Hammett Equation
Ionization of phenylacetic acids
Hammett Equation
s is
measure of a substituent's electron-donating or electron-withdrawing
effect.
+ s values are for electron-withdrawing groups; - are
electron-donating.
s is
dependent on inductive and resonance effects.
r values,
reaction constant, measure how reaction affected by
electron-withdrawing or electron-donating effects.
+ values of r
for reactions favored by electron-withdrawing groups and not by
electron-donating groups. Reverse with - values of
r.
Exercise: Predict if r > 0 or < 0
Table 1. Ionization of phenylacetic acids (taken from Streitwieser and Heathcock, 1985)
|
R |
pKam |
sm |
pKap |
s p |
|---|---|---|---|---|
|
H |
4.31 |
0 |
4.31 |
0 |
|
CH3 |
|
-0.069 |
4.37 |
-0.170 |
|
CH3O |
|
0.115 |
4.36 |
-0.268 |
|
F |
|
0.337 |
4.25 |
0.062 |
|
Cl |
4.14 |
0.373 |
4.19 |
0.227 |
|
Br |
|
0.391 |
4.19 |
0.232 |
|
I |
4.16 |
0.352 |
4.18 |
0.18 |
|
NO2 |
3.97 |
0.710 |
3.85 |
0.778 |
Using r to understand mechanisms
Possible pathways:
For each possible pathway, predict what the sign and magnitude (i.e., large or small) of r will be. Use your knowledge of what the actual product in this reaction is to decide which of the mechanisms can be eliminated on that basis. Is there any other data that you can use to decide on the correct mechanism?
A. Proton transfer to carbon 1.
B. Proton transfer to carbon 2.
C. Bromide transfer to carbon 1.
D. Bromide transfer to carbon 2.
E. Concerted additions.
