6/10/02
Property: s* (Taft's Polar Substituent Constant) for the CZ3 Groups)
References:
1. Hansch, C. and Leo, A.: Substituent Constants for Correlation Analysis in Chemistry and Biology, Wiley: New York, New York (1979)
2. Minamida, I., Ikeda, Y., Uneyama, K., Tagaki, W. and Oae, S.: Acid Dissociation, UV Spectra and Hydrolyses of Several a-Mercapto- and a-Alkoxyacetic Acids and their Ethyl Esters. Tetrahedron 24, 5293 (1968)
Related Links:
Summary:
s*= - (1.68±0.07) +.066cd + (0.0067±0.0006)q + (0.129±0.008)Ear + (0.138+0.002) pp
n = 35 s = 0.03676 r2 = 0.994
%cd = 44 %q = 10 %Ear = 11 %pp = 35
General Comments:
To begin our analysis, we transferred the four phosphorus(III) stereoelectronic parameters for the alkyl, aryl and the alpha-fluoroalkyl groups with exception of pp for the C-H bond: the pp value for the C-H bond was set to zero (the rationale for this is described elsewhere). We found a high correlation among the parameters; accordingly, we determined the coefficient for cd (0.066±0.003) by plotting the s* values for the CH2(p-XC6H4) groups versus cd in accordance with the QALE model. We imposed this coefficient on the analysis described below. We used the regression analysis of these aforementioned groups to determine the pp parameters for the remaining heteroatom substituted groups. It was our expectation that if the transference of parameters from phosphorus to carbon were not valid then the calculated pp parameters would not agree with the phosphorus values. Below, we show a plot of the calculated pp values versus the phosphorus(III) values. There is an excellent 1:1 correlation for all but the CH3-i(OR)i groups which have calculated pp values that are much larger than the phosphorus(III) values. But even here there is an excellent linear relationship with an intercept of zero. In the following analysis we used the new pp values for the CH3-i(OR)i groups among with the phosphorus(III) pp values for the other groups. The inclusion of the CH3-i(OR)i groups does not influence the overall analysis.
Data:
|
CZ3 |
s* |
cd |
q |
Ear |
pp |
ref |
|
CH3 |
0.000 |
17.00 |
87 |
0.0 |
0.00 |
1 |
|
CH2F |
1.100 |
22.30 |
93 |
0.0 |
4.40 |
1 |
|
CH2(OEt) |
0.570 |
16.60 |
94 |
0.4 |
3.3* |
2 |
|
CH2OMe |
0.600 |
17.30 |
94 |
0.3 |
3.3* |
1 |
|
CH2Me |
-0.100 |
14.20 |
97 |
0.0 |
0.00 |
1 |
|
CHF2 |
2.050 |
27.70 |
98 |
0.0 |
8.80 |
1 |
|
CH2Cl |
1.050 |
25.30 |
99 |
1.4 |
1.80 |
1 |
|
CH2(OPh) |
0.950 |
19.20 |
101 |
0.4 |
4.5* |
2 |
|
CH(OEt)2 |
1.140 |
16.20 |
102 |
0.8 |
6.8* |
2 |
|
CH2Pr |
-0.130 |
13.10 |
103 |
0.0 |
0.00 |
1 |
|
CH2Et |
-0.115 |
13.40 |
103 |
0.0 |
0.00 |
1 |
|
CH2CH2CH2CF3 |
0.120 |
15.80 |
103 |
0.0 |
0.77 |
1 |
|
CH2CF3 |
0.920 |
22.30 |
103 |
0.0 |
3.70 |
1 |
|
CF3 |
2.850 |
33.00 |
104 |
0.0 |
13.20 |
1 |
|
CH2(p-MeOC6H4) |
0.140 |
14.80 |
106 |
1.0 |
0.00 |
1 |
|
CH2(p-MeC6H4) |
0.170 |
15.20 |
106 |
1.0 |
0.00 |
1 |
|
CH2Ph |
0.215 |
15.80 |
106 |
1.0 |
0.00 |
1 |
|
CH2(p-ClC6H4) |
0.280 |
16.90 |
106 |
1.0 |
0.00 |
1 |
|
CHMe2 |
-0.190 |
11.40 |
107 |
0.0 |
0.00 |
1 |
|
CHClMe |
0.980 |
22.50 |
110 |
1.4 |
1.80 |
1 |
|
CH2CHMe2 |
-0.125 |
12.50 |
111 |
0.0 |
0.00 |
1 |
|
CHEtMe |
-0.210 |
10.60 |
112 |
0.0 |
0.00 |
1 |
|
CHCl2 |
2.100 |
33.70 |
112 |
2.7 |
3.60 |
2 |
|
CH(OPh)2 |
1.900 |
21.40 |
114 |
0.8 |
9.2* |
2 |
|
CHClEt |
1.050 |
21.80 |
114 |
1.4 |
1.80 |
1 |
|
CHEt2 |
-0.225 |
9.90 |
117 |
0.0 |
0.00 |
1 |
|
CHMePh |
0.110 |
12.90 |
117 |
1.0 |
0.00 |
1 |
|
CMe3 |
-0.300 |
8.55 |
118 |
0.0 |
0.00 |
1 |
|
CH2CMe3 |
-0.165 |
11.30 |
119 |
0.0 |
0.00 |
1 |
|
CHEtPh |
0.040 |
12.20 |
121 |
1.0 |
0.00 |
1 |
|
CHPh(CF3) |
1.110 |
21.10 |
123 |
1.0 |
3.70 |
1 |
|
CCl3 |
3.150 |
42.00 |
124 |
4.1 |
5.30 |
2 |
|
CHPh2 |
0.405 |
14.50 |
126 |
2.0 |
0.00 |
1 |
|
CHMe(t-Bu) |
-0.280 |
8.50 |
129 |
0.0 |
0.00 |
1 |
|
CPh3 |
0.560 |
13.25 |
145 |
2.7 |
0.00 |
1 |
*Values were determined herein.
Graphical Analysis:
Interpretation of Graphs:
• Plot of data versus cd for C(p-XC6H4)3(Graph A): The four CH2(p-XC6H4) points define a line with a slope +0.066±0.003.
• Slope of the CR3 line (Graph A):
The line defined by the alkyl points is less steep indicating a small steric effect that increases s* as q becomes larger.
• The point of intersection of the 2 lines in graph A
• The plot of s* versus 'i' for CH3-iZi (Graph B):The plots of the data for the four sets of substituted alkyl groups are linear indicating that the data are well behaved and suitable for a QALE analysis.
• Outliers: None that are obvious.
• Steric threshold: None that is obvious.
Statistical Analysis:
We began the analysis using all the data and a four parameter fit. The resulting regression equation is
The regression equation is
s*= - 1.68 +.066*cd + 0.00669q + 0.129Ear + 0.138 pp
|
Predictor |
Coef |
Stdev |
t-ratio |
p |
|
Constant |
-1.67638 |
0.06844 |
-24.49 |
0.000 |
|
cd |
0.066 |
|||
|
q |
0.0066874 |
0.0006371 |
10.50 |
0.000 |
|
Ear |
0.128974 |
0.007691 |
16.77 |
0.000 |
|
pp |
0.137999 |
0.002017 |
68.40 |
0.000 |
s = 0.03676 r2 = 0.994 r2 (adj) = 0.994
%cd = 44 %q = 10 %Ear = 11 %pp = 35
Stereoelectronic Profiles:
Discussion: Taft's polar substituent constant, s*, is a measure of the relative rates of base (b) and acid (a) effected hydrolysis of alkyl esters.
s* = {log(k/ko)b-log(k/ko)a}/2.48
s* is supposedly free of steric influences and dependent on only electronic factors. The QALE analysis largely supports these assertions. The steric parameter, q, is only a ten percent contributor to the overall variation in s*. This suggests that steric demands of the two transition states for the two reactions on which s* is based are slightly different. s* increases (the relative rates of the base effected hydrolysis increases) as cd and pp increase. This makes sense since we expect that the diminished electron donor ability (larger cd) of the alkyl groups would facilitate the development of negative charge in the transition state. Likewise, we expect that that the larger field effect and possibly p bonding (larger pp) would also facilitate the development of positive charge in the transition state.