Effects of neutral and charged substituents on the infrared carbonyl stretching frequencies in phenyl and alkyl benzoates in DMSO

Article abstract of DOI:10.1002/poc.3608

The carbonyl infrared stretching frequencies for 57 meta-, para- and ortho-substituted phenyl benzoates, C₆H₅CO₂C₆H₄-X and alkylbenzoates, C₆H₅CO₂R, containing besides neutral substituents the charged substituents in phenoxy and alkoxy part in dimethyl sulfoxide (DMSO) have been recorded. The carbonyl stretching frequencies, ν_CO, for meta- and para-substituted phenyl esters of benzoic acids in the case of neutral substituents were found to correlate well with the substituent constants, σ°. The ν_CO values for ortho derivatives correlated with the inductive substituent constants, σ_I, only. The values of constants for charged substituents, σ°_±, calculated on the basis of the ν_CO and the ¹³C NMR chemical shifts, δ_CO, in DMSO agree well with the σ°_± values for the corresponding ion pairs reported by Hoefnagel and Wepster and those determined from the log k values of the alkaline hydrolysis in 4.4 M NaCl solution at 50 °C. Thus, the values of substituent constants for ion pairs of charged substituents estimated on the basis of aqueous data could be successfully used in non-aqueous solution (DMSO) simultaneously with neutral substituents in case the charged substituents were not completely ionized and are in ion pair form. Copyright

Full text of DOI:10.1002/poc.3608

Research article
Received: 4 February 2016,
Revised: 24 May 2016,
Accepted: 25 May 2016,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3608
Effects of neutral and charged substituents on
the infrared carbonyl stretching frequencies in
phenyl and alkyl benzoates in DMSO
Vilve Nummerta, Mare Piirsalua, Signe Vahura, Lauri Toomb, Ivo Leitoa and
Ilmar A. Koppela*
The carbonyl infrared stretching frequencies for 57 meta-, para- and ortho-substituted phenyl benzoates, C6H5CO2C6H4-X and
alkylbenzoates, C6H5CO2R, containing besides neutral substituents the charged substituents in phenoxy and alkoxy part in
dimethyl sulfoxide (DMSO) have been recorded. The carbonyl stretching frequencies, νCO, for meta- and para-substituted phe-
nyl esters of benzoic acids in the case of neutral substituents were found to correlate well with the substituent constants, σ°.
The νCO values for ortho derivatives correlated with the inductive substituent constants, σI, only. The values of constants for
charged substituents, σ° , calculated on the basis of the νCO and the 13C NMR chemical shifts, δCO, in DMSO agree well with the
σ° values for the corresponding ion pairs reported by Hoefnagel and Wepster and those determined from the log k values of
the alkaline hydrolysis in 4.4 M NaCl solution at 50 °C. Thus, the values of substituent constants for ion pairs of charged sub-
stituents estimated on the basis of aqueous data could be successfully used in non-aqueous solution (DMSO) simultaneously
with neutral substituents in case the charged substituents were not completely ionized and are in ion pair form. Copyright ©
2016 John Wiley & Sons, Ltd.
Keywords: carbonyl carbon 13C NMR chemical shifts; charged substituents; infrared spectra; ortho effect; phenyl and alkyl benzoates;
SUP
C6H5CO2C6H4-X, containing substituents in phenyl part and al-
kyl benzoates, C6H5CO2R, in the present work, the IR spectra
INTRODUCTION
Infrared stretching frequency of the carbonyl group, νCO, has
been perhaps the most widely used infrared spectral feature
for elucidating interactions within and between molecules. Nu-
merous electronic, steric and solvation effects in a myriad of
compounds – ketones,[1–5] carboxylic acids,[6,7] esters,[8–10] metal
carbonyl complexes[11–13], etc. – have been studied via the car-
bonyl group stretching frequency.
for 42 meta-, para- and ortho-substituted phenyl benzoates
and 15 alkyl benzoates with neutral substituents and charged
substituents (X = 3-N+(CH3)3Iꢀ, 4-N+(CH3)3Iꢀ, 2-N+(CH3)3Iꢀ, 4-
SOꢀ3 Na+, 3-COOꢀN+Bu4, 4-COOꢀN+Bu4, 3-OꢀN+Bu4, 4-OꢀN+Bu4,
R = CH2N+(CH3)3Clꢀ, CH2CH2N+(CH3)3Iꢀ, CH2CH2CH2N+(CH3)3Iꢀ)
were recorded in dimethyl sulfoxide (their solubility in tetra-
chloromethane is too low).
We have previously recorded the infrared (IR) spectra of 33
substituted phenyl and alkyl esters of benzoic acid,
C6H5CO2C6H4-X, C6H5CO2R, and 22 phenyl esters of substituted
benzoic acids, X-C6H4CO2C6H5, in tetrachloromethane.[10] In
substituted phenyl benzoates, X-C6H4CO2C6H5, in the case of or-
tho substituents (X = Cl, Br, I, F, OCH3, CF3) in the benzoyl part,
doublet IR carbonyl stretching bands were observed. In phenyl
esters of ortho-substituted benzoic acids, the existence of cis
and trans isomeric conformations was supported by frequency
calculations with density functional theory (DFT) method at
B3LYP/6-311+ G** level.[10]
In our previous works in ortho-, meta- and para-substituted
phenyl and alkyl esters of benzoic acids (C6H5CO2C6H4-X, X-
C6H4CO2C6H5, C6H5CO2R), the influence of the substituent induc-
tive, resonance and steric effects, especially ortho effect, on the
carbonyl carbon 13C NMR chemical shifts, δCO, in CDCl3,[14] 17
O
NMR chemical shifts, δ(17O) for carbonyl oxygen and single-
bonded oxygen in CH3CN[15,16] and on the rates of the alkaline
hydrolysis in various media dependent of solvent electrophilicity
parameter were studied.[17–21] It was shown that the 13C NMR
chemical shifts, δCO, the 17O NMR chemical shifts, δ(17O), and
the log k values of the alkaline hydrolysis could be successfully
correlated with the carbonyl stretching frequencies νCO values
for the substituted phenyl and alkyl benzoates.[14–16,19–21] The
Previously,[10] we studied the influence of substituent effects
on the carbonyl stretching frequencies, νCO, in substituted phe-
nyl benzoates, C6H5CO2C6H4-X, X-C6H4CO2C6H5, and alkyl benzo-
ates, C6H5CO2R, in tetrachloromethane containing neutral
(uncharged) substituents in benzoyl and phenyl part in ben-
zene ring and alkyl chain. In the present paper, we extended
the study of the carbonyl stretching frequencies, νCO, depen-
dent on the substituent effects to cationic and anionic charged
substituents. To study the influence of charged (cationic and
anionic) and neutral substituent effects on the carbonyl
stretching frequencies, νCO, in substituted phenyl benzoates,
* Correspondence to: Ilmar A. Koppel, Institute of Chemistry, University of Tartu,
Ravila 14a, 50411 Tartu, Estonia.
E-mail: ilmar.koppel@ut.ee
a V. Nummert, M. Piirsalu, S. Vahur, I. Leito, I. A. Koppel
Institute of Chemistry, University of Tartu, Ravila 14a, 50411, Tartu, Estonia
b L. Toom
Institute of Technology, University of Tartu, Nooruse 1, 50411, Tartu, Estonia
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

V. NUMMERT ET AL.
obtained correlation equations enable to predict the NMR chem-
ical shifts, δCO and δ(17O), the IR νCO and log k values of the alka-
line hydrolysis for esters of benzoic acid.
ν
CO, in DMSO with those in CCl4, additionally, the carbonyl stretching
frequencies, νCO, for seven ortho-substituted phenyl benzoates and
four alkyl benzoates were measured in CCl4. The present data set
of carbonyl stretching frequencies, νCO, is the most comprehensive
data set of this type available in DMSO.
For substituted phenyl benzoates, C6H5CO2C6H4-X, and alkyl benzo-
ates, C6H5CO2R, in DMSO, no study of the ortho, meta, para and alkyl
substituent effects on the carbonyl stretching frequencies νCO values
in the case of neutral and charged (anionic and cationic) substituents
can be found in the literature. There are also no studies addressing the
carbonyl stretching frequencies, νCO, in DMSO for the substituted phe-
nyl esters of benzoic acid, except unsubstituted phenyl benzoate (in
DMSO νCO = 1733,[22]) available in the literature. Similarly, we could
EXPERIMENTAL
Infrared measurements
The IR spectra for 42 meta-, para- and ortho-substituted phenyl
benzoates, C6H5CO2C6H4-X, containing neutral and charged sub-
stituents in phenoxy part (Table 1) and 15 alkylbenzoates,
find only limited data on the IR carbonyl stretching frequencies, νCO
,
for alkyl benzoates, C6H5CO2R, with electronegative substituents mea-
sured in DMSO (R = CH2CN, CH2CF3, CH2CCl3, CH2CHCl2[16]). There are
more data in the literature on the carbonyl stretching frequencies, νCO
,
Table 1. Infrared carbonyl stretching frequencies, νCO, for or-
tho-, meta- and para-substituted phenyl benzoates,
C6H5CO2C6H4-X, in DMSO and in CCl4 (in brackets)
in DMSO for esters containing substituents in the acyl component.[22–
32] It was found[25–32] that the IR frequencies, νCO, in DMSO in the case
of anionic substituents in acyl part of esters could be correlated with
the Hammett equation using the σ+ substituent constants similarly
to neutral meta and para substituents.
Substituent X
νCO (cmꢀ1
)
Substituent X
νCO (cmꢀ1
)
4-NO2
3-NO2
4-SO2CH3
1741.1
1740.5
1740.5
2-NO2 1745.3
The substituent effects of charged substituents in aprotic sol-
vents are considerably less studied as compared with that for
neutral substituents. While the substituent effects of charged
substituents were found to be dependent on solvent effects
and processes considered, it was interesting to establish the sub-
stituent constants σ° for charged substituents on the basis of the
2,6-(NO2)2 1755.1 (1767)
2-SO2CH3 1748.5 (1759,
1745)
4-CF3
3-CF3
3-Br
3-Cl
1739.0
1738.0
1739.3
1737.5
1738.5
1737.6
1734.5
1736.5
1732.8
1733.2
1734.7
1733.7
1733.8
1731.8
1732.3
1732.3
1731.1
1729.4
1738.5
1741.4
1734.4
1734.5a
1734.5
1729.2b
1729.2b
2-CN 1746.2 (1755)
2-CF3 1745.3
1746.2
2-F 1744.7
2-Cl 1741.4
2-Br 1742.3 (1750)
2-I 1740.3
νCO values for phenyl and alkyl benzoates in DMSO and to com-
pare them with those obtained for other processes in DMSO and
aqueous solvents.
4-Cl
For aqueous solution, it was shown[33–37] that in the case of
charged substituents in processes involving changes in the ionic
charge at some structural units in course of reaction, activation
or excitation, the electrostatic correction should be taken into ac-
count. To describe the effects not only for the neutral (dipole) sub-
stituents but also those of charged substituents, Hoefnagel and
Wepster[33] proposed an extended Hammett equation (Eqn 1):
3-COOH
4-COOH
4-SO3 ꢀNa+
3-OH
2-COOH 1737.6
2-COCH3 1739.4, 1735.6
(1759, 1745.0)
2-OH 1734.0
2-OCH3 1734.4
1735.6
H
Δ ¼ ρσL þ δB
(1)
3-NH2
3-N(CH3)2
4-OCH3
2-CH3 1732.8
2,6-(CH3)2 1731.6 (1742)
2-C(CH3)3 1735.6, 1730.8
2-N(CH3)2 1733.3
2-N+(CH3)3Iꢀ 1748.3
where Δ = log KX ꢀ log KH, or log kX ꢀ log kH, ρ = ρm is the reaction
constant obtained with a set of standard meta σ values, σL is a “nor-
mal” σ value for neutral substituents, for charged substituents, σL is
σ value without electrostatic term, that is, the σ value for dipoles
equal to that for ion pairs, and δB is the electrostatic Coulombic
term.[38] In alkaline hydrolysis of substituted phenyl benzoates
and phenyl tosylates with charged substituents in concentrated
NaCl aqueous solutions (4.4M), the log k values attained a limiting
k∞ value, characteristic for ion pairs[34,39,40]: log k∞ ꢀ log k0 ≅ δB,
where k0 is the rate constant at infinite dilution.
4-OH
4-NH2
3-N+(CH3)3Iꢀ
4-N+(CH3)3Iꢀ
3-COOꢀN+Bu4
4-COOꢀN+Bu4
3-OꢀN+Bu4
4-OꢀN+Bu4
The aim of this work was to establish on the basis of carbonyl
stretching frequencies, νCO, of substituted phenyl benzoates,
C6H5CO2C6H4-X, and alkyl benzoates, C6H5CO2R, in DMSO the σ
substituent constants for charged substituents that could be ap-
plicable to correlate the influence of substituent effects on the
carbonyl stretching frequencies, νCO, of substituted phenyl and
alkyl benzoates in DMSO simultaneously with those of
electroneutral substituents. For comparison, the σ substituent
constants for charged substituents were estimated from the cor-
responding carbonyl carbon 13C NMR chemical shifts, δCO, in
DMSO, measured in this work.
aPhenyl benzoate prepared from the corresponding COOH
derivative, so by addition of equimolar (1:1) and twice lower
(1:0.5) amounts of dry sodium methoxide in methanol and
tetrabutylammonium
hydroxide
(tert-(C4H9)4OH
in
methanol), their spectra were recorded immediately (refer
to Supporting Information, Table S1).
bPhenyl benzoate prepared from the corresponding OH
derivative, so by addition of equimolar (1:1) and twice lower
(1:0.5) amounts of dry sodium methoxide in methanol and
tetrabutylammonium
hydroxide
(tert-(C4H9)4OH
in
methanol), their spectra were recorded immediately (refer
to Supporting Information, Table S1).
For more accurate comparison of substituent effects of
electroneutral substituents on the carbonyl stretching frequencies,
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EFFECTS OF CHARGED SUBSTITUENTS ON THE IR VCO VALUES IN BENZOATE
C6H5CO2R, with neutral and charged substituents in alkoxy part
(Table 2) in DMSO (Aldrich for IR spectroscopy) as well as 7
ortho-substitued phenyl benzoates and 4 alkyl benzoates in
CCl4 were recorded on Nicolet 6700 FT-IR spectrometer in the
region 225–4000 cmꢀ1 (resolution 2 cmꢀ1) in approximately
0.013 M solution. A KRS-5 cell with path length 1.02 mm was
used. The wavelength scale of the spectrometer was calibrated
using polystyrene film (0.013 mm thick). The day-to-day repro-
ducibility of the maxima was 0.3 cmꢀ1. Thermo Electron’s
OMNIC9 for FT-IR spectrometer software was used to collect
and process the IR spectra.
via the residual solvent signal (DMSO-d6, 2.50 and 49.52 ppm,
respectively). NMR signals were assigned using information from
gs-HSQC, gs-HMBC, gs-NOESY and TOCSY spectra. The reported
13C NMR data corresponds to the 1H-decoupled 13C NMR data.
Kinetic measurements
The rate constant for the alkaline hydrolysis of 2-N+(CH3)3Iꢀ-phe-
nyl benzoate in aqueous 4.4 M NaCl at 50 °C was determined: log
k = 1.353 (refer to Supporting Information).
DATA ANALYSIS
Synthesis of compounds
The values of the carbonyl stretching frequencies, νCO, for meta-,
para- and ortho-substituted phenyl benzoates, C6H5CO2C6H4-X,
in DMSO (Table 1) in the case of neutral substituents were
treated according to the Taft[46] and the Charton[47] equations
(Eqns 2–4):
The preparation procedure and characteristics of ortho-, meta-
and para-substituted phenyl benzoates, C6H5CO2C6H4X, and
R-substituted alkyl benzoates, C6H5CO2R, are given in Supporting
Information. Purity of the synthesized phenyl benzoates,
C6H5CO2C6H4X (X = 3-COOH, 4-COOH, 2-COOH,3-COOꢀN+(C4H9)
4, 4-COOꢀN+(C4H9)4, 4-SO3 ꢀNa+, 2-N+(CH3)3Iꢀ, 4-OH, 2-OH), was
confirmed by 1H and 13C NMR spectroscopy in deuterated DMSO
at 25 °C (refer to Supporting Information). In the case of
ðvCOÞm;p ¼ ðvCOÞ0 þ ρm;pσ°
(2)
B
ðvCOÞortho ¼ ðvCOÞ0 þ ðρIÞorthoσI þ ðρ°RÞorthoσ°R þ δorthoEs (3)
tetrabutylammonium
4-
and
3-benzoyloxybenzoate
(C6H5CO2C6H4COOꢀN+(C4H9)4) in H NMR spectra, the band for
1
ðvCOÞortho ¼ ðvCOÞ0 þ ðρIÞorthoσI þ ðρRÞorthoσ°R þ δortho (4)
υ
H in COOH group (δ = 13.06 and δ = 13.22) was not detected.
The values of the carbonyl stretching frequencies, νCO, for
substituted alkyl benzoates, C6H5CO2R, in DMSO were analyzed
with Eqn 5:
NMR measurements
The carbonyl carbon 13C NMR chemical shifts, δCO (in ppm), for
21 substituted phenyl esters of benzoic acid and 4 alkyl benzo-
ates, C6H5CO2R, in DMSO were determined (Table 3).
B
ðvCOÞAlk ¼ ðvCOÞCH3 þ ðρIÞAlkσI þ δAlkEs
(5)
For compound characterization, the H and 13C NMR spectra
1
were recorded at 700.1 or 400.1 MHz (1H) and 176.0 or
100.6 MHz (13C). All NMR measurements with the 700 MHz NMR
spectrometer were carried out at 20 °C, and NMR measurements
with the 400 MHz NMR spectrometer were carried out at 25 °C.
Chemical shifts (1H and 13C) were indirectly referenced to TMS
The Taft polar σ°,[46,48,49] inductive σI[49,50] and resonance σ°R
(σ°R = σ°p ꢀ σI)[49,51] scales were used for data analysis. In the case
of ortho substituents, two steric scales were employed: the EsB
constants[14,52–54] (EBs = log kHX+ ꢀ log kHH+, where kXH+ and kHH+ are
the rate constants for the acidic hydrolysis of ortho-substituted
and unsubstitued phenyl benzoates in water at 50 °C, EBs < 0)
and the Charton scale of υ.[55–57] The steric constants, EsB (EBs < 0)
determined on the basis of the acid hydrolysis of ortho-
substitued phenyl benzoates, C H5CO2C6H4-X,[52] in the case of
substituents X = H, F, Cl, Br, I, C6H3, C(CH3)3, CF3, were found to
be nearly linearly related to the υ values, calculated on the bases
of van der Waals radii, rv. The steric substituent constants,
EsB,[14,16,58,59] for the variable alkyl substituent in the alcohol com-
Table 2. Infrared carbonyl stretching frequencies, νCO, for al-
kyl benzoates, C6H5CO2R
Substituent
R
νCO (cmꢀ1
DMSO
1718.8
1713.4
)
CCl4
CH3
CH3CH2
ClCH2
NCCH2
F3CCH2
Cl3CCH2
ponent of esters, C6H5CO2R, (Eqn 5) were calculated as follows:
CH3
H+
EBs = (log kHR+ ꢀ log kCHH+3), where kRH+ and k
are the rate con-
1739.0, 1704.9
1731.8
stants for acid hydrolysis of R-substituted alkyl benzoates,
C6H5CO2R, in water.[59]
1734.8a
1732.3a
1726.5a
1723.4
1743.4
1741.2
1733.2
The results of the statistical data treatment are listed in Table
S3. The values of substituent constants σ°, σI, σ°R and EBs are given
in Table S4. For the data processing, a multiple-parameter linear
least-squares procedure[60] was used.
Cl2CHCH2
HC ≡ CCH2
ClCH2CH2
BrCH2CH2
C6H5CH2
CH3OCH2CH2
Clꢀ (CH3)3N+CH2
1718.9
1718.4a
1716.4
1729.3
DISCUSSION
1716.8
1742.5
1722.1
1717.3
Influence of substituents in meta and para position of
C6H5CO2C6H4-X
As can expected[22] on the basis of the high polarity of DMSO
compared with tetrachloromethane, all the νCO values for
ortho-, meta- and para-substituted phenyl benzoates as well as
I
ꢀ (CH3)3N+CH2CH2
Iꢀ (CH3)3N+CH2CH2CH2
aRef[16]
.
J. Phys. Org. Chem. (2016)
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V. NUMMERT ET AL.
Table 3. The carbonyl carbon 13C NMR chemical shifts, δCO (in ppm), for substituted phenyl benzoates, C6H5CO2C6H4-X, and alkyl
benzoates, C6H5CO2R, in DMSO
X
δCO
X or R
δCO
164.30
164.31
163.70
H
164.6
163.96
4-N+(CH3)3Iꢀ
4-NO2
4-OCH3
4-COOH
3-COOH
2-COOH
4-OH
3-OH
2-OH
4-NH2
4-Cl
3-N+(CH3)3Iꢀ
164.92, 164.91
164.2, 166.6 (COOH)
164.5, 166.5 (COOH)
164.7, 165.6 (COOH)
165.0
2-N+(CH3)3Iꢀ
4-SOꢀ3 Na+
164.63
4-COOꢀN+(C4H9)4
3-COOꢀN+(C4H9)4
CH2N+(CH3)3Clꢀ
CH2CH2CH2N+(CH3)3Iꢀ
CH2CH2N+(CH3)3Iꢀ
CH3
164.6 (167.5)
164.76 (167.0)
163.64
164.55
164.1
165.21
164.43
165.58
165.16, 165.1a
166.22, 166.18b
C6H5COOH
167.3c, 167.18d
167.48e
4-Cl-C6H5COOH
166.74e
aRef[41]
.
.
bRef[42]
cRef[43]
dRef[44]
.
.
.
eRef[45]
for alkylbenzoates in DMSO (Tables 1 and 2) in the case of neu-
tral substituents were ca 7–11 cmꢀ1 lower as compared with
the corresponding vCO values measured earlier in tetrachloro-
methane.[10] In the case of electroneutral ortho, meta, para and
alkyl substituents, the νCO values of phenyl benzoates in DMSO
(Tables 1 and 2) were found to be linearly correlated with the
corresponding νCO values in CCl4 as follows:
Â
Ã
σ°X
¼
ðvCOÞX ꢀ ðvCOÞ0 =ρm;p
(7)
The ρm,p constant in Eqn 7 was estimated from the depen-
dence of the νCO values on the Taft polar substituent constants,
σ°, for meta and para electroneutral substituents (Eqn 2) in
substituted phenyl benzoates, C6H5CO2C6H4-X, in DMSO (Eqn 8,
Table S3):
ðνCO
Þ
¼ ð82:8 37:6Þ þ ð0:947 0:022ÞðνCOÞCCl4
R2 ¼ D0M:S9O84; s ¼ 0:132; n=n0 ¼ 35=35
(6)
ðvCOÞm;p ¼ ð1733:5 0:4Þ þ ð10:3 0:6Þσ°
(8)
R2 ¼ 0:935; s ¼ 0:255; n=n0 ¼ 19=20
The value νCO = 1733.7 cmꢀ1 for phenyl benzoate in DMSO
(Table 1) agrees well with
CO = 1733 cmꢀ1 reported by
ν
Nyquist.[22] Similarly to previous works,[10,61–64] we found that
the carbonyl stretching frequencies, νCO, for benzoates in DMSO
increasing in the case of electron-withdrawing substituents and
decreasing when the electron-donating substituents are in-
cluded. The electron-donating substituents are considered to in-
crease the bond length and ionic character of the carbonyl bond.
The 13C NMR and 17O NMR data for the carbonyl group in
substituted phenyl benzoates[14,16,65] support this point of view.
It was found that the electron-withdrawing substituents shield
the carbonyl carbon and deshield the carbonyl oxygen as com-
pared with unsubstituted derivative.[14,16,65] The electron-
donating substituents act in the opposite direction.
To establish which σ substituent constants for charged substit-
uents (for ion pairs, they also contains an additional electrostatic
component) could be applicable to correlate the influence of
charged substituent effects on the carbonyl stretching frequen-
cies, νCO, of substituted phenyl benzoates in DMSO simulta-
neously with those of electroneutral substituents, the
substituent constants, σ°X , for meta- and para-charged substitu-
ents in DMSO were calculated with Eqn 7 (Table 4):
The νCO value for 4-COOH derivative was excluded first during
the data processing as the most deviating point. In literature,[76]
there was for 4-COOH-phenyl benzoate νCO = 1734 in DMSO
detected that was approximately the same as found in the pres-
ent work (νCO = 1734.5, Table 1). The lower νCO value for
4-benzoyloxybenzoic acid could be explained by formation of
the dimer form through hydrogen bond association of carboxyl
groups.[77]
The polar influence of meta and para substituents on the car-
bonyl stretching frequencies, νCO, in DMSO (ρ = 10.3, Eqn 8, Table
S3) was found to be about two units of ρ stronger than in
tetrachlorometane (ρ = 8.19,[10] Table S3). A good correlation of
the νCO values with the σ° constants for para-substituted phenyl
benzoates, once more, shows the absence of the through reso-
nance between the electron-donating substituents and the car-
bonyl group and that in phenyl esters, the carbonyl group was
found to be nearly perpendicular to the aroxy plane.[10,62,78,79]
The substituent constants, σ°X , for charged substituents esti-
mated with Eqn 7 using the corresponding carbonyl stretching
frequencies, νCO, in DMSO (Table 1) and the value of ρm,p from
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EFFECTS OF CHARGED SUBSTITUENTS ON THE IR VCO VALUES IN BENZOATE
Table 4. The values of substituent constants, (σ°)X , for charged substituents estimated from the infrared stretching frequencies,
νCO
C6H5CO2R, in aqueous 4.4 M NaCl solution at 50 °C
,
13C NMR carbonyl chemical shifts, δCO, in DMSO, and rates of alkaline hydrolysis of phenyl and alkyl benzoates, C6H5CO2C6H4-X,
a
b
X or R
σ°(νCO
)
σ° (δCO
)
σ° (4.4 M NaCl)c Recommended σ° for ion pairse σ° with electrostatic component
(in aqueous solution)f
d
(σ° )av
4-N+(CH3)3Iꢀ
0.65
0.46
0.78
0.08
0.07
0.44
0.44
0.53g
0.46g
0.54
0.45
0.76
0.09
0.03
0.53
0.59
—
0.26
0.24
1.025,g 0.88,i 0.96h
1.094,g 1.04,i 0.99h
1.15k
3-N+(CH3)3Iꢀ
2-N+(CH3)3Iꢀ
0.69i
0.14
0.83j
4-COOꢀN+(C4H9)4
3-COOꢀN+(C4H9)4
4-SO3 ꢀNa+
0.06g
ꢀ0.078,gꢀ0.122g
ꢀ0.194,g ꢀ0.195g
ꢀ0.037l
ꢀ0.02
0.04g
0.28
0.11
0.37m
0.25
0.51
4-OꢀN+(C4H9)4
3-OꢀN+(C4H9)4
ꢀ0.44
ꢀ0.37,–0.58g
ꢀ0.34,–0.57g
ꢀ0.41
ꢀ0.45
ꢀ0.833,g ꢀ0.815g
ꢀ0.886,g ꢀ0.874g
ꢀ0.44
ꢀ0.39
ꢀ0.28
Values
of σIn
CH2N+(CH3)3Clꢀ
(CH2)2N+(CH3)3Iꢀ
(CH2)3N+(CH3)3Iꢀ
0.51o
0.17o
0.09o
0.56
0.31
0.03
0.84p
0.42p
0. 24p
aThe σ° substituent constant values calculated using infrared carbonyl stretching frequencies, νCO, in DMSO for substituted phenyl
benzoates, C6H5CO2C6H4-X, (Table 1) with Eqns 7 and 8. In the calculation of σ° constant for 4-N+(CH3)3Iꢀ substituent, νCO = 1734.7
for phenyl benzoate was used.
bThe σ° values calculated from the carbonyl carbon 13C NMR chemical shifts, δCO, for substituted phenyl benzoates, C6H5CO2C6H4-
X, in DMSO (Table 3) according to Eqns 7 and 9.
cThe σ° values were calculated from the rate constants for the alkaline hydrolysis of substituted phenyl benzoates, C6H5CO2C6H4-X,
[66]
in aqueous 4.4 M NaCl solution at 50 °C with Eqn log k = log k0 + ρm,pσ° using the corresponding rate constants k∝
unsubstituted derivative log kH = 0.070 and ρ = 0.90 in aqueous 4.4 M NaCl.[67,68]
and for
dThe averaged values of the substituent constants, (σ°X )av, for ion pairs of charged substituents calculated on the bases of the
experimental data for substituted phenyl benzoates: the carbonyl stretching frequencies, νCO, and the carbonyl carbon 13C NMR
chemical shifts, δCO, in DMSO and log k values for the alkaline hydrolysis in aqueous 4.4 M NaCl solution.
eThe σ° values for ion pairs calculated by Wepster as free from the electrostatic interaction term.[33]
fThe σ° constants containing the electrostatic component for reactions between substrate containing charged substituent and the
ionic reagent. To calculate the σ° values, the rate constants or pK values were extrapolated to pure water were used.
gRef[66]
hRef[49]
.
.
iρortho = 1.47 was used.[14]
jlog k = 1.353 for the alkaline hydrolysis of 2-N+(CH3)3Iꢀ-phenyl benzoate in aqueous 4.4 M NaCl solution at 50 °C was used (refer to
Supporting Information). log kH = 0.070 and ρortho = 1.50[69] were used; influence of the steric term was considered minimal.
kThe σ° values calculated from the rate constants for the alkaline hydrolysis of substituted phenyl tosylates, CH3C6H4SO2OC6H4-X,
for pure water. log k0 = ꢀ2.940[70] and ρortho = 2.99[71] were used.
lThe σ° values calculated from the rate constants for the alkaline hydrolysis of substituted phenyl tosylates, CH3C6H4SO2OC6H4-X,
for pure water.[66]
mRef[72]
.
nThe σI values for X in alkyl chain R = CH2X. At data treatment with Eqn 5, σI = 0.56 for R = CH2N+(CH3)3Clꢀ,[33] σI = 0.20 for
R = CH2CH2N+(CH3)3Clꢀ (calculated as σI(CH2X) = σI(X)/2.8[49]) and σI = 0.05 for R = CH2CH2CH2N+(CH3)3Clꢀ[33] were used.
oEstimated with Eqn 12 (Table S3). The steric EsB constants EBs = ꢀ0.16, ꢀ0.15, ꢀ0.15 for R = CH2N+(CH3)3Clꢀ, CH2CH2N+(CH3)3Iꢀ,
R = CH2CH2CH2N+(CH3)3Iꢀ were used.
The steric constants, EBs , for charged alkyl substituents were calculated with the following equation: log k = log kCH3 + 3.39σI + 1.2EBs ,
log k = 0.684 for R = CH2N(CH3)3+Clꢀ, log k = ꢀ0.535 for R = CH2CH2N(CH3)3+Iꢀ, log k = ꢀ1.034 for R = CH2CH2CH2N(CH3)+3Iꢀ, from
Ref[73] and log k = ꢀ1.034 for R = CH3 in aqueous 5.3 M NaClO4 solution at 50 °C were used.[74,75]
pCalculated with the following equation: log k = ꢀ0.640 + 3.38σI + 1.05 EBs[73–75] for the alkaline hydrolysis of alkylbenzoates,
C6H5CO2R, in water at 50 °C using for ester with charged substituents the values of rate constants extrapolated to pure water.
ðδCOÞm;p ¼ ð164:72 0:04Þ ꢀ ð1:020 0:11Þσ°
Eqn 8 for neutral meta- and para-substituted phenyl benzoates,
C6H5CO2C6H4-X, in DMSO are listed in Table 4.
(9)
R2 ¼ 0:945; s ¼ 0:10; n=n0 ¼ 6=6
For comparison, the magnitudes of substituent constants, σ°X
,
for meta- and para-charged substituents were calculated on the
basis of the carbonyl carbon 13C NMR chemical shifts, δCO, for
substituted phenyl benzoates, C6H5CO2C6H4-X, in DMSO (Tables
3 and 4) using Eqn 9:
The magnitudes of the carbonyl carbon 13C NMR chemical
shifts, δCO, of substituted phenyl benzoates, C6H5CO2C6H4-X,
measured in the present work in DMSO (Table 3), appeared to
be approximately the same as compared with the corresponding
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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V. NUMMERT ET AL.
carbonyl carbon 13C NMR chemical shifts, recorded earlier in
deuterated cloroform (CDCl3).[14] The σ° constants for charged
substituents, σ°X , estimated with Eqn 9 using the carbonyl car-
bon chemical shifts, δCO, in DMSO (Table 3) are shown in
Table 4.
The substituent constants, σ°X , for charged substituents esti-
mated using the corresponding carbonyl stretching frequencies,
νCO (Table 1), and the carbonyl carbon chemical shifts, δCO
the carbonyl carbon chemical shifts, δCO, in DMSO with the
Hoefnagel and Wepster σ constants for ion pairs and those
obtained from the kinetic data of the alkaline hydrolysis of phe-
nyl benzoates in 4.4 M NaCl solution (Table 4) shows that in
substituted phenyl benzoates, C6H5CO2C6H4-X, the charged sub-
stituents X = 3-N+(CH3)3Iꢀ, 4-N+(CH3)3Iꢀ, 3-COOꢀN+Bu4, 4-
COOꢀN+Bu4, 3-OꢀN+Bu4, 4-OꢀN+Bu4 and 4-SOꢀ3 Na+ in DMSO
are not completely ionized and are in ion pair form. The corre-
sponding estimated σ° values for charged substituents do not
include the component of the electrostatic interaction between
charged substituent and reaction center as shown in Eqn 1. Sim-
ilarly, Kosugi and Takeuchi[43] concluded that in DMSO, benzoate
anions form tight ion pairs with sodium or tetraalkylammonium
cations.
(Table 3), in DMSO are in good agreement with constants, σ°X
,
for ion pairs proposed by Hoefnagel and Wepster[33] and those
calculated from the kinetic data of the alkaline hydrolysis of
substituted phenyl benzoates in concentrated aqueous NaCl so-
lution (4.4 M NaCl,[66] Table 4).
The averaged values of the substituent constants, (σ°X )av, for
charged substituents calculated on the basis of the experimental
data for substituted phenyl benzoates (the carbonyl stretching
frequencies, νCO, and the carbonyl carbon 13C NMR chemical
shifts, δCO, in DMSO and log k values for the alkaline hydrolysis
in aqueous 4.4 M NaCl solution (Table 4)) were proposed as “rec-
ommended” σ° substituent constants for ion pairs of charged
substituents (Table 4).
Correlation of data using the substituent constants (σ° )av for
charged substituents enables to evaluate the state of charged
substituents (compounds) in solution whether they are dissoci-
ated into ions, ion pairs or are they in non-dissociated form.
Influence of ortho substituents in C6H5CO2C6H4-X
In the IR spectra of phenyl benzoates with ortho substituents in
the aroxy part, C6H5CO2C6H4-X, in the present work in DMSO,
only one carbonyl stretching band was detected and splitting
of the carbonyl stretching bands was not observed similarly to
previous findings in CCl4.[1] In phenyl benzoates, the aromatic
plane in the phenoxy part is rotated out of the carbonyl plane
by nearly 90°,[10,62,78,79] and for ortho derivatives, there is only
one conformation possible.
When the carbonyl stretching frequencies, νCO, for ortho-
substituted phenyl esters of benzoic acid in DMSO in the case
of electroneutral substituents were correlated with the Charton
Eqns 3 and 4, the resonance and steric terms were excluded as
insignificant and a good correlation obtained with the inductive
constant, σI, only (Eqn 10), Table S3):
The magnitudes of the averaged (σ°X )av values for ion pairs of
charged substituents are quite close to the Hoefnagel and
Wepster σ° values[33] in the case of 3-N+(CH3)3Iꢀ, 4-N+(CH3)3Iꢀ,
3-OꢀN+Bu4 and 4-OꢀN+Bu4 substituents (Table 4). The σ° values
of 0.62 and 0.47 for 3-N+(CH3)3Iꢀ and 4-N+(CH3)3Iꢀ substituents
estimated from the pKa values of benzoic acids in DMSO
(9.36[80] and 9.79,[80] ρ = 2.6[81]), σ° = 0.62 for 4-N+(CH3)3Iꢀ calcu-
lated from pKa value of 4-N+(CH3)3Iꢀ-substitued phenol in DMSO
(14.7,[82] ρ = 5.3[81]) and of σ° = 0.01 for the 4-COOꢀN+Bu4 group,
estimated from the pK2 value of 4-phthalic acid in aqueous
7.75 m Bu4NBr solution (pK2 = 6.28[83] ρ = 2.09[84]), coincide as
well with the corresponding (σ° )av values found from our exper-
imental data (for 3-COOꢀN+Bu4 group (σ° )av = 0.03, Table 4).
In the case of substituents 3-COOꢀN+Bu4, 4-COOꢀN+Bu4 and
4-SOꢀ3 Na+, the constants (σ° )av were found to be less about
0.2 units of σ as compared with those estimated by Hoefnagel
and Wepster with Eqn 1 using pKa or log k values for aqueous
solutions. Probably, this difference is caused by the strong hydra-
tion of the ionic carboxylate and sulfonate groups by water mol-
ecules[85] that makes the negative charge on the carboxylate and
sulfonate anion less negative as assumed in calculation of the
electrostatic term with Eqn 1.
ðvCO
Þ
¼ ð1732:9 0:8Þ þ ð19:7 1:6ÞσI
R2 ¼ o0rt:h9o06; s ¼ 0:306; n=n0 ¼ 16=16
(10)
Almost the same correlation of the νCO values with σI con-
stants was obtained when 2-OH- and 2-COOH-substituted deriv-
atives were included in the data set as well (Table S3). The
influence of the ortho inductive term on the νCO values in DMSO
(Eqn 10) was found to be the same as in CCl4 (Eqn 11) in the case
the same substituents were included (in the present work, the
additional νCO values for five ortho-substituted phenyl benzoates
in CCl4 were determined (Table 1 and Table S3):
The νCO values for meta- and para-substituted phenyl benzo-
ates, C6H5CO2C6H4-X, in DMSO (Table 1) showed a good correla-
tion with the corresponding carbonyl carbon 13C NMR chemical
shifts, δCO, in DMSO (Table 3) in case both benzoates with the
charged substituents and electroneutral substituents were
included (Table S5, R2 = 0.925, s0 = 0, 271, n/n0 = 11/12).
ðvCO
Þ
¼ ð1741:8 0:4Þ þ ð19:9 1:2ÞσI
R2 ¼ o0rt:h9o51; s ¼ 0:224; n=n0 ¼ 16=16
(11)
Similarly, the νCO values for meta- and para-substituted phenyl
benzoates in DMSO (Table 1) correlated well with the corre-
sponding log k values for the alkaline hydrolysis of substituted
phenyl benzoates, C6H5CO2C6H4-X, in aqueous 4.4 M NaCl solu-
tion at 50 °C including both derivatives with the charged substit-
uents and electroneutral substituents (Table S5, R2 = 0.921,
s0 = 0.282, n/n0 = 10/10). When the log k values for pure water
were correlated with the νCO values, the data for charged substit-
uents deviated essentially because of additional electrostatic
interaction in the alkaline hydrolysis of phenyl benzoates (Table
S5, R2 = 0.817, s0 = 0.426).
In the case of ortho substituents, the resonance and steric fac-
tors appeared to be insignificant both in DMSO and CCl4 (Eqns
10 and 11), Table S3).
Approximately the same σI values for ortho N+(CH3)3Iꢀ substitu-
ent (Table 4) were found: with Eqn 10 using the νCO value in DMSO
in Table 1 (σI = 0.78), from the carbonyl carbon 13C NMR chemical
shifts, δCO, for 2-N+(CH3)3Iꢀ-substituted phenyl benzoate in DMSO
in Table 3 (σI = 0.69), from the rate constants for the alkaline hydro-
lysis of 2-N+(CH3)3I-substituted phenyl benzoate (σI = 0.83) and 2-
N+(CH3)3Iꢀ-phenyl tosylate (CH3C6H4SO2OC6H4-X) in aqueous
4.4 M NaCl solution at 50 °C (σI = 0.89, log k =ꢀ0.079,[86]
ρI = 3.02,[68] log kH = 2.78[87] were used).
The good agreement of the (σ° )av values for charged substit-
uents estimated using carbonyl stretching frequencies, νCO, and
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Copyright © 2016 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2016)

EFFECTS OF CHARGED SUBSTITUENTS ON THE IR VCO VALUES IN BENZOATE
For charged ortho substituent X = 2-N+(CH3)3I,ꢀ the estimated
σI values (Table 4) were essentially lower than the σI value for
X = 2-N+(CH3)3Iꢀ found for pure water (at zero ionic strenth
σI = 1.15, Table 4) and approximately twice larger as compared
with the corresponding (σ° )av constants for meta and para sub-
stituents (Table 4).
NMR chemical shifts, δCO, for 21 substituted phenyl benzoates
and alkyl benzoates with neutral and charged substituents in
DMSO have been recorded. Using carbonyl stretching frequen-
cies, νCO, and the carbonyl carbon 13C NMR chemical shifts, δCO
,
in DMSO for charged substituents, the polar substituent con-
stants σ° were calculated. These substituent constants σ° for
charged substituents in DMSO showed a good agreement with
ones obtained from the kinetic data of the alkaline hydrolysis
of phenyl benzoates in 4.4 M aqueous NaCl solution and the
Wepster and Hoefnagel σ values for ion pairs of the charged sub-
stituents estimated for aqueous solutions (Table 4). The esti-
mated polar substituent constants σ° for the charged
substituents characterize the reactivity of ion pairs in DMSO
and constitute a single scale with σ° and σI constants for the un-
charged electronegative substituents.
The carbonyl stretching frequencies, νCO, for meta- and para-
substituted phenyl esters of benzoic acid in DMSO in the case
of neutral substituents were found to correlate well with the sub-
stituent constants, σ°, the νCO values for ortho derivatives with
the inductive substituent constants, σI, only. The νCO values for
substituted alkyl benzoates in DMSO in the case of neutral sub-
stituents showed excellent correlation with the inductive con-
stants σI (for substituent X in R = CH2X) and the steric EsB
constants.
This shows that in the case of ortho N+(CH3)3Iꢀ substituent in
processes considered, the plus charge on nitrogen atom is less
efficiently neutralized by the Iꢀ ion as compared with that in
the case of corresponding meta and para substituents. In the
case of ortho N+(CH3)3Iꢀ substituent because of the steric
consequencies, the distance of the iodide anion from the reac-
tion center could be essentially greater as compared with the
same distance of the ortho N+(CH3)3 substituent, and the plus
charge on the nitrogen atom is not completely neutralized (sol-
vent separated ion pairs).[34] Similarly, the ortho polar substituent
effects were found to be about twice less dependent on the sol-
vent electrophilicity parameters than that in the case of meta
and para substituents.[18,88]
Influence of substituents in alkyl groups of C6H5CO2R
Similar to phenyl benzoates, all the νCO values of substituted alkyl
benzoates in DMSO (Tables 2) in the case of neutral substituents
were ca 8–9cmꢀ1 lower as compared with the corresponding vCO
values measured earlier in CCl4.[10] The carbonyl stretching frequen-
cies, νCO, of substituted alkyl benzoates, C6H5CO2R, in DMSO
(Table 2) in the case of neutral substituents showed excellent corre-
lation with the inductive constants σI (for substituent X in R= CH2X)
and the steric EBs constants (R2 = 0.986, s= 0.120, n/n0 = 12/12,
Table S3). The influence of the inductive and steric term of alkyl
substituents on the carbonyl stretching frequencies, νCO, in DMSO
(ρI = 59.8 and δ = 36.4, Table S3) was found to be approximately
the same as that in CCl4 (ρI = 60.8 and δ = 35.8 Table S3).
Acknowledgements
This work was supported by the Centre of Excellence HIGHTECHMAT
(SLOKT117T), by the targeted financing SFO180089008 and the in-
stitutional funding IUT20-14 (TLOKT14014I) from the Ministry of Ed-
ucation and Research of Estonia.
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