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From Vogels Practical Organic Chemistry
COOLING OF REACTION MIXTURES
It is often necessary to obtain temperatures below that of the laboratory.
Finely
crushed ice is used for maintaining the temperature at 0-5 °C; it is usually best to
use a slush of crushed ice with sufficient water to provide contact with the vessel
to be cooled and to stir frequently. It is of course essential to insert a ther-
mometer into the reaction mixture to ensure that the desired temperature is
attained.
For temperatues below 0°C, the commonest freezing mixture is an
intimate mixture of common salt and crushed ice: a mixture of one part of
common salt and three parts of ice will produce a temperature of about —5 to
—18 °C.
Greater cooling may be obtained by the use of crystalline calcium
chloride; temperatures of —40 to — 50 °C may be reached with five parts of
CaCl2.6H2O and 3.5-4 parts of crushed ice.
If ice is temporarily not available, advantage may be taken of the cooling
effect attending the solution of certain salts or salt mixtures in water. Thus a
mixture produced by dissolving 1 part of NH4C1 and 1 part of NaNO3 in 1-2
parts of water causes a reduction in temperature from 10 to — 15 °C to — 20 °C;
3 parts of NH4C1 in 10 parts of water from 13 to -15°C; 11 parts of
Na2S2O3.5H2O in 10 parts of water from 11 to - 8 °C; and 3 parts of NH4NO3
in 5 parts of water from 13 to — 13°C.
Solid carbon dioxide (Dry Ice, Drikold, Cardice) is employed when very low
temperatures are required. The commercially available blocks are stored in spe-
cially insulated containers. Since frostbite may result from handling solid carbon
dioxide, it is advisable to either wear gloves or to cover the hands with a thick
cloth. Conveniently small-sized lumps may be obtained by hammering, with a
wooden or polyethylene mallet, a suitable large piece wrapped in a cloth or con-
tained within a stout canvas bag.
The small pieces are carefully added to either
ethanol or acetone in a plastic bowl until the lumps of solid carbon dioxide no
longer evaporate vigorously. The temperatures attained are in the region of — 50
to — 70 °C according to the efficiency of the lagging around the freezing bath. In
order to keep the freezing mixture for hours or overnight, it should be prepared
in a Dewar flask.
The use of cooling baths employing other solvents with solid carbon dioxide
enables other temperatures to be attained.
An extensive list has been pub-
lished21
from which the following have been selected: ethylene glycol/Cardice,
- 1 5 °C; acetonitrile/Cardice, -4 2 °C; diethyl ether/Cardice, -10 0 °C. A steady
state temperature cooling bath may also be obtained by adding solid carbon
dioxide to o-xylene: m-xylene mixtures22; the volume fraction of o-xylene deter-
mines the temperature of the bath. For example, m-xylene/Cardice, — 72 °C; o-
xylene (0.4): m-xylene (0.6), -58°C; o-xylene (0.8): m-xylene (0.2), -32°C.
The attainment of temperatures lower than —100 °C requires the use of baths
employing liquid nitrogen,23
either alone, or admixed with other solvents. The
hazards of using liquid nitrogen are pointed out in Section 2.3.2, p. 38, and the
use of such cooling baths is not advised except in the hands of experienced
workers.
From Vogels Practical Organic Chemistry
COOLING OF REACTION MIXTURES
It is often necessary to obtain temperatures below that of the laboratory.
Finely
crushed ice is used for maintaining the temperature at 0-5 °C; it is usually best to
use a slush of crushed ice with sufficient water to provide contact with the vessel
to be cooled and to stir frequently. It is of course essential to insert a ther-
mometer into the reaction mixture to ensure that the desired temperature is
attained.
For temperatues below 0°C, the commonest freezing mixture is an
intimate mixture of common salt and crushed ice: a mixture of one part of
common salt and three parts of ice will produce a temperature of about —5 to
—18 °C.
Greater cooling may be obtained by the use of crystalline calcium
chloride; temperatures of —40 to — 50 °C may be reached with five parts of
CaCl2.6H2O and 3.5-4 parts of crushed ice.
If ice is temporarily not available, advantage may be taken of the cooling
effect attending the solution of certain salts or salt mixtures in water. Thus a
mixture produced by dissolving 1 part of NH4C1 and 1 part of NaNO3 in 1-2
parts of water causes a reduction in temperature from 10 to — 15 °C to — 20 °C;
3 parts of NH4C1 in 10 parts of water from 13 to -15°C; 11 parts of
Na2S2O3.5H2O in 10 parts of water from 11 to - 8 °C; and 3 parts of NH4NO3
in 5 parts of water from 13 to — 13°C.
Solid carbon dioxide (Dry Ice, Drikold, Cardice) is employed when very low
temperatures are required. The commercially available blocks are stored in spe-
cially insulated containers. Since frostbite may result from handling solid carbon
dioxide, it is advisable to either wear gloves or to cover the hands with a thick
cloth. Conveniently small-sized lumps may be obtained by hammering, with a
wooden or polyethylene mallet, a suitable large piece wrapped in a cloth or con-
tained within a stout canvas bag.
The small pieces are carefully added to either
ethanol or acetone in a plastic bowl until the lumps of solid carbon dioxide no
longer evaporate vigorously. The temperatures attained are in the region of — 50
to — 70 °C according to the efficiency of the lagging around the freezing bath. In
order to keep the freezing mixture for hours or overnight, it should be prepared
in a Dewar flask.
The use of cooling baths employing other solvents with solid carbon dioxide
enables other temperatures to be attained.
An extensive list has been pub-
lished21
from which the following have been selected: ethylene glycol/Cardice,
- 1 5 °C; acetonitrile/Cardice, -4 2 °C; diethyl ether/Cardice, -10 0 °C. A steady
state temperature cooling bath may also be obtained by adding solid carbon
dioxide to o-xylene: m-xylene mixtures22; the volume fraction of o-xylene deter-
mines the temperature of the bath. For example, m-xylene/Cardice, — 72 °C; o-
xylene (0.4): m-xylene (0.6), -58°C; o-xylene (0.8): m-xylene (0.2), -32°C.
The attainment of temperatures lower than —100 °C requires the use of baths
employing liquid nitrogen,23
either alone, or admixed with other solvents. The
hazards of using liquid nitrogen are pointed out in Section 2.3.2, p. 38, and the
use of such cooling baths is not advised except in the hands of experienced
workers.
