Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

382 CATALYTIC HYDKOGENATION
platinum metals, platinum or palladium, in the form of platinum
sponge, palladium black, platinum oxide or the metals precipitated in
finely divided form on chemically indifferent carriers.
Before metallic catalysts were used as carriers there was no possibility
of adding elementary hydrogen directly to the carbon-carbon
double bond. In such catalysts we have agents by means of which
it is possible to saturate with hydrogen practically all unsaturated
systems, and it is indeed the olefinic double bond which is most readily
attacked by catalytically activated hydrogen. Such hydrogen does not
react so quickly with the carbonyl group of aldehydes and ketones,
whilst it leaves carboxyl and ester groups intact.
The solvents which, in the scientific laboratory, have by far the
greatest importance for catalytic hydrogenation in the cold are glacial
acetic acid, ethyl acetate, the alcohols, ether, and water. The success
of a hydrogenation depends, in a manner not yet quite clear, on the nature
of the solvent. In general the most powerful effect is obtained with
platinum oxide in glacial acetic acid. Since the solubility of hydrogen'
in all solvents is low, the catalyst, in suspension or colloidal solution,
must be continuously kept in contact with the gas phase by shaking, so
that it can always take up further amounts of hydrogen and give them
up again to the substance undergoing hydrogenation. Instead of the
bulb-shaped hydrogenation vessel described above (Willstatter and
Waser), a " shaking duck " may be used with equally good results.
Frequently a hydrogenation, after proceeding well at the start, comes to
a stop before the uptake of hydrogen is complete; in many cases the
catalysts can then be reactivated by shaking with air (Willstatter). In
this connexion it must be borne in mind that a mixture of hydrogen
and air is ignited by the finely divided metallic catalyst, and that, therefore,
before reactivation is undertaken, the hydrogen present in the
hydrogenation vessel must be replaced by nitrogen or, more simply,
removed by evacuation.
Only completely pure substances should be subjected to catalytic
hydrogenation. This rule is based on the fact that the catalyst is
inactivated especially by substances containing sulphur and often also
by those containing halogen ; not infrequently incalculable influences
interfere with hydrogenation. The most certain means of avoiding
such troubles is to use pure materials and also pure solvents.
The same catalysts which permit the addition of elementary hydrogen
to a double bond are able to accelerate the opposite process—dehydrogenation,
or elimination of hydrogen—when the temperature is
altered. Thus cyclohexane is decomposed into benzene and hydrogen
when passed over nickel or palladium black at about 300° (Sabatier,
Zelinsky). The equilibrium
lies on the right side of the equation at the lower temperature, but at