M. Matić et al., Nucleofugality o fAliphatic Carboxylates in Mixtures ofAprotic Solvents and Water
381
Table 3. Statistical data for the correlation of AG* for hydroly-
sis of dianisylmethyl carboxylates in different solvents with
the corresponding AG*,model obtained at the IEFPCM-M06-
2X/AUG-cc-pVTZ level of theory
tions 2. MAEs for N f k values, which are in average
for 0.1-0.2 larger than the standard errors for the exper-
imental Nf values given in Table 2, can be taken as a
verification of the suitability of the model used. Also the
fact that the range of nucleofugalities for carboxylates
determined here for each solvent is about 7 units, while
the corresponding MAEs are 0.22, 0.24, and 0.26, re-
spectively, represents a further proof for reliability of
the presented model for the semiquantitative determina-
tion of the solvolytic reactivity of a given carboxylate.
fi)
Solvent00
60A40W
Slope(b)
Intercept(b’c)
-9.36 ± 1.10
-10.68 ± 1.25
-10.10 ± 1.07
MAE(c,e)
0.28
B®
10
10
11
0.99 ±0.04
0.994
0.994
0.994
80AN20W 1.05 ±0.04
60AN40W 1.00 ±0.04
0.33
0.30
00 Binary solvents are expressed as volume fractions at 25 °C:
A = acetone, AN = acetonitrile, W = water. Free energies of
activation for the model reaction (AG*>modeI) used in correla-
tions are given in Table S3 of Reference 6.
<b)Errors shown are standard errors.
Reactivity of Other Carboxylates
Once a method has been verified, we have estimated
barriers and rate constants for solvolysis of 34 other
dianisylmethyl carboxylates in aqueous acetonitrile and
acetone. We used the computed AG*,model values for 34
different 2,3-dihydroxycyclopropyl /ram-carboxylates
(published in Reference 6) to interpolate and extrapolate
the Gibbs free energies of activation (in a given solvent)
for solvolysis of the corresponding 34 dianisylmethyl
carboxylates from the AG*,exp vs. AG*,model correlation
lines presented above (Figure 3 and Table 3). The esti-
mated reaction rates of various dianisylmethyl carbox-
ylates in 60 %, and 80 % aqueous acetonitrile and in 60
% aqueous acetone are given in Table 5. The linear
correlations between the logarithms of solvolysis rates
and the sr parameters (given by Equations 2a, 2b, and
2c) enabled estimating the slope parameter (s“‘lim) of a
given nucleofuge in a given solvent. Similarly, as it is
presented above, from the rate constants (log /balc) for
dianisylmethyl derivatives and the corresponding s(eslim
parameters, the nucleofugality for each new carboxylate
in aprotic solvents/water mixtures has been determined
from Equation 1. The results are presented in Table 5.
In Figure 4 the nucleofugalities of the aliphatic car-
boxylates determined here in 60 % aq. acetone (experi-
mental and calculated) are compared with nucleofugali-
ties of some selected leaving groups published earlier.
For example, it can be seen that the most reactive carbox-
ylate leaving group examined here is trinitroacetate, while
the least reactive is the malonate leaving group. Further,
monohalogenated acetates fall in a narrow range of reac-
tivity between p-nitrobenzoate and 3,5-dinitrobenzoate.
Figure 4 also shows that the effects of the substituents are
cumulative. Each additional halogen atom (experimental
Nt) or nitro group (calculated Nf) introduced onto the
carboxylate moiety increases the nucleofugality for ap-
proximately the same number of units of Nf, due to more
pronounced inductive effects.
<c)In kcal mol-1.
(d) Correlation coefficient.
(<0Mean absolute error.
<f) The number of correlation data points. Experimental AG*
for solvolysis of acetate in all solvents used were taken from
Reference 5. Experimental AG* for solvolysis of heptafluoro-
butyrate and trifluoroacetate in 60A40W were taken from
Reference 2.
Additional verification of the method can be ac-
quired examining individual deviations of the calculated
kinetic parameters (log&calc,s“t'm, andN f k) from the
experimental values given in Tables 1 and 2. Therefore,
from the AG*,exp/AG*,model correlation plots (Table 3,
Figure 3), log &°aIc for 11 reference dianisylmethyl car-
boxylates have been determined according to the equa-
tion: AG*,model = RT\\n{kfh) - ln(^calc/7) in which ks is
the Boltzmann constant, h is the Planck constant, and R
is the gas constant. Also, from the correlations given
with Equations 2, s®1™ for the reference carboxylate
leaving groups have been estimated for 60 % aq. ace-
tone, 80 % aq. acetonitrile and 60 % aq. acetonitrile, and
compared with the experimental values. Finally, from
log k ^ c and the corresponding
, values for Nf'c
for the reference carboxylates have been calculated
using Equation 1, and also compared with the corre-
sponding experimental values in the terms of indi-
vidual deviations. All calculated kinetic parameters
(logkcalc,i’“t'n’, and N f k) and the individual deviations
are presented in Table 4. To make the comparison be-
tween the calculated and experimental values easier,
from the individual deviations given in Table 4, the
mean absolute errors (MAE) have been calculated for
both log k°alc and N f k values for each of three aqueous
solvents. MAEs for log k°alc and N f k in 60 % aq. ace-
tone, 80 % aq. acetonitrile and 60 % aq. acetonitrile are
essentially the same (log lčMc: 0.21, 0.24 and 0.22;
A7alc: 0.22, 0.24 and 0.26, respectively), showing that
the contribution of s“tim to the error in N f k is negligi-
ble. Indeed, the comparison between the experimental
and estimated st values given in Table 4 reveals agree-
ments in the limits of experimental error, verifying the
validity of method for calculating .S'“ tlm values by Equa-
In summary, nucleofugalities (experimental and
calculated) are now available for more than forty differ-
ent aliphatic carboxylates in aqueous ethanol,3 acetone
and acetonitrile, which together with nucleofugalities of
substituted benzoates determined earlier,10 constitute an
Croat. Chem. Acta 87 (2014) 375.
Mati, Mirela
