Weygand/Hilgetag Preparative Organic Chemistry

326 Formation of carbon-oxygen bonds
temperature. Above this is a fractionating apparatus, in which the aldehyde product is separated
from unchanged alcohol; the latter is returned to the boiling flask, and the former passes
to a condenser and thence to a receiver; the hydrogen escapes. Detailed dimensions are
suited to the conditions. For instance, if the catalyst tube is 1 m long its diameter is 25-30 mm.
The copper catalyst may be prepared, e.g., from a small roll of copper-wire net filled with a
copper oxide paste to which 1 % of silver oxide has been added. This catalyst is placed in
position in the apparatus, dried in a stream of hydrogen while the temperature is raised slowly,
and the oxide is then reduced, also slowly, at 300°, the time required being 8-10 h. It is then
allowed to cool without exposure to the air. Atmospheric pressure can be used if the boiling
point of the aldehyde does not exceed 200°; if it does, diminished pressure must be used.
The catalyst space must be heated to a temperature slightly higher than the boiling point
of the aldehyde product.
Geraniol is converted into oc- and /?-citral at atmospheric pressure; phenethyl
alcohol gives phenylacetaldehyde; cinnamaldehyde is obtained from cinnamyl
alcohol in good yield by use of a silver catalyst at 200°/20 mm. 429 Bremner
et al. 430 describe a laboratory method for oxidation of tetrahydrofurfuryl
alcohol by air on silver wool.
The following is a generally applicable procedure, applied here to the
conversion of 1-nonanol into nonanal: 431
The success of the operation depends on careful preparation of the dehydrogenating
catalyst. An aqueous solution of pure copper nitrate is treated with the calculated amount
of pure sodium hydroxide solution, and the precipitated copper(n) hydroxide is washed by
decantation with lukewarm water until completely neutral. The excess of water is then sucked
off on a filter without allowing the hydroxide to become dry. This material may be stored as
a paste in closed vessels. The copper hydroxide paste is spread in a thin layer on pieces of
copper gauze 10 x 5 cm, which are then formed into a roll 10 cm long an4 placed in a
copper tube 80 cm long into which thirty such rolls can be packed. The copper gauze should
not have too fine a mesh, otherwise the catalytic effect is less. For reduction of the copper
hydroxide the whole system is first washed through with hydrogen that has been washed with
permanganate and dried by sulfuric acid. The catalyst tube is then heated electrically to
300°, whereby the hydroxide is converted into spongy metallic copper; this reduction requires
8 h; the product is allowed to cool in a stream of hydrogen unless it is to be used immediately.
If the material is to be kept overnight a constant slow stream of hydrogen is led through the
apparatus. Before the dehydrogenation is begun the temperature is raised again to 300°
for 0.5 h and the whole is washed through with hydrogen. Then 1-nonanol is brought into
the boiling flask, the hydrogen stream is throttled back, the apparatus is evacuated to 3 to
5 mm, the temperature in the catalyst space is adjusted to 240°, and the alcohol is brought to
the boiling point. The receiver is arranged for collection of various fractions in a vacuum;
so, if the rate of distillation becomes temporarily too fast so that unchanged nonanol passes
over (recognized by a rise in temperature at the column head), then that portion of the
distillate is collected separately.
When the operation is finished, the apparatus is filled with hydrogen and allowed to cool
without entry of air; the catalyst then retains its activity. The yield of nonanal amounts to
90%, the b.p. being 78°/3 mm.
More than monohydric alcohols can be only incompletely dehydrogenated,
and carbohydrate alcohols do not survive attempted dehydrogenation.
High-boiling.alcohols are sometimes better dehydrogenated in the liquid
phase in presence of a hydrogen-acceptor such as copper and nitrobenzene
or ra-dinitrobenzene. The preparation of o-chlorobenzaldehyde in 86% yield
from 2-chlorobenzyl alcohol at 205-210° may be cited as example. 432
429 C. Moureu and G. Mignonac, C. R. Hebd. Seances Acad. Set, 111, 652 (1920)
430 I. G. M. Bremner and co-workers, /. Chem. Soc, 1949, Suppl., 25.
431 A. Lewisohn, Perfum. Essent. Oil Rec, 15, 13 (1924).
432 F. Zetsche and P. Zala, Helv. Chim. Acta, 9, 288 (1926).