Chemistry 211
Fall 2008
Equilibrium Controlled Reactions:
Acid Base Reactions and Electron Energies – 6
SUMMARY OF CLASS DISCUSSION
A.
Table D
Conjugate Acid
Conjugate Base
O
O
CH3 C OH
CH3 C O -
O
O-
Cl
Conjugate Base
O
O
H C O-
H C OH
O O
4.7
Cl
ClCH2 C OH
3.7
-
OH
4.8
pKa
O
O
2.8
O
ClCH2 C O -
2.8
O
O
Cl
Cl
O
OH
O
O
4.5
O
Cl3C C OH
Cl3C C O -
O
O
0.9
O
O-
OH
O
Conjugate Acid
Cl
O
OH
Cl
4.8
O-
OH
Cl
pKa
Cl
O
4.1
F3C C OH
F3C C O -
0.0
Acid-Base Rxns & Electron Energy-6
2
B. Based upon the data in Table D,
Indicate the site of the highest energy electrons on each of the following ions. Then list the ions in order of decreasing energy of their highest energy
electrons. (Highest Energy to Lowest Energy)
-
-
CH3 CH NH
Cl
-
Cl CH2 CH2 NH
-
CH3 CH NH
CH3 CH2 NH
F
1. All structures above HEE on an that has a 2 carton saturated chain attached. The structural variations:
 Only H attached to carbon atoms.
 One F atom attached to the C next to atom with HEE.
 One Cl atom attached to either the C next to atom with HEE or one atom further from the atom with HEE.
So we must account for the effect of replacing an H with a halogen atom, the difference between replacing H with Cl or F, the variation in
energy of HEE when the distance between the halogen and HEE changes.
N-
Explanation on the basis of the data.
1. From the pattern of pKa values in Table D, replacing a proton with a more electronegative atom in an acid or base appears to
decrease the pKa of the conjugate acid.
Figure 1
Conjugate Acid
Acidity
Conjugate Base
O-
OH
O
Cl
Basicity
pKa
4.8
O
OH
Cl
O
4.7
O
O
Cl
g
n
si
ea
cr
In
Cl
OH
O
O
-
4.5
In
cr
ea
si
n
g
O
O-
OH
Cl
O
Cl
O
4.1
O
O
H C OH
H C
Cl
O
-
3.7
Cl
OH
O
O
O
2.8
Acid-Base Rxns & Electron Energy-6
3
The magnitude of the effect appears to increase:
a. As the bonded distance between the electronegative atom and the atom holding the HEE in the base decreases (See Figure 1).
b. As the electronegativity of the attached atom increases
(See Figure 2, pKas 0.9 & 0.0).
c. As the number of electronegative atoms increases
(See Figure 2, pKas 4.8, 2.8 & 0.9).
Figure 2
Conjugate Acid
Conjugate Base
O
CH3
C
O
OH
CH3
C
-
4.8
O
-
2.8
O
-
O
-
O
O
O
g
n
si
ea
cr
In
ClCH2
C
ClCH2
OH
C
Cl3C
OH
C
C
0.9
O
O
F3C
C
O
O
Cl3C
pKa
Basicity
In
cr
ea
si
n
g
Acidity
F3C
OH
C
0.0
Since, as we have seen before, the conjugate bases have one less bond than the conjugate acids, the conjugate bases should have higher
energy than the conjugate acids. (Bond Formation Principle) Applying the Relative Effect assumption as we have in our earlier analyses,
we can interpret the data with the assumption that the energies of higher energy species have been affected more by changes in structure
than have lower energy species. Thus, the data in the table can be explained by an effect of the electronegative element that lowers the
energy of the new lone pair of e-'s (HEE) in the conjugate bases more as the affecting atom becomes more electronegative (F better than
Cl), as the electronegative atom is moved closer (fewer intermediate bonds) to the new lone pair of e-'s and as the number of
electronegative atoms increases. So the base at the bottom of each table should be judged to have HEE with the lowest in energy and that
at the top of each table to have HEE with the highest in energy. This same trend leads to the order of energies shown below for the
substituted ethyl amide ions in this question.
CH3
CH2
-
NH
Cl
-
CH2 CH2 NH
CH3
CH
Cl
-
NH
CH3 CH
F
-
NH
Acid-Base Rxns & Electron Energy-6
C.
4
Suggest theoretical explanations for the effects illustrated In B. above.
Replacement of H with a more electronegative atom (one with higher effective nuclear charge) will tend to polarize the sigma-bonding
e-‘s attached to the electronegative atom toward the electronegative atom compared with the original C-H bonding e-‘s. This polarization
leads to lowered e- density on the carbon atom holding the electronegative atom. Consequently that carbon atom will attract its other
bonded e-'s more strongly than it would if it had an attached hydrogen atom. So the electronegative atom not only polarizes its own
bonds, but also induces the polarization of adjacent sigma-bonds.
The result of this Inductive Electron Withdrawing Effect is to allow more electron density to interact directly with the electronegative
atom's nucleus and to slightly decrease the electron density on the remote atoms holding HEE. Thus, the effective nuclear charge of the
electronegative atom is able to lower the energy of remote HEE by decreasing electron density on remote atoms thus diminishing e-- erepulsion on those atoms. Since this effect is transmitted through successive polarizations of sigma-bonds it decreases rapidly with the
bonded distance between the electronegative atom and the HEE.
In the examples in Figures 1 & 2 above, the electronegative chlorine causes a small decrease in energy of the lone pair e-'s of the neutral
fully bonded acid forms of the compounds. However, it has a larger effect on the HEE, the extra lone pair e-'s of the higher energy
negatively charged carboxylate conjugate bases. As the chlorine is moved closer to HEE on the negatively charged carboxylates, it has a
progressively greater e- withdrawing effect on the HEE lone pair e-'s, lowering their energy by decreasing their e-- e- repulsion more and
more as the bonded distance between the Cl and the carboxylate decreases. (See Figure 3, variation from right to left.) A similar
increased effect is seen when a more electronegative atom replaces a less electronegative atom (F vs. Cl) or when additional
electronegative atoms replace H atoms (CF3 vs. CCl3).
Figure 3: Inductive Effects on the conjugate bases due to the presence of Cl:
-Cl
-Cl
 -Cl
Base
Base
Base
G
G
G
G
Base
Rxn. Coord.
Rxn. Coord.
Cl
OH
OH
Rxn. Coord.
Cl
OH
Cl O
O
O
Increase
G
Increase
pKa
Energy of R
O
O
Energy of
Acid
Acid
Acid
Acid
R
O
H
O-
Very small increase
Increase
Rxn. Coord.
OH
O
Acid-Base Rxns & Electron Energy-6
5
It is important to note that when a hydrogen atom is replaced by an atom from group V (N), VI (O, S) or VII (F, Cl, Br, I) elements, the
bond polarization shifts toward the new element and the result is considered to be electron withdrawing (i.e. The change will tend to
lower the energy of the electrons in HEE by decreasing e-- e- repulsion.)
However when a hydrogen atom is replaced by a carbon atom, the effect seems to be opposite. (See Figure 4)
Figure 4: Inductive Donation Effect of CH3- vs. H-:
Conjugate Acid
Conjugate Base
O
O
H
H
C OH
O
CH3
C
O
pKa
-
3.7
-
4.8
O
C OH
CH3
C
O
To be consistent with the inductive effect theory described above, we must view the CH3 group as being e- donating compared with
hydrogen. Thus, the methyl group raises the energy of the HEE in the conjugate base by increasing e- - e- repulsion. Thus, the proton
loss is less energetically more favorable for formic acid (pKa 3.7) than for acetic acid (pKa 4.8). (See Figure 5)
Figure 5: Energy effect of Electron Donation by CH3Base
Base
G
G
Acid
Acid
Rxn. Coord.
H
OH
O
Rxn. Coord.
CH3 OH
O
Magnitude of Inductive Effects.
The overall pKa change for similar compounds that differ only in the inductive effect of one or more new electronegative atoms is ~2-5 pK
units. Thus, this is a much smaller effect than the three other effects we have already considered.