1. Introduction
The Grunwald-Winstein equation (
equation 1) was developed [
1] in 1948 for the correlation of
specific rates of solvolysis of reactions which proceed
via a rate-determining ionization (S
N1 + E1). The original standard substrate,
tert-butyl chloride, has been replaced by 1-adamantyl chloride [
2] and either 1- or 2- adamantyl derivatives have been used to set up a series of
YX scales for the X leaving group [
3]. Since, with both substrates, a double bond would develop at the bridgehead, which is energetically disfavored (Bredt’s Rule), only substitution reaction is observed. In
equation 1,
k and
ko represent the specific rates of solvolysis in the solvent under consideration and in an arbitrarily chosen [
1] standard solvent (80% ethanol),
m represents the sensitivity to changes in the solvent ionizing power value (
Y), and
c is a constant (residual) term.
It was realized that
equation 1 could not be expected to apply to solvolyses where the solvent also participates in the rate-determining step of a bimolecular process (S
N2 + E2) and it was proposed [
4] that an additional term governed by the sensitivity (
ℓ) to changes in solvent nucleophilicity (
N) [
5,
6] should be added (
equation 2).
Since for the commonly used solvents, such as aqueous ethanol, methanol, or acetone, the
N values tend to the increase in a fairly uniform manner as the
Y values decreases [
7], multicollinearity can be a major problem if solvents are restricted to these classes. Fortunately, commercially available fluoroalcohols [2,2,2–trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)] in varying proportions with water show very different relationships between
N and
Y values [
8,
9] decrease. These fluoroalcohol-containing solvents play an important role in studies designed to allow a meaningful application of
equation 2.
For reactions in which an aromatic ring is attached to the site of developing charge, dispersion in simple (
equation 1) plots is often observed independent of any dispersion that might arise due to nucleophilic solvation effects [
10–
13]. This can be treated by the development of similarity models, using similarly constituted standard substrates to develop new ionizing power scales [
14,
15]. This is, however, very labor intensive and a new scale must be developed based not only on each leaving group but also on the number of aromatic rings entering into conjugation with the developing positive charge [
16]. We have favored an alternative approach [
10–
13] in which one can continue to use an established
YX scale [
3], accompanied by the introduction of a term governed by the sensitivity (
h) to solvent-induced changes in an aromatic-ring parameter (
I), as shown in
equation 3. The
I parameter is independent of the leaving group and, indeed, it can be applied to solvolyses involving both anionic [
11–
13] and neutral [
17] leaving groups. It can also be applied to solvolyses in which an aryl group
migrates from the
β - to the
α -carbon (1,2-aryl shift) [
18]. In many application of the
hI term, the nucleophilic solution contribution is negligible (ℓ ≈ 0) and
equation 4 can be applied [
18].
We have carried out studies to examine the extent to which the
Y and
N scales developed for solvolytic substitution (sometimes accompanied by elimination) reaction to an sp
3 hybridized carbon can be applied to substitution at an acyl (sp
2-hybridized) carbon as in acyl halides [
19,
20], haloformate esters [
21–
26], and carbamoyl chlorides [
27–
29], and to displacement of chloride ion from sulfur or phosphorus during solvolyses of organosulfur [
8] and organophosphorus [
30,
31] substrates.
Our application of
equation 2 to solvolyses at sp
2-carbon has been followed by related studies in other laboratories. Usually these studies have included an extension to additional solvents. A reassessment of these solvolyses in all the solvents now available allows for a more thorough correlation analysis and the possibility of probing as to how robust the analyses are in terms of whether the initially reported sensitivities (
ℓ and
m) change appreciably on the incorporation of new data points. In one instance, new substrates, closely related to the earlier one, have been studied, allowing one to see the effect on the
ℓ and
m values of the structural variation.
In our earlier studies, we assumed that there was no appreciable transfer of charge to aryl groups present on the nitrogen of carbamoyl chlorides or within the aryloxy group of chloroformate esters. Accordingly, we did not consider it necessary to include the
hI term and the correlations were only in terms of
equations 1 and
2. This belief was based upon the absence of canonical resonance structures which could transfer developing positive charge to the aromatic ring. In his study of the solvolyses of
N,
N-diphenylcarbamoyl chloride, Liu [
32] has claimed that the use of
equation 2 with a similarity model based
Y scale (
YBnCl) [
15] suggests a non-canonical resonance, which can lead to the transfer of a large amount of the developing positive charge to the phenyl rings. Further, he claims that this transfer is supported by quantum calculations.
If such a non-canonical transfer is possible with an intervening nitrogen, presumably it will also be possible with an intervening oxygen or sulfur. Accordingly, we have decided to reinvestigate and extend the correlations of the solvolyses of arylmethylcarbamoyl chlorides (ArMeNCOCl, 1), diphenylcarbamoyl chloride (Ph2NCOCl, 2), phenyl chloroformates (PhOCOCl, 3), phenyl chlorothionoformate (PhOCSCl, 4) and phenyl chlorodithioformate (PhSCSCl, 5) using all of the data presently available.
We have used the
NT values, based on solvolyses of the
S-methyldibenzothiophenium ion [
6,
33] as the solvent nucleophilicity scale, in conjunction with
YCl values based on the solvolyses of 1-adamantyl chloride [
2,
3,
34], and
I value based on the specific rates of solvolysis of the
p-methoxybenzyldimethylsulfonuim ion relative to those of the 1-adamantyldimethylsulfonium ion [
10,
35].
3. Conclusions
The correlations presently carried out all support the concept of an ionization mechanism with assistance from nucleophilic solvation for the solvolyses of the carbamoyl chlorides. The
N-methyl-N-arylcarbamoyl chlorides have
ℓ values in the range of 0.44 to 0.58 when
equation 2 is applied. Application of
equation 3 increases the multiple correlation coefficient only by 0.001 and it appears that the
hI terms is not statistically significant and, indeed, the probabilities that the
hI term is statistically insignificant range from 0.068 to 0.591. The
m value is rather low for an ionization mechanism (0.66 to 0.69 when
equation 2 is applied) and this is consistent with the concept that there is considerable internal nucleophilic assistance from the lone pair on the nitrogen.
The correlations of
N,
N-diphenylcarbamoyl chloride give a lower value for
ℓ of 0.23 when
equation 2 is applied. This is raised to 0.40 when
equation 3 is applied and the
h value of 0.55 ± 0.19 is associated with a probability of only 0.007 that the
hI terms is statistically insignificant. It appears, consistent with the claim by Liu [
32], that there is moderately strong evidence for a detectable effect when two aromatic rings, rather than one, are present.
The number of solvents now available for the correlation of the specific rates of solvolysis of phenyl chloroformate has increased dramatically from the time of our earlier correlation [
21] from 21 to 49. The values for
ℓ and
m obtained on application of
equation 2 are, however, essentially unchanged, indicating the very robust character of this correlation. The values of 1.66 and 0.56, respectively, are consistent with the previously proposed addition-elimination pathway, with the addition step being rate-determining. Application of the three-term
equation 3 increases the multiple correlation coefficient only by 0.002 and the
h value of 0.35 ± 0.19 is accompanied by a 0.068 probability that the
hI term is statistically insignificant. These solvolyses are best correlated in terms of
equation 2. The solvolyses of
p-methoxyphenyl chloroformate have been correlated, but of the 31 solvents only two contain fluoroalcohol. The
ℓ and
m values are consistent with the proposed addition-elimination pathway but additional fluoroalcohol-containing solvents are need before a high degree of confidence can be assigned to the correlations.
With phenyl chlorodithioformate, the correlations support the claim by Queen [
40,
45] that the solvolyses involve an ionization mechanism. A good correlation is obtained across the full range of 31 solvents with either
equation 2 (
R = 0.987) or
equation 3 (
R = 0.990). The
ℓ values of 0.69 (
equation 2) or 0.80 (
equation 3) are quite large indicating a high degree of nucleophilic solvation to the ionization process, with
m values of 0.95 (
equation 2) or 1.02 (
equation 3). The
h value of 0.42 ± 0.15 obtained with use of
equation 3 has only a 0.009 probability that the
hI term is statistically insignificant.
The phenyl chlorothionoformate was previously [
42] found to proceed by both addition-elimination and ionization mechanisms. Which pathway dominated was determined by the solvent properties, with high solvent nucleophilicity and low solvent ionizing power favoring the bimolecular pathway. A poor overall correlation (
R = 0.706 with
equation 2 applied) can be considerably improved when the 9 solvents most favorable to addition-elimination (
R = 0.950) and the 12 solvents most favorable to ionization (
R = 0.980) are separately correlated.