods.
(v) Ozonolysis *
The addition of ozone to alkenes can also be looked upon essentially
as an electrophilic addition(XXVI), i.e. the DL-glycol will result from the original cis olefine,
confirming an overall trans hydroxylation (cf. addition of bromine to
maleic acid, p. 139).
Thus by suitable choice of reagent, the hydroxylation of olefines
can be stereospecifically controlled to proceed cis or trans at will.