Org. Synth. 1934, 14, 8
DOI: 10.15227/orgsyn.014.0008
Submitted by W. E. Bachmann
Checked by John R. Johnson and H. R. Snyder.
1. Procedure
A mixture of 150 g. (0.82 mole) of benzophenone (Note 1), one drop of glacial acetic acid (Note 2), and 665 g. (850 cc., 11 moles) of isopropyl alcohol (Note 3) in a 1-l. round-bottomed flask is warmed to 45°. The flask is closed with a tight cork firmly wired or tied in place, and is supported in an inverted position in a tripod and exposed to direct sunlight. After three to five hours of bright sunshine crystals of benzopinacol begin to appear; after eight or ten days of exposure, depending upon the intensity of the light (Note 4), the flask is filled with crystals of benzopinacol. The solution is chilled in ice and the crystalline product is filtered with suction, washed with a small quantity of isopropyl alcohol, and allowed to dry in the air. The filtrate is reserved for subsequent reductions (see below). The yield of practically pure benzopinacol, m.p. 188–190° (Note 5), is 141–142 g. (93–94 per cent of the theoretical amount). The product is sufficiently pure for most purposes. It may be crystallized by dissolving it in 1 l. of hot benzene, filtering, and adding 400 cc. of hot ligroin (b.p. 90–100°) to the hot filtrate. After cooling in ice and filtering there is obtained 129–130 g. of purified product. The melting point is not changed by this purification.
To the isopropyl alcohol filtrate is added another 150-g. portion of benzophenone, and the solution is exposed to sunlight as in the first reduction. The benzopinacol which separates is filtered and dried. The yield in the second and subsequent runs is 142–143 g. (94–95 per cent of the calculated amount). This procedure can be repeated with the same filtrate until six or seven portions (900–1050 g.) of benzophenone have been reduced.
2. Notes
1. Although a practical grade of benzophenone can be used in this preparation, it is better to have material that has been recrystallized from alcohol. Directions for preparing benzophenone are given in Org. Syn. Coll. Vol. I, 1941, 95.
2. No more than one drop of acetic acid should be used. The acid is added to ensure the removal of traces of alkali, which cause decomposition of the pinacol into benzophenone and benzohydrol.
3. If isopropyl alcohol is not available, absolute ethyl alcohol can be used. With ethyl alcohol the reaction is slower and a yellow solution is obtained; nevertheless, the crystals of benzopinacol are colorless.
4. About five clear bright days are required to complete the reduction. The reaction can be interrupted at any time, the crystals filtered, and the filtrate then exposed further.
5. Since the pinacol decomposes near its melting point the latter will vary with the rate of heating. The temperatures reported here were obtained by slow heating; if the tube is placed in a bath at 150° and heated rapidly, the observed melting or decomposition point is 193–195°.
3. Discussion
Benzopinacol has been prepared by the action of phenylmagnesium bromide on benzil1 or methyl benzilate.1 Usually it has been obtained by reduction of benzophenone, the reducing agents being zinc and sulfuric acid2 or acetic acid,3 aluminum amalgam,4 and magnesium and magnesium iodide.5 The present method is based on a study by Cohen6 of the photochemical reaction discovered by Ciamician and Silber.7
This preparation is referenced from:

References and Notes
  1. Acree, Ber. 37, 2761 (1904).
  2. Linnemann, Ann. 133, 26 (1865).
  3. Zagumenni, Ber. 14, 1402 (1881).
  4. Cohen, Rec. trav. chim. 38, 75 (1919).
  5. Gomberg and Bachmann, J. Am. Chem. Soc. 49, 241 (1927).
  6. Cohen, Rec. trav. chim. 39, 243 (1920).
  7. Ciamician and Silber, Ber. 33, 2911 (1900).

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



ethyl alcohol (64-17-5)

sulfuric acid (7664-93-9)

acetic acid (64-19-7)

Benzene (71-43-2)

magnesium (7439-95-4)

aluminum (7429-90-5)

Benzil (134-81-6)

Benzophenone (119-61-9)

zinc (7440-66-6)

isopropyl alcohol (67-63-0)

Phenylmagnesium bromide (100-58-3)

Benzopinacol (464-72-2)

methyl benzilate (76-89-1)

magnesium iodide