A.
Homophthalic acid. A solution of
243 g. (0.83 mole) of technical potassium dichromate in 3.6 l. of water and
1330 g. (725 ml., 13 moles) of concentrated sulfuric acid is prepared in a
5-l. three-necked flask fitted with a sealed mechanical stirrer, a thermometer, a 100-ml. dropping funnel, and a reflux condenser. The mixture is warmed to 65°, and
72 g. (72 ml., 0.56 mole) of technical 90% indene (Note
1) is added dropwise from the dropping funnel. The temperature must be kept at 65 ± 2°, and cooling by a
water bath is necessary during the addition. After the addition is complete the mixture is stirred for 2 hours at 65 ± 2° (Note
2). At the end of this period the mixture is cooled with stirring to 20–25° and then further cooled in an
ice-salt bath for 5 hours at 0°. The
homophthalic acid which separates is collected on a
10-cm. Büchner funnel with the aid of suction, then washed with two
75-ml. portions of ice-cold 1% sulfuric acid and once with 75 ml. of ice water (Note
3). The precipitate is then dissolved in
215 ml. of 10% sodium hydroxide solution, and the resulting solution is extracted with two
50-ml. portions of benzene (Note
4), which are discarded. The aqueous solution is added to
160 ml. of 33% sulfuric acid with vigorous stirring, and the mixture is chilled in an
ice-salt bath for 2–3 hours. The
homophthalic acid is collected on a 10-cm. Büchner funnel with the aid of suction, washed with three 25-ml. portions of ice water, and pressed and sucked as dry as possible. The acid is transferred to a
500-ml. distilling flask,
300 ml. of benzene is added, and the mixture is distilled from a
steam bath until about
250 ml. of distillate (benzene and water) has been collected (Note
5). The slurry of acid and
benzene is filtered with the aid of suction through a 10-cm. Büchner funnel, and the product is spread out on a porous plate to allow the last traces of
benzene to evaporate. The yield of white crystals of
homophthalic acid, melting at
180–181° (Note
6), amounts to
67–77 g. (
66–77% calculated on the basis of
90% indene content of the commercial
indene) (Note
7).