Org. Synth. 1962, 42, 38
DOI: 10.15227/orgsyn.042.0038
Submitted by Charles H. DePuy and K. L. Eilers1.
Checked by William G. Dauben, Robert A. Flath, and Gilbert H. Berezin.
1. Procedure
In a 250-ml. round-bottomed flask fitted for vacuum distillation with a short path distilling head (Note 1), a condenser, and a 250-ml. receiving flask is placed 100 g. (1.0 mole) of a mixture of cyclopentenediols.2 A few Carborundum boiling chips are added. The receiver is cooled in ice and the mixture heated to 50–55° (Note 2). At this time, the flask is opened momentarily and 1–2 g. of p-toluenesulfonic acid monohydrate is added. The flask is immediately closed and the pressure is reduced to 10–15 mm. Careful heating is continued, and a mixture of 2-cyclopentenone and water begins to distil with the temperature in the distilling head rising from 45° to 60° (Note 3). The temperature of the flask is gradually increased as necessary to maintain a reasonably rapid distillation rate.
The reaction is complete when approximately 10% of the original material remains in the distilling flask. The distillation normally requires 30–60 minutes.
The distillate, containing 2-cyclopentenone, water and varying amounts of cyclopentenediols, is dissolved in 150 ml. of methylene chloride and dried over anhydrous sodium sulfate. The solvent is carefully removed through a Vigreux column and the residue purified by distillation. After a forerun (b.p. 50–150°), there is collected 44–49 g. (53–60%) of pure 2-cyclopentenone, b.p. 151–154°. Cyclopentenediols may be recovered from the pot residue by distillation at 0.1 mm. The forerun contains appreciable amounts of cyclopentenone and should be added to a succeeding preparation before final distillation.
2. Notes
1. The distilling head should be short and unobstructed, for any attempt at fractionation at this stage leads to resinification of the 2-cyclopentenone by the acid. No capillary bleed is used, since the product is extremely sensitive to oxygen.
2. The submitters have found a 250-watt infrared heat lamp controlled by a Variac to be the most convenient source of heat. Occasionally, the reaction may become rapid and exothermic, and it is important to remove the heat source as quickly as possible. If an oil bath is used, the temperature is gradually increased until it approaches 150° at the end of the reaction.
3. If the temperature of the distillate rises much above 60° at this pressure, considerable amounts of diols co-distil and the yield of 2-cyclopentenone is diminished. If a reasonably rapid distillation does not occur with a head temperature below 60°, an additional gram of acid should be added after lowering the temperature of the distillation flask below 50°.
3. Discussion
Previous preparations of 2-cyclopentenone have involved the elimination of HCl from 2-chlorocyclopentanone3 or its ketal.4 The oxidation of 3-chloro-5 or 3-hydroxycyclopentene6 has been utilized as well as the direct oxidation of cyclopentene with H2O2.7 Cyclopentenone has also been prepared from 1-dicyclopentadienol.8
4. Merits of Preparation
The α,β-unsaturated ketone system in 2-cyclopentenone makes possible a wide variety of reactions of the Michael and Diels-Alder type. Thus 2-cyclopentenone is a versatile starting material for preparing compounds containing a five-membered ring. The availability of the dihydroxycyclopentene mixture (p. 414) makes the present procedure the method of choice for its preparation.
This preparation is referenced from:

References and Notes
  1. Department of Chemistry, Iowa State University, Ames, Iowa.
  2. M. Kovach, D. R. Nielsen, and W. H. Rideout, this volume, p. 414.
  3. E. J. Corey and K. Osugi, Pharm. Bull. (Tokyo), 1, 99 (1953).
  4. H. Wanzlick, G. Gollmer, and M. Milz, Ber., 88, 69 (1955).
  5. K. Alder and F. H. Flock, Ber., 89, 1732 (1956).
  6. E. Dane and K. Eder, Ann., 539, 207 (1939).
  7. W. Treibs, G. Franke, G. Leichsenring, and H. Roder, Ber., 86, 616 (1953).
  8. M. Rosenblum, J. Am. Chem. Soc., 79, 3179 (1957).

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



HCl (7647-01-0)

sodium sulfate (7757-82-6)

oxygen (7782-44-7)

methylene chloride (75-09-2)

Cyclopentene (142-29-0)

cyclopentenone (930-30-3)

2-chlorocyclopentanone (694-28-0)

p-toluenesulfonic acid monohydrate (6192-52-5)