HYDRIDE TRANSFER TO CARBONIUM IONS: I. THE MECHANISM OF THE REDUCTION OF TRIPHENYLMETHYL CARBONIUM ION IN FORMIC ACID

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Abstract
In an attempt to prove that reduction can take place by hydride transfer, the conversion of triphenyl carbinol in formic acid to triphenylmethane via the carbonium ion was examined. Kinetic and isotopic proof was obtained for the following mechanism:[Formula: see text][Formula: see text]The rate law based on the above mechanism is[Formula: see text]where R = C6H5, which leads to the integrated rate expression[Formula: see text]This equation was found to be obeyed under a variety of conditions.Anhydrous formic-d acid was synthesized in good yield by the glycerol catalyzed decomposition of oxalic acid-d2. The concentration of deuterium was shown by nuclear magnetic resonance spectroscopy to be greater than 99%. Use of this material in the reduction gave a kinetic isotope effect and led to isolation of triphenylmethane which had greater than 97% deuterium in the α-position, thus supporting the idea that a hydride ion was transferred from formate ion to the carbonium ion.The energy and entropy of activation for the rate controlling step have been found to be 18.3 kcal. per mole and −7.5 e.u. The negative ΔS‡ is presumably due to the less likely orientation for the transition state A as compared to B.[Formula: see text]

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