Kinetic studies on the reactivity of triphenylmethyl cations adsorbed on silica, alumina, and aluminosilicate

Article abstract of DOI:10.1021/jp0210780

The apparent rate constants ka?2 of different surface-mediated reactions of three nucleophiles 1,4-cyclohexadiene, triethylsilane, and isobutylvinyl ether with triphenylmethylium ions have been determined for three different solid acid catalysts and various triphenylmethylium precursors (R¹R²R³C-X, X = SCN, OH, Cl, or Br). Generation of triphenylmethylium ions [R¹R²R³C⁺] was used for kinetic measurements when R¹R²R³C-X derivates are chemisorbed to the solid acid catalysts. The catalysis of those surface-mediated reactions by a silica, an alumina, and an aluminosilicate has been studied in a slurry of dichloromethane at ambient temperature. The value of ka?2 increases in the order OH^- < SCN^- < Cl^- < Br^- and silica < alumina < aluminosilicate. The log ka?2 can be correlated with the pK_s of the corresponding HX from R¹R²R³C-X or the acidity of the solid acid (Adolph, S.; Spange, S.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6429-6438) indicating the importance of the effective surface concentration of [R¹R²R³C⁺] on the apparent rate constant. It is also shown that the specific rate constant of the surface-mediated reaction can be interpreted in terms of Mayr's nucleophilicity parameters of the ??-nucleophile (N) and the pK_R⁺ values of the carbenium ion derived from polar reactions in homogeneous solution.

Full text of DOI:10.1021/jp0210780

298
J. Phys. Chem. B 2003, 107, 298-305
Kinetic Studies on the Reactivity of Triphenylmethyl Cations Adsorbed on Silica, Alumina,
and Aluminosilicate
Stefan Spange,* Simone Adolph, Ralph Walther, and Yvonne Zimmermann
Department of Polymer Chemistry, Institute of Chemistry, UniVersity of Technology Chemnitz,
Strasse der Nationen 62, 09107 Chemnitz, Germany
ReceiVed: April 29, 2002
The apparent rate constants k′ of different surface-mediated reactions of three nucleophiles 1,4-cyclohexadiene,
triethylsilane, and isobutylvinyl ether with triphenylmethylium ions have been determined for three different
1
2
3
solid acid catalysts and various triphenylmethylium precursors (R R R C-X, X ) SCN, OH, Cl, or Br).
1
2
3
+
1
2
3
Generation of triphenylmethylium ions [R R R C ] was used for kinetic measurements when R R R C-X
derivates are chemisorbed to the solid acid catalysts. The catalysis of those surface-mediated reactions by a
silica, an alumina, and an aluminosilicate has been studied in a slurry of dichloromethane at ambient
-
-
-
-
temperature. The value of k′ increases in the order OH < SCN < Cl < Br and silica < alumina <
aluminosilicate. The log k′ can be correlated with the pK
acidity of the solid acid (Adolph, S.; Spange, S.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6429-6438)
indicating the importance of the effective surface concentration of [R R R C ] on the apparent rate constant.
It is also shown that the specific rate constant of the surface-mediated reaction can be interpreted in terms of
Mayr’s nucleophilicity parameters of the π-nucleophile (N) and the pK +
from polar reactions in homogeneous solution.
1
2
3
s
of the corresponding HX from R R R C-X or the
1
2
3
+
R
values of the carbenium ion derived
Introduction
questions remain whether a halide-carbenium ion pair adsorbed
on silanol or a silanolate-carbenium ion pair is present. (C6H5)3-
CCl chemisorbed on solid acids is likely present in an
equilibrium state similar to its heterolytic dissociation, which
is caused by weak Lewis acids (ZnCl2, SbCl3) or acceptor
The quantification of the catalytic activity of solid acid
catalysts and heterogeneously induced organic reactions is an
important field of materials research and catalysis.1-15 This area
of research has been stimulated by the application of improved
adsorption-calorimetric and solid-state NMR spectroscopic
3
1,33,34
solvents.
(Chart 1). Because the heterolytic dissociation
of the carbenium precursor R-X is incomplete (eq 2), even in
methods and techniques to well-established solid catalyst
materials in examining their surface properties.4
-10
Related to
K
+
-
R-X {
\
} [R ] + X
(2)
this topic, the mechanism of heterolytic dissociation of arylm-
ethyl compounds for producing carbenium ions on solid acids
was studied by several authors during the last 3 decades.
homogeneous solution often observed, then only the relative
1
6-23
reactivity, an apparent rate constant kapp, of the reaction can be
In 1939, Weitz first reported that chlorotriphenylmethane
becomes yellow-colored when adsorbed on silica from a benzene
solution.1 Leftin studied the chemisorption of various tri-
3
0
determined. The value of kapp includes the product of k2K
K from eq 2). If the equilibrium constant K of eq 2 is known,
(
6
then k2 can be calculated.
1
7
phenylmethane derivatives on aluminosilicates. Other authors
In the previous paper, we studied the kinetics of the surface-
mediated hydride transfer reaction of 1,4-cyclohexadiene (CHD)
with triphenylmethylium to be catalyzed by 30 different solid
1
studied the infrared, UV/vis, and H MAS NMR spectra of these
1
8-23
adsorbates.
Recently, “ship in the bottle” synthesis of
substituted triarylmethylium ions in HY-zeolites were reported
by several authors.
2
3
acid catalysts. We showed that the relative rate constant
2
4-28
Also, photochemically generated car-
+
(
k′ ) kapp), the product of k2K ) k2f[R ], depends significantly
bocations in zeolites have been used for studying their reactivity
on the acidity of the solid acid catalyst. The parameter f is a
with various nucleophiles.2
8,29
theoretical correction factor for transforming the volume
However, the absolute reactivity of a carbenium intermediate
generated on the surface of a catalyst is difficult to determine
because the rate of the reaction (RG) observed depends on three
independent parameters: the second-order rate constant (k2),
-
1
concentration (mol L ) into the surface concentration (mol
-
2
m ). As acidity parameters for the solid acids, we did make
use of Kamlet-Taft’s R and Gutmann’s acceptor number, AN.35
We suggested that the acidity of the solid acids is preferably
+
the concentration of the carbenium intermediate [R ], and the
responsible for generating an effective concentration of
concentration of the nucleophilic educt [N] for the reaction under
+
[
R ].
study (eq 1).3
0,31
Despite the well-documented chemisorption
Extensive and detailed kinetic studies by Mayr and others
showed that the rate constant of a polar reaction of an
electrophile (carbocation) (SN1-type) with several structurally
+
+
-
d[R ]/dt ) -d[N]/dt ) RG ) -k [R ][N]
(1)
2
different nucleophiles can be described by linear free energy
process of halogeno arylmethanes to silica,16,20-22,32 some
(LFE) relationships.
31,36-41
On the basis of many kinetic results
1
0.1021/jp0210780 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/06/2002

Reactivity of Triphenylmethyl Cations
J. Phys. Chem. B, Vol. 107, No. 1, 2003 299
CHART 1: Altenative Chemisorption Mechanisms of Chlorotriphenylmethane on Silica Or Alumina Surface Sites
+
43
CHART 2: Hydride Transfer Reaction of
,4-Cyclohexadiene with Surface-Coordinated
Triphenylmethylium
(pKRH ) -24). Therefore, the proton-transfer reaction occurs
faster than the first step, the hydride ion transfer reaction.
In ref 22, we have reported that chlorodiarylmethanes react
with CHD on silica yielding diarylmethane and benzene. The
HCl formed does not significantly accelerate the reaction rate.
1
22
In preliminary studies, we used diphenylcarbenium ions, which
undergo various reactions with olefins on silica. Unfortunately,
competing reactions disturb clean kinetic investigations. There-
fore, triphenylmethyl cations have been chosen for a deeper
kinetic study. In the discussion part, we will also make use of
four kinetic values taken from our previous paper.
and studies, the following LFE relationship for k2 (from eq 1)
has been established:
ln k ) s(E + N)
(3)
We expect to answer the following question: Is it possible
to find a reasonable correlation between kinetic data of Mayr
2
3
1,36
from LFE relationships in solution
and kinetic data (kapp)
where E is the electrophilicity parameter of an electrophile and
N is the nucleophilicity parameter of a nucleophile;
coefficient that approaches 1. The E parameters have been
reported for several compounds and intermediates, carbocations,
3
1,36
from a polar model reaction on the solid acid catalysts? For
these studies, the suitability of other π-nucleophiles for surface-
mediated polar reactions with carbenium ions will be investi-
gated from the kinetic point of view. The compounds chosen
for these checks are shown in Chart 3.
s is a
3
1,36
diazonium ions, aldehyde-Lewis acid adducts and others.
The N parameters are known for olefins, aromatics, amines,
silanes, and silyl enol ethers.30,31,36-38,40,41
For this work, we used those solid acid samples as investi-
In this work, we will
2
3
gated in the previous paper. The acidity of the solids ranges
investigate the influence of the counterion X of the triphenyl-
in terms of the Hammett acidity scale between Ho ) 4.4 (Aerosil
methylium precursor [(C6H5)3C-X; X ) Cl, OH, Br, NCS] and
®
+
300 ) and H ) -9.3 and -10.5 (aluminosilicates) and of the
of substituents of the triphenylmethylium [(4-RC6H4)3C , R )
o
Kamlet-Taft HBD scale between R ) 1.06-1.08 (silicas) and
R ) 1.7 (aluminosilicate).
-
OCH3, -Cl, -CH3] on the apparent rate constant of the model
3
5
reaction.
The surface-mediated hydride transfer reaction of 1,4-
cyclohexadiene (CHD), as the reagent, with (C6H5)3C has been
+
Experimental Section
used as model reaction. The hydride transfer reaction of CHD
Materials. The chemical composition and physical properties
of the solid acid catalysts used are summarized in Table 1. They
were commercially available products.
Triphenylmethanol (Merck) was recrystallized from benzene
before use.
+
-
with (C6H5)3C BF4 in dichloromethane solution, as well as
+
that of CHD with chemisorbed (C6H5)3C on solid acids yield
quantitatively (C6H5)3CH and benzene (Chart 2).2
1, 37, 42
The benzenonium intermediate has a strong BrØnsted acidity

300 J. Phys. Chem. B, Vol. 107, No. 1, 2003
Spange et al.
TABLE 1: Solid Acids Used in This Paper, Their Chemical Compositions, Physical Properties, Surface Polarity Parameters,
and Source
solid acid
sample
BET surface
specific pore
avg pore
diam [nm]
2
-1
3
-1
area [m g
]
vol [cm g
]
R
π
source
SiO
SiO
2
, Aerosil 300
KG SG 432
240
315
131
296
1.14
1.14
1.53
1.69
1.05
0.98
0.33
0.36
Degussa
Grace
Grace
2
1.42
0.72
0.46
nd
Al
2
O
3
, SP 18-8510
aluminosilicate, Siral 5
3.1
Condea
a
nd ) not determined.
CHART 3: Nucleophiles That Are Checked for Kinetic
Studies with Carbenium Ions Adsorbed on Solid Acid
Catalysts
elemental analyses. To remove the organic fraction, the silica
cook was treated with aqueous NaOH and stirred for 12 h. After
neutralization with aqueous HCl, the organic fractions were
extracted with three portions of 10 mL of dichloromethane, dried
over anhydrous Na2SO4, filtered, and concentrated by rotary
evaporation. The residual organic oil or solid was directly used
I
for the H NMR or HPLC analyses.
Identification of the Compounds. All compounds, [1:1]
adducts, their elimination products and indane derivatives, and
3
4a,b,50,52
their spectral data are known from the literature.
Kinetic Measurements. The procedures for kinetic measure-
ments have been reported in detail in ref 23.
UV/vis Spectroscopic Measurements. The UV/vis absorp-
tion maxima of the triphenylmethyl compounds adsorbed on
the solid acid catalyst were recorded using a diode array UV/
vis spectrometer (MCS 4 Carl Zeiss) with glass fiber optics.
The concentration of the triphenylmethyl compound was about
Chlorotriphenylmethane and bromotriphenylmethane (Merck)
were recrystallized from a benzene/hexane (20/80) mixture that
contains 2% acetyl chloride and bromide, respectively.
Triphenylmethylisothiocyanate was synthezised from chlo-
rotriphenylmethane and sodium rhodanide in acetone as sol-
vent.44
The halogenodiphenylmethane derivatives were synthesized
from the corresponding carbinols and dry HCl gas in the
presence of anhydrous CaCl2 in benzene according to ref 45.
Their purity was checked by 1H NMR spectroscopy. The
diphenylcarbinols were commercially available products (Lan-
caster, Aldrich, Merck). They were recrystallized before use.
1
-10 mg per gram of catalyst. The transparent silica/1,2-
dichloroethane suspensions were measured as previously re-
ported.
2
3
Brunauer-Emmett-Teller (BET) Measurements. The
BET surface area was measured with nitrogen at 77 K using a
Sorptomatik 1900 (Fisons).
Correlation Analyses. The correlation analyses were done
with the statistics program Microcal Origin, version 5.0, SR2
from Microcal Software.
1,4-Cyclohexadiene (CHD) was purchased from Merck, dried
over CaH2, and purity-checked by GC (gas chromatography)
before use. Triethylsilane (Merck), dihydroanthracene (Merck),
and 1-methoxy-1,4-cyclohexadiene were used as received.
Styrene, p-methoxystyrene, 1,3-cyclohexadiene, and isobutyl-
vinyl ether were distilled under reduced pressure in the presence
of CaH2 before use.
Dichloromethane (Merck, analytical grade) was freshly
distilled over CaH2 and stored under dried argon.
Synthetic Procedures. Soluble Fraction. Into a 25 mL
Schlenk-bottomed flask, 1 g of the solid acid catalyst (Aerosil
Results
UV/vis Spectroscopy. According to the question whether a
chemical reaction or a reversible chemisorption takes place, we
have investigated the influence of temperature on the triphenyl-
methyl ion concentration on the surface. For the experiments,
we have chosen dichloromethane as solvent. The intensity of
the corresponding carbenium UV/vis absorption on silica Aerosil
300 has been shown for (C6H5)3CCl and (4-CH3OC6H4)2-
CHCl(AnAnCHCl) adsorption in Figure 1.
3
00) was weighed, which had been equilibrated with the
The degree of heterolytic dissociation of chlorotriphenyl-
methane on silica in a dichloromethane slurry is strongly
temperature-dependent and reversible: with decreasing tem-
perature, the intensity of the visible triphenylmethylium absorp-
tion increases three times from 20 to -75 °C. The same result
is found for the adsorption of (4-CH OC H ) CHCl on Aerosil
atmosphere at 400 °C for at least 24 h. The flask was stoppered
and allowed to cool under dry argon atmosphere. Then, a
solution containing 15 mmol of the substrate (halogenoaryl-
methyl component) in 20 mL of dichloromethane was added.
The resulting suspension was stirred, and a 15 mmol amount
of the nucleophile was added.
After continued shaking for the indicated amount of time,
the suspension was filtered, and the filtrate was washed with
three 15 mL portions of dichloromethane. The silica cook was
separately investigated (see below). The combined organic
fractions were washed with aqueous NaHCO3 and water. Then,
the combined organic fraction was dried with anhydrous Na2-
SO4, filtered, and concentrated by rotary evaporation. The
3
6
4 2
300. Therefore, the heterolytic dissociation process of the central
carbon-halide bond of (C6H5)3CCl and (4-CH3OC6H4)2CHCl
on silica occurs similarly for both compounds. Accordingly, the
behavior of R-X on solid acids is related to the heterolytic
dissociation process of R-X caused by Lewis acids in solu-
3
3,46,47
tion.
1
2
3
Studies on the Reaction of R R R CCl on Silica with
π-Nucleophiles. As possible nucleophilics for kinetic measure-
ments, we have tested aromatics, olefins, triethylsilane, and
hydride ion donating π-electron systems, because n-donors such
as amines can also evidently interact with the solid acid, which
1
residual organic oil was directly used for the H NMR analyses.
Functionalized Adsorbent (Solid Acid). The silica cook
obtained after filtration and washing was carefully dried in a
vacuum. Then the carbon content was determined by quantitative
4
8
leads to a partial deactivation of the catalyst. Also, surface-

Reactivity of Triphenylmethyl Cations
J. Phys. Chem. B, Vol. 107, No. 1, 2003 301
TABLE 2: Measured log k′ Values for the Model Reaction
of Several Nucleophiles with (C
6
H
5
3
) CX on Three Different
Solid Acid Catalystsa
reaction
solid acid
silica
silica
log k′ pK
s
(of HX)b Nc
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
(C
6
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
5
H )
3
CCl + CHD
CCl + TES
CCl + IBVE
CCl + CHD
CCl + TES
CCl + IBVE
CBr + CHD
-6.475
-5.16
-4.75
-5.79
-4.69
-4.09
-6.31
-7.87
-8.072
-7
-7
0.09
3.02
4.00
0.09
3.02
4.00
0.09
0.09
0.09
0.09
0.09
0.09
0.09
3
3
3
3
3
3
3
3
3
3
3
3
silica
-7
-7
-7
-7
-9
alumina
alumina
alumina
silica
CNCS + CHD silica
1.9
COH + CHD silica
15.74
-7
CCl + CHD
CBr + CHD
aluminosilicate -5.701
aluminosilicate -5.456
-9
CNCS + CHD aluminosilicate -6.376
COH + CHD aluminosilicate -7.32
1.9
15.74
a
] ) 25 mL; T ) 295 K. b pK
Figure 1. Intensity of the triphenylmethylium and bis(4-methoxyphen-
yl)methylium carbenium UV/vis absorption on Aerosil 300 as function
of the temperature in a dichloromethane suspension: (filled points)
while cooling; (unfilled points) while warming up (1, 0.2683 g Aerosil
[(C
6
H
5
)
3
CCl] ) 0.5 g; [CH
2
Cl
2
s
values
c
for HX were taken from ref 56. Nucleophilicity parameters for 1,4-
cyclohexadiene, triethylsilane, and isobutylvinyl ether were taken from
refs 36-38.
300, 0.14 g chlorotriphenylmethane, 15 mL dichloromethane; 2, 0.2785
1
2 3
g Aerosil 300, 0.066 g chlorobis(4-methoxyphenyl)methane, 15 mL
dichloromethane). The lines are a guide for the eye of the reader.
rate for producing the [1:1] adduct of olefins with R R R CCl
on silica is quite low because of the three sterically hindered
5
3
aryl groups. It seems that the disturbing steric influence of
the third aryl group on the addition reaction with a 1-olefin is
even enhanced for triphenylmethylium when adsorbed on a
surface. Therefore, only the addition reaction of triphenylm-
ethylium with the fairly strong nucleophile isobutylvinyl ether
has been considered for the kinetic studies. (C6H5)3CCl reacts
quantitatively with vinyl ethers to the [1:1] addition product
when a large excess of (C6H5)3CCl is used (see kinetic
procedure). The apparent rate constant determined for the
mediated reactions of electrophiles are suppressed completely
in moderate HBA (hydrogen bond accepting) solvents such as
THF.49
Aromatic compounds do not well react with diarylmethylium
ions on silica. Only the highly reactive aromatic compounds
1
,3-dimethoxybenzene and 1,3,5-trimethoxybenzene give the
50
Friedel-Crafts product in good yield. However, they do the
same reaction when the corresponding carbinols (R R CHOH)
are used in acetic acid as solvent because only a very low
1
2
+
reaction of (C6H5)3C with isobutylvinyl ether (N ) 4) on silica
51
-5 -1
-2
concentration of the carbocation is required for such reactions.
is k′ ) (1.77 ( 0.1) × 10
s
m
(log k′ ) -4.75).
Thus, for highly reactive aromatics, the heterogeneous catalyst
affords no progress. However, it should be mentioned at this
point that in some cases an unprecedented electron donor-
acceptor (EDA) complex formation between the carbenium ion
on silica and the arene component takes place.50 Those EDA
complex formations on silica suppress the Friedel-Crafts-
product formation and are associated with a strong color
formation of the silica particle slurry.50 Therefore, aromatic
compounds have not been used for the kinetic measurements.
Reaction of Hydride Donors with Carbenium Ions on
Solid Acids. In extension to our further studies, we have also
investigated the reaction of other dienes such as 1-methoxy-
1,4-cyclohexadiene (MCHD), dihydroanthracene (DHA), and
1,3-CHD (see the previous section) with several R R R CX
compounds on silica. Also, triethylsilane has been used for
comparison.
1
2 3
Usually, MCHD or 1,3-CHD reacts with carbocations under
formation of the [1:1] addition product according to ref 31. We
have found that the silica surface-mediated reaction of the
3
3
Para-substituted styrenes [R C6H4CHdCH2] (R ) H, -CH3)
1
2
+
react with chlorodiarylmethanes (R R HCCl) to the covalent
1:1]-product [R R CH-CH2-CHCl-C6H4R ] on silica. De-
AnAnCH with either MCHD or 1,3-CHD gave a mixture of
1
2
3
[
several products. However, the reaction responsible for the
fraction of the main product is the hydride transfer reaction from
spite the good yield (85-90%) and high purity of the [1:1]
product isolated from the surrounding solution phase, about 10-
+
either MCHD or 1,3-CHD to AnAnCH . With MCHD about
1
5% of byproducts remain strongly fixed on the silica particles
80% AnAnCH2 and benzene are observed beside oligomers and
26,25,32
+
after the heterogeneously induced reaction.
Indane deri-
the [1:1] product. For the reaction of 1,3-CHD with AnAnCH
vates and allylium ions remain strongly fixed on the solid acid
on Aerosil 300, a share of 50% hydride transfer products
26
1
catalyst, as also reported by Garcia and Corma. Because of
these competition reactions,52 a clean kinetic study is not really
possible.
(AnAnCH2 and benzene) is observed by the H NMR analyses
+
of the product mixture. The reaction of AnAnCH with
dihydroanthracene (DHA) proceeds smoothly to AnAnCH2 and
anthracene. No side products are detectable in the mixture.
1
,3-Dienes such as isoprene give the [1:1] product with
+
+
AnAnCHCl on silica in low yield, but cyclohexa-1,3-diene (1,3-
CHD) gives two products: the [1:1] adduct (20%) and
Unfortunately, the reaction of (C6H5)3C , AnAnCH , An-
+
+
PhCH , and TolTolCH (Tol ) 4-CH3C6H4-) on silica with
DHA does not go to completion. The reaction comes to a dead
stop at about 50-60% conversion, and an equilibrium state
between the products and educts is observed. Therefore, the
reaction of R R R CCl or R R CHCl with DHA on silica was
not suitable for clean kinetic studies. Triethylsilane reacts
52a
AnAnCH2 (50%) and benzene. This result is in contrast to
its reaction in solution in which only the [1:1] adduct formation
31a
occurs. Therefore, 1,3-CHD should be considered as a hydride
donor rather than as a 1,3-diene when used in surface-mediated
reactions with carbenium ions (see below).
1
2
3
1 2
1
2
3
1 2 3
Chlorotriphenylmethane [(C6H5)3CCl] on silica does not react
with 1-octene (N ) -2.03). With styrene (N ) 0.78) on silica,
only a slow reaction has been observed within a reaction time
of 24 h at room temperature in dichloromethane as solvent. The
quantitatively with R R R CCl on silica and alumina to R R R -
CH (Table 2). The apparent rate constants determined for the
1
2 3
reactions of CHD and triethylsilane with R R R CCl on silica
and aluminosilicate, respectively, are compiled in Table 2.

3
02 J. Phys. Chem. B, Vol. 107, No. 1, 2003
Spange et al.
TABLE 3: Apparent Rate Constants (log k′) for the
CHART 4: Disproportionation Reaction of Chlorobis(4-
methoxyphenyl)methane with Bis(4-methoxyphenyl)-
methylium on a Silica Surface to Bis(4-methoxyphenyl)-
ketone and Bis(4-methoxyphenyl)methane
Silica-Catalyzed Hydride Transfer Reaction of
1
,4-Cyclohexadiene with Various Chlorodi- and
1 2 3
Chlorotriarylmethane Derivates (R R R C-Cl) in
Dichloromethane at 295 K
pKR+a
R
1
R
2
R
3
log k′
C
6
H
5
C
4-CH
C
4-CH
C
C
6
H
5
H
-7.37b
-13.17
-9.92
-8.16
-5.57
-6.63
-6.02
-4.54
-3.40
b
4-CH
4-CH
4-CH
3
3
3
C
OC
OC
6
H
4
3
C
6
H
4
H
H
H
-7.34
b
6
H
H
4
H
6 5
-7.23
-6.44
b
6
4
3
OC
6
H
4
C
H
6 5
6
H
H
5
C
C
C
C
6
H
5
H
5
H
5
H
5
-6.47
-6.78
-7.76
-7.88
4
4
4
-CH
-CH
-CH
3
3
3
C
OC
OC
6
H
4
6
5
6
6
6
6
H
H
4
6 4
4-ClC H
C H
6 5
6
4
a
The pK + values for the corresponding di- and triarylmethylium
R
b
ions were taken from refs 57 and 58. Values were taken from ref 22.
reaction with CHD decreases. This is a clear indication that the
rate of the reaction in this section of carbenium reactivity is
controlled by the electrophilicity of the carbenium (k2) and not
1
2 3 +
by the concentration of [R R R C ]. That means that the amount
+
of k2 contributes stronger to k′ than [R ].
In a previous paper, we have studied the reaction of R R -
CHCl with CHD and determined the relative reactivity for this
1
2
1
2 3
Rate and yield of R R R CH formed decrease with decreasing
the nucleophilicity parameter of the hydride donor used as
follows: triethylsilane > CHD > DHA.37 For each triphenyl-
methyl ion used with CHD, a complete conversion to R R R -
CH and benzene has been established. Therefore, we have used
the hydride transfer reaction of CHD with surface-mediated
carbocations as reference reaction to determine the reactivity
of the carbenium.
22
reaction. There is a distinct difference in the kinetic plot of
the hydride transfer reaction of CHD with carbenium ions
1
2 3
1
2
3
+
1
2
+
whether R R R C or R R CH is investigated. The kinetic plot
1
2
3
+
23
1 2
for R R R C gives a straight line up to 90% conversion. R R -
+
CH carbenium ions also react fairly well with CHD, but the
kinetic plot approaches a straight line only in the first stage of
the reaction up to 30% conversion of CHD. Therefore, we have
only utilized this part of the curve in determining the apparent
rate constant. The apparent rate constant for the reaction of
However, there exists another problem for halogenodiaryl-
methanes on silica, because AnAnCHCl itself can react as a
+
hydride donor with AnAnCH (see Chart 4). This type of
+
AnAnCH (with AnAnCHCl as the precursor) on silica with
reaction is a disproportionation yielding finally AnAnCO and
AnAnCH2.
-7 -1
-2
CHD amounts to k′ ) 3.65 × 10
s
m
(log k′ ) -6.43).
This rate constant is slightly larger than that for the reaction of
+
AnAnCO is formed by the reaction of AnAnCCl with silanol
+
-7 -1
-2
(
C6H5)3C with CHD, k′ ) 3.35 × 10
s
m
(log k′ )
groups. It could be independently shown that the dichloride of
AnAnCO, AnAnCClCl, reacts spontaneously with silica forming
-
6.47) determined under the same experimental conditions. We
think that the sterical influence of the phenyl rings upon the
rate of the hydride transfer reaction is not so dramatic when
the reaction occurs on solid surfaces as observed for the olefin
addition with di- and triarylmethyl carbenium ions, which have
similar electrophilicity. The reason for this result is likely that
the mobility and accessibility of the carbenium is retarded as a
whole, which averages the steric influences. Therefore, we have
used the results from triphenylmethylium as the reference system
despite Mayr’s argument that electrophilicity parameters for
+
54
+
first AnAnCCl (λ ) 530 nm) and HCl. Then AnAnCCl
reacts with silanol groups, and a chlorinated silica surface and
2
2
AnAnCO are formed after 15 min. A similar disproportionation
reaction of AnAnCHOH to AnAnCH2 and AnAnCO occurs
when AnAnCHOH has been treated with hexamethyldisilazane
and trifluoromethane sulfonic acid.55 The disturbing reaction
(Chart 4) becomes of importance when such nucleophiles are
used that are weaker nucleophiles than the precursor halogeno
1
2
component, R R CHCl. Within 12 h reaction time, AnAnCHCl
undergoes no conversion to AnAnCH2.
37
triarylmethylium ions cannot be determined.
Influence of the Leaving Group X. The rate constant of
the reaction of (C6H5)3CX with CHD to be catalyzed by silica
and aluminosilicate, respectively, is strongly affected by the
Discussion
According to the theory of the kinetic approach from eq 3,
the logarithm of the rate constants are used for linear correlation
analyses between the E and N parameters and the kinetic
-
nature of X as shown in Table 2. On increase of the basicity
-
-
-
-
-
of X (OH > SCN > Cl > Br ) (pKs values for the
corresponding acid HX of the anions are also given in Table
), k′ decreases significantly. This result has been found
31
parameter k′.
5
6
2
An important question remained from our previous paper:
Is it justifiable to separate the influence of the part of the k2
independently of the nature of the solid acids, which differ
significantly in their surface acidity.
+
term and the [R ] term from the k′ value for a surface-mediated
2
3
Influence of the Structure of the Carbocation. As men-
reaction? Because of the multiple adsorption mechanism of
(C H ) CX on surfaces of solid acids containing both Lewis
1
2 3 +
tioned, CHD reacts very well with R R R C carbenium ions
on silica. The specific rate constants for the reaction of several
6
5 3
and BrØnsted acid sites, it is still not clear whether the two
1
2
3
+
+
R R R C cations with CHD to be catalyzed with silica are
compiled in Table 3.
terms k and [R ] adequately reflect their influence upon an
2
individual k′ for a distinct surface site.
For triphenylmethylium ions, on increase of the stability of
R R R C as expressed by their pKR values, the rate of the
Feigel and Kessler investigated the influence of the nature
of X on the heterolytic dissociation of several triphenylmethyl
1
2
3
+
+
57
-

Reactivity of Triphenylmethyl Cations
J. Phys. Chem. B, Vol. 107, No. 1, 2003 303
Figure 4. Relationships between the apparent rate constant of the
model reaction (log k′) of 1,4-cyclohexadiene with chlorodiarylmethanes
Figure 2. Relationships between the relative rate constant (log k′) for
(
O) on silica (Aerosil 300) (from ref 22) and chlorotriphenylmethanes
the reaction of 1,4-cyclohexadiene with (C
6
H
5
)
3
CX as function of the
basicity of the leaving anionic group X (expressed by their corre-
sponding pK values) for two solid acid catalysts, a silica (SG 432)
(
b) on silica (Grace) and the pKR+ of the corresponding carbenium.
-
s
with R ) 1.14 and an aluminosilicate (Siral 5) with R ) 1.69.
1
2 3 +
the elementary step or the concentration of R R R C , depend-
ing on the nature of the solid acid catalyst, but also the nature
1
2 3 +
of R R R C must be considered in interpretation of the results.
For the model reaction in dichloromethane, the rate constant
1
2 3 +
(
log k′) of the reaction of CHD with R R R C (X ) Cl) and
1
2
+
R R CH (X ) Cl), respectively, have been determined for
silica as solid acids. Figure 4 shows the log k′ values determined
+
as function of the electrophilicity (pKR ) of the carbenium
generated, because Mayr’s E parameters for triphenylmethylium
3
7
ions are still not available. The log k′ values for the reaction
1
2
+
of CHD with R R CH (X ) Cl) have been taken from our
2
2
previous paper.
The largest apparent rate constant for the hydride transfer
reaction of CHD with carbenium ions on silica has been found
Figure 3. Relationships between the relative rate constant (log k′) for
the reaction of 1,4-cyclohexadiene, triethylsilane, or isobutylvinyl ether
+
for AnAnCH (X ) Cl). For the initiation of the cationic surface
polymerization of isobutylvinyl ether on silica, the same result
with (C
nucleophile for two solid acid catalysts, a silica (KG SG 432) with R
1.14 and an alumina (Al SP 18-8510) with R ) 1.53.
6 5 3
H ) CCl as function of the nucleophilicity parameter E of the
20b
+
has been found. AnAnCH is also the most effective initiator
on MCM-41 for initiating the cationic host-guest polymeriza-
)
2 3
O
4
5
tion of cyclohexyl vinyl ether.
3
7
According to Mayr, Gorath, and Lang, it is expected that
and tropylium ions in solution.46 They showed that the equi-
librium constant from eq 2 significantly increases for R-X in
decreasing the pKs of the corresponding acid HX. The correla-
k2 should steadily increase from (4-CH3OC6H4)(C6H5)2CCl
+
+
(pKR ) -3.4) to (C6H5)2CHCl (pKR ) -13.17) from the right
to the left. They used BCl3 as catalyst, which is evidently a
much stronger Lewis acid than silica or alumina. Therefore,
the chlorodiarylmethanes are almost completely ionized with
BCl3. The value of K from eq 3 decreases in the same order for
silica and BCl3. For (C6H5)2CHCl and chlorobis(4-methyl-
phenyl)methane (TolTolCHCl), the characteristic UV/vis ab-
sorptions of the corresponding carbenium ions are expected at
-
56
tion of log k′ with pKs of X
for the reaction of CHD with
(C6H5)3CX is shown in Figure 2 for silica and aluminosilicate.
It is clearly seen that for each solid acid used a separate curve
is obtained. Both plots occur parallel to each other. Assuming
that k2 is independent of the solid acid used, then the differences
+
in the catalytic activity are only caused by the [R ] term. This
5
9
presumption seems justified because the dipolarity/polarizability
λ ) 435 nm and λ ) 462 nm, respectively. They cannot be
23
term (π* parameter), which should affect k2, for the two solid
observed on silica. This is an indication that the surface
acids is quite similar.35 Consequently, the decrease of k′ in
+
+
concentration of (C6H5)2CH or TolTolCH on silica is below
-
-7
-1
+
increasing the basicity of X is evidently a concentration effect.
10 g mol . As one is moving further left in Figure 4 (pKR
The stronger the acidity of the solid acid is and the lower the
bonding energy of the C-X bond is, the faster occurs the
reaction of the carbenium with an external nucleophile. This is
also an indication that the nucleophile does not influence the
heterolytic dissociation process of R-X. Figure 3 shows the
plots of the log k′ values for three different reactions as function
of the nucleophilicity parameter N value for two different solid
acid catalysts. The influence of the nucleophilicity of the
nucleophilic component on the apparent rate constant appears
almost identical for different solid acids. This shows that the
constant selectivity principle is also valid for heterogeneously
induced polar reactions.
< -7), one will be dealing with predominantly covalent
material. Now, the apparent rate constant will decrease with
increasing Lewis acidity of the cations because structural
variation affects the equilibrium constant to a greater degree
6
0
than the rate constants. Consequently, for carbocations with
high reactivity, the surface-mediated hydride transfer reaction
is controlled by the concentration of the carbenium intermediate.
+
For carbocations with low reactivity (pKR > -6), the reaction
is controlled by the intrinsic reactivity of the carbenium it-
self.
Therefore, calculations of multiple linear correlations between
log k′ and N, pKs, and the surface acidity R are only reasonable
for an individual carbocation. The result of the multiple least-
squares analysis for log k′ versus the R, pKs, and N parameters
for the reaction of (C6H5)3CX with nucleophiles catalyzed by
solid acids (SA) is shown in Figure 5 and eq 4 (r ) correlation
As mentioned, the k′ value involves both the rate constant k2
of the elementary step and the concentration of immediately
generated triphenylmethylium per square meter solid acid
catalyst. Therefore, not only these options, the variation of either

304 J. Phys. Chem. B, Vol. 107, No. 1, 2003
Spange et al.
The main problem arisis from the choice of suitable nucleo-
philes to study kinetics of heterogeneous reactions, because
seldom a clean kinetic plot is observed because interfering
reactions occur.
Acknowledgment. Financial support in particular for this
project by the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie is gratefully acknowledged. We thank
Condea GmbH, Merck AG Darmstadt, and Degussa for the
samples of the solid acid catalysts. Mrs. Berger and Prof. D.
Hoenicke, Department of Technical Chemistry, University of
Technology, Chemnitz, performed the BET measurements.
Figure 5. Calculated versus measured log k′ values according to eq 4
for the influence of three independent parameters (pK of HX, the
s
Kamlet-Taft acidity R of the solid acid catalyst, and the nucleophilicity
parameter N of the educt [N]) on the reaction of (C H ) CX with N
6 5 3
References and Notes
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(
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6
5 3
-
0
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(
4
1
3
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1
16, 10188-10195.
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(
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