The mechanism of the chromic acid cooxidation of 2-hydroxy-2-methylbutyric acid (HMBA) and 2-propanol (ROH) was investigated by using deuterium and carbon-13 isotope effects. The carbon isotope effect associated with the oxidative decarboxylation of HMBA was determined from the mass ratio 44:45 (12C02:13C02), the deuterium isotope effect from kinetic measurements. When the acidity is increased from [H+] = 0.02 or 0.05 M to [H+] = 1.86 M, the deuterium isotope effect,

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Communications to the Editor assumed 1:2 reaction (eq 5) would produce a highly (CH3)2SiCI2 + =S i (OH) 2 =SiT i (CH3)2 + 2HCI (5strained four-membered cyclic siloxane, a compound reported to be highly unstable.24 It seems, therefore, reasonable to assume that with geminal silanediols a 1:1 reaction (eq 6) will occur, as with single hydroxyls.The question remains whether the surfaces of commercial silica gel, and the silica gel plates used in this study, are identical. From Tables I and II, it can be seen that band shape and band position are the same for both types of silica. The reason why earlier workers with silica gel did not find a fine structure in the O-H stretching band might be insufficient resolution, the use of too large silica gel particles, or recording in the transmission instead of absorbance mode. Surface-hydroxyl concentrations, measured by following the weight change of silica gel samples on silanization with hexamethyldisilazane, gave essentially the same results for a silica gel plate and two commercial silica gels.25We conclude that the silica aerogel samples, used in this study, and commercial silica gel are closely similar in surface properties. Silica gel and silica powder are obviously not similar, neither in bulk structure, nor in surface properties. Therefore, results obtained from studies on silica powder should not be extended directly to practical systems, e.g., catalysts and drying agents, that are based on silica gel.

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