Electrochemical, ESR and quantum chemical study of 1-substituted naphthalenes and their radical anions

Article abstract of DOI:10.1002/poc.1288

Electrochemical reduction and oxidation of a series of 1-substituted naphthalenes (1-X-naphthalenes) have been studied by the method of cyclic voltammetry (CV). The first reduction peak of the majority of these compounds corresponds to a one-electron transfer to form the relatively stable radical anion (RA). For these species, ESR spectra have been registered and interpreted, the life time has been estimated. The first oxidation peaks of 1-X-naphthalenes are irreversible and correspond to a transfer of two or more electrons. Copyright

Full text of DOI:10.1002/poc.1288

Research Article
Received: 12 July 2007,
Revised: 21 September 2007,
Accepted: 24 September 2007,
Published online in Wiley InterScience: 6 December 2007
(www.interscience.wiley.com) DOI 10.1002/poc.1288
Electrochemical, ESR and quantum chemical
study of 1-substituted naphthalenes and their
radical anionsy,z
a
a
a
a,b
*
*
N. V. Vasilieva , I. G. Irtegova , T. A. Vaganova and V. D. Shteingarts
Electrochemical reduction and oxidation of a series of 1-substituted naphthalenes (1-X-naphthalenes) have been
studied by the method of cyclic voltammetry (CV). The first reduction peak of the majority of these compounds
corresponds to a one-electron transfer to form the relatively stable radical anion (RA). For these species, ESR spectra
have been registered and interpreted, the life time has been estimated. The first oxidation peaks of 1-X-naphthalenes
are irreversible and correspond to a transfer of two or more electrons. Copyright ß 2007 John Wiley & Sons, Ltd.
Keywords: electrochemical reduction; electrochemical oxidation; radical anions; radical cations; ESR spectroscopy; cyclic
voltammetry; naphthalene derivatives
(X ¼ H (1), CH3 (2), OCH3 (3), CO2CH3 (4), CONH2 (5), CON(CH3)2
(6), CN (7), COPh (8), COCH3 (9), CHO (10), NO2 (11)) in CH3CN, the
estimation of the substituent X influence on the electrochemical
potential values and stability of the resulting RIs and elucidation
of their electronic structure by means of ESR spectroscopy.
INTRODUCTION
One-electronic reduction and oxidation of aromatic compounds
with the formation of rather stable radical ions (RIs) is the
perspective approach to arene activation for various transform-
ations as occurrence of powerful reactivity factors such as a
charge and an unpaired electron is combined with conservation
of structural integrity.[2] For purposeful use of this approach,
electrochemical characteristics of activated compounds and data
on the electronic structure of their RIs are important. From this
point of view, the study of one-electron reduction and oxidation
of substituted arenes by the cyclic voltammetry (CV) method is
relevant to obtain electrochemical potential values reflecting
their frontier MO energies and, besides, to estimate a stability of
RIs thus formed. Additionally, their ESR spectra give an infor-
mation on the spin density distribution. All these characteristics
are useful for the interpretation of their reactivity and create the
basis for a tentative prediction of arene reaction ways with the RI
intermediacy.
Benzene derivatives were electrochemically studied quite
intensively.[3] At the same time, the higher electron affinity and
the smaller ionization potential of naphthalene in comparison
with benzene are favourable to a wider substituent variation in
the RIs which are stable enough to be generated in preparative
concentrations and suitable as synthons. There are the numerous
publications on the electrochemical reduction of 1-X-naphtha-
lene derivatives,[4] but these data are incommensurable because
of the great variety of experimental conditions. The oxidation
potential values are available mainly for naphthalene derivatives
with electron-donating substituents,[5] and from those with
electron-withdrawing substituents – only for naphthalenes with
the CN, NO2 and COOH groups.[6] The ESR data are known only for
radical anions (RAs) of 1-X-naphthalenes with X ¼ H, CH3, CN
and NO2.[7–9] Thus, stability of formed RAs in most of the cases
was not characterized.
EXPERIMENTAL
CV measurements on 2 ꢀ 10ꢁ3 mol/L solutions of 1–11 were
performed using CVA-1BM potentiostat equipped with a LAB-
MASTER analogue-to-digital converter with multifunctional
interface (Institute of Nuclear Physics, Russian Academy of
Sciences, Novosibirsk), which enables complete digital control
of the system. The measurements were carried out in a mode of
triangular pulse potential sweep in three-electrode electroche-
mical cell, at 295 K in an argon atmosphere in absolute DMF
or CH3CN solution of 0.1 M (n-C4H9)4NBF4 as supporting electro-
lyte at a stationary platinum electrode (S ¼ 8 mm2), with sweep
rates of 0.01–100 V/s. Peak potentials are quoted with reference
to a saturated calomel electrode. DMF was twice vacuum distilled
over P2O5. CH3CN was distilled over KMnO4 and twice over P2O5.
ESR spectra were recorded on Bruker ESP-300 spectrometer
(MW power 265 mW, modulation frequency 100 KHz, modulation
*
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of
the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.
E-mails: vasileva@nioch.nsc.ru; shtein@nioch.nsc.ru
a
N. V. Vasilieva, I. G. Irtegova, T. A. Vaganova, V. D. Shteingarts
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of
the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation
b
V. D. Shteingarts
Novosibirsk State University, 630090 Novosibirsk, Russian Federation
yDedicated to the memory of Academician N. N. Vorozhtsov on the occasion of
his 100th birthday.
zReductive activation of arenes. Part 23; for Part 22 see Ref. [1].
In this connection, the goal of the present work is the CV study
of electrochemical reduction and oxidation of 1-X-naphthalenes
J. Phys. Org. Chem. 2008, 21 73–78
Copyright ß 2007 John Wiley & Sons, Ltd.

N. V. VASILIEVA ET AL.
Table 1. Experimental redox potentials of 1-X-naphthalenes 1–11 in CH3CN (v ¼ 100 mv/s), their gas phase DFT BLYP/3z first
electron affinities (EA1) and adiabatic ionization energies (IE1)
Compound (X)
E1pC (V)
EA1 (eV)
nrev (ꢃV/s)a
E2pC (V)
E1pOx (V)
IE1 (eV)
1 (none)
2 (CH3)
ꢁ2.61
ꢁ2.66
ꢁ2.74
ꢁ1.99
ꢁ2.16
ꢁ2.38
ꢁ1.96
ꢁ1.78
ꢁ1.86
ꢁ1.69
ꢁ1.66
ꢁ1.12
0.312
0.289
0.390
ꢃ0.05
ꢃ0.10
ꢃ0.10
0.02b
ꢃ0.10
ꢃ2
—
—
—
—
—
—
—
ꢁ2.27
ꢁ2.44
—
1.72
1.61
1.33
1.94
1.89
1.80
2.11
1.88
1.88
1.98
ꢁ5.274
ꢁ5.147
ꢁ4.844
ꢁ5.462
ꢁ5.415
ꢁ5.187
ꢁ5.809
ꢁ5.434
ꢁ5.464
ꢁ5.691
3 (OCH3)
4 (CO2CH3)
5 (CONH2)
6 (CON(CH3)2)
7 (CN)
8 (COPh)
9 (COCH3)
10 (CHO)
In DMF
ꢁ0.523
ꢁ0.411
ꢁ0.239
ꢁ0.625
ꢁ0.737
ꢁ0.744
ꢁ0.763
0.02b
0.01b
0.02b
ꢃ0.20
0.01b
0.01b
ꢁ2.31
11 (NO2)
ꢁ1.292
ꢁ1.92
2.13
ꢁ5.950
a The sweep rate at which a anode peak 1A in the reverse part of reduction CV becomes observable.
b Anode peak 1A at this scan rate is sufficiently intensive, at a smaller scan rate the experiment is not carried out.
amplitude 0.005 mT) equipped with rectangular double reso-
nator and BST-100/700 temperature control unit. For ESR
measurements, DMF solutions (1–5 ꢀ 10ꢁ3 M) of 1-X-naphtha-
lenes were reduced on a Pt electrode in anaerobic conditions at
the first peak reduction potential and T ¼ 295 K (for 2, 4, 7–11,
Table 1), T ¼ 243 K (for 3) with 0.1 M Bu4NBF4 as supporting
electrolyte. Digital simulation of ESR spectra was carried out using
the Winsim 2002 program. Accuracy in calculating a values is
ꢂ0.0001 mT. The RA half-life values (t1/2) were calculated by
adjusting the time dependences of the ESR integral intensities
(starting from the moment of potential switching-off) to an
1-Carbomethoxynaphthalene was prepared by the esterifica-
tion of 1-naphthoic acid with MeOH in the presence of H2SO4,
m.p. 58–598C (from hexane), lit.[15] m.p. 58–608C.
1-Acetylnaphthalene was prepared by the interaction of
naphthyl magnesium bromide with CH3CN according to the
procedure[16] and purified by chromatography, m.p. 9–108C
(from hexane), lit.[11] m.p. 10.58C.
1-Benzoylnaphthalene was prepared by the interaction of
naphthalene with benzoyl chloride in the presence of AlCl3
according to the procedure[11] and purified by two-fold crystal-
lization from EtOH, m.p. 73–73.58C, lit.[14] m.p. 738C.
equation for the first-order RA decay (I ¼ Aeꢁkt þ B, t1/2
¼
0.69 kꢁ1). DFT BLYP/3z calculations of full and frontier MO
energies of neutral compounds and RAs were performed
using the program ‘Nature 2.02’[10] with full optimization of
molecular geometry. The orbital basis sets of size {3,1,1/1}
for H and {6,1,1,1,1,1/1,1} for C, N, O were used for DFT
calculations.
RESULTS AND DISCUSSION
The values of reduction and oxidation potentials of 1-X-
naphthalenes 1–11 (X ¼ H, CH3, OCH3, CO2CH3, CONH2,
CON(CH3)2, CN, COPh, COCH3, CHO, NO2) obtained in the present
work by using the CV method in CH3CN on a stationary Pt
electrode are listed in Table 1. As an example, the cyclic
voltamograms of reduction (Fig. 1a) and oxidation (Fig. 1b) of 7
are presented in Fig. 1. Depending on the X substituent, one or
two reduction peaks are observed on a cathodic curve of the
voltammogram. For the most of compounds investigated the first
reduction peak 1C is diffusionally controlled (ipnꢁ1/2 ¼ const,
where ip is the peak current and n is a potential sweep rate),
1-X-Naphthalenes were prepared and purified as described
below.
1-Methoxynaphthalene was purified by high vacuum (0.01 mm
Hg) distillation.
1-Methylnaphthalene was purified by distillation, b.p. 58–608C/
1 mm Hg, lit.[11] b.p. 1118C/12 mm Hg.
1-Naphthonitrile was purified by sublimation, m.p. 37–388C,
lit.[11] m.p. 388C.
1-Nitronaphthalene was purified by crystallization from EtOH,
m.p. 60–618C, lit.[11] m.p. 57.88C.
1C
reversible or quasireversible (E1pAꢁEp1C ¼ 0.06 V, E ꢁEp1C ¼ 0.06 V)
p=2
and corresponds to the one-electron transfer.
1-Naphthaldehyde was purified through the formation of
bisulphate derivatives and distillation, b.p. 111–1138C/1 mm
Hg, lit.[11] b.p. 1448C/4 mm Hg, purity 97.8%, 2-naphthaldehyde
admixture 1.9%.
1-Naphthamide was prepared according to the procedure,[12]
m.p. 2058C (from EtOH), lit.[11] m.p. 2058C.
Depending upon substituents, the first reduction peak
potential changes from ꢁ1.12V (X ¼ NO2) to ꢁ2.74V (X ¼OCH3).
A good linear correlation (r ¼ 0.988) exists between the E1pC values
and the sꢁpara constants for electron-accepting substituents, the
point for X ¼CN being only deviating. The same situation we
*
observed for the benzene derivatives. Respectively, a satisfac-
N,N-Dimethyl-1-naphthamide was prepared by the interaction
of naphthoyl chloride with DMF according to the procedure,[13]
m.p. 62–62.58C (from hexane), lit.[14] m.p. 62–638C.
ꢁ
*
The values of peak potentials of C6H5X and spara – constants of substitutients
were taken from Ref. [17].
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 73–78

1-SUBSTITUTED NAPHTHALENE RADICAL ANIONS
studied sweep rates that testifies to instability of corresponding
dianions.
The experimental values of peak 1C potentials of 1-X-
naphthalenes correlate well with their DFT BLYP calculated
LUMO energies (r ¼ 0.986) and adiabatic electron affinities (r ¼
0.980) (EAad ¼ ENꢁERA, where EN and ERA – full energies of neutral
1-X-naphthalene and its RA, accordingly) listed in Table 1. This
probably means that the molecular structures of 1-X-naphtha-
lenes 1–11 and their RAs calculated with DFT BLYP are close to
that in solution, and the influence of solvation on the energy of
one-electron reduction is insignificant.
1-X-Naphthalene RAs stable under CV conditions have been
electrochemically generated at the peak 1C potential in the
resonator of an ESR spectrometer (for example Fig. 2). The
experimental hyperfine interaction (hfi) constants and the t1/2
values of RAs, along with DFT BLYP calculated hfi constants and
the respective literature data for RAs of 1, 2, 7, 11 are shown in
Table 2.
In DMF at 295 K, the most long-lived are RAs of 1-X-naphtha-
lenes 7 (X ¼CN, t1/2 ¼ 729 s) and 11 (X ¼ NO2, t1/2 ¼ 213 s). The
RAs with X ¼CH3, COOCH3, COPh, COCH3 are somewhat less
stable (t1/2 ¼ 70–130 s). Even less stable is a RA of 10 (X ¼CHO,
t1/2 ¼ 30 s), and a RA of 3 (X ¼OMe) was possible to be registered
only at T ꢄ 253 K as at higher temperature it is unstable. The RA of
5 (X ¼CONH2) was unstable and undetectable by ESR even at
243 K. Since the sweep rate at which a anode peak 1A of 6
(X ¼CON(CH3)2) in the reverse part of reduction CV becomes
observable much more, than that for RA of 5 (nrev ꢃ 2 V/s), there
was no point to generate electrochemically AR of 6. As a
whole, the data obtained are in line with the above CV estimation
of the RA relative stability in CH3CN.
The experimental hfi constants (a) of 1-X-naphthalene RAs
were assigned on the basis of comparison with the correspond-
ing values calculated by the DFT BLYP method with full geometry
optimization. There is a good correspondence between the
experimentally obtained and calculated hfi constants. An
exception is the discrepancy between the experimental and
calculated aN values for the RA of 1-nitronaphthalene, the first
value being ꢅ4 times larger than the second one. This fact may
be due to the underestimation of the exchange interactions for
the planar p-radicals with the spin-polarization mechanism of the
isotropic hyperfine coupling on 14N nucleus.[18] Nevertheless, the
assignment of this constant in the ESR spectrum is unambiguous
due to a characteristic 1:1:1 triplet due to coupling with
14N nucleus.
Figure 1. CV of 1-naphthonitrile (7) at scan rates of 0.10 V/s: reduction (a)
and oxidation (b)
tory linear correlation (r ¼ 0.978) between the values of the first
reduction peak potentials of 1-X-naphthalenes (X ¼ H, CHO,
COCH3, CO2CH3, COPh, CN, NO2) and corresponding benzene
derivatives points to the uniformity of substituent influences on
the reduction potentials in these two series of compounds.
As follows from the n values at which the peak of the RA
oxidation (peak 1A) to appear, the most stable are the RAs of
1-X-naphthalenes with X ¼ NO2, CN, COPh, COCH3 and CO2CH3. In
all these cases, the peak 1A is intensive at n ¼ 0.02 V/s. For the
compounds with X ¼CH3, OCH3 and CONH2, it appears at n ¼
0.1 V/s, and for naphthalene itself – at n ¼ 0.05 V/s. The least
stable is the RA with X ¼CON(CH3)2: in this case peak 1A appears
only at n ¼ 2 V/s, though, apparently, this electron-accepting
group should stabilize a RA more effectively, than the electron-
donating methyl and methoxy groups.
Upon reduction of 10 in CH3CN, the distinctly discerned peak
1A appears at the n ¼ 0.2 V/s, and in DMF – at n < 0.01 V/s, that
testifies to greater stability of the RA in the latter case. Unlike this,
transition from CH3CN to DMF does practically not affect the
stability of the RA of 6.
The second reduction peak (peak 2C), admittedly attributed to
the reduction of a RA to a dianion, was observed in cases of
X ¼ NO2, COPh, COCH3 and also CHO in DMF (Table 1). For all
substances, the second reduction peak is irreversible with all
According to the DFT BLYP results, the naphthalene frag-
ment in a RAs is flat, the substituents X ¼ NO2, CHO, COCH3,
COPh, CO2CH3 are in co-planarity with it thus promoting a
—
negative charge and spin density delocalization. The C O bond
—
of substituents X ¼CHO, COCH3, COPh, CO2CH3 is transoid to the
naphthalene C1—C2 bond. In the RAs with X ¼CONH2 and
CON(CH3)2, the carbonyl group also is aside to a non-substituded
ring, in so doing the amide group being out-of-plane deviated by
108 and 208, respectively.
For naphthalene 1 RA, a(Ha)/a(Hb) ¼ 2.6 (Table 2). An electron-
withdrawing substituent in the naphthalene position 1 disperses
a part of RA spin density and redistributes it inside a naphthalene
framework. So, substituents X ¼CHO, COCH3, COOCH3, CN
increase a(H2) 2.5–3 times and a(H4) ꢅ1.5 times (a(H4) in RA of
11 is increased only slightly). In contrast, the a(H3) constants in
the specified RAs are 2.8–4.7 times decreased as compared to the
naphthalene RA.
J. Phys. Org. Chem. 2008, 21 73–78
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N. V. VASILIEVA ET AL.
Figure 2. Experimental (a) and simulated (b) ESR spectra of RAs from electrochemical reduction of 1-X-naphthalenes 3, 4, 9 in DMF
As a whole, for the non-substituted naphthalene fragment of
RAs with substantially electron-accepting X substituents the
tendency is observed to distinctly decrease the a(H) values as
compared with the naphthalene RA. The hfi constant a(H5) in RAs
of 7 (X ¼CN), 4, 9, 10 (X ¼CO2CH3, COCH3, CHO) and 11 (X ¼ NO2)
is 1.5–3 times decreased. The ratios of a(H8) and a(H6) values in RA
of 1 to a(H8) and a(H6) in its 1-X-substituted derivatives depend
on the nature of X more specifically, being raised from ꢅ2–3.4 for
RAs of 7 and 11 up to ꢅ7 for a(H8) in RAs of 4, 9, 10 and to 17 for
a(H6) in RA of 9. At the same time, the influence of electron-
accepting substituents on the a(H7) value is slight and ambi-
guous: in RA of 7 (X ¼CN) it is practically the same, in RA of 11
(X ¼ NO2) ꢅ1.3 times reduced and in RAs of 4, 9, 10 (X ¼CO2CH3,
COCH3, CHO) ꢅ1.3 times increased as compared to naphthalene
RA.
Unlike electron-accepting substituents, electron-donating
ones influence on hfi constants more moderately: methyl and
methoxy groups in the naphthalene position 1 render a(H5) and
a(H8) 5–20% larger and a(H4) 20–30% smaller.
Some conclusions about the influence of the nature of
substituent X on the distribution of unpaired electron density
(UED) in RAs of 1-substituted naphthalenes could be done on the
basis of hfi constants presented in Table 2. The UED in RAs with
electron-donating substituents CH3 and OCH3, such as in the
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 73–78

1-SUBSTITUTED NAPHTHALENE RADICAL ANIONS
Table 2. Experimental and calculated hfi constants of ESR spectra of 1-X naphthalene (1–4, 7–11) RAs and their half-life times t1/2
(T ¼ 295 K)
R
t1/2, s
hfi constants, mT
H
—
Lit.7: H1, H4, H5, H8 0.479; H2, H3, H6, H7 0.184
DFT BLYP: H1, H4, H5, H8 0.526; H2, H3, H6, H7 0.191
CH3
69
Lit.8 a: H2 0.153; H3 0.223; H4 0.416; H5 0.549; H6 0.149; H7 0.196; H8 0.523; H (CH3) 0.361
DFT BLYP: H2 0.166; H3 0.196; H4 0.490; H5 0.542; H6 0.136; H7 0.244; H8 0.513; H (CH3) 0.406
Experimental: H2 0.157; H3 0.194; H4 0.432; H5 0.530; H6 0.135; H7 0.231; H8 0.497; H (CH3) 0.378
DFT BLYP: H2 0.186; H3 0.170; H4 0.485; H5 0.529; H6 0.196; H7 0.192; H8 0.547; H (CH3) 0.023
Experimental: H2 0.179; H3 0.128; H4 0.370; H5 0.546; H6 0.208; H7 0.184; H8 0.587; H (OCH3) 0.010
DFT BLYP: H2 0.439; H3 0.034; H4 0.684; H5 0.280; H6 0.018; H7 0.214; H8 0.152; H (CH3) 0.078
Experimental: H2 0.514; H3 0.038; H4 0.792; H5 0.271; H6 0.012; H7 0.237; H8 0.077; H (CH3) 0.062
Lit.9: H2 0.227; H3 0.042; H4 0.785; H5 0.465; H6 <0.008; H7 0.170; H8 0.308; N (CN) 0.128
DFT BLYP: H2 0.376; H3 0.081; H4 0.678; H5 0.348; H6 0.076; H7 0.211; H8 0.266; N (CN) 0.088
Experimental: H2 0.444; H3 0.047; H4 0.770; H5 0.313; H6 0.015; H7 0.180; H8 0.225; N (CN) 0.122
Unresolved specter
DFT BLYP: H2 0.401; H3 0.028; H4 0.663; H5 0.251; H6 0.056; H7 0.217; H8 0.099; H (CH3) 0.304
Experimental: H2 0.512; H3 0.039; H4 0.757; H5 0.235; H6 0.017; H7 0.229; H8 0.070; H (CH3) 0.337
DFT BLYP: H2 0.380; H3 0.047; H4 0.677; H5 0.262; H6 0.008; H7 0.226; H8 0.106; H (CHO) 0.450
Experimental: H2 0.508; H3 0.057; H4 0.791; H5 0.246; H6 0.011; H7 0.237; H8 0.065; H (CHO) 0.525
Lit.9 H2 0.544; H3 0.062; H4 0.552; H5 0.165; H6 0.070; H7 0.141; H8 0.137; N (NO2) 0.772
DFT BLYP: H2 0.412; H3 0.013; H4 0.545; H5 0.200; H6 0.014; H7 0.181; H8 0.062; N (NO2) 0.191
Experimental: H2 0.541; H3 0.065; H4 0.549; H5 0.166; H6 0.065; H7 0.140; H8 0.140; N (NO2) 0.756
OCH3
CO2Me
CN
379
(243 K)
120
729
COPh
COMe
132
99
CHO
NO2
30
213
a Reduction with Na in THF.
naphthalene RA, is located mainly in a-positions of the naph-
thalene nucleus. In so doing, it is a little bit larger in positions
5 and 8 and a little bit smaller in position 4 as compared with the
respective positions in the naphthalene RA, that testifies to
forcing out a small part of UED from an X-substituted ring into a
non-substituted ring. In contrast, introduction of electron-
accepting substituents draw up UED into the substituted ring
along with the redistribution of UED between positions 2 and 4 in
favour of the first, the redistribution being the larger with the
substituent X is more electron accepting: the ratio a(H4)/a(H2) is
1.73 for RA of 7 (X ¼CN), ꢅ1.5 for RA of 4, 9, 10 (X ¼COR) and ꢅ1
for RA of 11 (X ¼ NO2). The similar UED redistribution is character-
istic of an unsubstituted ring: in going from naphthalene RA to
RAs of 4, 7–11 the UED in position 7 increases relatively those in
positions 5 and, especially, 8. The overall pattern of UED redistri-
bution as depending on a nature of X substituent demonstrates
two types of unpaired electron MOs in the RAs under consi-
deration: the first is characteristic for RAs of unsubstituted
naphthalene and its derivatives with electron-donating sub-
stituents X, the second is mainly due to the resonance conju-
gation of electron-withdrawing substituent X with positions 2, 4,
5 and 8 of naphthalene nucleus.
first oxidation peak potential varies with substituent X from 1.33 V
(X ¼OCH3) to 2.13 V (X ¼ NO2). The influence of X nature is well
consistent with its electronic effect which is illustrated by a good
linear correlation between E1pOx values and substituent spꢁX
–
constants (r ¼ 0.982), the points of the electron-donating substi-
tuents fit to this correlation only with electrophilic sþp constants.
The first oxidation peaks are diffusionally controlled and for the
most of compounds investigated correspond to transfer of two or
more electrons (I1pOx ꢃ 2.8 Ip1C). Reversible oxidation peaks of 1-X-
naphthalenes including those with electron-donating substitu-
ents could not been observed even at high electrode polarization
rate (v ¼ 50–100 V/s). Thus, 1-X-naphthalene radical cations are
Ring positions in RA, bearing a significant spin density, are the
potential sites of radical recombination, including dimerization.
So, according to data,[19] dimerization of a 1-naphthonitrile RA
occurs at the position para to the CN group. Hence, one can
believe, that diverse radical reactions of 1-X-naphthalene RAs
with electron-accepting X will exhibit the same regiochemistry.
Besides reduction potentials, oxidation potentials of 1-X-naph-
thalenes have also been measured with the use of the CV method
and oxidation peaks for all compounds under investigation have
been registered in an accessible potential range. The value of the
Figure 3. Comparison of the electrochemical oxidation and reduction
potentials of 1-X-naphthalenes
J. Phys. Org. Chem. 2008, 21 73–78
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N. V. VASILIEVA ET AL.
extremely unstable and rapidly undergo subsequent reactions
(the fast ECE process), where the second and subsequent
electrons are transferred from the products. Unfortunately, one
cannot compare the values obtained of 1-X-naphthalene oxi-
dation potential with similar data for corresponding benzene
derivatives because of the lack of such data for benzenes with
electron-accepting substituents.
There are satisfactory linear correlations between oxidation
and reduction potentials of the compounds under investigation
(Fig. 3), different for compounds with electron-accepting and
with electron-donating substituents. So the E1pC values are more
sensitive to the nature of the electron-accepting substituent,
whereas the E1pOx – to the nature of the electron-donating
substituent. The only point not obeying the linear relationship is
that of 1-naphthonitrile which is likely due to its reduction
potential (vide supra).
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Thus, electrochemical reduction and oxidation potentials of a
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CV method in unified conditions. The first reduction peak of
the most of compounds investigated has been established to be
due to a one-electron transfer to form respective stable RAs. For
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