4678 J . Org. Chem., Vol. 62, No. 14, 1997
Gandler et al.
are almost the same (32.611 and 36,11 respectively, at 25
°C), for the deprotonation reactions carried out in these
solvents, the contribution from a classical solvent effect
to the observed solvent effect is expected to be small.11
The change in the intrinsic rate constant for the reaction
carried out in these solvents should therefore be a more
direct measure of the solvent effect that can be attributed
to transition state imbalances (PNS effects). The analy-
sis of the solvent effect reported here (a 104-105 increase
in the intrinsic rate constant for reaction in acetonitrile
compared to methanol) is consistent with a solvent effect
that results from transition state imbalance (PNS ef-
fects), and a transition state hydrogen bond between the
carbon acid and base that is stronger in acetonitrile than
in methanol solution.
Buffer solutions were prepared by dissolving the buffer acid
and its potassium salt in methanol.
Equ ilibr iu m Con sta n ts. Acidity constants for the arylni-
tromethanes in methanol were determined spectrophotometri-
cally relative to benzoic acid (pKa ) 9.4).2 Equilibrium acidity
constants were determined in benzoate buffer solutions with
buffer ratios (PhCOOH/PhCOO-) ranging from 0.0758 to 0.152
for (4-nitrophenyl)nitromethane, from 0.0710 to 0.155 for (3-
nitrophenyl)nitromethane, and from 0.100 to 0.590 for (3,5-
dinitrophenyl)nitromethane. The pKa values of the substrates
were evaluated using the equation KCH ) Ka{[C-]/[CH]}({[BH]/
[B]}, were KCH is the acid dissociation constant of the
arylnitromethane, Ka is the acid dissociation constant of
benzoic acid in methanol (Ka ) 1.98 × 10-10 M),11 {[C]/[CH]}
is the arylnitromethyl anion/arylnitromethane ratio, and
{[BH]/[B]} is the benzoic acid/benzoate ion buffer ratio.
Tr a n sfer Activity Coefficien ts. The transfer activity
coefficient from methanol to acetonitrile solution of the (3-
nitrophenyl)nitromethyl anion was calculated using the ex-
Exp er im en ta l Section
M
pression, log MγAN(C-) )
∆
ANpKCH - log MγAN(H+) + log
M
MγAN(CH), where
∆
ANpKCH is the difference in pKa of the
Ma ter ia ls. Methanol was purified by distillation using a
2 ft fractionating column. Potassium benzoate, 3-bromoben-
zoate, 3,4-dimethylbenzoate, and 3,4-dichlorobenzoate were
prepared following the general procedure described by Kolthoff
and Chantooni.12 These salts were dried by several azeotropic
distillations with toluene as cosolvent and then in vacuo over
P2O5. The purity of the benzoate salts were determined by
potentiometric titration against 0.0600 M HCl in 90 volume
percent DMSO in water. The purities determined in this way
for potassium benzoate, m-bromobenzoate, 3,4-dimethylben-
zoate, and 3,4-dichlorobenzoate were 103%, 97%, 98%, and
98%, respectively.
arylnitromethane in methanol relative to acetonitrile and MγAN
is the transfer activity coefficient of the arylnitromethyl anion
(C-), the lyonium ion (H+), and the arylnitromethane (CH),
respectively. The transfer activity coefficients are equal to
∆G°/2.303RT, where ∆G° is the standard free-energy of
transfer of a molecule or ion from methanol to acetonitrile
solution. The transfer activity coefficient of (3-nitrophenyl)-
nitromethane was determined from solubility measurements
as described by Kolthoff.17 The solubility was measured by
saturating 1.00 mL of methanol with (3-nitrophenyl)ni-
tromethane. This solution was made 0.001 M in HCl to
supress ionization of the substrate. The saturated solution of
the substrate was kept in a shaker bath at 25.1 °C for 24 h. A
1 or 2 µL amount of this solution was then injected into 3.00
mL of acetonitrile, and the absorbance values of the resulting
solutions were determined at 258 nm. At 258 nm, in aceto-
nitrile solution, (3-nitrophenyl)nitromethane has a molar
(3,5-Din itr op h en yl)ben zyl Br om id e.13 In
a 250 mL
three-necked flask, under nitrogen, was dissolved 7.5 g of 3,5-
dinitrobenzyl alcohol in 75 mL of CHCl3. To this solution was
added dropwise, over a 30 min period, 10.25 g of PBr3 dissolved
in 15 mL of CHCl3. After the addition was complete, the
solution was heated at reflux for an additional 2 h and then
left overnight at room temperature. After workup, the final
product was recrystallized from ethanol to yield 7.2 g of yellow
crystals, mp 97-98 °C (lit. mp13 65-66 °C and mp14 97-99
°C). 1H NMR (CDCl3) δ 4.63 (2H, s), 8.63 (2H, s), 9.10 (1H, s).
(3,5-Din itr op h en yl)ben zyl Iod id e.14 With a Na vapor
lamp as the only light source, 3.5 g of 3,5-dinitrobenzyl bromide
was dissolved in dry acetone. This solution was slowly added,
with stirring, to 90 mL of a 10% w/w solution of NaI in acetone.
The resulting reaction was instantaneous. After workup the
product (3.72 g) was recrystallized from ethanol, mp 89-90.5
°C (lit.14 mp 91.5-93 °C). 1H NMR (CDCl3) δ 4.63 (2H, s), 8.63
(2H, s), 8.95 (1H, s).
absorbtivity constant of 0.770 × 104 M-1 cm-1 7
. The solubility
of (3-nitrophenyl)nitromethane determined in this way was
0.219 M.
Resu lts
The deprotonation of an arylnitromethane by benzoate
ions in methanol solution can be described by eqs 1 and
2, where eqs 1 and 2 represent the deprotonation (re-
protonation) reactions promoted by benzoate ion (and
benzoic acid) and methoxide ion (and methanol), respec-
tively.
(3,5-Din itr op h en yl)n itr om eth a n e.15 In dry ether, 3.7 g
of 3,5-dinitrobenzyl iodide was added over a 1 h period to a
slurry of 4.7 g of AgNO2 in ether. The mixture was continu-
ously stirred and kept at 1 °C for 12 h. After evaporation of
the ether, the crude product was recrystallized from ethanol
(0.2 g yield), mp 128-129 °C (lit.16 mp 129-130 °C). 1H NMR
(CDCl3) δ 5.68 (2H, s), 8.72 (2H, s), 9.18 (1H, s).
Kin etics. Kinetics were carried out by following spectro-
photometrically the appearance of the arylnitromethane an-
ions at the λmax of the anions: 312 nm (ꢀ 2.19 × 104 M-1 cm-1),
394 nm (ꢀ 1.80 × 104 M-1 cm-1), and 305 nm (ꢀ 1.55 × 104 M-1
cm-1) for the anions of (3-nitrophenyl)nitromethane, (4-nitro-
phenyl)nitromethane, and (3,5-dinitrophenyl)nitromethane,
respectively. Reactions were carried out at 25.0 °C in metha-
nol under pseudo-first-order conditions, in the presence of
excess buffer base and acid. Plots of ln(A∞ - A) against time
(using the program Enzfitter) obeyed pseudo-first-order kinet-
ics and were monitored for at least two to three half-lives.
k1
-
ArCH2NO2 + Ar′CO2
{ }
k-1
ArCHdNO2- + Ar′CO2H (1)
M
ArCH2NO2 + MeO- { k1 } ArCHdNO2- + MeOH (2)
M
k-1
Under pseudo-first-order conditions, kobsd is given by
eq 3.
kobsd ) k1[Ar′CO2-] + k-1[Ar′CO2H] +
M
k1M[MeO-] + k-1 (3)
Equation 3 may be rearranged to eq 4, where r ) [BH]/
[B-], the benzoate buffer ratio, and Keq ) k1/k-1
.
(12) Kolthoff, I. M.; Chantooni, M. K. J . Phys. Chem. 1966, 70, 856.
(13) Krohnke, F.; Schmeiss, H. Chem. Ber. 1939, 72, 440.
(14) Palmer, C. A. M.S. Thesis, San Francisco State University,
1978.
(15) Kornblum, N.; Smiley, R. A.; Blackwood, R. K.; Iffland, D. C.
J . Am. Chem. Soc. 1955, 77, 6269.
kobsd/(1 + r/Keq) ) k1[Ar′CO-] + {k1M[MeO-] +
k-1M}/(1 + r/Keq) (4)
Figure 1 shows a plot of kobsd/(1 + r/Keq) against
benzoate ion concentration for the deprotonation reac-
(16) Feiser, L. F.; Gates, M. J . Am. Chem. Soc. 1946, 68, 1122.
(17) Chantooni, M. K.; Kolthoff, I. M. J . Phys. Chem. 1973, 77, 527.