Selected Papers
Gas-Phase Acidities of ¡- and ¡,¡-SO
2
CF
3
-Substituted Toluenes.
Varying Resonance Demand in the Electron-Rich System
Min Zhang,
1
Md. Mizanur Rahman Badal,
1
Ilmar A. Koppel,
2
and Masaaki Mishima*
1
1
Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki,Higashi-ku, Fukuoka 812-8581
2
Institute of Chemistry, University of Tartu, Ravila 14a Street, 50411, Tartu, Estonia
Received February 20, 2013; E-mail:mishima@ms.ifoc.kyushu-u.ac.jp
The gas-phase acidities (GA) of aryl(trifluoromethylsulfonyl)methanes (ArCH
2
SO
2
CF
3
; 1) and arylbis(trifluoro-
methylsulfonyl)methanes (ArCH(SO
2
CF
3
)
2
; 2) were determined by measuring proton-transfer equilibria. Substituent
effects for acidities of a series of ArCH(R
1
)R
2
including 1 and 2 have been analyzed successfully in terms of the Yukawa-
Tsuno equation. The resonance demand parameter r
¹
value was found to decrease linearlywith increasing acidity of
the GA values of the unsubstituted parent carbon acids, and the change of the r
¹
value was correlated with the acidifying
effect of the phenyl group (R = Ph) in the RCH
2
(R
1
)R
2
. In addition, the geometric features and natural charges of the
conjugate anions calculated at B3LYP/6-311+G(d,p) were found to be correlated linearlywith the r
¹
values. Such
behavior of the resonance demand parameter in the electron-rich system, ArC
¹
(R
1
)R
2
, is completely consistent with that
observed for the electron-deficient system, ArC
+
(R
1
)R
2
, revealing that the resonance demand is contingent upon the
structure of carbanions and carbocations. Furthermore, it was found that the μ values also decreased with increasing
acidity of the GA values of the unsubstituted parent carbon acids. This would be related to the distribution of the charge
between the aromatic moiety and the C(R
1
)R
2
moiety.
Gas-phase proton-transfer experiments have played a key
role for separating structural effects from medium effects
on organic reactivity, providing new insights into the intrin-
sic nature of the resonance demand in the substituent effect
which is one of the most important concepts in physical
organic chemistry.
1-5
The substituent effects on the thermo-
dynamic stability (¦G° in kcal mol
¹1
)of benzylic carbocation,
XC
6
H
4
C
+
(R
1
)R
2
, were found to be described in terms of
the Yukawa-Tsuno eq 1 using a gas-phase set of substituent
constants of which values differ slightly from the standard
values determined in aqueous solution.
6-11
¤G
¼ μð·
þ r
þ
·
þ
R
Þ ð1Þ
where · ° and ·
þ
R
are the normal substituent constant and
the resonance substituent constant, respectively, and r
+
is the
resonance demand parameter representing the degree of the
³-delocalization of the positive charge into the aryl ³-system.
These correlation results revealed that the resonance demand
parameter (r
+
) varies significantlywith the stability of the
unsubstituted parent carbocations (X = H), i.e., the r
+
value is
correlated linearly as the following eq 2.
11
r
þ
¼ 0:026G
ðX=HÞ
þ 1:00 ð2Þ
where ¦¦G°
(X=H)
is the stability of the unsubstituted parent
benzylic carbocation relative to ¡-cumyl cation. Furthermore,
the r
+
value was described in terms of substituent constants for
fixed substituents, R
1
and R
2
,
r
þ
¼ 0:45·
þ 0:40 ·
þ
R
þ 1:28 ð3Þ
where -· ° and ·
þ
R
are sums for · ° and ·
þ
R
of R
1
and R
2
,
respectively, indicating that the change of the r
+
value can be
related quantitativelywith both field/inductive and resonance
effects of R
1
and R
2
substituents. These results suggest that
the degree of ³-delocalization in the benzylic carbocation
is determined by the intrinsic properties of the structure of
the unsubstituted parent carbocation. Similar correlations were
observed for the gas-phase basicities of benzoyl compounds,
ArCOR,
12
and arylacetylenes, ArC¸CR,
13-15
of which the
conjugate acids are benzylic carbocations, ArC
+
(OH)R and
ArC
+
=CHR, respectively. In addition, similar relationships
were observed for the substituent effects of electron affinities
of aromatic compounds, XC
6
H
4
R where R isa fixed sub-
stituent.
16,17
Therefore, it is interesting to study whether the
resonance demand parameter in the substituent effect shows
similar behavior for the electron-rich system in which the
³-delocalization effect of para-³-acceptor should be exalted.
The gas-phase substituent effects in such electron-rich systems
themselves are important for understanding the intrinsic nature
of resonance effects because strong ³-acceptors such as NO
2
,
NO, and CHO groups at para-position in solution are signifi-
cantly influenced by specific hydrogen-bonding solvation,
i.e., substituent solvation assisted resonance (SSAR) effects
as noted by Taft et al.
2-4
We thus decided to examine the effects
of ¡-substituents on the resonance demand for the gas-phase
acidity of ¡,¡-substituted toluene derivatives, ArCH(R
1
)R
2
,
as a measure of the thermodynamic stabilities of benzylic
carbanions. The gas-phase acidity values are known for several
series of benzylic carbon acids.
18,19
In this study, the gas-phase
© 2013 The Chemical Society of Japan
Published on the web April 13, 2013; doi:10.1246/bcsj.20130052
Bull. Chem. Soc. Jpn. Vol. 86, No. 7, 813-820 (2013) 813