Hydrochloric Acid → 2-Thiopheneglyoxylic acid

Participants

Reaction Steps

A solution of sodium nitrite (35g; 0.5 mole) in water (100 ml.) was added steadily over 95 minutes to a stirred mixture of 2-acetylthiophene (25.1g; 0.2 mole) in 6N-hydrochloric acid (200ml) at 65°. 1.3M Trisodium phosphate (160 ml.) was then added over ca 3 minutes to raise the pH of the mixture to 1.8. Simultaneously, addition of a second portion of sodium nitrite (28g; 0.4 mole) in water (70 ml.) was commenced which took 75 minutes to complete. The pH was kept below 3.5 by addition of hydrochloric acid as required, during the nitrite addition, and was then lowered to 2. Urea (2g.) in water (10 ml.) was then added to decompose any excess of nitrite. The mixture was cooled to 20° and extracted first at pH 5.7 with methylene chloride (2 × 100 ml.) to give crude unreacted 2-acetylthiophene (3.6g; 14%), then at pH 2.7 with methylene chloride (3 × 100 ml.) to give crude thienoic acid (1.4g; 6%), and finally at pH 0.2 with methylene chloride (5 × 100 ml) followed by ethyl acetate (3 × 100 ml). These latter extracts were washed with saturated sodium chloride, and evaporated to dryness. Crystallisation of the residue from methylene chloride -- light petroleum (b.p. 40°-60°) gave thien-2-ylglyoxylic acid as two crops of bright yellow prisms [19.8g (63.5%), m.p. 92°-94°, and 3.0g (9.6%) m.p. 91°-93°].

Provenance & Evidence