BULLETIN OF THE
Article
Determination of Aromaticity Indices of Five-membered Monoheterocycles
KOREAN CHEMICAL SOCIETY
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The most important observation from the values of the
slopes in Table 3 is for the β-Cs. The β-C atom is bonded
directly to the benzene and the heteroaryl rings. Therefore,
the values of the slopes should be considered as a measure
of the overall influence of the heteroaryl ring that is com-
pared to the benzene ring. If we define the aromaticity as
the overall influence of the benzene ring, the relative mag-
nitude of the slope of a plot for a heteroaryl system against
a benzene system many be considered as the index of the
aromaticity. Such logic may lead to the values of the slopes
0.92, 0.76, and 0.76 for the β-Cs of 2/1, 3/1, and 4/1,
respectively, should be a reflection of the aromaticity indi-
ces for thiophene, pyrrole, and furan, respectively, if the
index of benzene is set to 1.00.
The values of the slopes from the plots of the chemical
shift values of the carbonyl carbon atoms in 6–8 against
those in the benzene compounds 5 also seem to be consid-
ered as the indices of aromaticity for the heterocycles. The
values 0.86, 0.64, and 0.52 from 6/5, 7/5, and 8/5, respec-
tively, should reflect the magnitude of the direct influence
of the heterocyclic rings in comparison with the benzene
ring. The values obtained for α-C and β-C from the slopes
of the plots 6/5, 7/5, and 8/5 may be considered as the indi-
ces of the aromaticity for the heterocycles. Although they
are slightly different, it is certain that thiophene is most like
benzene whereas furan is least like benzene.
In conclusion, it is possible to evaluate the degree of aro-
maticity of heterocyclic compounds numerically from the
slopes of the plot of the 13C NMR chemical shift values of
the heterocyclic derivatives of chalcones against the
chalcone derivatives with same substituents Figure S1–S16.
Acknowledgments. We thank Dr. Gary Kwong for help in
preparing the manuscript. This study was supported by
2016 Research Grant from Kangwon National University
(No. 520160084).
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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1. See comprehensive reviews: P. Schleyer, Chem. Rev. 2001,
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