ATOOCV1 3 13 The Hammett Equation and Linear Free Energy Relationship
ATOOCV1 3 13 The Hammett Equation and Linear Free Energy Relationship
ATOOCV1 3 13 The Hammett Equation and Linear Free Energy Relationship
𝑘 (23)
log = 𝜎𝜌
𝑘0
where k and k0 are the constant for the group X ≠ H and X = H; ρ and σ are the constants for reaction conditions
and substituent X, respectively.
➢ Derivation of Hammett Equation
To derive the Hammett equation, we need to recall the quantitative relationship between the structure
and reactivity first. To do so, we need to find some mathematical parameter that can be used to represent the
combined magnitude of inductive and resonance effects of different substituents. This can be achieved by
considering the hydrolysis of a series of different benzoic acids as given below.
𝐾𝑎 (24)
XC6 H4 COOH + H2 O ⇌ XC6 H4 COO− + H3 O+
Where X is a substituent at the m- or p-position and Ka is the dissociation constant. As expected, the
dissociation constant was found to be different for differently substituted substrates.
Since an electron-withdrawing group will better stabilize the conjugate base (i.e., XC 6H4COO−),
resulting in a larger magnitude of Ka (lower pKa). On the other hand, an electron-donating group will
destabilize the conjugate base (i.e., XC6H4COO−), resulting in a smaller magnitude of Ka (higher pKa).
Therefore, we can say that the electronic effect (inductive plus mesomeric effect) of a substituent can be
represented as the difference between the pKa value of its benzoic acid derivative and the pKa value of benzoic
acid itself; mathematically, we can say
Where the parameter σX (or simply σ) is called as substituent constant; and was found for several different
groups just subtracting its benzoic acid derivative’s pKa value from pKa value of benzoic acid.
Table 1. pKa values and substituent constants for XC6H6COOH using benzoic acids p(Ka)0 = 4.21.
Using log 𝑚 − log 𝑛 = log 𝑚/𝑛, equation (36) can also be written as
(𝐾𝑎 ) (26)
log =𝜎
(𝐾𝑎 )0
Now if we plot a curve between log(𝐾𝑎 )/(𝐾𝑎 )0 vs σ, we will definitely get a straight line with a slope = 1.
Now we need to check if these σ values (i.e., of substituted benzoic acids) can also be used for other
meta- or para-substituted benzene derivatives. To do so, consider two series of reactions; the first one is the
acid dissociation of phenyl phosphonic acid, and the second one is the base hydrolysis of substituted ethyl
benzoate. Here we will find if different substituents affect their dissociation constants or rates in the same
manner as affected in the case of substituted benzoic acid. Also, we did not use ortho-substituents or
substituents in the aliphatic system because they also contain steric factors and don’t not linear variation.
The experimental log(𝐾𝑎 )/(𝐾𝑎 )0 for the reaction-I and experimental log 𝑘/𝑘0 for reaction-II are given below.
Table 2. Experimental values of log(𝐾𝑎 )/(𝐾𝑎 )0 and log 𝑘/𝑘0 for the acid dissociation of phenyl
phosphonic acid and base hydrolysis of substituted ethyl benzoates, respectively.
Br 0.29 0.23 − −
OCH3 − − − −0.67923
When plotted the experimental log(𝐾𝑎 )/(𝐾𝑎 )0 for the reaction-I and experimental log 𝑘/𝑘0 for reaction-II vs
the substituent constants obtained for the substituted benzoic acids, we get the following curves.
Figure 18. log(𝐾𝑎 )/(𝐾𝑎 )0 and log 𝑘/𝑘0 vs σ for reaction-I and reaction-II.
It is obvious that the plots are still linear like in Figure 3 but the slope has changed. This implies that the order
and relative effects for different substituents on both reactions remain the same though the magnitude has been
changed which can be attributed to the different nature reaction considers from ‘base reaction’.
Therefore, our aim, in this case, should be the determination of the slope (let us say ρ). Since on the
vertical side we have ‘[log(𝐾𝑎 )/(𝐾𝑎 )0 ]𝑠𝑝𝑝𝑎 ’ for reaction-I (acid dissociation of phenyl phosphonic acid) and
on the horizontal side we have ‘𝜎 𝑜𝑟 [log(𝐾𝑎 )/(𝐾𝑎 )0 ]𝑠𝑏𝑎 ’ for base reaction (hydrolysis of substituted benzoic
acid), the slope should be
or
But from equation (26), we know that [log(𝐾𝑎 )/log(𝐾𝑎 )0 ]𝑠𝑏𝑎 = 𝜎; and therefore, equation (40) takes the form
(𝐾𝑎 ) (29)
[log ] = 𝜌𝜎
(𝐾𝑎 )0 𝑠𝑒𝑏
Similarly, on the vertical side we have ‘log 𝑘/𝑘0 ’ for reaction-II (base hydrolysis of substituted ethyl benzoate)
and on the horizontal side we have ‘𝜎 𝑜𝑟 [log(𝐾𝑎 )/(𝐾𝑎 )0 ]𝑠𝑏𝑎 ’ for base reaction (hydrolysis of substituted
benzoic acid), the slope should be
or
But from equation (26), we know that [log(𝐾𝑎 )/log(𝐾𝑎 )0 ]𝑠𝑏𝑎 = 𝜎; and therefore, equation (32) takes the form
𝑘 (33)
[log ] = 𝜌𝜎
𝑘0 𝑠𝑝𝑝𝑎
The results given by equation (29, 30, 33, 34) are called as Hammett’s equations; which shows that the rates
of ortho and para-substituted benzene derivatives can be obtained if the substituent contents for substituted
benzoic acid are known. Now we will discuss the substituent and reaction constants in more detail
➢ Linear Free Energy Relationship (LFER)
The Hammett equation is a linear free energy relationship that can be proved for any group X by
recalling the kinetics of organic reactions is in the framework of “Activated complex Theory”, which states
that the rate constant (k) for a typical reaction is
𝑅𝑇 −𝛥𝐺 ∗ (35)
𝑘= 𝑒 𝑅𝑇
𝑁ℎ
Where ΔG* is the free energy change of the activation step at temperature T. The symbols R, N, and h are the
gas constant, Avogadro number, and Planck’s constant, respectively. Similarly, for k0 we have
𝑅𝑇 −𝛥𝐺0∗ (36)
𝑘0 = 𝑒 𝑅𝑇
𝑁ℎ
After putting the value of equation (35) and equation (36) in Hammett equation (23), we get
𝑅𝑇 −𝛥𝐺
∗
(37)
𝑒 𝑅𝑇
log 𝑁ℎ 𝛥𝐺 ∗ = 𝜎𝜌
𝑅𝑇 − 𝑅𝑇0
𝑒
𝑁ℎ
𝛥𝐺 ∗ (38)
𝑒 − 𝑅𝑇
log 𝛥𝐺0∗
= 𝜎𝜌
𝑒 − 𝑅𝑇
Multiplying both sides by 2.303, we have
𝛥𝐺 ∗ (39)
𝑒 − 𝑅𝑇
2.303 log 𝛥𝐺0∗
= 2.303 𝜎𝜌
𝑒 − 𝑅𝑇
𝛥𝐺 ∗ (40)
𝑒 − 𝑅𝑇
ln 𝛥𝐺0∗
= 2.303 𝜎𝜌
𝑒 − 𝑅𝑇
𝛥𝐺 ∗ 𝛥𝐺 ∗
− 𝑅𝑇0 (41)
ln 𝑒 − 𝑅𝑇 − ln 𝑒 = 2.303 𝜎𝜌
or
𝛥𝐺 ∗ 𝛥𝐺0∗ (42)
(− ln 𝑒) − (− ln 𝑒) = 2.303 𝜎𝜌
𝑅𝑇 𝑅𝑇
or
𝛥𝐺 ∗ 𝛥𝐺0∗ (43)
− + = 2.303 𝜎𝜌
𝑅𝑇 𝑅𝑇
Which implies
𝛥𝐺0∗ 𝛥𝐺 ∗ (44)
− = 2.303 𝜎𝜌
𝑅𝑇 𝑅𝑇
or
𝛥𝐺0∗ − 𝛥𝐺 ∗ (45)
= 2.303 𝜎𝜌
𝑅𝑇
or
or
Hence, the variation of negative of the free energy of activation varies linearly with slope 2.303RTρ and −𝛥𝐺0∗
as intercept.
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