Eighty-four grams (1 mole) of thiophene (p. 578) is dissolved in
340 cc. of acetic anhydride, and
80 g. (1.2 moles) of fuming nitric acid (sp. gr. 1.51) is dissolved in
600 cc. of glacial acetic acid (Note
1). Each solution is divided into two equal parts. One-half of the
nitric acid solution is introduced into a
2-l. three-necked, round-bottomed flask, provided with a
thermometer, a
motor stirrer, and a
separatory funnel. The mixture is cooled to 10°. Then with moderate stirring one-half of the
thiophene solution is introduced, drop by drop, and at such a rate as to prevent the heating of the reaction mixture above room temperature. A rapid rise of temperature will occur during the addition of the first fraction of the
thiophene solution. In cool weather the temperature is controlled by dipping the nitrating flask into a bath of cold tap water. Cooling to a very low temperature is not necessary, but it is important to avoid superheating the reaction mixture (Note
2). After the addition of the first half of the
thiophene, the temperature of the reaction mixture is reduced to 10° and the remainder of the
nitric acid solution is rapidly introduced into the flask. Nitration is continued by the gradual addition of
thiophene. Throughout the nitration the solution should show a permanent light brown color. The appearance of a pink or dark red color indicates oxidation. The product is allowed to remain at room temperature for two hours. It is then treated with an equal weight of finely crushed ice with rapid shaking.
Mononitrothiophene separates in pale yellow crystals. More crystals form if the mixture is allowed to remain in the
ice chest for twenty-four hours or longer. The solid is filtered (Note
3) on a
Büchner funnel or a
Jena glass filter plate at a low temperature, washed thoroughly with ice water, pressed, and dried in a
brown desiccator or in the absence of light (Note
4).
The filtrate and the washings contain in solution a small quantity of the product. This is recovered by distillation with steam. The acid distillate consists of snow-white crystals (if it is protected from light) and a solution of the compound. The solid is removed by filtration and washed. The filtrate is made neutral with
sodium carbonate and extracted with
ether. Upon drying and evaporating, the ethereal layer yields
mononitrothiophene contaminated with
dinitrothiophene (Note
5).
The total yield is
90–110 g. (
70–85 per cent of the theoretical amount) of a product which melts at
44–45°. If this material is steam-distilled and then repeatedly crystallized from
petroleum ether (b.p.
20–40°) it is obtained as colorless crystals melting at
45.5° (Note
6) and (Note
7).