Determination of Anomalies of Gas Chromatographic Retention Indices for Homologs Based on Their Homologous Increments

Autor: I. G. Zenkevich
Rok vydání: 2021
Předmět:
Zdroj: Russian Journal of Physical Chemistry A. 95:1720-1728
ISSN: 1531-863X
0036-0244
DOI: 10.1134/s003602442107030x
Popis: Possible anomalies of gas chromatographic retention indices (RIs) for homologs are difficult to determine as they are masked by the “normal” homologous dependence (~100 index units per CH2 homologous difference). A comparison of the RIs of homologs in the form of their homologous increments iRI = RI – 100x (х = int(M/14), where int is the integer part of the number equivalent to M = 14x + yМ, where уМ is the number of the homologous group of the compound; yМ ≡ M( $$\bmod 14$$ )) revealed two anomalies of chromatographic retention, which were not discussed previously. One of these is abnormal decrease in iRI of di-n-alkyl phthalates at increased number of carbon atoms in alkyl fragments, which is not associated with variations in their polarity or polarizability. The most probable reason is the position of the conformational equilibria of the alkyl fragments of molecules (predominantly “folded” into Gaussian coils) in the molecules of ortho-disubstituted aromatic compounds. Among the numerous theoretically possible structures of this type, the largest number of homologs with different numbers of carbon atoms, which makes it possible to trace the anomalies of their gas chromatographic properties, was characterized only in the series of dialkyl phthalates. The second anomaly relates to polymethyl-substituted aromatic compounds containing substituents at ortho-positions relative to each other. The presence of this structural fragment leads to an anomalous increase in the iRI values of homologs, which is also not related to their polarity or polarizability, but can be explained by steric hindrances to the intramolecular rotation of substituents relative to С(sp2)–C(sp3) ordinary bonds (“gear effect”). Both effects are compared with the so-called “propyl group effect” discussed in the 1980s, whose value does not exceed 10 index units for aliphatic dienes; in the given cases, the RI anomalies reach 160 units for dialkyl phthalates and 150 index units for methylbenzenes and methylnaphthalenes.
Databáze: OpenAIRE
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