Org. Synth. 1935, 15, 20
DOI: 10.15227/orgsyn.015.0020
[Pyrrole, 2,4-dimethyl-]
Submitted by Hans Fischer
Checked by C. R. Noller
1. Procedure
A solution of 270 g. (4.8 moles) of potassium hydroxide in 150 cc. of water is prepared in a 3-l. round-bottomed flask, 120 g. (0.5 mole) of crude 2,4-dimethyl-3,5-dicarbethoxypyrrole (p. 202) and a pinch of sand are added, and the whole is mixed thoroughly by shaking. The flask is fitted with a reflux condenser, and the mixture is heated in an oil bath at 130° for two to three hours with occasional shaking until the thick paste has become partially liquefied owing to the formation of dimethylpyrrole.
The flask is next fitted for distillation with superheated steam and with a separatory funnel for the introduction of water into the center of the flask. A 3-l. round-bottomed flask fitted with a vertical condenser is used as a receiver (Org. Syn. Coll. Vol. I, 1941, 479). The temperature of the oil bath is raised to 160°, and superheated steam at 220–250° is introduced. The temperature of the oil bath is then gradually raised to 200°. If foaming becomes too great, a few drops of water are added from the separatory funnel, care being taken that the water does not strike the hot glass walls (Note 1). Steam distillation is continued until no more dimethylpyrrole comes over. This takes from one to two hours, and the distillate amounts to 2.5–3 l. The distillate is extracted once with 200 cc. of ether and three times with 100-cc. portions, and the extract is dried for two hours over 20 g. of anhydrous potassium carbonate. The ether is removed by distillation from a 100-cc. modified Claisen flask having a 15-cm. fractionating side arm, the solution being added gradually through a separatory funnel. After the ether is removed the residue is distilled and the fraction boiling at 160–165° is collected. The yield is 27 to 30 g. (57–63 per cent of the theoretical amount) (Note 2) and (Note 3).
2. Notes
1. If the contents of the flask cake or become semi-solid, the temperature of the oil bath should be lowered and the rate of flow of superheated steam gradually decreased.
2. There is practically no fore-run, but if repeated or larger batches are made it is possible to obtain about 2 g. more of product per run from the higher-boiling fractions. If recrystallized ester is used a higher yield of dimethylpyrrole with less high-boiling products is obtained but the over-all yield is lower.
3. 2,4-Dimethylpyrrole is very readily oxidized in the air to a red resinous substance. If it is not used immediately, it should be stored under nitrogen or sealed in a glass vial under a vacuum.
3. Discussion
2,4-Dimethylpyrrole has been obtained by a large number of reactions, but the method of Knorr1 and the condensation of acetone with aminoacetone2 are the only ones of preparative interest. The procedure given above is one modification of Knorr's method. In another modification, the pyrrole ester and alkali are heated in a steel bomb and the product is distilled in an inert atmosphere; a yield of 95 per cent is reported.3
This preparation is referenced from:

References and Notes
  1. Knorr, Ann. 236, 326 (1886).
  2. Piloty and Hirsch, ibid. 395, 65 (1913).
  3. Corwin and Krieble, J. Am. Chem. Soc. 63, 1830 (1941).

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

potassium carbonate (584-08-7)

ether (60-29-7)

nitrogen (7727-37-9)

acetone (67-64-1)

potassium hydroxide (1310-58-3)

aminoacetone (298-08-8)

Pyrrole, 2,4-dimethyl- (625-82-1)


2,4-Dimethyl-3,5-dicarbethoxypyrrole (2436-79-5)