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Determination of the Position of the Conformational Equilibrium of a Trans 1,2-Disubstituted Cyclohexane by NMR Spectroscopy. An Experiment in Physical Organic Chemistry for Undergraduate Students

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Andrei G. Kutateladze and Joseph M. Hornback

Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208-2436

J. Chem. Educ., 2001, 78 (1), p 81

DOI: 10.1021/ed078p81

Publication Date (Web): January 1, 2001

Section:

History, Education, and Documentation

Abstract

An experiment using NMR spectroscopy to determine the relative amounts of the two chair conformations of trans-1-chloro-2-(2-nitrophenythio)cyclohexane is described. The cyclohexane derivative is prepared in an NMR tube by the addition of 2-nitrophenylsulfenyl chloride to cyclohexene. The coupling constants for the hydrogens adjacent to the chlorine and sulfur are used to calculate the percentage of each conformer in the equilibrium mixture. The coupling constants for these hydrogens in the individual conformers are obtained by application of the Karplus equation. The needed dihedral angles are obtained by a molecular mechanics calculation, although idealized angles can also be used. The reaction is conducted in two solvents, chloroform-d and acetonitrile-d₃, so the effect of solvent polarity on the equilibrium is also investigated.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Organic Chemistry

Keywords (Pedagogy):

Hands-On Learning / Manipulatives

Keywords (Subject):

Equilibrium

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