Hammett Sigma Constants*

GroupσmetaσparaσIσvπEsMR
H 0.00 0.00 0.00 0.00 0.00 0.00 1.03
CH3 -0.07 -0.17 -0.04 0.52 0.56 -1.24 5.65
C2H5 -0.07 -0.15 -0.05 0.56 1.02 -1.31 10.30
n-C3H7 -0.07 -0.13 -0.03 0.68 1.55 -1.60 14.96
i-C3H7 -0.07 -0.15 -0.03 0.76 1.53 -1.71 14.96
n-C4H9 -0.08 -0.16 -0.04 0.68 2.13 -1.63 19.61
t-C4H9 -0.10 -0.20 -0.07 1.24 1.98 -2.78 19.62
H2C=CH** 0.05 -0.02 0.09 2.11 0.82   10.99
C6H5** 0.06 -0.01 0.10 2.15 1.96 -3.82 25.36
CH2Cl 0.11 0.12 0.15 0.60 0.17 -1.48 10.49
CF3 0.43 0.54 0.42 0.91 0.88 -2.40 5.02
CN 0.56 0.66 0.53 0.40 -0.57 -0.51 6.33
CHO 0.35 0.42 0.25   -0.65   6.88
COCH3 0.38 0.50 0.29 0.50 -0.55   11.18
CO2H** 0.37 0.45 0.39 1.45 -0.32   6.93
Si(CH3)3 -0.04 -0.07 -0.13 1.40 2.59   24.96
F 0.34 0.06 0.52 0.27 0.14 -0.46 0.92
Cl 0.37 0.23 0.47 0.55 0.71 -0.97 6.03
Br 0.39 0.23 0.50 0.65 0.86 -1.16 8.88
I 0.35 0.18 0.39 0.78 1.12 -1.40 13.94
OH 0.12 -0.37 0.29 0.32 -0.67 -0.55 2.85
OCH3 0.12 -0.27 0.27 0.36 -0.02 -0.55 7.87
OCH2CH3 0.10 -0.24 0.27 0.48 0.38   12.47
SH 0.25 0.15 0.26 0.60 0.39 -1.07 9.22
SCH3 0.15 0.00 0.23 0.64 0.61 -1.07 13.82
NO2** 0.71 0.78 0.76 1.39 -0.28 -2.52 7.36
NO 0.62 0.91 0.37   -0.12   5.20
NH2 -0.16 -0.66 0.12   -1.23 -0.61 5.42
NHCHO 0.19 0.00 0.27   -0.98   10.31
NHCOCH3 0.07 -0.15 0.26   -0.37   16.53
N(CH3)2 -0.15 -0.83 0.06 0.43 0.18   15.55
N(CH3)3+ 0.88 0.82 0.93 1.22 -5.96   21.20

* σmeta, σpara = Hammett constants; σI = inductive sigma constant; σv = Charton's v (size) values; p = hydrophobicity parameter; Es = Taft size parameter; MR = molar refractivity (polarizability) parameter.

** indicates that the group is in the most sterically hindered conformation.

Reference: C. Hansch and A. Leo, "Substituent Constants for Correlation Analysis in Chemistry and Biology," Wiley-Interscience, NY, 1979.