Kinetics of the solvolyses of benzhydryl derivatives: Basis for the construction of a comprehensive nucleofugality scale

Article abstract of DOI:10.1002/chem.200500845

A series of 21 benzhydrylium ions (diarylmethylium ions) are proposed as reference electrofuges for the development of a general nucleofugality scale, where nucleofugality refers to a combination of leaving group and solvent. A total of 167 solvolysis rate constants of benzhydrylium tosylates, bromides, chlorides, trifluoroacetates, 3,5-dinitrobenzoates, and 4-nitroben-zoates, two-thirds of which have been determined during this work, were subjected to a least-squares fit according to the correlation equation log k _25°C = S_f(N_f + E_f), where s _f and N_f are nucleofuge-specific parameters and E _f is an electrofuge-specific parameter. Although nucleofuges and electrofuges characterized in this way cover more than 12 orders of magnitude, a single set of the parameters, namely s_f, N_f, and E _f, is sufficient to calculate the solvolysis rate constants at 25°C with an accuracy of ± 16%. Because s_f ≈ 1 for all nucleofuges, that is, leaving group/ solvent combinations, studied so far, qualitative discussions of nucleofugality can be based on N_f.

Full text of DOI:10.1002/chem.200500845

DOI: 10.1002/chem.200500845
Kinetics of the Solvolyses of Benzhydryl Derivatives: Basis for the
Construction of a Comprehensive Nucleofugality Scale
Bernard Denegri,[a] AndrØ Streiter,[b] Sandra Juric´,[a] Armin R. Ofial,[b] OlgaKronja, [a] and
Herbert Mayr*[b]
This and the following article are dedicated to Professor Henning Hopf on the occasion of his 65th birthday
Abstract: A series of 21 benzhydrylium
ions (diarylmethylium ions) are pro-
posed as reference electrofuges for the
development of a general nucleofugali-
ty scale, where nucleofugality refers to
a combination of leaving group and
solvent. A total of 167 solvolysis rate
constants of benzhydrylium tosylates,
bromides, chlorides, trifluoroacetates,
3,5-dinitrobenzoates, and 4-nitroben-
zoates, two-thirds of which have been
determined during this work, were sub-
jected to a least-squares fit according
though nucleofuges and electrofuges
characterized in this way cover more
than 12orders of magnitude, a single
set of the parameters, namely sf, Nf,
and Ef, is sufficient to calculate the sol-
volysis rate constants at 258C with an
accuracy of ꢁ16%. Because sf ꢂ1 for
all nucleofuges, that is, leaving group/
solvent combinations, studied so far,
qualitative discussions of nucleofugality
can be based on Nf.
to the correlation equation logk258C
=
sf(Nf + Ef), where sf and Nf are nucleo-
fuge-specific parameters and Ef is an
electrofuge-specific parameter. Al-
Keywords: carbocations · kinetics ·
nucleophilic substitution · reaction
mechanisms · solvent effects
Nucleofuge: A leaving group that carries away the bonding
Introduction
electron pair[43] (for example, Xꢀ in Equation (1) below).
Electrofuge: A leaving group that does not carry away the
bonding electron pair[43] (for example, R+ in Equation (1)
below, or H+ in the nitration of benzene by NO2+).
The knowledge of leaving group abilities is of similar impor-
tance for the understanding of nucleophilic substitution re-
actions as the knowledge of nucleophilic properties. While
the latter aspect has been the topic of numerous investiga-
tions for more than half a century,[1] much less attention has
been paid to the quantification of nucleofugalities,[2,3] and
most textbooks confine themselves to the qualitative rule
that leaving group abilities of Xꢀ increase with increasing
acidities of the conjugate acids HX.[4]
It is obvious that a general nucleofugality scale cannot
exist. Relative leaving group abilities do not only depend on
the mechanism—SN1 or SN2—but even within one type of
mechanism they depend on substrate and solvent.
Nucleofugality and electrofugality are kinetic terms describ-
ing leaving group abilities. They are related to the kinetic
terms nucleophilicity and electrophilicity and the thermody-
namic terms Lewis basicity and Lewis acidity, respectively.
Specifically in this work nucleofugality is defined for combi-
nations of leaving groups and solvents, while differential ef-
fects of solvents on electrofugality are neglected.
In SN1-type solvolyses, for example, large leaving groups
(tosylates) come off particularly fast from bulky substrates,
due to steric repulsions in the ground state (back strain).[5]
Furthermore, geminal interactions between the leaving
group and other substituents at the reaction center are vari-
able (e.g., anomeric effect)[6] and, therefore, account for the
dependence of relative nucleofugalities on the nature of the
electrofuges.
The observation that benzhydryl bromides solvolyze
about 30 times faster in ethanol than the corresponding
chlorides (see below), while the leaving group abilities of
bromide and chloride are comparable in trifluoroethanol, il-
lustrate the dependence of relative leaving group abilities
on the solvent. Even more pronounced solvent effects on
´
[a] B. Denegri, Dr. S. Juric, Prof. Dr. O. Kronja
Faculty of Pharmacy and Biochemistry, University of Zagreb
ˇ ´
A. Kovacica 1, 10000 Zagreb (Croatia)
[b] A. Streiter, Dr. A. R. Ofial, Prof. Dr. H. Mayr
Department Chemie und Biochemie der
Ludwig-Maximilians-Universität München
Butenandtstrasse 5–13 (Haus F), 81377 München (Germany)
Fax : (+49)89-2180-77717
E-mail: Herbert.Mayr@cup.uni-muenchen.de
Supporting information for this article is available on the WWW
under http://www.chemeurj.org or from the author.
1648
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1648 – 1656

FULL PAPER
relative leaving group abilities are encountered when anion-
ic and neutral leaving groups are compared.[7,8] Thus, a
change from ethanol to 20% aqueous ethanol increases the
solvolysis rate of 1-adamantyl chloride by a factor of 106.6
(at 258C),[9] whereas the same variation of solvents acceler-
ated the solvolysis of the 1-adamantyldimethylsulfonium ion
by a factor of only 1.7 (at 70.48C).[8]
For these reasons, comparisons of leaving group abilities
have to refer to certain reference reactions, for example, the
rate constants for the reaction in Equation (1) for a given R
in a certain solvent at a certain temperature.
In this way, the steric requirements of the electrofuges are
kept constant. Furthermore, the use of benzhydrylium ions
as reference electrophiles in electrophile/nucleophile combi-
nations and as reference electrofuges in heterolysis reactions
will allow us to elucidate the relationships between the two
types of scales.
Combinations of poor leaving groups (weak nucleofuges)
with highly stabilized benzhydrylium ions (good electrofug-
es), for example, A, and combinations of good leaving
groups (good nucleofuges) with destabilized benzhydrylium
ions (poor electrofuges), for example, B, give substrates that
solvolyze at rates that can be measured conveniently.[17]
oC
RꢀX EtOH=25 Rþ þ Xꢀ ! Products
ð1Þ
ꢀꢀꢀꢀꢀꢀ!
k
However, nucleofugality scales derived in this way cover
only narrow ranges. For the determination of solvolysis
rates, the substrate has to be dissolved in the corresponding
medium, and because of the mixing problem, it is very diffi-
cult to investigate solvolysis reactions with k>102 sꢀ1. While
rate constants of this order of magnitude may be obtained
for solvent mixtures by means of stopped-flow techniques,
the limitation for pure solvents is even lower. On the other
hand, the study of reactions with k<10ꢀ5 sꢀ1 takes a long
time, and even when investigations at variable temperature
are considered, it is difficult to develop nucleofugality scales
based on single reference electrofuges R+, which exceed ten
orders of magnitude. In practice, this range is even smaller,
and the core of the well-known nucleofugality scale by
Noyce,[2a] which is based on solvolysis rates of 1-phenylethyl
derivatives in 80% aqueous ethanol, covers only six orders
of magnitude. Noyce extended this scale to 14 orders of
magnitude by including substituted 1-phenylethyl derivatives
and assuming constant reactivity ratios.
Because of the variable solvent effects on the leaving
group abilities of different groups X, the nucleofuge-specific
parameters sf and Nf, defined by Equation (3), will generally
refer to combinations of leaving groups and solvents, for ex-
ample, Clꢀ in EtOH or dinitrobenzoate in 80% aqueous
acetone.
log ks ¼ sfðNf þ EfÞ
ð3Þ
sf, Nf: nucleofuge-specific parameters,
Ef: electrofuge-specific parameter.
We now report that a comprehensive nucleofugality scale
may be constructed by the same procedure that had ren-
dered the most extended nucleophilicity scales presently
available.[10]
As previously discussed for Equation (2),[10,11] the special
types of the linear free-energy relationships [Eq. (2) and
Eq. (3)], which define N (or Nf) as the negative intercepts
on the abscissa (E or Ef axis), render nucleophilicity param-
eters N or nucleofugality parameters Nf that are not strongly
dependent on the slopes. This is because the intersections of
the correlation lines with the abscissa are within or close to
the experimental range and never require long-ranging ex-
trapolations, which would be inevitable if nucleophilicity or
nucleofugality would be defined as intercepts on the ordi-
nate.
Results and Discussion
By using highly stabilized benzhydrylium ions for character-
izing strong nucleophiles and non- or weakly stabilized
benzhydrylium ions to characterize weak nucleophiles, the
method of overlapping reactivity series had allowed us to
quantitatively compare nucleophiles as weak as benzene
with nucleophiles as strong as malonate and thiolate
ions.[11–16] Equation (2), where s and N are nucleophile-spe-
cific parameters and E is an electrophile-specific parameter,
presently includes electrophiles and nucleophiles that differ
by almost 30 orders of magnitude.
By selecting common leaving groups (LG), for example,
p-tosylate (OTs), bromide, chloride, trifluoroacetate, 3,5-di-
nitrobenzoate (DNB), and 4-nitrobenzoate (PNB), and
common solvents (ethanol, methanol, 80% aqueous ethanol,
80 and 90% aqueous acetone, and trifluoroethanol) for our
investigations (Scheme 1), it is possible to employ a mani-
fold of literature data for the correlations.
log k ¼ sðN þ EÞ
ð2Þ
In analogy to this procedure, which employed benzhydry-
lium ions as the reference electrophiles, we now use p- and
m-substituted benzhydrylium ions as reference electrofuges.
Kinetics: All solvolysis rate constants determined during
this work were derived from conductivity measurements.
Whenever possible, the experiments were carried out at
Chem. Eur. J. 2006, 12, 1648 – 1656
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1649

H. Mayr et al.
258C. If the reactions were too fast or too slow at this tem-
perature, the kinetics were determined at lower or at elevat-
ed temperatures, and the Eyring equation was used to ex-
trapolate to 258C.
Table 1[18–30] summarizes all solvolysis rate constants ks
(sꢀ1) that were employed to calculate the electrofugality and
nucleofugality parameters according to Equation (3). In
cases where different solvolysis rate constants for the same
substrate under the same conditions are given in the litera-
ture, only the ks that is closest to the calculated value from
the correlation equation (kcalcd) is given in Table 1. All avail-
able rate constants are listed in Table S1 of the Supporting
Information.
The solvolysis rate constants given in Table 1 were sub-
jected to a least-squares fit according to Equation (3): ꢀD2,
as defined in Equation (4), was minimized[31] to yield the op-
timized parameters Nf and sf (Table 2) as well as Ef
(Table 3). In order to link Equation (3) to Equation (2), Ef
Scheme 1. Solvolysis reactions of X,Y-substituted benzhydryl derivatives
with different leaving groups.
Table 1. Solvolysis rate constants of X,Y-substituted benzhydryl derivatives in different solvents (258C).
[a]
Nucleofuge
Electrofuge
X,Y
ks [sꢀ1
]
kcalcd [sꢀ1
]
Ref.
Leaving group
Solvent[b]
90A10W
TsO
H
3.01”10ꢀ1
2.60”10ꢀ3
2.05”10ꢀ4
6.24”10ꢀ2
6.86”10ꢀ2
4.08”10ꢀ3
3.60”10ꢀ4
1.87”10ꢀ4
6.45”10ꢀ5
1.83”10ꢀ3
1.11”10ꢀ4
9.78”10ꢀ6
5.07”10ꢀ6
1.44”10ꢀ2
7.97”10ꢀ4
4.95”10ꢀ5
9.42”10ꢀ4 [c]
1.01”10ꢀ4 [c]
2.87”10ꢀ5 [c]
2.74”10ꢀ1
3.31”10ꢀ3
1.77”10ꢀ4
6.98”10ꢀ2
5.87”10ꢀ2
4.40”10ꢀ3
3.50”10ꢀ4
1.88”10ꢀ4
6.40”10ꢀ5
1.79”10ꢀ3
1.16”10ꢀ4
9.47”10ꢀ6
5.11”10ꢀ6
1.52”10ꢀ2
7.12”10ꢀ4
5.26”10ꢀ5
9.33”10ꢀ4
1.04”10ꢀ4
2.82”10ꢀ5
–
–
–
–
–
–
–
–
–
–
–
–
[18]
–
–
–
–
3-Cl, 4’-Cl
3-Cl, 3’-Cl
4-Me, 4’-Me
4-OPh
4-Me
4-F
H
4-Cl
4-Me, 4’-Me
4-Me
4-F
Br
Cl
90A10W
90A10W
H
CF3CO2
DNB
90A10W
90A10W
4-OMe
4-Me, 4’-Me
4-Me
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
–
–
TsO
Br
80A20W
80A20W
3-Cl, 4’-Cl
3-Cl, 3’-Cl
3,5-(Cl)2, 3’-Cl
4-OPh
1.41”10ꢀ2
1.07”10ꢀ3
4.05”10ꢀ5
2.59”10ꢀ1
4.54”10ꢀ2
8.99”10ꢀ3
5.19”10ꢀ3
3.71”10ꢀ3
4.08”10ꢀ3
2.03”10ꢀ3
6.81”10ꢀ4
4.60”10ꢀ4
2.72”10ꢀ4
4.42”10ꢀ5
3.33”10ꢀ6
2.79”10ꢀ2
2.64”10ꢀ2
1.76”10ꢀ3
2.25”10ꢀ4
2.05”10ꢀ4
1.34”10ꢀ4
1.45”10ꢀ2
1.01”10ꢀ3
4.16”10ꢀ5
3.54”10ꢀ1
3.45”10ꢀ2
6.62”10ꢀ3
4.73”10ꢀ3
3.55”10ꢀ3
3.53”10ꢀ3
2.02”10ꢀ3
7.69”10ꢀ4
5.58”10ꢀ4
3.09”10ꢀ4
6.14”10ꢀ5
2.67”10ꢀ6
2.96”10ꢀ2
2.55”10ꢀ2
1.74”10ꢀ3
2.65”10ꢀ4
1.81”10ꢀ4
1.30”10ꢀ4
–
–
–
[19a]
–
[19b]
[20]
–
[19a]
–
–
[19a]
–
[19a]
[21]
–
4-Me
3,5-(Me)2
4-OPh, 4’-NO2
4-F
3-Me
H
4-Cl
4-Br
4-Cl, 4’-Cl
3-Cl
4-NO2
4-Me, 4’-Me
4-OPh
4-Me
3,5-(Me)2
4-OPh, 4’-NO2
4-F
Cl
80A20W
–
–
[22]
[20]
–
1650
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1648 – 1656

Solvolysis Kinetics
Table 1. (Continued)
FULL PAPER
[a]
Nucleofuge
Electrofuge
X,Y
ks [sꢀ1
]
kcalcd [sꢀ1
]
Ref.
Leaving group
Solvent[b]
80A20W
Cl
3-Me
H
4-Cl
4-Br
4-Cl, 4’-Cl
3-Cl
4-Me, 4’-Me
4-Me
4-F
H
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
4-OMe
H
1.22”10ꢀ4
6.82”10ꢀ5
2.31”10ꢀ5
1.66”10ꢀ5
8.40”10ꢀ6
1.20”10ꢀ6
3.77”10ꢀ3
2.85”10ꢀ4
3.24”10ꢀ5
2.04”10ꢀ5
2.99”10ꢀ3 [c]
3.34”10ꢀ4 [c]
1.08”10ꢀ4 [c]
1.85”10ꢀ5 [c]
4.39”10ꢀ10
1.29”10ꢀ4
6.86”10ꢀ5
2.27”10ꢀ5
1.58”10ꢀ5
8.03”10ꢀ6
1.27”10ꢀ6
3.59”10ꢀ3
3.11”10ꢀ4
3.32”10ꢀ5
1.91”10ꢀ5
3.09”10ꢀ3
3.79”10ꢀ4
1.09”10ꢀ4
1.47”10ꢀ5
4.65”10ꢀ10
[22]
–
[23]
[18]
[18]
[24]
–
CF3CO2
DNB
80A20W
80A20W
–
–
[19a]
–
–
–
–
[19a]
TsO
Br
ethanol
ethanol
3-Cl
5.57”10ꢀ2
2.83”10ꢀ2
1.83”10ꢀ3
9.65”10ꢀ5
5.81”10ꢀ6
2.95”10ꢀ2
3.55”10ꢀ3
2.38”10ꢀ3
1.34”10ꢀ3
5.00”10ꢀ4
2.10”10ꢀ4
1.58”10ꢀ5
5.72”101
5.47”10ꢀ2
2.47”10ꢀ2
2.12”10ꢀ3
1.12”10ꢀ4
5.06”10ꢀ6
2.67”10ꢀ2
3.45”10ꢀ3
2.56”10ꢀ3
1.44”10ꢀ3
5.32”10ꢀ4
2.08”10ꢀ4
1.48”10ꢀ5
7.36”101
1.15”101
3.81
–
–
–
–
–
–
[20]
–
–
3-Cl, 4’-Cl
3-Cl, 3’-Cl
3,5-(Cl)2, 3’-Cl
3,5-(Cl)2, 3’,5’-(Cl)2
4-Me
4-OPh, 4’-NO2
4-F
H
4-Cl
–
–
–
4-Cl, 4’-Cl
3-Cl, 4’-Cl
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
4-OMe
4-Me, 4’-Me
4-OPh
4-Me
3,5-(Me)2
4-OPh, 4’-NO2
4-F
3-Me
H
4-Cl
4-Br
4-Cl, 4’-Cl
3-Cl
3-Cl, 4’-Cl
4-NO2
4-OMe
4-Me, 4’-Me
4-Me
4-F
H
Cl
ethanol
[25]
[25]
[25]
[25]
–
[25]
–
[26b]
[20]
–
1.52”101
5.14
5.00”10ꢀ1
2.17”10ꢀ2
2.22”10ꢀ2
1.54”10ꢀ3
2.25”10ꢀ4
2.67”10ꢀ4
1.07”10ꢀ4
1.12”10ꢀ4
5.54”10ꢀ5
2.06”10ꢀ5
1.61”10ꢀ5
8.07”10ꢀ6
1.25”10ꢀ6
5.03”10ꢀ7
4.24”10ꢀ8
2.86”10ꢀ2
1.89”10ꢀ3
1.40”10ꢀ4
1.84”10ꢀ5
9.37”10ꢀ6
5.79”10ꢀ3 [c]
7.52”10ꢀ4 [c]
1.88”10ꢀ4 [c]
3.22”10ꢀ5 [c]
6.44”10ꢀ1
2.48”10ꢀ2
2.09”10ꢀ2
1.55”10ꢀ3
2.46”10ꢀ4
1.69”10ꢀ4
1.23”10ꢀ4
1.22”10ꢀ4
6.57”10ꢀ5
2.23”10ꢀ5
1.56”10ꢀ5
8.05”10ꢀ6
1.33”10ꢀ6
4.58”10ꢀ7
4.03”10ꢀ8
2.95”10ꢀ2
1.73”10ꢀ3
1.54”10ꢀ4
1.69”10ꢀ5
9.81”10ꢀ6
5.64”10ꢀ3
7.23”10ꢀ4
2.14”10ꢀ4
3.01”10ꢀ5
[26a]
–
[26a]
[26a]
[26b]
[26a]
[26c]
[26a]
–
–
–
–
–
CF3CO2
DNB
TsO
ethanol
ethanol
80E20W
80E20W
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
4-OMe
–
–
–
–
3-Cl
6.54”10ꢀ1
2.72”10ꢀ1
1.59”10ꢀ2
9.13”10ꢀ4
3.64”10ꢀ5
4.72”10ꢀ2
5.88”10ꢀ2
3.04”10ꢀ2
1.21”10ꢀ2
5.99”10ꢀ1
2.60”10ꢀ1
1.96”10ꢀ2
8.93”10ꢀ4
3.45”10ꢀ5
6.75”10ꢀ2
4.99”10ꢀ2
2.78”10ꢀ2
1.01”10ꢀ2
–
–
–
–
3-Cl, 4’-Cl
3-Cl, 3’-Cl
3,5-(Cl)2, 3’-Cl
3,5-(Cl)2, 3’,5’-(Cl)2
4-OPh, 4’-NO2
4-F
–
Br
[20]
[27]
[27]
–
H
4-Cl
Chem. Eur. J. 2006, 12, 1648 – 1656
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1651

H. Mayr et al.
Table 1. (Continued)
Nucleofuge
[a]
Electrofuge
X,Y
ks [sꢀ1
]
kcalcd [sꢀ1
]
Ref.
Leaving group
Solvent[b]
80E20W
Br
4-Cl, 4’-Cl
3-Cl
3-Cl, 4’-Cl
4-NO2
3-Cl, 3’-Cl
3,5-(Cl)2, 3’-Cl
4-Me
3,5-(Me)2
4-OPh, 4’-NO2
4-F
3-Me
H
4-Cl
4-Cl, 4’-Cl
3-Cl
3-Cl, 4’-Cl
4-NO2
4-Me, 4’-Me
4-Me
4-F
H
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
4-OMe
4-OMe, 4’-OMe
4-OMe, 4’-OPh
4-OMe, 4’-Me
4-OMe
4.81”10ꢀ3
6.48”10ꢀ4
2.59”10ꢀ4
2.63”10ꢀ5
8.99”10ꢀ6
3.84”10ꢀ7
4.64”10ꢀ2
5.84”10ꢀ3
4.26”10ꢀ3
3.67”10ꢀ3
3.40”10ꢀ3
2.04”10ꢀ3
7.29”10ꢀ4
2.86”10ꢀ4
3.56”10ꢀ5
1.53”10ꢀ5
1.30”10ꢀ6
2.30”10ꢀ2
2.10”10ꢀ3
3.27”10ꢀ4
1.55”10ꢀ4
3.93”10ꢀ2
5.59”10ꢀ3
1.98”10ꢀ3
3.50”10ꢀ4
2.65”10ꢀ3
3.35”10ꢀ4
1.11”10ꢀ4
2.38”10ꢀ5
1.19”10ꢀ6
4.50”10ꢀ9
3.90”10ꢀ3
7.24”10ꢀ4
2.67”10ꢀ4
2.74”10ꢀ5
1.23”10ꢀ5
3.10”10ꢀ7
4.26”10ꢀ2
6.98”10ꢀ3
4.83”10ꢀ3
3.52”10ꢀ3
3.50”10ꢀ3
1.91”10ꢀ3
6.60”10ꢀ4
2.43”10ꢀ4
4.14”10ꢀ5
1.45”10ꢀ5
1.33”10ꢀ6
2.23”10ꢀ2
2.28”10ꢀ3
2.83”10ꢀ4
1.69”10ꢀ4
3.78”10ꢀ2
5.95”10ꢀ3
1.99”10ꢀ3
3.40”10ꢀ4
2.12”10ꢀ3
3.69”10ꢀ4
1.31”10ꢀ4
2.46”10ꢀ5
1.15”10ꢀ6
4.32”10ꢀ9
–
[27]
–
[27]
–
–
Cl
80E20W
–
[22]
[20]
–
[22]
–
–
–
[24]
–
[24]
–
–
–
–
–
–
–
–
–
CF3CO2
DNB
80E20W
80E20W
80E20W
PNB
–
–
–
[28]
[2c]
4-Me, 4’-Me
H
Br
Cl
methanol
methanol
4-OPh, 4’-NO2
4-F
H
3-Cl
4-NO2
4-Me
3,5-(Me)2
4-OPh, 4’-NO2
4-F
3-Me
H
4-Cl
4-Br
4-Cl, 4’-Cl
3-Cl
3-Cl, 4’-Cl
4-NO2
4.74”10ꢀ2
2.92”10ꢀ2
1.76”10ꢀ2
4.22”10ꢀ4
1.14”10ꢀ5
1.98”10ꢀ2
3.87”10ꢀ3
2.18”10ꢀ3
1.38”10ꢀ3
1.74”10ꢀ3
8.33”10ꢀ4
2.97”10ꢀ4
2.39”10ꢀ4
1.15”10ꢀ4
1.93”10ꢀ5
8.21”10ꢀ6
5.50”10ꢀ7
4.47”10ꢀ2
3.26”10ꢀ2
1.76”10ꢀ2
3.78”10ꢀ4
1.21”10ꢀ5
2.01”10ꢀ2
3.29”10ꢀ3
2.28”10ꢀ3
1.66”10ꢀ3
1.65”10ꢀ3
8.97”10ꢀ4
3.11”10ꢀ4
2.18”10ꢀ4
1.14”10ꢀ4
1.94”10ꢀ5
6.83”10ꢀ6
6.25”10ꢀ7
[20]
[27]
[27]
[27]
[27]
[24]
[26b]
[20]
[26a]
[26a]
[29]
[26a]
[26a]
[26b]
[24]
[26c]
[30]
–
–
TsO
Br
TFE[f]
TFE[f]
3,5-(Cl)2, 3’-Cl
3,5-(Cl)2, 3’,5’-(Cl)2
H
4-Cl, 4’-Cl
3-Cl
4.36”10ꢀ2
1.07”10ꢀ3
1.10
4.36”10ꢀ2[d]
1.07”10ꢀ3 [d]
1.36
–
–
–
–
[27]
–
–
–
[20]
–
1.52”10ꢀ1
6.62”10ꢀ2
1.40”10ꢀ2
2.89”10ꢀ4 [e]
6.70”10ꢀ4
1.99”10ꢀ5
3.41”10ꢀ7
6.34”10ꢀ1
6.17”10ꢀ1
6.40”10ꢀ2
1.52”10ꢀ2
4.85”10ꢀ3
1.97”10ꢀ1
3.76”10ꢀ2
1.41”10ꢀ2
3-Cl, 4’-Cl
4-NO2
(1.50”10ꢀ3
)
3-Cl, 3’-Cl
3,5-(Cl)2, 3’-Cl
3,5-(Cl)2, 3’,5’-(Cl)2
4-OPh, 4’-NO2
H
4-Cl, 4’-Cl
3-Cl
3-Cl, 4’-Cl
6.82”10ꢀ4
1.82”10ꢀ5
4.00”10ꢀ7
8.60”10ꢀ1
3.94”10ꢀ1
6.98”10ꢀ2
1.58”10ꢀ2
6.59”10ꢀ3
Cl
TFE[f]
–
–
–
1652
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1648 – 1656

Solvolysis Kinetics
Table 1. (Continued)
FULL PAPER
[a]
Nucleofuge
Electrofuge
X,Y
ks [sꢀ1
]
kcalcd [sꢀ1
]
Ref.
Leaving group
Solvent[b]
TFE[f]
Cl
4-NO2
3-Cl, 3’-Cl
5.44”10ꢀ5 [e]
7.20”10ꢀ4
(8.87”10ꢀ4
4.38”10ꢀ4
)
[24]
–
Table 2. Nucleofugality parameters Nf and sf for leaving groups (LG) in various solvents.[a]
LG
Nf/sf
90A10W
80A20W
EtOH
80E20W
MeOH
TFE
TsO
Br
Cl
CF3CO2
DNB
PNB
5.42/0.89
2.31/1.00
0.69/0.99
0.12/0.94
ꢀ2.57/1.18
–
5.94/0.81
3.04/0.90
1.98/1.021.87/1.00
0.70/0.88
ꢀ2.23/1.13
–
6.05/0.75
2.97/0.92
7.46/0.79
4.39/0.94
3.28/0.98
–
9.82/0.89[b]
6.20/0.92
5.56/0.82
–
–
–
4.27/0.98
2.95/0.98
–
–
–
[c]
0.30/0.87
ꢀ2.04/1.10
–
1.46/0.82
ꢀ1.43/0.99
ꢀ2.84/0.94
[a] Mixtures of solvents are given as (v/v); solvents: A=acetone, E=ethanol, TFE=2,2,2-trifluoroethanol, W=water. [b] Only two solvolysis rate con-
stants were available for a tentative determination of Nf and sf. [c] The predefined slope parameter for chloride in ethanol (sf =1.00) was kept constant
during the optimization procedure.
X
X
X
2
2
D2
¼
ðlog ksꢀlog kcalcd
Þ
¼
ðlog ksꢀsfðNf þ EfÞÞ
ð4Þ
for the dianisylcarbenium ion (X, Y = 4-OMe, 4’-OMe) was
set to 0.00 (as was E for this carbocation), and sf for the
leaving group/solvent combination chloride/ethanol was set
to 1.00. Details of the optimization procedure are given in
the Supporting Information.
The good agreement between experimental and calculat-
ed rate constants shown in Table 1 and the graphical repre-
sentation of some of the linear correlations (Figure 1) show
that Equation (3) is indeed well-suited for correlating the
solvolysis rate constants listed in Table 1.
Table 3. Electrofugality (Ef) and electrophilicity (E) parameters of X,Y-
substituted benzhydryl derivatives.
No.
X
Y
Ef
E[a]
1
2
3
4
5
6
7
8
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-CH3
4-OPh
4-CH3
3,5-(CH3)2
4-OPh
4-F
3-CH3
H
4-Cl
4-Br
4-Cl
4-OCH3
4-OPh
4-CH3
H
4-CH3
H
H
H
4-NO2
H
H
H
H
H
4-Cl
H
4-Cl
H
3-Cl
3-Cl
3,5-(Cl)2
0.00[b]
ꢀ0.81
0.00
0.61
1.48
2.11
3.63
2.90
4.59
–
ꢀ1.29
ꢀ2.06
ꢀ3.47
ꢀ3.55
ꢀ4.68
ꢀ5.48
ꢀ5.64
ꢀ5.78
ꢀ5.78
ꢀ6.05
ꢀ6.52–
ꢀ6.67
ꢀ6.96
ꢀ7.74
ꢀ8.21
ꢀ9.26
ꢀ9.63
ꢀ11.34
ꢀ13.14
9
–
5.60
–
10
11
12
13
14
15
16
17
18
19
20
21
5.90
–
6.02
–
–
–
–
–
–
3-Cl
3-Cl
4-NO2
3-Cl
3,5-(Cl)2
3,5-(Cl)2
Figure 1. Plot of logks (258C, from Table 1) vs Ef (from Table 3) for the
solvolysis reactions of X,Y-substituted benzhydryl derivatives in various
solvents. Mixtures of solvents are given as (v/v); solvents: W=water, A=
acetone, E=ethanol, M=methanol, TFE=2,2,2-trifluoroethanol. The
~
)
solvolysis rate constant for the 4-nitrobenzhydryl chloride in TFE (
was not used for the correlation. In order to avoid overlaps, only 11 of
the 26 correlations are shown; all correlation lines are depicted in the
Supporting Information.
[a] From ref. [10a]. [b] The predefined electrofugality Ef =0 was kept
constant during the optimization procedure.
Chem. Eur. J. 2006, 12, 1648 – 1656
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1653

H. Mayr et al.
The similarity of the slopes sf implies that variation of the
carbocations has similar effects on the ionization rates of
different esters in different solvents. So far, we have not
been able to interpret the small differences in sf, which are
neglected in the following qualitative treatments.
Figure 2illustrates the increase of nucleofugality from 4-
nitrobenzoate to tosylate in 80% aqueous ethanol by more
Figure 3. Plot of Nf(LG) vs Nf(Cl) in different solvents. Mixtures of sol-
vents are given as (v/v); solvents: 90A=90% aq. acetone, 80A=80%
aq. acetone, E=ethanol, 80E=80% aq. ethanol.
calization of charge density in the picrate leaving group. Be-
cause of the small number of data points for the correlations
in Figure 3, we refrain from a quantitative discussion of the
slopes.
Figure 2. Nucleofugalities Nf for typical leaving groups in the reference
solvents used in this work (data from Table 2). Mixtures of solvents are
given as (v/v); solvents: 90A=90% aq. acetone, 80E=80% aq. ethanol,
100E=ethanol, TFE=2,2,2-trifluoroethanol.
The linear correlation between the electrofugality param-
eters Ef versus the Hammett s[37] constants, with a slope of
1=ꢀ4.39 (Figure 4), is in agreement with previous analyses
by Tsuno and Fujio, who systematically studied the devia-
tions of donor/acceptor-substituted benzhydrylium systems
from this correlation.[38]
than ten orders of magnitude. Since, for most leaving
groups, the reactivities in 100% ethanol and 80% aqueous
acetone are very similar (Tables 1 and 2), the latter parame-
ters could not be included in the graphical presentation of
Figure 2. Our method of assigning nucleofugality parameters
to combinations of leaving groups and solvents is an alterna-
tive to previous approaches that employ different Y scales
for different leaving groups.[3c,32–34]
The solvent effect on ionization rates, which is illustrated
by Nf in Table 2and Figure 2, is alternatively illustrated in
Figure 3. It can be seen that the solvent dependence is stron-
gest for Cl and decreases in the order Cl>BrꢂOTs>
CF3CO2 >DNB. This order correlates qualitatively with the
localization of charge in the anionic leaving group, that is,
the necessity of anion solvation, and not with the nucleofu-
gality Nf. In agreement with this observation, Bentley and
Roberts reported a slope of 0.7 when solvolysis rates of 1-
adamantyl trifluoroacetate in a variety of solvents were plot-
ted against the solvolysis rates of 1-adamantyl chloride
(258C).[35] An analogous correlation between the solvolysis
rate constants for 1-adamantyl picrate[36] and 1-adamantyl
chloride,[9] with a slope of 0.5, reflects the even better delo-
Figure 4. Correlation of the electrofugality parameters Ef of benzhydryli-
um ions (Table 3) with Hammett
s constants (from ref. [37]). Ef =
ꢀ4.39ꢀsꢀ6.14, n=20, r2 =0.9917; the deviating point of 4-nitro-4’-phe-
*
noxybenzhydrylium ( ) is not included in the correlation.
1654
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1648 – 1656

Solvolysis Kinetics
FULL PAPER
3,3’,5-trichlorobenzhydrol by treatment with n-butyllithium at ꢀ788C and
subsequent addition of tosyl chloride. All other benzhydryl tosylates
were prepared according to the procedure published by Cheeseman and
Poller[42] by addition of silver tosylate to solutions of the benzhydryl
chlorides in diethyl ether. Detailed procedures and characterizations of
the new compounds are given in the Supporting Information.
A comparison of the electrofugality parameters Ef with
the available electrophilicity parameters E (Table 3) shows
that in most cases Ef ꢂꢀE, but that the 4-phenoxy- and 4,4’-
dichloro-substituted benzhydrylium ions are poorer electro-
fuges than expected on the basis of their electrophilicities
(Figure 5). The reason for these deviations is not clear at
present. It now has to be examined whether the good corre-
lation between electrofugality Ef and electrophilicity E
shown in Figure 5 also holds for other types of carbocations
and for systems with Ef >0 and Ef <ꢀ6.
Kinetics: Solvolysis rates of benzhydrylium derivatives (Table 1) were
monitored by following the increase in the conductivity of the reaction
mixtures (conductimeters: WTW LF530 or Tacussel CD 810, Pt elec-
trode: WTW LTA 1/NS). In the cases of 4-nitrobenzoates, 3,5-dinitroben-
zoates, and trifluoroacetates, organic bases were added to ionize the
weak acids produced by the solvolysis reactions. Details of the kinetic
measurements are given in the Supporting Information.
Acknowledgements
We thank Prof. D. N. Kevill for helpful discussions, and Dr. B. Kempf
and Dr. U. Wiesner for technical assistance. Financial support by the
Deutsche Forschungsgemeinschaft (Ma-673/20-1), the Ministry of Sci-
ence, Education and Sport of the Republic of Croatia, and the Fonds der
Chemischen Industrie is gratefully acknowledged.
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electrophilicity parameters E (Ef =ꢀ1.02E+0.04, n=8, r2 =0.9962; elec-
trofuges 6 and 15 are not considered in the correlation).
Conclusion
The excellent correlations shown in Figure 1 indicate that
our goal of developing a basis for a general nucleofugality
scale has indeed been achieved. In the succeeding paper, we
will show how this method can be employed for characteriz-
ing other nucleofuges and electrofuges and for estimating
absolute solvolysis rate constants.
Experimental Section
Caution: One or more of the chloro-substituted benzhydryl compounds
cause severe irritation of the skin and should be handled with extreme
care.
Benzhydryl derivatives: Chloro-substituted benzhydrols were prepared
by reactions of substituted benzaldehydes with Grignard reagents, which
were obtained from bromoarenes. Treatment of benzhydrol solutions in
benzene with 4-nitrobenzoyl chloride or 3,5-dinitrobenzoyl chloride in
the presence of pyridine furnished benzhydryl 4-nitrobenzoates and 3,5-
dinitrobenzoates, respectively.[39,40] Benzhydryl trifluoroacetates were pre-
pared from benzhydrols and trifluoroacetic anhydride in dry diethyl
ether according to the procedure published by Bunton and Hadwick.[41]
Benzhydryl chlorides were obtained from the reaction of benzhydrols
with thionyl chloride in dichloromethane. Benzhydryl bromides were pre-
pared by treatment of benzhydrols with phosphorus tribromide in di-
chloromethane. 3,3’,5-Trichlorobenzhydryl tosylate was obtained from
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1655

H. Mayr et al.
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[31] Parameters Ef, Nf, and sf have been calculated by minimizing ꢀD2,
where D2 =(logksꢀlogkcalcd)2 =(logksꢀsf(Nf+Ef))2, by using the pro-
gram “WhatꢀsBest! 7.0 Professional” by Lindo Systems.
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in detail in ref. [34].
´
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