vogel_practical_ochem_3.pdf

758 PRACTICAL ORGANIC CHEMISTRY [IV,
15 per cent, sulphuric acid (74 g. or 40 ml. of concentrated sulphuric acid
in 400 ml. of water). Do not add the acid to the alkaline solution, for in
this way the acid is liable to be contaminated by the sodium salt. Filter
the purified acid at the pump, wash it thoroughly with cold water, and
dry it in the steam oven. The yield of j»-nitrobenzoic acid, m.p. 237°,
is 48 g. : this is sufficiently pure for most purposes. Upon recrystallisation
from benzene or from glacial acetic acid, the m.p. is raised to 239°.
IV,155. 2:4: 6-TRINITROBENZOIC ACID
In a 1 litre bolt-head flask, mounted in an empty water bath, place
720 g. (392 ml.) of concentrated sulphuric acid and add 72 g. of commercial
trinitrotoluene. Stir the mixture mechanically with a powerful stirrer
and introduce powdered crystallised sodium dichromate (Na2Cr2O7,2H20)
in small quantities until the temperature of the mixture reaches 40° :
now fill the empty water bath with cold water and add the sodium
dichromate at such a rate that the temperature is maintained at 45-55°.
The total weight of sodium dichromate required is 110 g. Stir the
mixture for a further 2 hours at 45-55°, and pour into a beaker containing
800 g. of crushed ice. Filter off the trinitrobenzoic acid and wash it
carefully with cold water until free from chromium salts (the acid is
appreciably soluble in water). Transfer the solid to a 1-litre bolt-head
flask, provided with a stirrer and containing 400 ml. of water. Add 15 per
cent, sodium hydroxide solution from a separatory funnel, with vigorous
stirring, until a faint red colour persists for 5 minutes ; discharge the
colour with a few drops of acetic acid. Filter off the unattacked trinitrotoluene
and wash it with a little water. Precipitate the trinitrobenzoic
acid from the filtrate by the addition of a slight excess of 50 per cent,
sulphuric acid. Cool in ice, filter the acid at the pump and wash it free
from salts with ice-cold water. Dry the 2:4: 6-trinitrobenzoic acid in
the air : it melts at 228° (decomp.) and the yield is 55 g.
IV,156. 2 :4-DINITROPHENYLACETIC ACID
Place 25 g. of phenylacetic acid (Section IV,160) in a 500 ml. roundbottomed
flask, cool the latter in running water and add 250 ml. of fuming
nitric acid, rather slowly at first and then more rapidly. The addition
occupies about 15 minutes. Attach a condenser to the flask, reflux the
solution for 1 hour, and pour into about 500 ml. of cold water. When
cold, filter the crude 2 : 4-dinitrophenylacetic acid at the pump and wash
it with cold water : the resulting acid, after drying at 100°, is almost pure
(m.p. 181°) and weighs 31 g. Recrystallise it from 300 ml. of 20 per cent,
alcohol. Collect the first main crop (25 g.), and allow the mother liquor
to stand overnight when a further 2 g. of pure acid is obtained ; dry at
100°. The yield of pure 2 : 4-dinitrophenylacetic acid, m.p. 183°, is 27 g.
IV,157. o-CHLOROBENZOIC ACID
Method 7. Place 1250 ml. of water, 75 g. of pure potassium permanganate
and 50 g. of o-chlorotoluene (Section IV,61) in a 2-5-litre threenecked
flask equipped with a mechanical stirrer and reflux condenser.