Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

378 USB OF PALLADIUM
bond is nevertheless taken up in three hours (at 20° and a barometric
pressure of 740 mm., 840 c.c). The flocculated palladium is removed
by filtration, the methyl alcohol is evaporated, and the hydrogenated
acid is recrystallised as described on p. 234.
If undiluted methyl alcohol is used as a solvent a secondary catalytic
action of the palladium results in the production of ester. 1 In this case
5 g. of potassium hydroxide are added, the solution is concentrated by
heating, and hydrocinnamic acid precipitated with dilute hydrochloric
acid.
Calculation of the Amount of Hydrogen Required
One mole of substance requires for each double bond 224 litres of
hydrogen under standard conditions. The volume of a mole of hydrogen
under the experimental conditions which prevail can be calculated from
the formula
Y Y 760T
~ °273&'
where b is the observed barometric pressure less the vapour pressure of
water at the temperature concerned, and T is the absolute temperature.
Generally (for b = 760 mm. and t = 15°) it amounts to 24 litres. Five
grammes of cinnamic acid (mol. wt. 148) =5/148 mole ; the volume of
hydrogen required is therefore 24 x 5/148 litre =811 c.c.
Preparation of Palladium on Charcoal
Suspend 2 g. of animal charcoal in 100 c.c. of water in a hydrogenation
bulb (capacity about 300 c.c). By means of a rubber stopper attach a
dropping funnel with bent stem to the side opening of the bulb, open the
cock on the funnel and pass hydrogen into the bulb until a sample of the
escaping gas, collected in a test-tube, burns with a steady flame. Now
close the cock on the dropping funnel, lower the levelling vessel and run
in gradually from the funnel (with continuous mechanical shaking) a
solution of 0-1 g. of palladium chloride in 10 c.c. of approximately 0-1 N
hydrochloric acid. When the solution has lost its colour, open the bulb
and let the hydrogen escape. Filter the mixture at the pump, collecting
the catalyst on a filter plate, and wash with much water keeping the
catalyst always covered since gentle ignition occurs on exposure to the
air. When acid can no longer be detected in the filtrate, rapidly wash
twice with alcohol and absolute ether, immediately transfer the material,
still moist with ether, to a desiccator, and evacuate. After 24 hours
cautiously admit nitrogen or carbon dioxide before opening. The
completely dry catalyst does not glow in air and can readily be kept.
1 Cf. E. Waser, Helv. Chim. Ada, 1925, 8, 117.