A Raman Spectral Study of the Kinetics of Hydrolysis of Acetonitrile Catalyzed by Hg(II)

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Abstract
Raman spectroscopy has been employed to follow the relatively slow rate of hydrolysis of acetonitrile, catalyzed by mercury(II). Raman lines at 2275 and 2305 cm−1 are characteristic of CH3CN bound to Hg2+, and are distinct from lines of bulk solvent. The intensities of these new lines decrease with time. From the intensities, concentrations of bound acetonitrile, [CH3CN]B were calculated for a time span of 400 min. The data fit a second order rate law: Rate = k[CH3CN]B[H2O]. The specific rate constant, k, obtained from four sets of data for the system Hg(ClO4)2–CH3CN–H2O equals 1.05 ± 0.06 × 10−4 mol−1 1 min−1 at 25 °C. The energy of activation is 18.9 kcal mol−1. In the proposed mechanism water molecules attack acetonitrile molecules which are bound to Hg2+ and form a mercury(II)–acetamide complex. Raman lines characteristic of this species are observed. This species slowly converts to mercury(II) ammine complexes and acetic acid. Anions which coordinate with Hg2+ more strongly than CH3CN, such as nitrate or acetate, slow or prevent the hydrolysis reaction.

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