Org. Synth. 1936, 16, 77
Submitted by Donald Starr and R. M. Hixon.
Checked by John R. Johnson and H. B. Stevenson.
(A) Preparation of Palladous Oxide.—In a 350-cc. casserole, 2.2 g. (0.02 gram atom) of palladium metal is dissolved in a small amount of aqua regia, and the solution (Note 1) is treated with 55 g. of c. p. sodium nitrate and enough distilled water to make a thick paste. The substances are thoroughly mixed and then heated gently to drive off the water. The heating is increased until the mixture melts (about 270–280°) and continued cautiously. Just above the melting point the mixture must be stirred and heated carefully as oxides of nitrogen are evolved and foaming occurs. After the evolution of gases is nearly complete (about five minutes) the full flame of a Bunsen burner is applied for about ten minutes. The entire time of heating should be about one-half hour. As the mass cools, the casserole is rotated to allow the melt to solidify on the sides of the dish. After digestion with about 200 cc. of distilled water until the sodium salts are completely dissolved, the dark brown precipitate of palladous oxide is filtered and washed thoroughly with 1 per cent sodium nitrate solution (Note 2). The oxide must not be washed with pure water since it shows a marked tendency to become colloidal. After drying in a vacuum desiccator the palladous oxide weighs 2.3–2.4 g. (91–95 per cent of the theoretical amount) (Note 3).
(B) Tetrahydrofuran.—In the pressure bottle of an apparatus for catalytic reduction (Note 4) are placed 10 g. of pure furan (Note 5) and 0.2 g. of palladous oxide. The bottle is swept out with hydrogen (Note 6), and an initial hydrogen pressure of about 7 atm. (100 lb.) is applied (Note 7). After a lag of about ten minutes the reduction proceeds smoothly, and in an hour the theoretical amount of hydrogen is absorbed; the reaction is noticeably exothermic. After the reaction has ceased 20 g. of furan and 0.2 g. of palladous oxide are added (Note 6), the bottle swept out with hydrogen, and the hydrogen pressure raised to 7 atm. After this addition the lag is short and the reaction proceeds somewhat more rapidly than before; the temperature rises to 40–50°. When the reaction nears completion, 30 g. of furan and 0.2 g. of palladous oxide are added, and the reduction is continued. Successive portions of 30 g. of furan and 0.2 g. of palladous oxide are added in the same manner, until the bottle is about two-thirds filled. To ensure complete reduction, another portion of palladous oxide is added and the mixture shaken until no more hydrogen is taken up. The catalyst is allowed to settle (Note 8), and the tetrahydrofuran is decanted through a filter into a flask for distillation. The reduction product distils completely at 64–66°.
The reduction of 120 g. (128 cc., 1.76 moles) of furan requires about fifteen to twenty hours, depending upon the purity of the furan and the activity of the catalyst (Note 9). The yield of redistilled tetrahydrofuran is 114–118 g. (90–93 per cent of the theoretical amount). Since reduction is practically quantitative, the yield is determined largely by the care exercised in handling the volatile furan and tetrahydrofuran.
An aqueous solution of the equivalent quantity of Eimer and Amend's c.p. or Merck's reagent palladium chloride
may be used.
The filtrates should be clear and colorless; if they show a yellow-orange opalescence, some of the oxide has become colloidal. The palladium
may be recovered1
as the oxide by evaporating the filtrates to dryness and re-fusing, or as palladium black
by rendering the filtrates slightly alkaline with sodium carbonate
and heating with formaldehyde
A small amount of palladous oxide
adheres to the casserole and cannot be removed by the ordinary means. The oxide is not dissolved readily by aqua regia
but is easily removed by boiling with 48 per cent hydrobromic acid
If initial pressures of 6–7 atm. are to be applied, the ordinary apparatus for catalytic reduction (Org. Syn. Coll. Vol. I, 1941, 61)
must be modified by using a flexible coil of copper tubing instead of rubber tubing for the connection between the hydrogen tank
and the reduction bottle. To avoid dangerous accidents and loss of materials, it is advisable to cover the reduction bottle at all times with a screen of wire mesh and to test the bottle at the higher pressures before use. Brass fittings are used to hold the mouth of the bottle against a rubber gasket;
before use the gasket is treated with alkali, washed thoroughly, and dried.
As in all catalytic reductions, the purity of the starting material is of great importance. Redistilled furan
, b.p. 31–32°
, prepared by the method of Wilson (Org. Syn. Coll. Vol. I, 1941, 274)
is quite satisfactory. It has been reported that furan
prepared by the method of Gilman and Louisinian2
should be dried over calcium chloride
and fractionated carefully.3
It is advisable to redistil the furan
shortly before use and to avoid contact with rubber stoppers
Owing to the high volatility of furan
the bottle is not evacuated, as is customary, before the hydrogen pressure is applied; instead, the air is displaced by hydrogen
. For the same reason, appreciable losses will occur if the reduction bottle is not cooled before the hydrogen pressure is released for the introduction of fresh portions of furan
and catalyst. Effective and rapid cooling may be obtained by directing a jet of ether
, from a wash bottle
, over the surface of the reduction bottle while the shaking motor is in operation.
The hydrogenation of furan
may be carried out with initial pressures of only 3 atm. (45 lb.), but the reduction is slower.
The catalyst may be removed, dried in a vacuum desiccator over sulfuric acid
, and used again. The second reduction usually proceeds much more slowly, and it is advantageous to use one or two portions of fresh oxide along with the recovered material. Spent catalyst may be regenerated by conversion to the oxide as described in Part (A
In the high-pressure apparatus of Adkins, 120 g. of furan
can be hydrogenated with 10 g. of Raney nickel
in a single run. Using pressures of 100–150 atm. and a temperature range of 100–150° the reduction is extremely rapid and is strongly exothermic. Platinum oxide-platinum black
is not a satisfactory catalyst for the reduction of furan
The preparation of palladous oxide-palladium black
and its use as a catalyst in the reduction of organic compounds have been studied by Shriner and Adams.1 Palladium black
and colloidal palladium
have been widely used as hydrogenation catalysts.5
has been prepared by the reduction of furan
in the vapor phase with a nickel
catalyst at 170°,6
in butyl alcohol
at 50° with Raney nickel
with palladous oxide-palladium black
in the absence of a solvent,3
and with an osmium-asbestos
by the reduction of an alkyl succinate over a copper chromite
and by the dehydration of 1,4-butyleneglycol
This preparation is referenced from:
Chemical Abstracts Nomenclature (Collective Index Number);
oxides of nitrogen
platinum oxide-platinum black
Palladous oxide-palladium black
Eimer and Amend's c.p. or Merck's reagent palladium chloride
calcium chloride (10043-52-4)
sulfuric acid (7664-93-9)
HYDROBROMIC ACID (10035-10-6)
sodium carbonate (497-19-8)
butyl alcohol (71-36-3)
Raney nickel (7440-02-0)
Furan, tetrahydro- (109-99-9)
Copyright © 1921-, Organic Syntheses, Inc. All Rights Reserved