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Journal of Interdisciplinary Undergraduate Research

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Abstract

Accurate rate constant values are critical for understanding chemical reaction pathways involving cyclopentenyl and cyclohexenyl cation intermediates. One approach for determining this essential kinetic information follows the time rate of change of such cations using UV-visible spectrophotometry. However, the obtained absorption spectra often show asymmetric irregularities such as shoulders due to interfering side reactions. The aim of this exploratory study was to develop a method of interpreting UV-visible spectra that provides accurate rate constant values in addition to clarifying the cause of signal abnormalities. In this investigation, changes in spectra of the cation intermediates were followed over the course of time using UV-visible spectrophotometry for acid-catalyzed reactions for 3-methylcyclopentanol and 4-methylcyclohexanol. Guggenheim treatment of spectral data were then applied to obtain rate constant values at intervals of wavelength across the range of the spectra. From analysis of 3-methylcyclopentanol, the observed rate constants were highly accurate over a range of wavelengths (wavelengths 285 nm to 300 nm): the actual rate constant values remained constant at around 0.099 s-1 , associated with coefficients of determination that were consistently high at 0.99. Hence, consistent and quality observed rate constants were obtained in the region of the absorbance spectra farthest away from asymmetric anomaly, not just at the maximum wavelength. The data from analysis of 4-methylcyclohexanol supports the aforementioned conclusion but goes beyond 3-methylcyclopentanol’s analysis by providing support for the nature of the mechanism and relationship between two intermediates. It is anticipated that the developed method has potential for resolving more complex cases of time-series absorbance spectra showing deviations stemming from reactions involving cyclopentenyl and cyclohexenyl intermediates.

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