vogel_practical_ochem_3.pdf

872 PRACTICAL ORGANIC CHEMISTRY [VI,
An example of the application of the Raney nickel catalyst is given in
Section IV,35 (p-phenylethylamine from benzyl cyanide).
VI,6. COPPER-CHROMIUM OXIDE CATALYST
This catalyst is prepared by the decomposition of basic copper ammonium
chromate ; the main reactions may be written as :
2Cu(N03)2 + Na2Cr207 + 4NH3 + 3H20
-^•2CuNH4(OH)Cr04 + 2NaN03 + 2NH4N03
300°
• CuO.CuCr204 + N2 + 5H20
The most active varieties contain barium chromite, which is incorporated by
adding barium nitrate in the preparation of the basic copper ammonium
chromate : this is sometimes referred to as barium-promoted copper-chromium
oxide. The barium in the catalyst gives protection against sulphate poisoning
and is said to confer stabilisation against reduction. Copper-chromium oxide
is an approximately equimolecular combination of cupric chromite and cupric
oxide CuCr204.CuO ; the barium-promoted catalyst contains some barium
chromite. It is not a mechanical mixture of cupric chromite and cupric oxide
nor is it a simple copper chromite ; the catalytic activity is not due to copper
chromite alone since removal of the cupric oxide with an acid leaves cupric
chromite which is inactive as a catalyst for the hydrogenation of esters. Reduction
of the black cupric oxide to red copper results in deactivation, as does
also excessive heating which leads to cuprous chromite Cu2Cr204 and oxygen.
For these reasons the name " copper chromite catalyst ", which is sometimes
used, is misleading.
Hydrogenations with copper-chromium oxide catalyst are usually carried
out in the liquid phase in stainless steel autoclaves at pressures up to 5000-
6000 lb. per square inch. A solvent is not usually necessary for hydrogenation
of an ester at 250° since the original ester and the alcohol or glycol produced
serve as the reaction medium. However, when dealing with small quantities
and also at temperatures below 200° a solvent is desirable : this may be methyl
alcohol, ethyl alcohol, dioxan or methylq/c/ohexane.
The catalyst, which may be regarded as complementary to Raney nickel
(Section VI ,5) is largely used for the hydrogenation of esters (esters of monobasic
and of dibasic acids to alcohols and glycols respectively) :
RCOOR' + 2H2
^±RCH2OH + R'OH
Two mechanisms have been given :—(i) hydrogenation—
O OH O O
II I II II
RCOR' + H2^RCH—OR' ^RCH -f R'OH ; RCH + H2 ^ RCHaOH
(ii) hydrogenolysis—
O O O
II II I
RCOR' + H2^ RCH + R'OH ; RCH + H2^ RCH2OH
Other applications include :
(a) reduction of aldehydes and ketones to alcohols ;
(b) reduction of amides to amines (RCONH2 + 2H2 -
pome side reactions occur : dioxan is the best medium)
(c) reduction of lactones to glycols.
RCH2NH2 + H20;