Acid-base properties of substituted naphthoic acids in nonaqueous media

Article abstract of DOI:10.1135/cccc19971737

Thirteen substituted 1-naphthoic acids have been prepared and their dissociation constants, along with those of twenty-five substituted 2-naphthoic acids, have been measured potentiometrically in methanol, N,N-dimethylformamide, pyridine, and acetonitrile. The pK_HA values obtained have been treated by linear regression using four sets of substituent constants. The experimental data have also been interpreted by statistical methods using latent variables. The first latent variable calculated by these methods can be used as a new set of substituent constants for describing substituent effects in naphthalene skeleton.

Full text of DOI:10.1135/cccc19971737

Substituted Naphthoic Acids
1737
ACID-BASE PROPERTIES OF SUBSTITUTED NAPHTHOIC ACIDS
IN NONAQUEOUS MEDIA
1
2
Patrik PARIK and Miroslav LUDWIG
Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice,
532 10 Pardubice, Czech Republic; e-mail: 1 parik@hlb.upce.cz, 2 ludwig@hlb.upce.cz
Received May 14, 1997
Accepted July 21, 1997
Thirteen substituted 1-naphthoic acids have been prepared and their dissociation constants, along with
those of twenty-five substituted 2-naphthoic acids, have been measured potentiometrically in methanol,
N,N-dimethylformamide, pyridine, and acetonitrile. The pKHA values obtained have been treated by
linear regression using four sets of substituent constants. The experimental data have also been inter-
preted by statistical methods using latent variables. The first latent variable calculated by these
methods can be used as a new set of substituent constants for describing substituent effects in naphthalene
skeleton.
Key words: Naphthoic acids; Dissociation; Substituent effects; Solvent effect; Chemometrics.
Hammett1 adopted the dissociation of substituted benzoic acids in water for a model
process in study of substituent effects. A replacement of benzene ring by another
hydrocarbon skeleton, e.g. naphthalene, results in a different basic model. As compared
with benzene ring, naphthalene skeleton provides room for studies of a more extensive
set of substituent effects. The present communication is focused on the dissociation of
monosubstituted naphthoic acids.
Many papers have been published dealing with the dissociation of both unsubstituted
and substituted naphthoic acids in water or in mixed aqueous–organic solvents. The
reports on the acid-base properties of major sets of substituted naphthoic acids involve
the potentiometric determination of dissociation constants of five substituted 1-naph-
thoic acids in 78% aqueous ethanol2, thirteen nitronaphthoic acids in 50% aqueous
2-butoxyethanol3, and the dissociation constants of 1- and 3-halo-2-naphthoic acids in
71% aqueous ethanol determined by conductometry4. Spectrophotometry was adopted
to study the dissociation of a series of four methylnaphthoic acids in 20% aqueous
dioxane5, conductometry was applied to a series of five methylnaphthoic acids in 50%
aqueous ethanol6, and again spectrophotometry was used to study a series of 6-sub-
stituted-2-naphthoic acids in 50% aqueous ethanol7. (Methylsulfonyl)naphthoic acids
were investigated potentiometrically in 50% aqueous ethanol8, 50% aqueous 2-butoxy-
ethanol9, and 80% aqueous 2-methoxyethanol9. The dissociation constants of five
chloro-1-naphthoic acids were determined in 50% aqueous ethanol10. Aminonaphthoic
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Parik, Ludwig:
acids undergo zwitterionic equilibrium reactions in aqueous methanolic media. These
reactions were studied potentiometrically and spectrophotometrically11. Considerable
attention was given to 8-substituted-1-naphthoic acids in 50% aqueous ethanol12, 80%
aqueous 2-methoxyethanol13,14, and in 80% aqueous 2-butoxyethanol15. The most ex-
tensive and comprehensive works concern the dissociation of thirty four substituted
1-naphthoic acids in 50% aqueous ethanol16 and of six substituted 1-naphthoic acids
and forty five substituted 2-naphthoic acids in the same medium17. Dewar and Grisdale16
suggested an expression for a quantitative description of substituent effects in naph-
thalene and pointed out possible different behaviour in various solvents. They also
stressed the necessity of more experimental work on the systems different from benzene.
Wells and Adcock17 described further factors affecting the behaviour of the system:
substituent–naphthalene skeleton–reaction centre. An overall analysis of substituent ef-
fects in naphthalene based on treatment of experimental data obtained by measurements
in aqueous and aqueous–organic media was presented by Wells, Ehrenson and Taft18.
The dissociation of naphthoic acids in organic solvents is dealt with in a polaro-
graphic study of 1- and 2-naphthoic acids in methanol and N,N-dimethylformamide19,
in a spectrophotometric study of 1-naphthoic acid in N,N-dimethylformamide20, and in
a spectral study of 1- and 2-naphthoic acids in ethanol21.
The above survey (which is not complete and neglects the dissociation of naphthoic
acids in water) indicates the absence of studies dealing with the acid-base behaviour of
substituted naphthoic acids in nonaqueous media.
The main aim of the present communication is to create a set of experimental data
and, by their mathematical-statistical treatment, to contribute to the quantification of
substituent effects in naphthalene skeleton in organic solvents and to discuss the effects
observed.
EXPERIMENTAL
The model substances, except for 4-cyano-1-naphthoic acid and 4-chloro-1-naphthoic acid, were pre-
pared by reported procedures. Their structure was verified by 1H NMR spectra (a Bruker AMX 360
spectrometer). The dissociation constants, pKHA, at 25 °C in methanol (MeOH), acetonitrile (AN),
N,N-dimethylformamide (DMF), and pyridine (Py) were determined by potentiometric titration using
an automatic titrator TITRALAB3 (Radiometer) and the same experimental and electrode arrange-
ment as in our previous communications22–24. The solvents used were purified by standard proce-
dures.
Synthesis of 4-Cyano-1-naphthoic Acid
4-Bromo-1-naphthoic acid25 was esterified with dimethyl sulfate in basic medium, using the method
of Short and Wang26. The methyl 4-bromo-1-naphthoate obtained was transformed into
methyl 4-cyano-1-naphthoate by a modified procedure of Dewar and Grisdale27: a mixture of methyl
4-bromo-1-naphthoate (2.6 g), cuprous cyanide (0.9 g), N,N-dimethylformamide (20 ml), and py-
ridine (3 drops) was heated at 180 °C for 5.5 h. The hot solution was poured in an ammonia solution
(52 ml concentrated ammonia and 52 g ice), and the resulting suspension was extracted with chloro-
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Substituted Naphthoic Acids
1739
form (2 × 70 ml); the organic layer was washed with dilute hydrochloric acid and water and dried
(Na2SO4). The solvent was evaporated to leave 2.4 g of crude product which was recrystallized from
50% aqueous ethanol with charcoal to give 1 g of methyl 4-cyano-1-naphthoate, m.p. 105–107 °C.
A mixture of methyl 4-cyano-1-naphthoate (1 g), acetic acid (20 ml), hydrochloric acid (15 ml),
and water (20 ml) was refluxed for 4.5 h. After cooling, the separated crystals were collected by
filtration and washed with water. The yield was 0.6 g of the crude product; recrystallization from
ethanol with charcoal afforded 0.3 g of 4-cyano-1-naphthoic acid, m.p. 237–240 °C (ref.27 gives m.p.
236–237 °C).
Synthesis of 4-Chloro-1-naphthoic Acid
A mixture of 4-acetylamino-1-naphthonitrile28 (5 g), acetic acid (85 ml), and hydrochloric acid (85 ml)
was refluxed for 3 h. After cooling, the separated solid was collected by filtration and washed with
water, yield 2.8 g of 4-amino-1-naphthonitrile.
A suspension of 4-amino-1-naphthonitrile (2.8 g) in hydrochloric acid (10 ml) and water (85 ml)
was diazotized by adding a solution of sodium nitrite (2.5 g) in water (20 ml) at 0 °C. The solution
formed was poured into a stirred solution of cuprous chloride (6.7 g) in hydrochloric acid (60 ml) at
0 °C with stirring. Then the mixture was heated to 90 °C and cooled to give a yellow solid which
was collected by filtration and washed with water. The yield was 2.7 g of the crude 4-chloro-1-naph-
thonitrile.
A mixture of the crude 4-chloro-1-naphthonitrile (2.7 g), acetic acid (30 ml), sulfuric acid (30 ml),
and water (30 ml) was refluxed for 13 h. After cooling, the solution was poured onto crushed ice (ca
200 g). The separated solid was collected by filtration, washed with water, and redissolved in a sol-
ution of sodium carbonate (10 g) in water (100 ml). The alkaline solution was extracted with toluene
(2 × 50 ml) and with ether (2 × 50 ml), and then it was acidified with hydrochloric acid. The precipi-
tate was filtered off, washed with water, and recrystallized from an ethanol–water mixture (2 : 1) with
charcoal to give 1.4 g of 4-chloro-1-naphthoic acid, m.p. 222–225 °C (ref.27 gives m.p. 220–221 °C).
RESULTS AND DISCUSSION
Tables I and II summarize the average dissociation constants pKHA (along with the
corresponding standard deviations) for individual substrates and respective solvents.
Comparison of the results obtained in different media is difficult since it is impossible
to define an absolute pH scale. For easier orientation at least, we give the dissociation
constants of benzoic acid which was used as the standard. The solvents chosen cover
the basic types usually adopted in potentiometric studies (the values in brackets are the
dissociation constants of benzoic acid): amphiprotic methanol (9.41; ref.29), dipolar
aprotic protophobic acetonitrile (20.70; ref.30), dipolar aprotic protophilic N,N-di-
methylformamide (12.27; ref.30), and pyridine (9.80; ref.31). The substituents chosen
involved those with predominant inductive effects (CH3, Cl, Br), those with distinct
mesomeric effects (NO2, CN, OCH3, NH2), and, of course, the standard substituent –
hydrogen.
The experimental data obtained were analyzed using mathematical-statistical
methods. All the pKHA values (each measurement was repeated three times) were sub-
jected to both simple and multiple linear regressions. On the other hand, the methods
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Parik, Ludwig:
working with latent variables, in particular the principal component analysis32 (PCA)
and the conjugated deviations analysis33 (CDA) treated the pKHA values. The calculations
using simple linear regression and the classic Hammett relation (pKHA = pK 0HA – ρσ)
were carried out for all the four media studied with four sets of substituent constants: σ
(the constants proposed by Exner34), σP6 and σF6 (the constants designed for non-
aqueous media35), and ∆calc (the constants designed for description of substituent effects
in naphthalene skeleton17). One can presume that for the alternating and nonalternating
positions it will be more precise to adopt the σp and σm constants, respectively. Table III
summarizes the results of calculations by simple linear regression using the σ values by
Exner and ∆calc values. From the regression characteristics it follows that the presump-
tion mentioned is not always satisfactorily fulfilled (e.g. for the position 7β in all the
four solvents). In the case of 2-naphthoic acids it seems more appropriate to use the σm
values for the substitution at 4β and 5β positions and σp values for 6β, 7β, and 8β
positions. All the four sets of substituent constants give comparable results. Hence,
only the application of σ (by Exner) and ∆calc will be discussed below.
TABLE I
The average dissociation constants (pKHA) and their standard deviations (s) of substituted 1-naphthoic
acids in various solvents
___
pKHA (s)
X
MeOH
DMF
Py
AN
H
8.98 (0.02)
8.05 (0.04)
9.58 (0.01)
8.66 (0.04)
8.68 (0.02)
7.86 (0.03)
9.13 (0.01)
8.62 (0.02)
8.58 (0.03)
8.30 (0.01)
8.32 (0.04)
8.38 (0.02)
8.55 (0.05)
12.00 (0.02)
10.35 (0.08)
12.58 (0.05)
11.31 (0.03)
11.28 (0.04)
10.46 (0.02)
12.60 (0.02)
11.31 (0.03)
11.19 (0.07)
10.96 (0.02)
10.86 (0.07)
11.93 (0.06)
11.47 (0.03)
9.49 (0.06)
8.10 (0.03)
10.15 (0.05)
8.89 (0.02)
8.88 (0.03)
8.07 (0.04)
9.97 (0.03)
8.98 (0.04)
8.96 (0.08)
8.90 (0.01)
8.55 (0.06)
9.40 (0.04)
9.23 (0.03)
20.13 (0.01)
18.74 (0.06)
20.97 (0.06)
19.95 (0.06)
19.91 (0.00)
19.14 (0.02)
20.36 (0.08)
19.73 (0.07)
19.80 (0.12)
19.32 (0.02)
19.24 (0.08)
19.70 (0.06)
19.58 (0.07)
3-NO2
4-OCH3
4-Cl
4-Br
4-CN
5-NH2
5-Cl
5-Br
5-CN
5-NO2
8-Cl
8-NO2
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Substituted Naphthoic Acids
TABLE II
1741
The average dissociation constants (pKHA) and their standard deviations (s) of substituted 2-naphthoic
acids in various solvents
___
pKHA (s)
X
MeOH
DMF
Py
AN
H
9.36 (0.03)
9.39 (0.03)
8.87 (0.04)
8.85 (0.03)
9.03 (0.02)
8.82 (0.03)
9.54 (0.00)
9.44 (0.02)
9.13 (0.02)
9.13 (0.04)
8.77 (0.05)
8.73 (0.01)
9.59 (0.05)
9.39 (0.03)
9.41 (0.02)
9.10 (0.01)
9.10 (0.01)
8.88 (0.04)
8.79 (0.02)
9.49 (0.02)
9.32 (0.05)
9.11 (0.02)
9.07 (0.03)
8.86 (0.02)
8.85 (0.02)
12.11 (0.05)
12.26 (0.05)
11.24 (0.02)
11.20 (0.04)
11.66 (0.06)
11.40 (0.03)
12.47 (0.01)
12.31 (0.03)
11.99 (0.05)
11.87 (0.00)
11.49 (0.04)
11.37 (0.04)
12.79 (0.04)
12.28 (0.07)
12.27 (0.02)
11.89 (0.05)
11.85 (0.05)
11.52 (0.03)
11.46 (0.05)
12.41 (0.03)
12.15 (0.06)
11.72 (0.02)
11.76 (0.05)
11.39 (0.07)
11.33 (0.05)
9.74 (0.04)
9.89 (0.04)
8.98 (0.07)
8.98 (0.07)
9.29 (0.04)
8.98 (0.06)
9.95 (0.04)
9.87 (0.04)
9.39 (0.05)
9.39 (0.05)
8.99 (0.07)
8.86 (0.07)
10.32 (0.02)
9.78 (0.03)
9.74 (0.02)
9.43 (0.07)
9.33 (0.05)
9.06 (0.07)
8.97 (0.06)
9.70 (0.05)
9.63 (0.06)
9.33 (0.04)
9.23 (0.04)
8.98 (0.06)
8.99 (0.02)
20.30 (0.05)
20.52 (0.02)
19.73 (0.05)
19.76 (0.03)
20.49 (0.06)
19.94 (0.10)
20.77 (0.01)
20.47 (0.04)
20.73 (0.03)
20.48 (0.01)
19.79 (0.05)
19.91 (0.06)
20.60 (0.06)
20.46 (0.10)
20.50 (0.07)
20.18 (0.04)
20.02 (0.12)
19.76 (0.04)
19.45 (0.14)
20.47 (0.09)
20.46 (0.02)
20.32 (0.10)
20.32 (0.06)
20.05 (0.15)
20.01 (0.06)
4-CH3
4-Cl
4-Br
5-Br
5-NO2
6-OCH3
6-CH3
6-Cl
6-Br
6-CN
6-NO2
7-NH2
7-OCH3
7-CH3
7-Cl
7-Br
7-CN
7-NO2
8-OCH3
8-CH3
8-Cl
8-Br
8-CN
8-NO2
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1742
Parik, Ludwig:
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

Substituted Naphthoic Acids
1743
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

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Parik, Ludwig:
For the 2-naphthoic acids we also carried out the calculations by fivefold linear re-
gression of pKHA vs σ (σm for 4β, 5β, and σp for 6β, 7β, 8β) and pK vs ∆calc, the results
of which are summarized in Table V. There are no substantial differences between the
applications of the two sets of substituent constants. Due to the nonexistence of univer-
sal pH scale, comparisons of pK 0 values in the individual media are difficult. However,
the order of pK 0 values agrees with that of pKHA values of benzoic acids in all the
media studied, increasing in the series: MeOH, Py, DMF, AN.
The PCA and CDA calculations were carried out for the series of substituted 2-naphthoic
acids. The average values of dissociation constants were arranged in the form of a
matrix with 25 lines (substituents) and 4 columns (solvents). The matrix was com-
pletely filled. Both the methods lead to the same results. The first latent variable, t1,
(see Table IV) describes 92.6% variability of the data matrix and represents a new set
of substituent constants describing the substituent effects in naphthalene skeleton. This
fact is confirmed by the results of fivefold linear regression of pKHA vs t1 summarized
in Table V. As the score vector t1 is standardized (the values within the interval from
0 to 1), Table V does not give the pK 0 and ρ values. The correlations are closer than
those using the sets of σ and ∆calc constants. The second latent variable, t2, stands at the
limit of statistical significance and describes 6.1% variability of the data matrix. Its
existence can be ascribed to the lower accuracy of the measurements in acetonitrile
and, hence, greater variability of data describing the dissociation in this solvent.
In conclusion it should be stated that the experimental data need extension with re-
spect to the number of both model compounds (further substrates) and media (further
organic solvents).
TABLE IV
The score vector t1 from PCA calculation for substituted 2-naphthoic acids
X
t1
X
t1
X
t1
H
0.624
0.737
0.042
0.035
0.387
0.100
0.884
0.810
0.548
6-Br
0.475
0.073
0.036
1.000
0.705
0.710
0.417
0.356
0.120
7-NO2
8-OCH3
8-CH3
8-Cl
0.000
0.745
0.629
0.396
0.371
0.132
0.112
4-CH3
4-Cl
6-CN
6-NO2
7-NH2
7-OCH3
7-CH3
7-Cl
4-Br
5-Br
8-Br
5-NO2
6-OCH3
6-CH3
6-Cl
8-CN
8-NO2
7-Br
7-CN
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

Substituted Naphthoic Acids
TABLE V
1745
Fivefold linear regressions of pKHA vs σ (σm for 4β, 5β, and σp for 6β, 7β, 8β), pKHA vs ∆calc, and
pKHA vs t1 for substituted 2-naphthoic acids in various solvents
Subst.
const.
Solvent
MeOH
pK0 (s)
ρ4β (s)
ρ5β (s)
ρ6β (s)
ρ7β (s)
ρ8β (s)
s (R)
σ
9.28
1.15
0.67
0.72
0.59
0.59
0.049
(0.01)
(0.05)
(0.04)
(0.03)
(0.02)
(0.03)
(0.984)
∆
9.36
0.83
0.92
1.05
0.98
0.68
0.054
calc
(0.01)
(0.04)
(0.05)
(0.04)
(0.04)
(0.03)
(0.980)
t1
–
–
–
–
–
–
0.043
(0.988)
DMF
σ
12.11
(0.01)
2.41
(0.10)
1.05
(0.06)
0.92
(0.04)
0.90
(0.38)
1.03
(0.04)
0.084
(0.982)
∆
12.20
(0.02)
1.62
(0.07)
1.37
(0.08)
1.31
(0.07)
1.30
(0.07)
1.13
(0.05)
0.092
(0.975)
calc
t1
–
–
–
–
–
–
0.074
(0.987)
Py
σ
9.63
1.78
0.91
0.96
0.89
0.85
0.107
(0.01)
(0.12)
(0.08)
(0.06)
(0.05)
(0.06)
(0.965)
∆
9.71
1.23
1.21
1.38
1.26
0.98
0.087
calc
(0.01)
(0.06)
(0.08)
(0.06)
(0.07)
(0.05)
(0.972)
t1
–
–
–
–
–
–
0.088
(0.977)
AN
σ
20.40
(0.03)
1.79
(0.22)
0.46
(0.14)
0.68
(0.10)
0.89
(0.09)
0.46
(0.10)
0.189
(0.871)
∆
20.53
(0.03)
1.31
(0.12)
0.80
(0.15)
1.01
(0.12)
1.71
(0.13)
0.64
(0.10)
0.169
(0.899)
calc
t1
–
–
–
–
–
–
0.154
(0.916)
The authors are indebted to Prof. W. Adcock (The Flinders University of South Australia, Adelaide,
Australia) who kindly supplied the series of substituted 2-naphthoic acids.
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