Abstract
Gas chromatographic measurements of the retention times of alkyl naphthalenes on packed columns with polar and non-polar stationary phases have proven that the logarithm of the relative retention time increases bi-linearly (not linearly) with the number of carbon atoms in a molecule. This is caused by a strong inclination of alkyl side chains toward intramolecular cyclization. A FTIR spectral analysis has shown that longer alkyl side chains of alkyl naphthalenes are cyclized through an interaction between the terminal CH3 group and the aromatic ring. Conventional aromatic-aliphatic molecules thus become new molecules with quasi-alicyclic rings. This, however, alters the effect of non-covalent van der Waals attractive forces both inside and outside the molecules, which is reflected in an exponential increase of the retention times of alkyl naphthalenes with a side chain longer than propyl and in the bi-linearity of the logarithmic dependence of the relative retention times on the number of carbons in the molecule.
Highlights
Classic considerations on the dependence of retention times (Rt,rel) on the number of carbons in a molecule (z) in different homologous series of gas-chromatographically separated substances always focus on purely linear de-How to cite this paper: Straka, P., Novotná, M., Buryan, P. and Bičáková, O. (2014) The Cyclization of Alkyl Side Chains of Naphthalenes: The GC/Potential Energies/FTIR Approach
The bi-linearity detected in the dependence log Rt,rel = az + b [1] [2] suggests that the behavior of molecules of the compounds in question during separations in stationary phases is more complicated than has been assumed by the classic considerations
In the case of n-alkyl phenols and n-alkyl benzenes, this effect is strong with the molecules having the number of carbons higher than nine. This results in bi-linearity in the retention characteristics of these compounds, observed in the dependence of the logarithm of the relative retention time on the total number of carbons in the molecule
Summary
Classic considerations on the dependence of retention times (Rt,rel) on the number of carbons in a molecule (z) in different homologous series of gas-chromatographically separated substances always focus on purely linear de-. This results in bi-linearity in the retention characteristics of these compounds, observed in the dependence of the logarithm of the relative retention time on the total number of carbons in the molecule It is not clear whether the cyclization of the side chains of alkyl phenols and alkyl benzenes is their inherent feature or it occurs only during their motion through the column. The question of the cyclization of the alkyl side chains as an inherent property of aromatic-aliphatic molecules is resolved on the basis of the FTIR spectra interpretation of n-alkyl phenols as model compounds Another aim of the presented work is to explain the bi-linearity in the logarithmic dependence of the relative retention times on the number of carbons in the molecule in the case of n-alkyl naphthalenes by molecular mechanics modeling and to clarify the behavior of n-alkyl naphthalenes in the environment of different stationary phases of packed GC columns. The potential energies of the bonds and non-covalent interactions have been considered
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