1024 Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Alternative Mechanistic Scheme for Salt Effects
Olah26 reviewed the significance of carbocations in chem-
istry; in general, stable carbocations are prepared in superacidic
media. Schaller and Mayr27 made it possible to monitor
photometrically the formation of the carbocation of 4,4¤-
dimorpholinobenzhydryl carboxylates in aqueous acetone and
acetonitrile. However, we have discovered that stable carboca-
tions can be produced from trityl halides by addition of the
perchlorates salts of Li+, Na+, Mg2+ (Ca2+, Sr2+), and Ba2+ in
acetonitrile;28-30 which have been detected by UV-visible,28
1H and 13C NMR spectroscopy.29,30a For a fluoran-based black
color former in acetonitrile, the color has been developed by
the addition of the alkali metal (M+) or alkaline earth metal
(M2+) perchlorates.30 The extraordinary chemical reactions
with alkali metal and alkaline earth metal ions in higher relative
permittivity media, which possess lower solvation abilities,
have been reviewed.31 It is an easy task for us to produce the
CIPs and SSIPs separately from nitrophenols in the presence of
appropriate bases in acetonitrile: the addition of MClO4 and
M(ClO4)2 causes the formation of CIPs between the phenolate
and M+ or M2+ ions while Et4NX (X = Cl or Br) produces the
free phenolate ions or SSIPs.32
In order to confirm further our proposal in previous
solvolytic studies,17-21 we examine the influence of various
types of salts, MClO4, M(ClO4)2, and R4NX (R = Et, Pr, or
n-Bu; X = ClO4, Cl, Br, or OTs) on the solvolysis reactions
of typical SN1 and SN2 substrates in a 75% (v/v) DMSO/H2O
solvent mixture. The relative permittivity and basicity of
DMSO are rather high, ¾r = 46.533 and Gutmann’s donor
number (DN) = 29.8,34 while the acidity of the solvent is low
or small, e.g., the acceptor number (AN) = 19.3.34 Solubilities
of substrates are insufficient in a 50% (v/v) DMSO/H2O
mixture, therefore, a system with a higher DMSO content is
utilized. The substitution (or “anion exchange”) reactions of
leaving groups with added salt anions for typical SN2 substrates
are examined in the same binary solvent system. A linear
relationship is observed between the stabilities of carboca-
tions (¦G○) and the Mg(ClO4)2 effects on the solvolysis rate
constants of the various substrates (SN1, SN1-SN2 borderline,
and SN2). In a previous study,21 the relation between the
LiClO4 effects vs. ¦G○ was examined.
+ M+, M2+
+
δ+
X
X δ -
M+, M2+
Scheme 1. The proposed mechanism for SN1 solvolysis in
the presence of MClO4 and M(ClO4)2.
compounds.17-21 Exponential increases in rate constants of
typical SN1 substrates are observed with increasing concen-
tration of added alkali metal (M+) and alkaline earth metal
(M2+) perchlorates: in most cases, the cation effects increase
in the order Na+ < Li+ < Mg2+, Ba2+ in 80% (v/v) MeOH/
H2O (¾r = 49),17 and 50% (v/v) organic solvent systems of
acetone/H2O (¾r = 55),18 1,4-dioxane/H2O (¾r = 44),19 sulfo-
lane/H2O,20 and DMF/ or DMA/H2O.21 Citing our papers,
Ji et al.22 have mentioned that salt effects by LiClO4 on the
reaction rates can be used to distinguish between SN1 and SN2
processes. We have also established that the solvolysis rate
constants of typical SN1 substrates should decrease in the pres-
ence of nonmetallic salts at higher concentrations because of a
decrease in water activity.17-21 Judging from the rate increase
¹3
[log(k1/k0)] in the presence of 1.0 mol dm LiClO4, we can
indicate that a phenyl group of the substituted halomethane
acts just as two methyl groups would on the carbon center.19
A correlation has been observed between the ·+ values in the
Hammett equation1c and the ¦log(k/s¹1) for substituted benzyl
halides upon the addition of 1.0 mol dm LiClO4, as well as
between the log(kx/kH) values themselves.19
¹3
The mechanism of the metal cation effect on a SN1 substrate
can be illustrated by Scheme 1. In an “aqueous medium,”
the direct chemical interaction should operate between added
metal cations and the leaving-group anion. However, some
difficulties may arise from the following two points: (a) The
coordination ability of alkali metal and alkaline earth metal
ions should be much weaker than that of transition-metal ions
because of the lack of partly filled d- or f-shells. (b) In aqueous
solution, strong solvation (hydration) may shade completely
the chemically interacting sites not only on alkali metal or
alkaline earth metal cations but also on anions, even if they
have some potential for such chemical interaction in addition
to the electrostatic interaction.
Nevertheless, we have postulated that a very small but direct
“chemical” interaction can operate between M+ or M2+ and
simple anions, such as halide ions, even in aqueous or organic-
aqueous solutions when water molecules are modified to
“dihydrogen ether” ([R](H)-O-(H)[R])17-21 conditions. In a
review article, Fromm23 stated that “the coordination chemistry
of group 1 and 2 metal compounds with organic ligands in
the widest sense has been, until relatively recently, largely
unknown compared to transition-metal coordination networks.”
Reichardt et al.24 described, citing our papers17,19 that, at high
salt concentrations (c > 5 mol dm¹3), region C of ion solvation,
according to Frank and Wen,25 can be abolished and only
regions A and B survive, resulting an aqueous solvent called
“dihydrogen ether.”
The nitration of phenols in reversed micelle systems has
been reported and an enhanced oxidation ability of diluted
nitric acid (<2.0 molarity) has also been demonstrated in
concentrated salt (bulk) solutions.35 Apparent contradictions
between NMR and Raman evidence have been discussed and
integrated successfully in terms of a distortion of the bulk water
structure in the presence of high salt concentrations, such as
Li+ and Na+ as well as Mg2+ and Ca2+, in aqueous solution.35
Results and Discussion
Metal Salt Effects on a Typical SN1 Substrate. Figure 1
shows the changes in the solvolysis rate of 1-bromoadamantane
with the addition of alkali metal or alkaline earth metal
perchlorates in the 75% (v/v) DMSO/H2O solvent mix-
ture at 50 °C. The “pseudo” first-order reaction rate constant
(k/s¹1) increases exponentially with increasing concentration
of LiClO4, NaClO4, Mg(ClO4)2, or Ba(ClO4)2, i.e., linearity is
observed between log(k/s¹1) and the concentration of each
metal perchlorate. The effects of metallic ions enlarge in the