17O NMR study of ortho and alkyl substituent effects in substituted phenyl and alkyl esters of benzoic acids

Article abstract of DOI:10.1135/cccc2011100

¹⁷O NMR spectra for 44 ortho-, meta- and para-substituted phenyl and alkyl benzoates (C₆H₅CO₂C ₆H₄-X, C₆H₅CO₂R) at natural abundance in acetonitrile were re

Full text of DOI:10.1135/cccc2011100

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1737
17O NMR STUDY OF ortho AND ALKYL SUBSTITUENT EFFECTS
IN SUBSTITUTED PHENYL AND ALKYL ESTERS OF BENZOIC ACIDS
1
2
3
4
Vilve NUMMERT , Vahur MÄEMETS , Mare PIIRSALU , Signe VAHUR and
5,
Ilmar A. KOPPEL *
Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia;
1
2
3
e-mail: vilve.nummert@ut.ee, vahur.maemets@ut.ee, mare.piirsalu@ut.ee,
4
5
signe.vahur@ut.ee, ilmar.koppel@ut.ee
Received May 9, 2011
Accepted August 10, 2011
Published online January 13, 2012
17O NMR spectra for 44 ortho-, meta- and para-substituted phenyl and alkyl benzoates
(C6H5CO2C6H4-X, C6H5CO2R) at natural abundance in acetonitrile were recorded. Sub-
stituent effects on the 17O NMR chemical shifts, δ(17O), of the carbonyl oxygen and the
single-bonded phenoxy (OPh) and alkoxy (OR) oxygens have been studied. The δ(17O) val-
ues of the carbonyl oxygen for para derivatives showed a good correlation with the σ° con-
stants. The δ(17O) values of carbonyl oxygen for ortho derivatives were found to be described
B
well with the Charton equation containing the inductive, σI, resonance, σ°R, and steric, Es
,
substituent constants in case the data treatment was performed separately for electron-
donating +R and electron-attracting –R substituents. The electron-donating +R ortho and para
substituents in substituted benzoates caused shielding and the electron-withdrawing –R sub-
stituents produced deshielding of the O signal. The steric interaction of ortho substituents
with the ester group decreased the electron density at the carbonyl oxygen. In alkyl
benzoates the δ(17O) values were found to be described well with the inductive, σI, and the
steric, EsB, substituent constants.
Keywords: NMR spectroscopy; Substituent effects; Structure–activity relationships; 17O NMR
spectra; ortho Effect; Phenyl benzoates; Alkyl benzoates.
We have previously investigated the contribution of the inductive, reso-
nance and steric effects of ortho, meta, para and alkyl substituents into the
rates of the alkaline hydrolysis in various media (as shown in refs1–11), the
carbonyl carbon 13C NMR chemical shifts12, δCO, and the infrared stretch-
ing frequencies of the carbonyl group13, νCO, in substituted phenyl
benzoates, X-C6H4CO2C6H5, C6H5CO2C6H4-X, and alkyl benzoates,
C6H5CO2R. In the recent paper14 we studied the ortho substituent effect
on the 17O NMR chemical shifts, δ(17O), of the carbonyl oxygen and the
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763
© 2011 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2011100

1738
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
single-bonded phenoxy oxygen in substituted phenyl benzoates, contain-
ing substituents in benzoyl moiety, X-C6H4CO2C6H5.
For comparison, in the present paper we report 17O NMR spectra for 44
ortho-, meta- and para-substituted phenyl esters, C6H5CO2C6H4-X, and alkyl
esters, C6H5CO2R, of benzoic acid containing substituents in phenyl and
alkyl components (Chart 1).
CHART 1
On the bases of determined 17O NMR chemical shifts, δ(17O), the influ-
ence of the substituent inductive, resonance and steric effects on the car-
bonyl and single-bonded oxygen 17O NMR chemical shifts, δ(17O), from the
phenyl and alkyl components was studied.
The studies regarding the investigation of the ortho, meta and para as
well as alkyl substituents effects on the 17O NMR chemical shifts, δ(17O), us-
ing correlation equations for phenyl benzoates and alkyl benzoates involv-
ing substituents in phenyl and alkyl moiety appear to be missing in the
literature. To the best of our knowledge there are practically no data on
the 17O NMR chemical shifts, δ(17O), for substituted phenyl benzoates,
C6H5CO2C6H4-X, except unsubstituted phenyl benzoate14–18, X = H, avail-
able in the literature. One could find in the literature the values of the
17O NMR chemical shifts, δ(17O), only for phenyl acetates15,16,19,20
,
CH3CO2C6H4-X, with X = H, 4-NO2, 4-OCH3, 2-CH3, 2,6-(CH3)2, phenyl
formate21 (HCO2C6H4-X), with X = H, meta-substituted phenyl naph-
thoates22, and para-substituted phenylthiol acetates23, containing variable
substituents in phenoxy part (OPh). In substituted phenyl benzoates the
influence of the substituent effects from the aryl moiety on the carbonyl
carbon 13C NMR chemical shifts more thoroughly was investigated in
refs24–27. There are only few chemical shifts, δ(17O), for substituted alkyl
benzoates, (C6H5CO2R), involving electronegative substituents available. In
the case of alkyl benzoates15,16,28–33, (C6H5CO2R), the chemical shifts,
δ(17O), have been reported for only R = CH3, CH2CH3, C(CH3)3, CH2Ph and
CH2CF3. The influence of substituents on the chemical shifts, δ(17O), of the
oxygen attached to the phenyl group have been studied in the case of para
substituents for substituted phenols34–36, anisoles36–39, vinyl aryl ethers39,
phenoxyethyl derivatives40 and trialkoxysilanes41. The influence of the
electronegative substituents on the chemical shifts, δ(17O), of the single-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1739
bonded oxygen attached to the alkyl group was investigated for alco-
hols42–46, alkoxygermanes46 and alkoxysilanes43. The δ(17O) values in
alkoxysilanes were found to be dependent on the inductive and steric
substituent constants and showed a good linear relationship with the
δ(17O) values for the corresponding alcohols43. It was found that the
substituent-induced chemical shifts of the single-bonded oxygen in ethers
ROR′ correlate well with the 13C chemical shifts for the methylene groups
of the corresponding alkanes39,47,48 RCH2R′ and the substituent-dependent
chemical shifts of the double-bonded oxygen in carbonyl group depend on
the same factors as the 13C chemical shifts of the β-carbon of alkenes28,48,49
.
The behavior of the ether oxygen atoms (2p level) was found to be similar
to that of sp3 carbons, whereas carbonyl oxygen atoms exhibit the same
trends as sp2 carbon atoms28,48,49
.
In the previous paper14 in substituted phenyl benzoates, containing sub-
stituents in benzoyl moiety, (X-C6H4CO2C6H5), we studied the influence of
the ortho, meta and para substitutent effects on the 17O NMR chemical
shifts, δ(17O), of the carbonyl oxygen and the single-bonded phenoxy oxy-
gen. The δ(17O) values for para and meta derivatives with substiuents in
benzoyl moiety gave good correlation with the σ+ and σm constants, respec-
tively. The δ(17O) values for the ortho-substituted phenyl benzoates,
X-C6H4CO2C6H5, as well as ortho-substituted nitrobenzenes, acetophenones
and benzoyl chlorides showed excellent correlation with the Charton equa-
B
tion50 using the inductive, σI, resonance, σ+ , and steric, Es , substituent
R
constants in case the data treatment was performed separately for deriva-
tives containing the electron-donating +R substituents and electron-
attracting –R substituents14.
Following our studies on the ortho effect in the current work we check
the applicability of the Charton equation50 to describe the influence of
ortho substituents on the chemical shifts, δ(17O), in substituted phenyl
esters of benzoic acid, C6H5CO2C6H4-X. It was of interest to compare the
contributions of the inductive, resonance and steric effects to the δ(17O)
values in phenyl benzoates containing substituents in phenoxy part,
C6H5CO2C6H4-X, with those containing substituents in benzoyl moiety
(X-C6H4CO2C6H5). Similarly, it was of interest to compare the substituent
effects in the carbonyl oxygen δ(17O) values for substituted phenyl
benzoates, C6H5CO2C6H4-X, and alkyl benzoates, C6H5CO2R, with the cor-
responding substituent-induced carbonyl 13C NMR chemical shifts12, ∆δCO
,
the infrared stretching frequencies13, ∆νCO, and the rates of the alkaline
hydrolysis5,51,52, ∆log k.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1740
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
B
To estimate the steric substituent constants Es for alkyl substituents R =
CH2CF3, CH2CCl3, CH2CHCl2, CH2CH2Br, the infrared stretching frequen-
cies of the carbonyl group, νCO, for corresponding substituted alkyl
benzoates, C6H5CO2R, were determined.
EXPERIMENTAL
NMR Measurements
17O NMR spectra for 32 ortho-, meta- and para-substituted phenyl esters of benzoic acid,
C6H5CO2C6H4-X (X = 4-NO2, 4-CN, 4-SO2CH3, 4-CF3, 4-F, 4-Cl, 4-CH3, 4-OCH3, 4-NH2,
3-NO2, 3-CN, 3-CF3, 3-F, 3-Cl, 3-Br, 3-CH3, 3-NH2, 3-N(CH3)2, 2-NO2, 2,6-(NO2)2, 2-CN,
2-CF3, 2-SO2CH3, 2-F, 2-Cl, 2-Br, 2-OCH3, 2-CH3, 2-C(CH3)3, 2,6-(CH3)2, 2-CO2CH3,
2-COCH3), and 12 R-substituted alkyl esters of benzoic acid, C6H5CO2R (R = CH3, CH2CH3,
CH2Cl, CH2CN, CH2CF3, CH2C6H5, CH2C≡CH, CH2CHCl2, CH2CCl3, CH2CH2OCH3,
CH2CH2Cl, CH2CH2Br), were recorded at natural abundance in acetonitrile (Tables I and II).
NMR spectra were recorded on a Bruker Avance II 200 spectrometer equipped with 10 mm
BBO for 17O or 5 mm QNP-z probe for 1H and 13C NMR spectra. For 17O NMR the benzoates
were dissolved in the mixture of 1 ml CD3CN and 2 ml CH3CN. Acetonitrile-d3 ′Special
HOH′ 99.8 atom % D (Aldrich) and Acetonitrile SPECTRANAL (Riedel-de Haën) were used,
respectively. Resonance frequency for 17O measurements was 27.13 MHz. Depending on the
type of the benzoate, the concentration of the benzoate varied between 0.3 and 1 molal.
Temperature of the measurements was 50 °C, and water at 50 °C was used as an external ref-
erence (17O chemical shift was taken 0 ppm). Other measurement parameters were the same
as described in previous work14. The standard 1H and proton-decoupled 13C NMR spectra
were recorded at a room temperature in CDCl3 solution (with 1% TMS added for the spec-
tral referencing) at 200.13 and 50.33 MHz, correspondingly. 2D HSQC and HMBC spectra
were also recorded for assignment of the 1H and 13C chemical shifts of some benzoates.
IR Measurements
The IR spectra of 4 alkyl-substituted phenyl esters, C6H5CO2R (R = CH2CH2Br, CH2CF3,
CH2CHCl2, CH2CCl3) were registered in the region 225–4000 cm–1 on Nicolet 6700 FT-IR
spectrometer in approximately 0.013 M solution of tetrachloromethane (Aldrich for IR spec-
troscopy). 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). Thermo Electron’s
OMNIC software for FT-IR spectrometer was used to collect and process IR spectrum. The
day-to-day reproducibility of the adsorption maxima was 0.3 cm–1. The values of (νCO max
)
for esters C6H5CO2R (R = CH2CH2Br, CH2CF3, CH2CHCl2, CH2CCl3) are shown in Table II.
Synthesis of Compounds
The preparation procedure and characteristics for the most ortho-, meta- and para-substituted
phenyl esters of benzoic acid, C6H5CO2C6H4-X, and alkyl esters of benzoic acid, C6H5CO2R,
have been previously described9,52–55. The phenyl benzoates, C6H5CO2C6H4-X, (X =
2-SO2CH3, 2-COCH3, 2-CF3, 3-Cl, 4-SO2CH3, 4-CF3) were prepared by the addition of
benzoyl chloride to the corresponding substituted phenol in pyridine with stirring at 0 °C
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1741
(the Einhorn method)56–58. The phenyl benzoates, C6H5CO2C6H4-X, (X = 3-Br, 3-N(CH3)2,
3-CF3, 2-Br) and 2,6-dimethylphenyl benzoate, were prepared by addition the benzoyl
chloride to the mixture of the corresponding substituted phenol in aqueous 10% sodium
hydroxide (the Schotten–Baumann method)59,60. 2,6-Dinitrophenyl benzoate was synthe-
sized by the addition of the thionyl chloride to the mixture of benzoic acid and 2,6-di-
nitrophenol in pyridine with stirring at 0 °C 61,62. Purity of synthesized phenyl benzoates,
C6H5CO2C6H4-X (X = 2-SO2CH3, 2-COCH3, 2-CF3, 2-Br, 2,6-(CH3)2, 2,6-(NO2)2, 3-Cl, 3-Br,
3-N(CH3)2, 3-CF3, 4-SO2CH3, 4-CF3) and 2,2,2-trichloroethyl benzoate was confirmed by 1H
and 13C NMR spectroscopy in deuterated chloroform at 25 °C (see Supplementary material).
DATA PROCESSING AND RESULTS
The values of chemical shifts, δ(17O), for the carbonyl oxygen and the single-
bonded oxygen in the ortho-, para- and meta-substituted phenyl esters of
benzoic acid, C6H5CO2C6H4-X, given in Table I were treated according to
the Taft and Charton equations using the Taft σ° 63,64, inductive σI 64,65 and
the resonance σ°R (σ°R = σP – σI)64,66 substituent constants12 (see Supplemen-
tary material). In the case of ortho substituents, two steric scales were em-
B
ployed: the Es constants6,12,67 and the Charton scale68–70 of υ. The steric
B
constants, Es , determined on the basis of the acid hydrolysis of ortho-sub-
stituted phenyl benzoates67, C6H5CO2C6H4-X, in the case of substituents X
= H, F, Cl, Br, I, CH3, C(CH3)3, CF3, were found to be nearly linear function
of the υ values, calculated on the bases of van der Waals radii, rv.
In the case of the carbonyl and single-bonded oxygen chemical shifts,
δ(17O), for alkyl benzoates, C6H5CO2R, given in Table II, Eq. (1) was applied.
B
δ(17O)Alk = δ(17O)H + (ρI)Alk σI + δAlk(Es )Alk
(1)
B
The steric substituent constants10,12, Es , for the variable alkyl substituent
in the alcohol component of esters, C6H5CO2R, (Eq. (1)) were calculated as
B
R
R
CH3
follows: Es = (log kH+ – log kH+CH3), where kH+ and kH+
are the rate
constants for acid hydrolysis of R-substituted alkyl benzoates, C6H5CO2R,
in water71.
The results of correlations of the carbonyl oxygen and the single-bonded
oxygen δ(17O) values for ortho-, meta- and para-substituted phenyl esters of
benzoic acid, C6H5CO2C6H4-X, and alkyl benzoates, C6H5CO2R (given in
Tables I and II) with the Taft and Charton equations (see Supplementary
material, Eqs (1)–(5)), are listed in Table III.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1742
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
TABLE I
17O NMR chemical shifts, δ(17O), (in ppm) of the carbonyl oxygen, the single-bonded
oxygen and the oxygen in substituent X for substituted phenyl esters of benzoic acid,
C6H5CO2C6H4-X, in CH3CN at 50 °C
a
b
X
δ(17O)C=O δ(17O)OPh
δ(17O)X
cester (m) δ(13C)C=O
log kc
4-NO2
348.6
347.9
349.1
348.3
346.1
346.3
345.5
345.7
344.0
345.7e
345.8e
346.9
346.9
346.8
346.8
347.7
346.7
346.9
345.4
344.9
345.2
347.5
350.2
349.5
348.6
348.6
347.6
349.6
188.4
190.1
188.5
187.9
183.4
184.6
185.5
183.8
182.9
187.3e
187.5e
185.7
185.6
185.7
186.8
186.6
185.7
186.4
187.3
187.6
188.8
178.0
166.1
185.5
180.0
179.9
169.2
183.3
570.5
0.3
0.5
0.524
0.320
0.29d
4-CN
4-SO2CH3
4-CF3
4-F
156.0
0.5
0.5
0.5
0.5
0.5
0.5
0.3
165.72
164.66
–0.015d
–0.507
–0.389
–0.886
–0.857
–1.215
–0.775
4-Cl
4-CH3
4-OCH3
4-NH2
H
48.5
0.4
0.5
0.5
0.5
0.5
1.0
0.5
0.5
3-NO2
3-CN
3-CF3
3-F
573.9
0.230
0.145d
–0.116d
0.441d
–0.260
164.78
164.66
3-Cl
3-Br
0.496d
–0.866
–0.955
–1.028d
–0.060
0.791d
0.320
3-CH3
3-NH2
3-N(CH3)2
2-NO2
2,6-(NO2)2
2-CN
0.5
0.5
165.24
163.21
600.1
607.5
2-CF3
–0.406
–0.533d
–0.481
–0.613
2-SO2CH3
2-F
161.0
0.3
164.37
2-Cl
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1743
TABLE I
(Continued)
a
b
X
δ(17O)C=O δ(17O)OPh
δ(17O)X
cester (m) δ(13C)C=O
log kc
2-Br
349.5
346.1
345.4
347.4
347.7
347.5
189.2
173.8
184.6
183.3
189.5
184.7
164.13
–0.635d
–1.309
–1.226
–1.891d
–1.740
–1.03f
2-OCH3
37.8
2-CH3
2,6-(CH3)2
2-C(CH3)3
2-CO2CH3
2-COCH3
164.31
165.1
354; 135.8
574.0
345.7
183.1
–0.528d
a
Approximately 1 molal solutions of ortho, meta and para derivatives in mixture of CH3CN
b
and CD3CN (2 ml + 1 ml) were used, except where shown otherwise. The carbonyl carbon
13C NMR chemical shift (in ppm) in CDCl3. Unpublished kinetic data for the alkaline hy-
c
d
drolysis of C6H5CO2C6H4-X in aqueous 0.5 M Bu4NBr at 25 °C. The log k values calculated
from equations5 log km,p = –0.754 + 1.45σ° and log kortho = –0.727 + 1.61σI + 1.37σ°R
+
B
e
f
1.38Es
.
Data from ref.14 Supporting information for ref.12
DISCUSSION
Influence of para and meta Substituents
The carbonyl oxygen chemical shifts, δ(17O), for the para- and meta-
substituted phenyl esters of benzoic acid, C6H5CO2C6H4-X (Table I), correla-
ted well so with the single parameter treatment using the Taft σ° as the dual
substituent method using the Taft σI and σ°R (see Supplementary material).
δ(17O)para = (345.8 0.2) + (3.74 0.40)σ°
R = 0.952, s = 0.507, n/n0 = 10/10
(2)
δ(17O)para = (346.1 0.4) + (3.25 0.72)σI + (4.34 0.76)σ°R
R = 0.951, s = 0.507, n/n0 = 10/10
(3)
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1744
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
δ(17O)meta = (345.6 0.1) + (2.43 0.32)σ°
R = 0.929, s = 0.301, n/n0 = 10/10
(4)
TABLE II
17O NMR chemical shifts, δ(17O), (in ppm) of the carbonyl oxygen, the single-bonded oxy-
gen and the oxygen in substituent R for substituted alkyl esters of benzoic acid, C6H5CO2R,
in CH3CN at 50 °C
a
Bc
R
δ(17O)C=O
δ(17O)OR
δ(17O)R and νCO
σIb
Es
d
CH3
338.5
338.2
349.2
345.9
342.0
339.7
342.3
339.6
339.6
338.1
340.8
341.3
128.1
158.4
165.7
134.8
128.2
156.3
148.1
148.2
151.3
148.1
146.7
149.4
0
0
e
CH2CH3
CH2Cl
–0.05
0.47
0.58
0.41
0.11
0.27
0.17
0.17
0.07
0.31
0.41
0
–0.17
–0.55
–0.214g
–0.17
–0.27
–0.23
–0.22h
–0.20
–0.348i
–0.413j
CH2CN
f
CH2CF3
νCO = 1743.4
CH2C6H5
CH2C≡CH
CH2CH2Cl
CH2CH2Br
CH2CH2OCH3
CH2CHCl2
CH2CCl3
νCO = 1729.3
–24
νCO = 1733.2
νCO = 1741.2
a
b
The IR stretching frequency of carbonyl group (in cm–1) in tetrachloromethane. Data
from ref.66
.
Data from refs11,12
.
δ(17O)C=O = 340 and δ(17O)OR = 128 in acetonitrile at
c
d
75 °C 33; δ(17O)C=O = 337.3 and δ(17O)OR = 127.7 in acetonitrile at 40 °C 28; δ(17O)C=O
=
337.3 and δ(17O)OR = 127.7 in acetonitrile at 40 °C 31
.
δ(17O)C=O = 339 and δ(17O)OR = 159
e
in acetonitrile at 75 °C 16; δ(17O)C=O = 337.0 and δ(17O)OR = 157.4 in acetonitrile at 40 °C 31
.
f
g
δ(17O)C=O = 340.3 and δ(17O)OR = 126.6 in deuterochloroform at 40 °C 15
.
For R = CH2CF3
B
B
derivative Es = –0.214 was calculated as the average value from Es = –0.23112, –0.20712 and
B
–0.205; Es = –0.205 was calulated from equation13 (νCO) = 1743.4 = 1726.7 + 56.8σI
+
B
h
B
32.2Es
.
For R = CH2CH2Br derivative Es = –0.22 was calculated from equation13 (νCO) =
B
i
B
1729.3 = 1726.7 + 56.8σI + 32.2Es
.
For R = CH2CHCl2 derivative Es = –0.348 was calcu-
B
B
lated as the average value from Es = –0.35012 and –0.345; Es = –0.345 was calculated from
B
j
B
equation13 (νCO) = 1733.2 = 1726.7 + 56.8σI + 32.2Es
.
For R = CH2CCl3 derivative Es
=
B
B
–0.413 was calculated as the average value from Es = –0.51812 and –0.308; Es = –0.308 was
B
calculated from equation13 (νCO) = 1741.2 = 1726.7 + 56.8σI + 32.2Es
.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1745
δ(17O)meta = (345.6 0.2) + (2.47 0.47)σI + (0.93 0.48)σ°R
R = 0.913, s = 0.330, n/n0 = 10/10
(5)
The calculated positive sensitivities towards the substituent-induced induc-
tive (ρI) and resonance (ρR) effects (Eqs (2)–(5), Table III) for the carbonyl
oxygen chemical shifts, δ(17O), indicate that the electron-withdrawing sub-
stituents cause high-frequency shift of the 17O signal showing deshielding
effect of the carbonyl O atom. The electron-donating substituents induce
an opposite effect resulting in low-frequency shift and shielding of the O
atom. This is in accord with the reverse substituent-induced inductive and
resonance effects observed in the carbonyl carbon 13C NMR chemical shifts,
(δCO), for substituted phenyl benzoates where the electron-withdrawing
substituents increased and the electron-donating substituents decreased the
electron density at the carbonyl carbon12. Consequently, the substituent ef-
fects so in the carbonyl oxygen chemical shifts, δ(17O), as in the carbonyl
carbon 13C chemical shifts, (δCO), prove the increased double bond charac-
ter of the carbonyl group produced by an electron-withdrawing groups and
an increase in single bond character resulting from the electron-donating
substituents48,72–76. In substituted phenyl benzoates the increased double
bond character of the carbonyl group due to electron-withdrawing substitu-
ents was proved earlier by the increased infrared stretching frequencies of
the carbonyl group, νCO 13, as well.
In the carbonyl oxygen chemical shifts, δ(17O), the susceptibility to the
inductive effect of para substitutents from the phenyl side (ρI = 3.74, ρI =
3.25; Eqs (2) and (3), Table III) was found to be ca. 2.3 times weaker than
that from the benzoyl side (ρ+ = 9.07, ρI = 7.2114). Similarly, the magnitude
of the para inductive term for the carbonyl carbon 13C NMR chemical
shifts, (δCO), from the phenyl side (ρI = –1.10 12) was twice smaller, com-
pared to that from the benzoyl moiety (ρI = –2.18 12). In the alkaline hydro-
lysis of para- and meta-substituted phenyl benzoates in water the polar
influence from the phenyl side was ca. 1.6 times weaker than the corre-
sponding influence from the benzoyl moiety i.e. (ρI)PhCO/(ρI)PhO = 1.6 6.
With the generally accepted views in the substituted benzoyl derivatives,
inductive effect increased due to shorter distance between the substituent
and carbonyl carbon atom, compared to that in phenyl side. Similar to the
inductive effect, the contribution of the para substituent resonance effect to
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1746
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
TABLE III
Correlation of the 17O NMR chemical shifts, δ(17O), of the carbonyl oxygen and single-
bonded oxygen with the Taft and Charton equations
2 b
d
Scale
δ(17O)H
ρI or ρ°
ρ°R
δ
Ra
(R0
)
sc
n/n0
ortho-substituted phenyl benzoates
+R substituents
C=17O
B
σI, σ°R, Es
σI, σ°R, υ
345.6 0.5 9.10 0.82
345.6 0.5 8.84 0.89
345.5 0.5 8.08 0.81
9.36 1.41 –5.74 0.88 0.977 0.286 0.362 7/8e
9.03 1.54 –6.16 0.91 0.968 0.286 0.401 8/8f
7.66 1.44
2.85 0.91 0.969 0.286 0.390 8/8f
–R substituents
B
B
σI, σ°R, Es
346.1 1.4 8.94 1.57
346.2 1.7 7.53 1.78
–19.0 5.2
–14.8 6.1
0
0
0.942 0.618 0.585 7/7g,h
0.889 0.617 0.741 8/8h
17OC6H5
+R substituents
σI, σ°R, Es
σI, σ°R, υ
187.1 0.7 5.35 1.25
187.0 5.2 9.28 8.11
187.1 0.6 4.34 1.23
186.7 4.7 7.83 7.17
43.6 1.9
57.1 14.5
41.8 1.9
54.1 13.1
–10.1 1.2 0.996 0.056 0.468 6/6i,j
–16.0 9.1 0.848 0.088 3.88
8/8j
4.75 0.56 0.996 0.056 0.481 6/6i,j
7.85 3.93 0.866 0.088 3.66
8/8j
–R substituents
B
σI, σ°R, Es
188.5 0.8 –17.0 1.2
0
0
0.985
1.18
7/8k
C=17
O
para-substituted phenyl benzoates
σ°p
345.8 0.2 3.74 0.40
346.1 0.4 3.25 0.72
–
–
–
0.952
–
0.507 10/10
σI, σ°R
4.34 0.76
0.951 0.531 0.507 10/10
17OC6H5
σ°p
185.0 0.6 4.80 1.27
–
–
–
–
0.773
–
1.61 10/10
σI, σ°R
187.3 0.3
187.0 0.2
0
0
9.86 1.14
9.00 0.74
0.944 0.071 0.832 10/10
0.974
0.929
–
0.510 9/10l
C=17
O
meta-substituted phenyl benzoates
σ°
345.6 0.1 2.43 0.32
345.6 0.2 2.47 0.47
–
–
–
–
0.301 10/10
σI, σ°R
0.93 0.48
0.913 0.776 0.330 10/10
17OC6H5
σ°
187.5 0.2 –3.07 0.54
187.2 0.2 –2.53 0.41
–
–
–
–
–
0.881
0.907
–
–
0.509 10/10
0.348 9/10m
σI, σ°R
187.2 0.3 –2.48 0.69 –2.14 0.69
0.894 0.579 0.482 10/10
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1747
TABLE III
(Continued)
2 b
d
Scale
δ(17O)H
C=17
ρI or ρ°
ρ°R
δ
Ra
(R0
)
sc
n/n0
O
R-substituted alkyl benzoates
B
σI, Es
σI, Es
339.1 0.7
27.4 1.6
–
17.1 2.1
0.987
0.958
0.771
0.572
2.49
10/10n
9/9o
17OR
B
163.0 3.4
35.4 7.2
–
85.1 9.5
–0.010
a
b
R – correlation coefficient. R0 – zeroth correlation coefficient (R for correlation with one
c
d
descriptor σI). s – standard deviation (in ppm). n0 reflects the total number of data in-
volved in the correlation; n – the number of points remaining after exclusion of signifi-
cantly deviating points. The 2,6-(CH3)2 derivative was excluded. For 2,6-(CH3)2 derivative,
double values of resonance and steric constants were used and the inductive constant was
e
f
g
h
equal to zero. The 2-CO2CH3 derivative was omitted before data treatment. For 2-COCH3
i
substituent σ°Rj = 0.16 and 2-CO2CH3 σ°R = 0.14 were used. The 2-F and 2-Br derivatives
were omitted. For 2,6-(CH3)2 derivative, double values of inductive, resonance and steric
k
l
constants were used. The 2-CN derivative was excluded. The 4-CN derivative was ex-
cluded.
m
n
The 3-NH2 derivative was excluded.
Benzoates with R = CH2CF3 and R =
CH2CCl3 were omitted. The inductive and steric constants from Table II were used.
o
Benzoates with R = CH2CF3, R = CH2CCl3 and R = CH3 were omitted.
the carbonyl δ(17O) values from the phenyl side was about 2.5 times smaller
than the corresponding influence from the benzoyl side ((ρR)PhCO/(ρR)PhO
=
9.71 14/4.34 = 2.24; Table III). A good correlation obtained for the carbonyl
oxygen chemical shifts, δ(17O), with σ°, σI and σ°R constants (Eqs (2) and
(3), Table III) for para-substituted esters, C6H5CO2C6H4-X, indicated the ab-
sence of the through-resonance between substituents in the phenyl moiety
and the carbonyl oxygen. In phenyl benzoates containing para-substituents
in the benzoyl side, X-C6H4CO2C6H5, the contribution of the through-
conjugation between substituent and the carbonyl oxygen (or carbonyl
group) was found to be significant so to the carbonyl oxygen as in the
single-bonded chemical shifts14, δ(17O), the infrared stretching frequencies
of the carbonyl group13, νCO, and the log k values of the alkaline hydro-
lysis6. This once more prove that in phenyl benzoates the benzene ring in
benzoyl part of the ester is coplanar with the carbonyl bond plane while
the aromatic plane in the phenoxy part is twisted nearly perpendicular rela-
tive to the carboxyl bond plane13,77–81. In contrast to this for phenyl benzo-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1748
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
ate and benzoates with ortho substituents in phenoxy moiety nearly planar
conformers were proved as well82–84. The primary condition for the reso-
nance interaction between two groups is the mutual coplanarity of these
groups.
In meta-substituted phenyl benzoates, C6H5CO2C6H4-X, the contribution
of the inductive effect to the carbonyl oxygen chemical shifts, δ(17O), from
the phenyl side was about one unit lower ((ρI)m = 2.47; Eq. (5), Table III),
compared to that for para substituents ((ρI)p = 3.25; Eq. (3), Table III). The
influence of the meta inductive effect from the phenyl side was approxi-
mately 2.4 times weaker than the same effect from the benzoyl component
((ρI)m = 6.16 14).
Influence of ortho Substituents
The carbonyl oxygen δ(17O) values measured for fourteen ortho-substituted
phenyl esters of benzoic acid, C6H5CO2C6H4-X (Table I), except CH3,
showed high frequency shift of the 17O signal relative to unsubstituted
phenyl benzoate, C6H5CO2C6H5. The carbonyl oxygen chemical shifts,
δ(17O), for ortho-substituted phenyl esters of benzoic acid, C6H5CO2C6H4-X,
estimated in the present work (Table I) correlated well with the Charton
equation in case the data treatment was carried out separately for deriva-
tives with +R substituents and with –R substituents. In the case of +R
substituents (X = H, 2-OCH3, 2-CH3, 2,6-(CH3)2, 2-C(CH3)3, 2-F, 2-Cl, 2-Br),
the influence of ortho substituents to the carbonyl 17O NMR chemical shifts,
δ(17O), was expressed as follows (Table III).
(δ17O)ortho = (345.6 0.5) + (9.10 0.82)σI + (9.36 1.91)σ°R –
B
– (5.74 0.88)Es
(6)
R = 0.977, s = 0.362, n/n0 = 7/8
It follows from Eq. (6) that in the case of ortho derivatives with +R substitu-
ents, the carbonyl oxygen is deshielded due to the –I inductive and steric
effects and shielded by the +I inductive and +R resonance effects of ortho
substituents.
For ortho-substituted phenyl esters of benzoic acid containing electron-
withdrawing –R substituents (X = H, 2-NO2, 2-CN, 2-CF3, 2-SO2CH3,
2-CO2CH3, 2-COCH3, 2,6-(NO2)2), the steric factor appeared to be insignifi-
cant (Table III).
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1749
(δ17O)ortho = (346.1 1.4) + (8.94 1.57)σI – (19.0 5.2)σ°R
R = 0.942, s = 0.585, n/n0 = 7/7
(7)
Unexpectedly, for ortho derivatives with –R substituents in the phenyl
moiety the negative resonance effect ρ < 0 was obtained. In the case of
–R substituents the carbonyl oxygen appeared to be deshielded by the in-
ductive effect but shielded by resonance effect similar to that for the ortho
derivatives containing +R substituents. The oxygens of the ortho NO2
groups also revealed the deshielding effect, compared to oxygens in the
para NO2 group (Table I).
The correlation using the modified Charton steric constants, υ, gave
B
nearly the same results as with the steric constants Es . In the case of the
Charton steric constants, υ, the susceptibility to the steric effect, δortho, was
positive and the magnitude of δortho was ca. twice smaller, compared to the
B
B
δortho value obtained with the Es steric constants (Es < 0). For phenyl
benzoates with ortho +R substituents, we obtained Eq. (8) (Table III).
(δ17O)ortho = (345.5 0.5) + (8.08 0.81)σI + (7.66 1.44)σ°R +
+ (2.85 0.91)υ
(8)
R = 0.969, s = 0.390, n/n0 = 8/8
The influence of the ortho inductive effect in the case of +R substituents
(ρI = 9.10) and –R substituents (ρI = 8.94) was found to be approximately
twice higher than the corresponding influence from the para position (ρI =
3.74, 3.25) (Table III). In the case of ortho substituents in phenyl moiety
similar to para and meta derivatives the inductive effect appeared to be ca.
twice weaker than that in the case of ortho substituents in benzoyl moiety
(ρI = 17.3 14).
Results of correlations (see Table III) show that the steric requirements of
B
ortho +R substituents (δorthoEs > 0) caused an increase in the carbonyl oxy-
gen δ(17O) values (high frequency shift). Due to the steric effect of substitu-
ents in phenyl moiety, carbonyl oxygen is more deshielded, compared to
the unsubstituted derivative. In the case of +R substituents in the phenyl
moiety, the steric influence on the carbonyl oxygen δ(17O) values was re-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1750
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
duced ca. 13.5 times, compared to that involving substituents in the
benzoyl moiety (δortho = –81.2 14). We suggested that the observed steric ef-
fects of ortho +R substituents in phenyl moiety are caused by the van der
Waals deshielding effect85–88. In this mechanism the π-electron density
around the CO oxygen in an ortho-substituted ester would be reduced by
electrostatic repulsion between the orbitals of X and those of the carbonyl
CO bond with the effect increasing as X increases in size.
Influence of Substituents on the δ(17O) values of Single-Bonded
Phenoxy Oxygen
The substituent effects on the δ(17O) values of the single-bonded phenoxy
oxygen in phenyl benzoates, C6H5CO2C6H4-X, from the phenyl side ap-
peared to be different from those found for the carbonyl oxygen chemical
shifts, δ(17O), and from the substituent effects from the benzoyl moiety.
The substituent effects from the benzoyl part of esters on the δ(17O) values
of the single-bonded phenoxy oxygen were found to be ca. twice smaller
than that on the chemical shifts, δ(17O), of carbonyl oxygen14.
In para-substituted phenyl benzoates, the positive value of ρR = 9.00
(Eq. (9), Table III) indicated that due to resonance effect of the para
electron-donating substituents, the single-bonded phenoxy oxygen is
shielded and the electron-withdrawing substituents are deshielded it, com-
pared to the unsubstituted derivative.
δ(17O)para = (187.0 0.2) + (9.00 0.74)σ°R
R = 0.974, s = 0.510, n/n0 = 9/10
(9)
Similar resonance effect exerted para substituents on the carbonyl oxygen
chemical shifts, δ(17O), only the resonance effect on the carbonyl oxygen
chemical shifts, δ(17O), was more than twice weaker (ρR = 4.34; Eq. (3),
Table III), compared to that on the single-bonded oxygen (ρR = 9.00). Due
to the resonance and inductive effects, the oxygen of the para NO2
substituent in phenyl benzoate, C6H5CO2C6H4-NO2, (δ(17O) = 570.5; Table I)
appeared to be shielded, compared to that for unsubstituted nitrobenzene,
C6H5-NO2 (δ(17O) = 573.3 14).
Due to polar effect, the electron-withdrawing meta substituents in the
phenyl side attract electrons from the carbonyl oxygen, increasing the
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1751
electron density at the carbonyl carbon12 and the single-bonded phenoxy
oxygen, compared to the unsubstituted derivative (Table III).
δ(17O)meta = (187.2 0.2) – (2.53 0.41)σ°
R = 0.907, s = 0.348, n/n0 = 9/10
(10)
For ortho-substituted phenyl benzoates with +R substituents we found
Eq. (11).
(δ17O)ortho = (187.1 0.7) + (5.35 1.25)σI + (43.6 1.9)σ°R – (11)
B
– (10.1 1.2)Es
R = 0.996, s = 0.468, n/n0 = 6/6
It follows from Eq. (11) that in the case of the electronegative +R ortho sub-
stituents capable of electron-withdrawal by inductive effect (I-effect > 0),
B
the inductive effect and the steric effect (Es < 0) deshielded single-bonded
phenoxy oxygen atom while the resonance effect shielded it (σ°R < 0). The
contribution of the ortho resonance term to the single-bonded oxygen
chemical shifts was ca. five times stronger than that to the carbonyl oxygen
δ(17O) (ρR = 9.36; Eq. (6), Table III). An increased sensitivity of the δ(17O)
values to the resonance effect of substituents was detected also in anisoles
and aromatic ethers39 (ρR >> ρI). The influence of the steric term of ortho
substituents on the single-bonded oxygen chemical shifts, δ(17O), was ca.
twice stronger, compared to that on the carbonyl oxygen δ(17O) (δortho
=
–5.74; Table III).
In ortho-substituted phenyl benzoates with electron-attracting –R sub-
stituents in phenyl moiety increase the shielding of the single-bonded oxy-
gen by the substituent inductive effect only (Eq. (12), Table III).
(δ17O)ortho = (188.5 0.8) – (17.0 1.2)σI
R = 0.985, s = 1.18, n/n0 = 7/8
(12)
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1752
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
Influence of Substituents on the Phenyl ipso-Carbon
13C MNR Chemical Shifts
We found that in substituted phenyl benzoates, C6H5CO2C6H4-X, substitu-
ents in para, meta and ortho positions in phenyl moiety induced the charge
at the phenyl ipso-carbon (C-1) similarly as it was found previously in the
case of the adjacent single-bonded phenoxy oxygen.
The ipso-carbon (C-1) in phenyl moiety is shielded with the para electron-
donor substituents and deshielded when the electron-attracting –R substitu-
ents were involved (Eq. (13)).
δ(C-1)para = (150.81 0.24) + (3.26 0.47)σI + (16.24 0.55)σ°R (13)
R = 0.997, s = 0.317, n/n0 = 8/8
In the case of para substituents the resonance affects the electron density
at the adjacent single-bonded phenoxy oxygen in the same way (Eq. (9),
Table III).
The substituents in the meta position in phenyl moiety show very small
effects on the phenyl ipso-carbon 13C MNR chemical shifts, whereas the res-
onance operates in the opposite sign to those for the corresponding para
substituents.
δ(C-1)meta = (151.12 0.13) + (0.67 0.26)σI – (1.68 0.26)σ°R
R = 0.905, s = 0.425, n/n0 = 10/10
(14)
The adjacent single-bonded phenoxy oxygen was found to be affected by
the reverse resonance effect of meta substituents as well (Eq. (10)).
In the case of electron-donating ortho +R substituents in phenyl moiety
due to the inductive effect a decrease in the electron density at ipso-carbon
(C-1) was observed while the resonance effect increased it.
δ(C-1)ortho = (152.24 0.88) + (4.65 1.85)σI + (31.5 3.4)σ°R
R = 0.968, s = 0.953, n/n0 = 7/7
(15)
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1753
The electron density at the adjacent single-bonded phenoxy oxygen was
exerted by inductive and resonance effects of ortho +R substituents in the
same way (Eq. (11)). In the case of ortho +R substituents the contribution of
the resonance effect in the shielding effect of the ipso-carbon (C-1) was
twice stronger than that for para substituents.
In the case of ortho –R substituents in phenyl moiety we obtained Eq. (16)
B
δ(C-1)ortho = (150.73 1.45) – (14.4 1.4)σI – (8.50 2.26)Es
R = 0.980, s = 0.870, n/n0 = 7/8
(16)
and without the steric factor Eq. (17).
δ(C-1)ortho = (152.6 1.1) – (10.2 1.7)σI
R = 0.924, s = 1.66, n/n0 = 7/8
(17)
The electron density at the adjacent of single-bonded oxygen was af-
fected by the ortho –R substituent inductive effect only (Eq. (12)).
The influence of the ortho electron-attracting –R substituents in the phen-
yl moiety is different from that for para –R substituents. The electron-
attracting –R para substituents due to resonance effect reduce the electron
density at the ipso-carbon (C-1) and the adjacent single-bonded phenoxy
oxygen. In the case of –R ortho substituents, similar to +R ortho substituents,
the increase in the electron density at the phenyl ipso-carbon (C-1) and the
adjacent single-bonded phenoxy oxygen was observed.
It appeared that –R substituents in ortho position in substituted acet-
anilides89, CH3CONHC6H4-X, show the shielding effect on the phenyl
ipso-carbon C-1 with respect to unsubstituted derivative similarly we found
in ortho-substituted phenyl benzoates, C6H5CO2C6H4-X (X = H, CN, COOMe,
CF3, NO2).
δ(C-1)PhCOOC6H4-X = (0.825 0.087)δ(C-1)CH3CONHC6H4-X
+
(18)
+ (36.38 12.09)
R = 0.978, s = 0.677, n/n0 = 5/5
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1754
Effect of Alkyl Substituents
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
In substituted alkyl benzoates, C6H5CO2R, the substituent-induced effect on
the carbonyl oxygen chemical shift, δ(17O), (Table II) was described by the
inductive and steric effects as follows (Table III).
B
(δ17O)Alk = (339.2 0.7) + (27.4 1.6)σI + (17.1 2.1)Es
R = 0.987, s = 0.572, n/n0 = 10/10
(19)
Good correlation (Eq. (19)) was obtained when esters with R = CH2CF3,
CH2CCl3 were not included into the data treatment. Similar to the carbonyl
oxygen chemical shifts, the single-bonded oxygen chemical shifts, δ(17O),
B
for C6H5CO2R showed good correlation with the inductive σI and steric Es
substituent constants in case esters with R = CH3, CH2CF3, CH2CCl3 were
omitted before the data treatment (Table III).
B
(δ17O)Alk = (163.0 3.4) + (35.4 7.2)σI + (85.1 9.5)Es
R = 0.958, s = 2.49, n/n0 = 10/10
(20)
In substituted alkyl benzoates, C6H5CO2R, the carbonyl oxygen and
single-bonded oxygen are deshielded by –I inductive substituent effect and
shielded by +I inductive and steric substituent effects. We suggested that in
the case of substituted alkyl benzoates the steric effect so on the carbonyl
oxygen chemical shifts, δ(17O), as the single-bonded oxygen chemical
shifts, δ(17O), is caused by the attractive van der Waals interaction between
the orbitals of the oxygen and substituent with shielding effects85,86. The
influence of the steric factor on the single-bonded oxygen chemical shifts,
δ(17O), was five times stronger (Eq. (20)), compared to the steric contribu-
tion to the carbonyl oxygen chemical shifts (Eq. (19)). The chemical shifts,
δ(17O), for alkyl benzoates, C6H5CO2CH2CF3 and C6H5CO2CH2CCl3, deviate
from correlations given by Eqs (19) and (20). The EsB values for R = CH2CF3
and R = CH2CCl3 shown in Table II are too low to correlate the δ(17O) val-
B
ues with Eqs (19) and (20). The Es values for R = CH2CF3 and R = CH2CCl3
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1755
calculated from Eqs (19) and (20) are –0.487, –0.582 and –0.526, –0.330, re-
spectively.
We found that the single-bonded oxygen chemical shifts, δ(17O), for
C6H5CO2R could be correlated well with the corresponding δ(17O) values
for substituted alcohols ROH 42,43 when R = CH3, CH2CH3, CH2Ph,
CH2CH2Cl, CH2CF3, CH2C≡CH as follows.
δ(17O)PhCO2R = (150.6 1.8) + (0.720 0.098)δ(17O)ROH
R = 0.954, s = 3.92, n/n0 = 6/6
(21)
Correlation of the Carbonyl Oxygen δ(17O) Values with the Substituent-Induced
Carbonyl Carbon 13C NMR Chemical Shifts, ∆δCO, and IR Carbonyl Stretching
Frequencies, ∆νCO, and Rates of the Alkaline Hydrolysis, ∆log k
The carbonyl oxygen chemical shifts, δ(17O), for ortho-, para- and meta-
substituted phenyl esters of benzoic acid, C6H5CO2C6H4-X, and alkyl
benzoates, C6H5CO2R, were correlated with the corresponding substituent-
induced carbonyl carbon 13C NMR chemical shifts, ∆δCO, the IR carbonyl
stretching frequencies, ∆νCO, and the rates of the alkaline hydrolysis, ∆log k
in pure water and in aqueous 0.5 M Bu4NBr (see equations in Supplemen-
tary material). In the case of ortho derivatives we obtained good correlations
with the corresponding substituent-induced carbonyl carbon 13C NMR
chemical shifts, ∆δCO, IR carbonyl stretching frequencies, ∆νCO, and the
∆log k values of the alkaline hydrolysis in case the additional resonance and
steric terms were involved and the data treatment was carried out sepa-
rately for esters containing +R substituents and –R substituents (Table IV;
0.983 > R > 0.900). Correlation equations obtained (Table IV) enables one
to predict the carbonyl carbon 13C NMR chemical shifts, ∆δCO, IR carbonyl
stretching frequencies, ∆νCO, and the ∆log k values of the alkaline hydro-
lysis using the carbonyl oxygen chemical shifts, δ(17O), for the esters con-
sidered.
For phenyl esters of ortho-substituted phenyl benzoates containing +R
substituents we obtained Eqs (22)–(24) (Table IV).
(δ17O)ortho = (345.8 0.4) – (4.00 0.34)∆δCO
+ (7.56 1.07)σ°R – (6.49 0.77)Es
+
(22)
B
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1756
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
TABLE IV
Correlation of the carbonyl oxygen 17O NMR chemical shifts, δ(17O), with the carbonyl car-
bon 13C NMR chemical shifts, ∆δCO, the carbonyl IR stretching frequencies, ∆νCO and the
∆log k values of the alkaline hydrolysis (see Supplementary material, Eqs (6)–(9))
c
Scale
δ(17O)H
a1
a2
a3
Ra
sb
n/n0
ortho-substituted phenyl benzoates
+R substituents
C=17O
B
B
∆δCO, σ°R, Es
345.8 0.4
345.5 1.3
345.5 0.6
345.2 1.0
–4.00 0.34
0.457 0.130 7.96 3.66 –5.67 2.25 0.827 0.885
7.56 1.07 –6.49 0.77 0.983 0.297 6/6d
∆νCO, σ°R, Es
6/6e
7/7f
7/7g
B
B
∆log k1, σ°R, Es
∆log k2, σ°R, Es
5.94 0.67
5.19 1.03
3.88 1.38 –10.8 1.4
–12.1 2.6
0.965 0.447
0.897 0.757
0
–R substituents
B
∆δCO, σ°R, Es
346.8 0.4
346.3 0.4
345.7 0.9
345.8 0.7
–1.98 0.52
–2.47 0.50
0
0
0
0
0.809 0.954
8/8
B
∆δCO, σ°R, Es
0.894 0.781 7/7h
B
B
∆log k1, σ°R, Es
∆log k2, σ°R, Es
5.60 0.60 –22.5 3.5
5.18 0.44 –20.6 2.7
–5.56 0.80 0.978 0.364 7/8i,k
–5.49 0.64 0.986 0.292 7/8j,k
C=17
O
para-substituted phenyl benzoates
∆δCO, σ°R
346.4 0.2
345.5 0.2
345.6 0.2
345.6 0.2
–2.85 0.49
0.353 0.056
3.25 0.41
2.63 0.28
0
0
0
0
–
–
–
–
0.951 0.533
8/8l
∆νCO, σ°R
0.920 0.561 8/8m
0.936 0.581 10/10n
0.952 0.507 10/10o
∆log k1, σ°R
∆log k2, σ°R
C=17
O
meta-substituted phenyl benzoates
∆δCO, σ°R
345.7 0.1
345.8 0.1
345.5 0.2
345.5 0.2
345.6 0.1
–2.81 0.35
–2.52 0.28
0.233 0.024
2.13 0.34
1.65 0.21
0
0
0
0
0
–
–
–
–
–
0.936 0.285 10/10
0.954 0.211 9/10p
0.969 0.203 7/7q
0.901 0.352 10/10r
0.933 0.293 10/10s
∆νCO, σ°R
∆log k1, σ°R
∆log k2, σ°R
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1757
TABLE IV
(Continued)
c
Scale
δ(17O)H
C=17
a1
a2
a3
Ra
sb
n/n0
O
R-substituted alkyl benzoates
B
∆δCO, Es
336.0 0.9
340.0 0.4
339.9 1.0
340.2 1.1
–4.55 0.67
0.421 0.048
7.21 0.76
6.10 0.78
–
–
–
–
0
0.913
0.945
0.964
0.948
1.49
1.20
0.974
1.16
10/10
10/10
10/10t
10/10u
B
∆νCO, Es
0
B
B
∆log k1, Es
∆log k2, Es
8.99 2.92
8.03 3.43
a
b
c
R – correlation coefficient. s – standard deviation (in ppm). n0 reflects the total number
of data involved in the correlation; n – the number of points remaining after exclusion of
d
e
significantly deviating points. The 2,6-(CH3)2 and 2-CH3 derivatives were omitted. The
∆νCO values for 2,6-(CH3)2 and 2-Br derivatives were not available. ∆log k1 = log kX – log kH
f
for the alkaline hydrolysis of C6H5CO2C6H4-X in water at 25 °C. The log k1 value in water at
25 °C for 2-Br derivative was calculated from equation5 log kortho = –0.377 + 1.54σI
+
B
g
0.95σ°R + 1.09Es
.
∆log k2 = log kX – log kH for the alkaline hydrolysis of C6H5CO2C6H4-X
in aqueous 0.5 M Bu4NBr at 25 °C. The log k2 value in aqueous 0.5 M Bu4NBr at 25 °C for
2-Br derivative was calculated from equation5 log kortho = –0.727 + 1.61σI + 1.37σ°R
+
B
h
i
1.38Es
.
The 2-CO2CH3 derivative was omitted. The ∆log k1 values for 2-SO2CH3,
f
2-CO2CH3, 2-COCH3 and 2,6-(NO2)2 derivatives were calculated from equation in note
.
j
The ∆log k2 values for 2-SO2CH3, 2-CO2CH3, 2-COCH3 and 2,6-(NO2)2 derivatives were cal-
g
k
l
culated from equation in note
and 4-NH2 derivatives were omitted.
.
The 2-CO2CH3 derivative was excluded. The 4-SO2CH3
m
The 4-SO2CH3 and 4-CF3 derivatives were not in-
cluded. There are no νCO values for 4-SO2CH3 and 4-CF3 derivatives available in literature.
n
The log k1 values in water at 25 °C for 4-CF3 and 4-SO2CH3 derivative were calculated from
equation5 log km,p = –0.377 + 1.11σ°. The log k2 values in aqueous 0.5 M Bu4NBr at 25 °C
o
for 4-CF3 and 4-SO2CH3 derivatives were calculated from equation5 log km,p = –0.754 +
p
q
1.45σ°. The 3-NH2 derivative was excluded. The 3-Br, 3-CF3 and 3-N(CH3)2 derivatives
were not included. The νCO values for 3-Br, 3-CF3 and 3-N(CH3)2 derivatives are not avail-
able. The log k1 values in water at 25 °C for 3-Br, 3-CF3 and 3-N(CH3)2 derivatives were cal-
r
culated from equation5 log km,p = –0.377 + 1.11σ° (see note n). The log k2 values in
s
aqueous 0.5 M Bu4NBr at 25 °C for 3-Br, 3-CF3 and 3-N(CH3)2 derivatives were calculated
t
from equation5 log km,p = –0.754 + 1.45σ° (see note o). The log k1 values for alkaline hydro-
lysis of esters C6H5CO2CH2CH2Br and C6H5CO2CH2CHCl2 in water were calculated from
B
u
equation11 log k = –1.272σI + 0.98 Es
.
The log k2 value for alkaline hydrolysis of ester
C6H5CO2CH2CH2Br in aqueous 0.5 M Bu4NBr was calculated from equation11 log k =
B
–1.613σI + 1.17Es
.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1758
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
R = 0.983, s = 0.297, n/n0 = 6/6
(δ17O)ortho = (345.5 1.3) + (0.457 0.130)∆νCO
+
(23)
(24)
B
+ (7.96 3.66)σ°R – (5.67 2.25)Es
R = 0.827, s = 0.885, n/n0 = 6/6
(δ17O)ortho = (345.5 0.6) + (5.94 0.67)∆log k +
B
+ (3.88 1.38)σ°R – (10.8 1.4)Es
R = 0.965, s = 0.447, n/n0 = 7/7
In the case of substituted phenyl benzoates containing –R substituents,
we found Eqs (25) and (26) (Table IV).
(δ17O)ortho = (346.3 0.4) – (2.47 0.50)∆δCO
R = 0.894, s = 0.791, n/n0 = 7/7
(25)
(δ17O)ortho = (345.7 0.9) + (5.60 0.60)∆log k –
(26)
B
– (22.5 3.5)σ°R – (5.56 0.80)Es
R = 0.978, s = 0.364, n/n0 = 7/8
For alkyl benzoates, Eqs (27)–(29) were obtained (Table IV).
(δ17O)Alk = (336.0 0.9) – (4.55 0.67)∆δCO
R = 0.913, s = 1.49, n/n0 = 10/10
(27)
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1759
(δ17O)Alk = (340.0 0.4) + (0.421 0.048)∆νCO
(28)
R = 0.945, s = 1.20, n/n0 = 10/10
(δ17O)Alk = (339.9 1.0) + (7.21 0.76)∆log k + (8.99 2.92)Es
R = 0.964, s = 0.974, n/n0 = 10/10
B
(29)
The negative values of the parameter a1 = ρI(17O)/ρI(δCO) (see Supplemen-
tary material) in correlation of the δ(17O) values with the carbonyl carbon
13C NMR chemical shifts, ∆δCO, show that in ortho-, meta- and para-substi-
tuted phenyl esters of benzoic acid, C6H5CO2C6H4-X, as well as substituted
alkyl benzoates, C6H5CO2R, the influence of the substituent-induced induc-
tive effect on the δ(17O) values is opposite to that in 13C NMR chemical
shifts, ∆δCO (Table IV). Earlier12 when studying the 13C NMR chemical shifts
in C6H5CO2C6H4-X we found the inductive effect of the electron-attracting
ortho, meta and para, as well as alkyl substituents to increase in the electron
density at the carbonyl carbon while the electron-donating +R substituents
deshielded the carbonyl carbon. The influence of –R and +R substituents on
the δ(17O) values (Table III) was reverse to that found in 13C NMR chemical
shifts, ∆δCO
.
When the carbonyl oxygen chemical shifts, δ(17O), were correlated with
the corresponding substituent-induced carbonyl carbon 13C NMR chemical
shifts, ∆δCO, the a1 values for ortho, para and meta derivatives were a1 =
ρI(17O)/ρI(δCO) = –4.00, –2.85 and –2.52, respectively. Approximately the
same values for the parameter a1 = ρI(17O)/ρI(δCO) = –3.62, –3.10 and –2.20
we found in the previous work14 for phenyl esters of ortho-, para- and
meta-substituted benzoic acids, X-C6H4CO2C6H5.
The positive values of the parameter a1 in Table IV prove that the
substituent-induced δ(17O) values, the ∆log k values of the alkaline hydroly-
sis and the IR carbonyl stretching frequencies, (νCO), grow with increase of
the inductive effects of substituent included.
CONCLUSIONS
The 17O NMR spectra for 44 ortho-, meta- and para-substituted phenyl esters
C6H5CO2C6H4-X, and alkyl esters, C6H5CO2R of benzoic acid containing
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

1760
Nummert, Mäemets, Piirsalu, Vahur, Koppel:
substituents in phenoxy OC6H4-X and alkoxy OR moieties were recorded.
The 17O NMR chemical shifts, δ(17O), of the carbonyl oxygen for para and
meta derivatives showed a good correlation with the σ° constants as the
dual substituent parameter treatment using the Taft inductive, σI, and reso-
nance, σ°R, substituent constants. This reveals the absence of through con-
jugation and coplanarity between the ester carboxyl group plane and the
aromatic plane in phenoxy part. The chemical shifts, δ(17O), for ortho deriv-
atives correlated well with the Charton equation containing the inductive,
B
σI, resonance, σ°R, and the steric (Es ) substituent constants in case the cor-
relation was carried out separately for esters with the electron-donating +R
substituents and –R substituents. The positive ρI and ρR values proved that
electron-attracting ortho, meta and para, as well as alkyl substituents caused
the deshielding effect (downfield shift) of the carbonyl and single-bonded
17O signal. The electron-donating +R and +I substituents resulted in shield-
ing effect (upfield shift). In ortho derivatives with +R substituents the car-
bonyl oxygen and single-bonded oxygen were found to be deshielded due
to the –I inductive and steric effects and shielded by the +R resonance
effect. The contribution of the ortho resonance term to the single-bonded
oxygen chemical shifts was ca. five times stronger than that to the carbonyl
oxygen. In the carbonyl oxygen chemical shifts, δ(17O), the susceptibility to
the inductive effect of ortho, meta and para substitutents from the phenyl
side was found to be ca. twice weaker than that from the benzoyl side. In
substituted alkyl benzoates, C6H5CO2R, for carbonyl oxygen and single-
bonded oxygen chemical shifts, δ(17O), the positive inductive (ρI > 0) and
B
negative steric effects (δAlkEs < 0) were detected. The carbonyl oxygen
chemical shifts, δ(17O), for ortho-substituted benzoates, C6H5CO2C6H4-X,
correlated well with the corresponding substituent-induced carbonyl car-
bon 13C NMR chemical shifts, ∆δCO, IR carbonyl stretching frequencies,
∆νCO, and the rates of the alkaline hydrolysis, ∆log k, in case the additional
resonance and steric terms were involved and the data treatment was carried
out separately for esters containing +R substituents and –R substituents.
SUPPLEMENTARY MATERIAL
The following supplementary material is available:
Synthesis of compounds. The preparation procedure and analyses of substi-
tuted phenyl benzoates and alkyl benzoate; the 1H and 13C NMR spectra for
2-(Methylsulfonyl)phenyl benzoate, 2-Acetylphenyl benzoate, 2-(Trifluoro-
methyl)phenyl benzoate, 2-Bromophenyl benzoate, 2,6-Dimethylphenyl
benzoate, 2,6-Dinitrophenyl benzoate, 3-Chlorophenyl benzoate, 3-Bromo-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1737–1763

Influence of ortho and Alkyl Substituents on Oxygen-17 NMR
1761
phenyl benzoate, 3-(Trifluoromethyl)phenyl benzoate, 3-(Dimethylamino)-
phenyl benzoate, 4-(Methylsulfonyl)phenyl benzoate, 4-(Trifluoromethyl)-
phenyl benzoate, 2,2,2-Trichloroethyl benzoate, 2-Bromoethyl benzoate.
Data processing. Equations for correlation of NMR chemical shifts, δ(17O),
with substituent constants (Eqs (1)–(5)) and for correlation of the carbonyl
oxygen 17O NMR chemical shifts, δ(17O), with the carbonyl carbon 13C NMR
chemical shifts, ∆δCO, the carbonyl IR stretching frequencies, ∆νCO and the
∆log k values of the alkaline hydrolysis using Eqs (6)–(9).
This work was supported by the grant No. 8162 of the Estonian Science Foundation.
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