EFFECTS OF SUBSTITUENTS AND MEDIUM ON ACIDITY OF α,α-BIS(BUTYLSULFONYL)TOLUENES

Article abstract of DOI:10.1135/cccc19940391

Seven substituted α,α-bis(butylsulfonyl)toluenes and three 2-aryl-1,1,3,3-tetraoxo-1,3-dithiolanes have been prepared and their 1H NMR, IR, and MS spectra measured.The pK_HA values of the substituted α,α-bis(butylsulfonyl)toluenes have been determined by potentiometric titration in dimethyl sulfoxide, dimethylformamide, and 80percent (v/v) mixture of dimethyl sulfoxide and water.The results are intterpreted by the Yukawa-Tsuno relationship (parameter r = 0.43 - 0.49) and classified in relation to analogous N- and O-acids by the methods with latent variables (the method of conjigated deviations).

Full text of DOI:10.1135/cccc19940391

Acidity of α,α-Bis(butylsulfonyl)toluenes
391
EFFECTS OF SUBSTITUENTS AND MEDIUM
ON ACIDITY OF α,α-BIS(BUTYLSULFONYL)TOLUENES
Miroslav LUDWIG, Jan PETRZILEK, Jiri KULHANEK and Oldrich PYTELA
Department of Organic Chemistry,
University of Chemical Technology, 532 10 Pardubice, The Czech Republic
Received January 27, 1993
Accepted June 26, 1993
Seven substituted α,α-bis(butylsulfonyl)toluenes and three 2-aryl-1,1,3,3-tetraoxo-1,3-dithiolanes
have been prepared and their 1H NMR, IR, and MS spectra measured. The pKHA values of the sub-
stituted α,α-bis(butylsulfonyl)toluenes have been determined by potentiometric titration in dimethyl
sulfoxide, dimethylformamide, and 80% (v/v) mixture of dimethyl sulfoxide and water. The results
are interpreted by the Yukawa–Tsuno relationship (parameter r = 0.43 – 0.49) and classified in rela-
tion to analogous N- and O-acids by the methods with latent variables (the method of conjugated
deviations).
From the point of view of substituent effects, benzyl cation represents an important
model of C-acids. Direct acidity measurements of substituted toluenes were carried out
in gas phase1,2 and in dimethyl sulfoxide3,4. Quantum-chemical calculations5 proved
that a study of substituent effects upon the dissociation of toluene in gas phase can
adopt aniline and phenol as models. As expected the dominant effect is the stabilization
of conjugated base, the sensitivity to substituents being increased in the order: aniline,
phenol, toluene. A more logic order of sensitivity was found in the correlations of
changes of equilibrium Gibbs energies in gas phase6 and dimethyl sulfoxide7 with the
substituent constants σF and σR where the sensitivity increases in the order: phenol,
aniline, toluene. A comparative study of substituent effects on the dissociation of
toluenes and anilines in dimethyl sulfoxide3 showed a roughly double sensitivity of
toluene (ρ ≈ 12) as compared with that of aniline (ρ ≈ 5.67). According to Taft and
Topsom7 the transition from gas phase to polar aprotic solvent (DMSO) is connected
with an additional substituent effect due to solvation, this effect being increased with
decreasing acidity of the weak acid. The behaviour of benzyl anion in protic solvents
has not been extrapolated.
The acidity of weak acids is increased by attaching strong electron acceptor groups
to the atom carrying the acidic proton, on the other hand the same effect decreases the
sensitivity, which can be documented e.g. by the toluene or aniline series8,9. A suitable
electron-acceptor substituent is the SO2R group, which possesses a distinct −M effect
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

392
Ludwig, Petrzilek, Kulhanek, Pytela:
(due to the large electronegativity difference between sulfur and oxygen) and, at the
same time, is relatively inert chemically. The evidence of acidifying properties of sul-
fonyl group is given in literature10 – 15. Some authors10,11,14 ascribe the acidity increase
not to the mesomeric effect but to a manifestation of d orbitals of sulfur, whereas
others16,17 take a skeptical view of this interpretation. The behaviour of benzyl anion
with attached sulfonyl groups was studied experimentally using substituted
α,α-bis(benzylsulfonyl)toluenes in the medium of 80% aqueous dimethyl sulfoxide18.
The substituent effects were interpreted by means of the σ−p constant, and the reaction
constant had the value ρ = 2.53. As compared with toluene in dimethyl sulfoxide (ρ ≈
12) the sensitivity decrease is considerable, being caused by both the presence of
benzylsulfonyl groups and the change of medium.
Acidity measurements of C-acids are complicated by the rate of splitting off of the
proton from carbon, hence it is possible to determine both the kinetic and the equili-
brium (thermodynamic) acidities19. The papers20 – 22 report the ionization of trisub-
stituted methanes HC(X)(SO2R)2 and HC(X)(CN)2 indicating the diffusion-controlled
deprotonation. If X = aryl, then the deprotonation rate is reduced, and this effect must
be taken into account in experimental measurements of thermodynamic acidities.
The aim of our work was to synthesize new C-acids and study the effects of medium
and substituents upon the acidity (pKHA) of these model substrates. For the model com-
pounds we chose the substituted α,α-bis(butylsulfonyl)toluenes (I) and 2-phenyl-
1,1,3,3-tetraoxo-1,3-dithiolanes (II). These compounds have not been studied yet
except for the syntheses of the parent α,α-bis(butylsulfonyl)toluene (Ia, refs23,24) and
dithiolanes24. The present paper continues our investigation of acidity of various series
of substituted weak N- and O-acids in various media9,25 – 28
.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Acidity of α,α-Bis(butylsulfonyl)toluenes
393
EXPERIMENTAL
The 1H NMR spectra of solutions of model compounds in deuteriochloroform with hexamethyldi-
siloxane as the internal reference were measured with a JEOL FX-100 or Bruker AM-400 apparatus.
The mass spectra were measured with a Kratos MS 25RSA apparatus. The sample was introduced by
the GC/MS technique or (in the case of sulfones) direct inlet. The IR spectra were measured with a
Perkin–Elmer PE 684 spectrophotometer using the KBr technique. The potentiometric acidity meas-
urements of model compounds in dimethylformamide, dimethyl sulfoxide, and 80% (v/v) aqueous
dimethyl sulfoxide were carried out on an automatic titrator Radiometer RTS-622 using the same
procedure, electrodes, and titrant as those in ref.25.
The substituted benzaldehydes were synthesized by known ways29,30. 3-Nitrobenzaldehyde was ob-
tained by the nitration of unsubstituted benzaldehyde, the other benzaldehydes were synthesized from
the corresponding benzyl bromides by the Sommelet reaction with urotropin (0.1 mol benzyl bro-
mide, 0.11 mol urotropin, 100 ml chloroform, reflux 1 h). 4-Chlorobenzaldehyde was a commercial
sample.
Synthesis of 2-Aryl-1,3-dithiolanes31
Dry hydrogen chloride was bubbled through an equimolar mixture of the respective substituted benz-
aldehyde and 1,2-ethanedithiol at ca 60 °C 2 h. Then water was distilled off under reduced pressure,
and the product was recrystallized from ethyl acetate. In this way obtained were the parent 2-phenyl-
1,3-dithiolane (yield 73%, m.p. 24 – 26 °C; ref.24 gives m.p. 29 °C), 2-(4-chlorophenyl)-1,3-dithio-
lane (yield 80%, m.p. 58 – 59 °C; ref.24 gives m.p. 62 °C), and 2-(4-nitrophenyl)-1,3-dithiolane
(yield 72%, m.p. 77 – 78 °C; ref.24 gives m.p. 79 °C).
Synthesis of 2-Aryl-1,1,3,3-tetraoxo-1,3-dithiolanes II
Method A: A solution of 2-aryl-1,3-dithiolane (0.1 mol) in acetic acid (500 ml) was heated to boil-
ing and 30% hydrogen peroxide (60 ml, 0.53 mol) was added thereto during 10 min. The reaction
mixture was evaporated and the dry residue was washed with saturated solution of sodium hydrogen-
carbonate.
Method B: A solution of 2-aryl-1,3-dithiolane (0.1 mol) in acetic acid (500 ml) was treated with
30% hydrogen peroxide (60 ml, 0.53 mol), and the reaction mixture was evaporated until dry.
Method C: The respective dithiolane (0.1 mol) was dissolved in ethanol (900 ml), and 2% aqueous
solution of sodium tungstate (100 ml) was added followed by 30% hydrogen peroxide (30 ml, 0.26 mol).
The mixture was heated to 80 °C, and another 30 ml peroxide was added. After filtration with char-
coal, concentration, and cooling, the crystalline products were collected by suction.
Method D: A saturated solution of potassium permanganate was added into a solution of dithiol-
ane (0.1 mol) in acetic acid (250 ml) with stirring until permanent violet colour. The reaction mixture
was filtered and the filtrate was evaporated until dry.
Method E: A solution of dithiolane (0.1 mol) in ethyl acetate (300 ml) was vigorously stirred, and
a solution of KMnO4 (41.2 g, 0.26 mol) in water (1 000 ml) and a solution of sulfuric acid (12.8 g,
0.13 mol) in water (200 ml) were added simultaneously during 2 h. The product was isolated as sub C.
The 2-aryl-1,1,3,3-tetraoxo-1,3-dithiolanes II were then washed with saturated solution of sodium
hydrogencarbonate and recrystallized from ethanol. The yields and products of the oxidation reac-
tions are given in Table I. The melting points of known substances do not agree with the literature
data, but the spectral analyses confirmed the structure of the compounds prepared. The IR spectra
contain strong signals due to SO2 group (1 350 and 1 150 cm−1). The mass spectra show a weak
molecular peak and fragments M − 155 (M − SO2C2H4 + H, 100%), M = 64 (SO2, ca 40%).
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

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Ludwig, Petrzilek, Kulhanek, Pytela:
Synthesis of Substituted α,α-Bis(butylthio)toluenes
The respective benzaldehyde (0.1 mol) and butanethiol (0.2 mol) were dissolved in benzene (300 ml),
and a little amount of 4-toluenesulfonic acid was added. In the course of reaction, water was
removed by azeotropic distillation 4 h. The mixture was filtered with charcoal, and the filtrate was
evaporated to give an oily residue. The structure and purity of the prepared compounds were checked
by elemental analyses and GC/MS method. The mass spectra of α,α-bis(butylthio)toluenes show the
following peaks: M+ (weak), M − 89 (M − SC4H9, 100%), M − 146 (M − SC4H9 − C4H9 − H, ca
40%).
Synthesis of Substituted α,α-Bis(butylsulfonyl)toluenes I
Method A: A solution of substituted α,α-bis(butylthio)toluene (0.1 mol) in 50% aqueous acetic
acid (500 ml) was cooled with intensive stirring and solid potassium permanganate was added until
permanent violet colour. After several days’ standing, the precipitated product was collected by suc-
tion, washed with water, extracted with ethanol in a Soxhlet apparatus, and 90% of solvent was evap-
orated from the extract.
Method B: The respective α,α-bis(butylthio)toluene (0.1 mol) was dissolved in ethanol (900 ml),
and 2% aqueous solution of sodium tungstate (100 ml) was added. To the suspension formed, 30%
hydrogen peroxide (30 ml, 0.26 mol) was slowly added, whereafter the mixture was heated to 80 °C
and another 30 ml peroxide was slowly added. After filtration with charcoal, concentration, and cool-
ing the crystalline products were collected by suction.
The obtained products were recrystallized from ethanol (compound Ig from a mixture of acetic
acid and heptane). The reaction conditions, yields, melting points, and results of elemental analyses
TABLE I
Reaction conditions, melting points and yields in syntheses of compounds II
Reaction
medium
Reaction
temp., °C
Yield
%
Compound
Method
M.p., °C
IIa
IIa
IIb
IIb
IIb
IIb
IIb
IIc
IIc
IIc
IIc
IIc
A
D
A
B
C
D
E
A
B
C
D
E
AcOH
AcOH–H2O
AcOH
116
212 – 213a
3
0
b
25 – 40
116
b
b
0
AcOH
25 – 40
80 – 85
25 – 40
25 – 40
116
0
EtOH–H2O
AcOH–H2O
benzene
178 – 180c
12
0
b
d
b
b
0
AcOH
0
AcOH
25 – 40
80 – 85
25 – 40
25 – 40
0
EtOH–H2O
AcOH–H2O
benzene
135 – 140
7
b
0
d
0
a Ref.32 gives m.p. 118 °C; b only the corresponding 4-substituted benzoic acid was isolated; c ref.32 gives
m.p. 218 °C; d the starting thiole was isolated.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Acidity of α,α-Bis(butylsulfonyl)toluenes
395
are given in Table II. The infrared spectra of all the products contained two strong bands correspond-
ing to sulfonyl group (1 360 – 1 320 cm−1, 1 150 – 1 130 cm−1), the bands of C−H bond vibration
of butyl group (2 960 – 2 860 cm−1), and signals of substituted aromatic ring. The mass spectra
showed the following fragments: M+ (10 – 47%), M − 121 (M − SO2C4H9, ca 50%), M − 241 (M − SO2C4H9
1
− SO2C4H9 + H, 70 – 100%), M − 241 + 16 (M − 241 + O, 20 – 60%). The H NMR spectra showed
four to five aromatic signals (δ ≈ 8.4 – 7.5) and signals of aliphatic protons of butylsulfonyl group
(δ ≈ 3.4 – 0.75). All the spectra contain a signal of acidic proton at δ = 5.3.
Treatment of Results
The results were treated with the help of our own mathematical-statistical programmes using a per-
sonal computer PC/AT.
RESULTS AND DISCUSSION
The syntheses of 2-aryl-1,1,3,3-tetraoxo-1,3-dithiolanes and α,α-bis(butylsulfonyl)-
toluenes were carried out in similar ways. From the standpoint of reaction products, the
synthesis and oxidation of α,α-bis(butylthio)toluenes were studied in more detail. The
analysis of mixture after the reaction of benzaldehydes with 1-butanethiol showed
beside the required α,α-bis(butylthio)toluenes also the starting materials, α-butylthio-
α-hydroxytoluenes, dibutyl disulfide, and α-butylthiotoluene. The oxidation of α,α-
bis(butylthio)toluenes with potassium permanganate in aqueous ethanol can provide the
non-overoxidized products. However, the products are not easily isolated, and therefore
we also adopted the oxidation with hydrogen peroxide catalyzed by sodium tungstate in
aqueous ethanol. This procedure gives comparable yields, and the isolation of products
is easier than that in the oxidation with potassium permanganate.
The average values of pKHA of the substituted α,α-bis(butylsulfonyl)toluenes I in
dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and 80% (v/v) aqueous di-
methyl sulfoxide (DMSO–W) are given in Table III.
For the interpretation of dissociation constants from Table III we adopted the
Yukawa–Tsuno equation and the correlations obtained were good except for dimethyl-
formamide (RDMF = 0.987, RDMSO = 0.993, RDMSO–W = 0.996), the residual standard
deviations (sDMF = 0.253, sDMSO = 0.156, sDMSO–W = 0.131) being comparable with the
experimental accuracy. The reaction constants found (ρDMF = 3.49, ρDMSO = 2.92,
ρDMSO–W = 3.15) correspond to the cognate α,α-bis(benzylsulfonyl)toluenes18 (ρ = 2.53).
The greater ρ value in dimethylformamide as compared with that in dimethyl sulfoxide
agrees with the results obtained for other weak acids (e.g. N-arylbenzenesulfonamides9
ρDMF = 3.69, ρDMSO = 3.28), the reason is probably in the lower basicity and polarity of
dimethylformamide33,34. The change of the reaction constant on going from dimethyl
sulfoxide to aqueous dimethyl sulfoxide is very small and, in addition – in contrast to
the expectation – opposite, although the difference is not statistically significant. Ac-
cording to the value of reaction constant ρDMSO the delocalization of the electron pair
is greater in the model studied than in the nitrogen analogue – N-phenyl-
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Ludwig, Petrzilek, Kulhanek, Pytela:
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Acidity of α,α-Bis(butylsulfonyl)toluenes
397
benzenesulfonamide9 (ρ = 3.28) or the phenol used for comparison5,35 (ρDMSO = 3.97).
The conclusions about electron delocalization are also confirmed by the value of the
parameter r in the Yukawa–Tsuno equation (rDMF = 0.48, rDMSO = 0.49, rDMSO–W = 0.43).
The values found for the parameter r indicate an about half extent of direct stabilization
of reaction centre as compared with phenols and anilines, being approximately identical
with those of N-arylbenzeneslfonamides9 (rDMF = 0.39, rDMSO = 0.42).
A summary of behaviour of substituted α,α-bis(butylsulfonyl)toluenes as compared
with substituted toluenes1,2,6,7, anilines (deprotonation8, protonation36), phenols35,37
,
and other suitable compounds such as N-arylbenzenesulfonamides9, diphenylamines
(deprotonation8, protonation38), and triazenes39 are given in Table IV. The data of
Table IV were obtained by treating the set described (substituents: H, 3-CH3, 3-NO2,
3-OCH3, 3-Cl, 4-CH3, 4-CF3, 4-CN, 4-NO2, 4-OCH3, 4-F, 4-Cl, 4-Br) by the method of
conjugated deviations40. The first latent variable interprets 98.9% of variability of set,
which indicates its high homogeneity and, hence, mutual similarity in behaviour during
substitution of the models compared. The results in Table IV show that the sensitivity
to substitution increases with decreasing electronegativity of the atom from which the
proton is split off. This expected relation remains maintained also when going from gas
phase into an aprotic solvent (dimethyl sulfoxide) and, probably, into a protic one, too.
The transition from gas phase into solvent is connected with a distinct lowering of
sensitivity to substituents, this lowering being greater with phenol than with aniline. If
these proportions are also valid for toluene, the value of reaction constant in dimethyl
sulfoxide could be expected about 13.5 and in water about 13, i.e. an obvious reduction
of the effect of medium upon the dissociation of C-acids. This conclusion is in accord-
ance with the small change in reaction constant on going from dimethyl sulfoxide to
TABLE III
Average values of pKHA and standard deviations s (from four measurements) of substituted
α,α-bis(butylsulfonyl)toluenes I in DMF, DMSO, and DMSO–W at 25 °C
pKHA (s)
Compound
DMF
DMSO
DMSO–W
13.10
Ia
Ib
Ic
Id
Ie
If
16.05
15.80
15.13
14.65
14.39
13.59
12.36
(0.05)
(0.03)
(0.09)
(0.05)
(0.04)
(0.04)
(0.06)
14.04
13.84
13.27
12.88
12.52
12.00
10.97
(0.12)
(0.03)
(0.05)
(0.05)
(0.02)
(0.02)
(0.05)
(0.03)
(0.03)
(0.06)
(0.05)
(0.09)
(0.03)
(0.00)
12.62
12.24
11.73
11.31
10.76
9.80
Ig
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

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Ludwig, Petrzilek, Kulhanek, Pytela:
80% aqueous dimethyl sulfoxide with the models studied by us. The presence of strong
acceptor groups (e.g. SO2) on the atom carrying the acidic hydrogen results in a distinct
reduction of sensitivity (to about one half with aniline, and obviously even more with
toluene, see Table IV), which indicates a significant delocalization of electron pair of
conjugated base into these groups. The substituents further connected to the electron-
acceptor group are probably less important (butyl is identical with benzyl) unless they
are able of participation in the conjugation (aryltoluenesulfonamides, difenylamines,
triazenes). The transmission coefficient of SO2 group is about 0.45 (ref.41), which
agrees with the results of Table IV. The sensitivity to substituents of dissociation of
conjugated acids of weak bases is markedly lower than that of the weak bases them-
selves (diphenylamine, Table IV). Hence the magnitude of reaction constant is con-
nected with the electron density and the extent of delocalization of electrons from the
chemical neighbourhood of the atom carrying the acidic proton. The connection with
electron density at this atom is indicated by the order obtained by the method of conju-
gated deviations from a set of chemical shifts of the atoms immediately attached to a
substituted aromatic nucleus (toluene42, styrene43,44, phenylacetylene45, aniline46, ani-
sol47, selenoanisol48, fluorobenzene49) and logarithm of dissociation constants of se-
lected models (aniline3, phenol33, N-arylbenzenesulfonamide9, α,α-bis(butylsulfonyl)-
toluene in dimethyl sulfoxide). The relative order given by the magnitude of regression
coefficient value in the first latent variable is as follows: δ 13C CH3 (0.000) < δ 13C
C≡ (0.244) < log K (phenol) (0.705) < log K (N-arylbenzenesulfonamide) (0.711) <
TABLE IV
Regression coefficientsa (R.c.) of dependence of log Ka (or ∆G) of model compounds in gas phase
(g) and in solutions upon the first latent variable obtained by CDA method
Medium
C-acids
R.c.
N-acids
R.c.
O-acids
R.c.
(g)
CH3C6H4X
17.02
NH2C6H4X
NH2C6H4X
13.67
5.54
HOC6H4X 12.61
HOC6H4X 3.02
HOC6H4X 2.23
DMSO
W
DMSO
(C4H9SO2)2CHC6H4X
1.84
1.94
C6H5SO2NHC6H4X 2.18
DMSO–W (C4H9SO2)2CHC6H4X
80% (v/v) (C6H5CH2SO2)2CHC6H4X 1.95
W
NH+3–C6H4X
2.27
3.77
2.04
W
C6H5NHC6H4X
C6H5NH+2–C6H4X
W
DMSO–
EtOH 20%
C6H5N=NNHC6H4X 1.19
a Recalculated with respect to the value ρ (phenol, water, 25 °C) = 2.23.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Acidity of α,α-Bis(butylsulfonyl)toluenes
399
log K (α,α-bis(butylsulfonyl)toluene) (0.726) < δ 77Se (0.780) < δ 17OCH3 (0.791) <
log K (aniline) (0.797) < δ 19F (0.916) < δ 13C C= (0.971) < δ 15N (1.000). As it is
obvious from the given order the greatest changes in the character of interaction of
substituents and the atom investigated are at a carbon atom where the splitting off of
the proton is accompanied by a considerable extent of rehybridization; the role of the
electronegativity connected with the hybridization cannot be neglected either. The
character of the interaction between substituent and reaction centre in the models stu-
died is comparable with that in anisole or phenol.
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Translated by J. Panchartek.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)