Thirty-three grams of aluminum granules is placed in a flat-bottom, 1-L, three neck
flask with a thin, ~1 cm layer of glass wool protecting the bottom. The flask
is equipped with a thermometer, a double-surface reflux condenser, and a dropping
funnel with pressure equalization containing 185 g bromine, previously dried by
shaking with concentrated sulfuric acid. The condenser is cooled by freezing water
from a large ice water reservoir with a small (~2 W) recirculating pump. The top of
the reflux condenser is protected from moisture by a CaCl2 tube, and prior to commencement
of reaction, the setup is flushed with dry nitrogen.
The reaction is started by allowing 10–15 drops of bromine to run into the flask,
and waiting up to several minutes for the evolution of white fumes, indicating a
reaction. Following this, a few more drops are cautiously added, observing the
gradual rise in temperature. The bromine reacts with the aluminum in sporadic
fashion, with the addition frequently accompanied by flashes and sparks lasting
several seconds; at other times, the addition produces no obvious sign of reaction.
With the bromine added in bursts of 5–10 drops, after about 10–15 min the temperature
in the flask should reach 210°C–220°C. After about half the bromine has
been added, vigorous refluxing commences in the bottom portion of the condenser
. This provides cooling and allows the addition rate of bromine to be increased, controlled by the reflux rate. Provided this regime is maintained,
there
should be no noticeable escape of bromine at the top of the condenser or deposition
of solid Al2Br6 in its middle section. The temperature at the end of the reaction adaptor with a shut-off valve. Vacuum is applied to the system and
the reagent
flask heated. Initially, bromine vapor fills the apparatus and is removed by the
aspirator. This is followed by some liquid bromine that distills when the flask
temperature rises above the mp of Al2Br6. This is also removed by the aspirator
without condensing in the receiving flask. Boiling commences at about 120°C–
140°C, and the first fraction collected is of reddish color. After about 10 g of
product has passed, the distillate becomes perfectly colorless, and this fraction is
gathered. If a multiple feed adaptor is not used and the receiver flask needs to be
replaced, the system must be flushed with nitrogen prior to reconnection. Toward
the end of the distillation, some Al2Br6 “icicles” may be observed sticking to the
walls and possibly plugging the adaptors. The glass can be gently warmed at that
point with an air gun until the product melts and drops into the receiver flask.
When all product has been collected, the vacuum is disconnected and the apparatus
filled with nitrogen. The receiver flask is now disconnected while the product
is still liquid and its contents are rapidly emptied into a preheated mortar contained
in a nitrogen-filled desiccator, making an effort to spread the liquid onto the mortar
walls as much as possible. When the mortar has cooled, the Al2Br6 can be ground
under nitrogen and the resulting powder stored in a glass-plugged bottle. The yield is
about 180 g, or 85% based on the bromine.
drops to just below 180°C; however, no external heating is required. The entire
process takes about 2 h.
At the end of the reaction, the insides of the flask are dark red, while white-red
flakes are condensed on the flask walls. The reflux condenser is now removed,
and the dropping funnel replaced by a downward-sloping distillation head leading
to a 250 mL, three-neck receiver flask. One neck of the flask is connected
through a small U-tube containing CaCl2 and an empty guard flask to the aspirator
vacuum; the other neck is connected to a nitrogen source through an inlet |
