ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 7, pp. 1178−1185. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © N.I. Belaya, A.V. Belyi, A.I. Pomeshchenko, K.V. Glushchenko, 2015, published in Zhurnal Prikladnoi Khimii, 2015, Vol. 88, No. 7,
ORGANIC SYNTHESIS AND INDUSTRIAL
Natural Phenolic Compounds in the Reaction
with a Nitrogen-Centered Radical in an Aprotic Solvent
N. I. Belaya
, A. V. Belyi
, A. I. Pomeshchenko
, and K. V. Glushchenko
Donetsk National University, 600-richya str., Vinnytsia, 21021 Ukraine
Litvinenko Institute of Physical-Organic Chemistry and Coal Chemistry, ul. Rozy Luxemburg 70, Donetsk, 83114 Ukraine
Received July 20, 2015
Abstract—Kinetics and mechanism of the reaction of vegetable phenols (PhOH) with 2,2'-diphenyl-1-picrylhy-
drazyl radical (DPPH
) in a polar aprotic solvent, dimethyl sulfoxide, were studied. The reaction of natural phe-
nols with DPPH
in dimethyl sulfoxide occurs in two stages. In the ﬁ rst stage, a proton-coupled electron transfer
(PCET) occurs from a PhOH molecule to DPPH
to give primary transformation products, phenoxyl radicals
) and diphenyl hydrazine (DPPH–H), and in the second, the hydrazyl radical is consumed in the reaction
transformation products, enolized dimers, which is conﬁ rmed by NMR spectroscopy. A relationship
was revealed between the antiradical activity of phenols in the reaction with DPPH
(ln k) and the ionization
potential of the phenolates being formed.
Vegetable phenols are widely used as biological
antioxidants  owing to their aromatic structure and
phenolic OH groups deactivating free radicals. There exist
a large number of in vitro methods for evaluating the an-
tiradical activity of phenol compounds by using artiﬁ cial
radicals. The simplest and the most widely used of these
is the 2,2'-diphenyl-1-picrylhydrazyl (DPPH
) radical. A
speciﬁ c feature of various groups of vegetable phenols
(PhOH) with DPPH
is the strong inﬂ uence exerted on the
process kinetics by the nature of a solvent. In nonpolar
media [2, 3], this reaction occurs by the hydrogen atom
transfer (HAT) mechanism:
+ PhO–H ↔ PhO
+ DPPH–H. (1)
Reaction (1) is rather slow because the low dielectric
constant of the medium hinders the solubility of both the
radical and phenols.
In polar solvents [4, 5], the given reaction has the
form of a proton-coupled electron transfer (PCET) and
may occur by different mechanisms [6, 7].
For polar solvents (S) maintaining the ionization of
phenols, the reaction occurs by the pathway of single-
electron transfer from the phenolate ion being formed
) to DPPH
[4, 5, 8], which can be characterized
as the mechanism of successive proton transfer–electron
PhO–H + S ↔ PhO
+ SH+ → DPPH–H + S.
In this case (2), the calculation of the reaction kinetics
is strongly complicated by the very fast reaction rate in the
initial stage and the need to rigorously take into account
the distribution of ionic forms of phenol, dependent on
the pH of the medium.
It is worth attributing polar solvents with low ion-
izing capacity [dimethyl sulfoxide (DMSO), dimethyl-
formamide, acetone, acid solutions) to a separate group.
Suppressing the deprotonation of phenol compounds,
these solvents hinder their reaction with DPPH
thereby enable a study of the process kinetics in early
stages of conversion of reagents [5, 9].