Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

HEATING UNDEK PEESSUEE 37
in hydrogen or carbon dioxide, the reaction vessel should not simply
be left connected to the cylinder. In order to relieve the pressure in
the apparatus the circuit includes a T-tube, one branch of which is
connected to a glass tube dipping into a cylinder with mercury or
water.
In such cases it is more convenient to use a Kipp or, when nitrogen
is employed, a gas-holder filled from a cylinder.
Experience shows that much gas is wasted because the beginner
rarely considers how much is approximately required for his reaction.
Nevertheless he should do so.
With the exception of nitrogen, all commonly used gases can,
if necessary, be prepared by simple, well-known methods.
HEATING UNDER PRESSURE
When it is desired to accelerate a reaction in solution or with
undissolved substances at a temperature above their boiling point,
the reactants must be shut off from the external atmosphere either
by sealing in a glass tube in which they are then heated, or in a closed
metallic vessel (autoclave). This is obviously necessary even when
we wish a reaction to take place in alcoholic solution at 100° or in
aqueous solution at about 120°. Thus the aim is entirely to increase
the temperature of the reaction; the accompanying increase in
pressure has no effect on the reaction velocity, for as a rule it is unaccompanied
by any significant change in the concentration.
Since solutions are most frequently heated in sealed tubes in
which the vapour pressure of the solvent determines the pressure, it
is necessary to reckon with quite considerable pressures at temperatures
which are appreciably above 100°. Gases which may
possibly be formed during the reaction will add to the pressure. An
estimate of the degree of pressure to be expected in a sealed tube
reaction should be made with the help of the vapour pressure curve
of the solvent used. In preparative reactions carried out in the
heated tube, the pressure is always that of the saturated vapour.
The pressure is therefore not dependent on the absolute amount of
the solution taken. But since particularly water in the liquid state,
and hence also solvents containing water, attack glass seriously at
high temperatures, the tube is generally not more than half rilled.
Naturally when a gas is produced during the reaction the amount of
free space available for it plays a part in determining the ultimate
pressure.