Organic Reactions Volume 4 - Sciencemadness Dot Org

274 ORGANIC REACTIONS
Most syntheses of benzoins have involved the use of a solvent which
contains water, and many of the theories on mechanism of formation
are based on the assumption that the reaction is ionic and that water
is necessary for the reaction. Very few of the early experiments were
carried out in such a way that ionization or hydrolysis could not occur.
However, that water is unnecessary was shown by Smith 26 in 1899, and
confirmed by other workers. 27,28 The reaction also takes place in anhydrous
petroleum ether. 27 This has led to the suggestion of a nonionic
mechanism, In the formation of unsymmetrical benzoins the elimina-
C6H5CH-ONa
C6H6CHO + NaCN -> C6H6CH-CN
ONa
C6H6CH-ONa HCN C6H6CHOH
I > NaCN + I ->
C6H6C-ONa C6H6C-ONa
C6H6C-ONa CN CN
CN
C6H6CHOHCOC6H6 + NaCN
tion of hydrogen cyanide in one or the other direction would determine
the structure of the product.
Other important discussions of the mechanism have been given
by Zincke, 29 Chalanay and Knoevenagel, 30 Bredig and Stern, 31 Staudinger,
16 Horbye, 32 Ekecrantz and Ahlqvist, 33 Lachman, 34 Hodgson and
Rosenberg, 20 Tiffeneau and L6vy, 19 and Hammett. 34a More recently
Wheeler, 28 Weiss, 28 and Watson 23 have reviewed the previous theories.
Weiss and Lachman make a comparison of the benzoin condensation
with the Cannizzaro reaction.
2(5 Smith, Am. Chem. J., 22, 249 (1899).
27 Morton and Stevens, J, Am. Chem. Soc, 52, 2031 (1930).
28 Nadkarni, Mehta, and Wheeler, J. Phys. Chem., 39, 727 (1935); Nadkarni and Mehta,
ibid., 39, 901 (1935); Nadkarni, ibid., 39, 907 (1935).
29 Zincke, Ber., 9, 1769 (1876).
30 Chalanay and Knoevenagel, Ber., 25, 289 (1892).
81 Stern, Z. physik. Chem,, 50, 513 (1905); Bredig and Stern Z. Elehtrochem., 10, 582
(1904).
32 Horbye, Dissertation, Dresden, 1917.
33 Ekecrantz and Ahlqvist, Arkiv Kemi, Mineral. GeoL, 3, No. 13, 1 (1908) (Chem.
Zentr., 1908,11, 1688).
34 Lachman, J. Am. Chem. Soc, 45, 1509, 1522, 1529 (1923); 46, 708 (1924).
Ua L. P, Hammett, Physical Organic Chemistry, McGraw-Hill, 1940, pp. 348-350.