K. Minksztym, A. Jarczewski / Journal of Molecular Structure 691 (2004) 203–209
205
Then, there was no need to use the B/BHþ buffer to
eliminate the noxious effect of common BHþ cation [27].
The increase of absorbance of the anionic product of the
proton transfer was monitored at the wavelength character-
istic for the product peak of the corresponding C-acid 1, 2,
and 3 and were measured under conditions of a large excess
of the base over C-acid (1 and 2 (2 £ 1025 M) and 3
(4 £ 1025 M)) for six temperatures within the range of
10–35 8C. The pseudo-first order rate constants, kobs were in
the range of stopped flow method and were fitted to equation
kobs ¼ k2 [B] þ int. Where k2 is the rate constant for the
forward reaction and intercept (int.) consists possible
contribution of backward reaction. The results of the kinetic
measurements are collected in Table 1.
into consideration of the C-acids activation seems to be
para-nitro group, as can be seen when comparing
diphenylacetonitrile pKa ¼ 28:2 and (4-nitrophenyl)phe-
nylcyanomethane 0 pKa ¼ 22:7 [19]. The introduction of
alkyls R (Me, Et, i-Pr) into 2,6 positions of the phenyl ring
(Scheme 1) reduces acidity by 3 pKa units [19] by two
effects: the steric inhibition of 4-nitrophenyl group
mesomerism and, in smaller extent, the electron donor
influence of alkyls.
Subsequent introduction of alkyls into 2,6-positions of
phenyl ring of 0 causes considerable change of the values of
lmax and molar absorbtivity coefficient of the anionic
product. The lmax ¼ 590 nm and the molar absorptivity 1 ¼
40; 000 for the product of reaction 0 with 1,1,3,3-
tetramethylguanidine (TMG) base in acetonitrile at 25 8C
change to lmax ¼ 597 nm and 1 ¼ 24; 000; when one ortho-
methyl group is introduced as in (2-methyl-4-nitrophenyl)-
phenylcyanomethane 4 [8,28,29]. The estimated angle of
the planarity deviation of the carbanion, based on the above
observation, is equal 398. The introduction of the second
methyl group causes batochromic shift to lmax ¼ 671 nm.
The molar absorptivity of the products of above reactions
with DBU base in acetonitrile decreases by 22,000 units [8].
In the 1, 2, 3 order the pKa increase is small, possibly
because deviation of carbanion planarity reaches maximum
limit and carbanion’s formation reduces steric interactions
of the substituents. Those last reflect the restricted rotation
of Et and i-Pr group of carbon acids 2 and 3 observed in the
1H NMR spectra. The signals of diastereotropic methylene
protons of 2 are two superimposed in small extent multi-
plets. One of them, at the lower field is well-defined septet
the second one is suppressed and blurred. Also, the triplet of
methyls reveals broadening and loss of intensity. On the
other hand, in the spectrum of the carbon acid 3 the signal of
the one diastereotropic methyl at the lower field is a well
resolved doublet and the second one is low intensity broad
singlet.
Unfortunately, the carbanionic product in each reaction
slowly decomposes, so special precautions were undertaken
to diminish this effect. Therefore, the solutions of reactants
were freshly prepared and kept under Argon free from
moisture, CO2 and O2. These precautions eliminate
decomposition in the region of minutes, while the kinetic
measurements were complete in less than 30 s for the
slowest reaction in the group 1, 2, and 3 (H) at 10 8C. To
eliminate eventual influence of destruction of the carbanion
formed in case of deuterated analogues 1 (D) and 3 (D), the
undistorted part of the kinetic curves were taken for
estimation of the rate constants kobs: As can be seen from
Table 1, the standard errors for deuteron transfer are in the
same range of accuracy as for the proton transfer.
The carbanionic product formed in the reaction systems
with C-acids activated by cyano and 4-nitrophenyl groups
(1, 2, 3) seems to be different comparing to nitroalkanes [16].
The bond length changes on the way from the parent
acids to their anions after the proton abstraction. With the
˚
nitriles, the C–CN bond shortens by ,0.05 A, depending
on the mother compound of nitrile, and the C ; N lengthens
˚
by 0.02 A [17]. In case of nitro-compounds, formation of the
˚
anion leads to a 0.15 A decrease in the C–N bond length and
In this series, small pKa changes are accompanied by
relatively high changes of lmax and 1: The replacement of
two methyls by two ethyls and later two isopropyls causes
again batochromic shift to lmax ¼ 695 and 730 nm,
respectively. The molar absorptivity of the products of the
reactions of 1, 2, 3 with MTBD in acetonitrile decreases in
the following sequence 26,000, 19,000 and 8000. The
confrontation of two series of carbon acids 0, 4, 1 and 1, 2, 3
shows lack of correlation of pKa values with their anions’
spectroscopic parameters lmax and 1: Additionally, the large
and declining values of the molar absorptivity coefficients
suggest that the delocalization of charge within nitrophenyl
group could not be neglected [16,17]. The phenyl
substituent brings on a moderate contribution (2.55 pKa
units) to the acidity as can be seen from the pKa ¼ 25:25 and
22.7 for 4-nitrophenylcyanomethane 5 and (4-nitrophenyl)-
phenylcyanomethane 0, respectively, [19] (Scheme 3).
Bernasconi [30,31] has pointed out that the trend toward
lower intrinsic rate constants k0 for the proton transfer
˚
a 0.05 A increase in the N–O length. So these changes
imply that there is extensive charge transfer from carba-
nionic carbon atom to electron withdrawing group via a p
interaction with nitro compounds [5], and to a much smaller
extent with the nitriles [17] (Scheme 2).
The NMR provides the p electron density at the
carbanion carbon although the majority of papers report
total atomic charges rather than their s and p contributions
[17]. Nevertheless, the activating contribution of the cyano
group accounts for large increase of acidity, going from
bis(4-nitrophenyl)methane pKa ¼ 26:05 to bis(4-nitrophe-
nyl)cyanomethane pKa ¼ 19 [19]. The next one, taking
Scheme 2.