Chemistry 211
Fall 2013
Equilibrium Controlled Reactions:
Acid-Base & Electron Energy-8
A. Comparison of the Magnitudes of Structural Effects on HEE Energies
Based on our studies of acid-base reactions we have concluded that:
 The pKa of an acid is determined by the energy of the conjugate base. (Relative Effect Assumption)
 The highest energy electrons (HEE) are the ones that determine the site of reaction of a structure (Relative Effect Assumption)
 We can use the pKa of the conjugate acid (pKaH of the conjugate base) as a measure of the energy of the HEE on a conjugate base.
Figure 1 below provides structures of conjugate bases with their pKaH values. These have been taken from Tables A-E in acid-base-3-7 activities.
Use these data and the following questions to explore the relative magnitudes of the five effects we have conceived.
1. Circle highest energy electrons on each of the structures in Figure 1.
Conjugate Base
~37
Conjugate Base
CH3 CH2: -
CH3
O:
~37
16
CH3
CH2
~42
CH2
CH
~40
HC
C
Conjugate Base
pKaH
-
CH2:
-
CH2 :
25
-
16
-
-1
O:
O:
H
Figure 1: Various Structural Effects on Electron Energies
~ 50
41
16
10
Conjugate Base
:O:
pKaH
- 4.8
CH3 C O:
:O:
- 3.7
H C O:
:O :
F3C C
: :
CH3 O
CH3
NH -
pKaH
: :
O:
10
CH3
~42
Conjugate Base
: :
-
: :
CH3
H
N
H
: : : :
H
pKaH
Inductive
: : : :
CH3 N
pKaH
Delocalization
Hybridization
Nuclear Charge
Net Charge
- 0.0
O:
2
Acid-Base & Electron Energy- 8
2. Estimate the energy differences (in pK units) between the HEE on each pair of structures below:
DG
pK units
vs.
CH3 CH2: -
H
H
CH3 O
H
CH3
CH3
vs.
CH3
NH -
vs.
CH3
H
N
H
vs.
CH3 O
NH -
CH3
CH2
vs.
CH2
CH
CH2
CH
vs.
HC
C
O:
: :
O:
: :
CH3
H
-
CH3 N
: : : :
vs.
CH3 N
DG
pK units
DG
pK units
H
DG
pK units
DG
pK units
O:
3. Below, list the effects in 2 in order of decreasing G:
-
O:
vs.
:O:
CH3 C
O:
H C
:O:
vs.
F3C C
-
O:
: :
vs.
-
CH3 C
: :
O:
CH2 :
: :
: :
-
vs.
:O:
:O:
: :
CH2:
-
: :
-
-
O:
Acid-Base & Electron Energy-8
3
B. Use of empirical benchmarks for competing structural effects on electron energies:
These and future applications require some empirical evidence, which provides benchmarks for relating effective nuclear charge (C vs. N vs.
O), net charge (- vs. neutral) and carbonyl delocalization effects when they do not reinforce each other. These pKaH values are ones that you
must KNOW.
Conjugate Base pKaH
Conjugate Base pKaH
: :
O
15.7 Isolated electrons on
:
CH3
Delocalized carbanion electrons
alpha to C=O
10
NH2
O:
-
-
: :
:OH
20
: :
: :
-
-
: :
C CH2 :
CH3
O
10
Delocalized O electrons
alpha to benzene ring
O
Isolated electrons on neutral N
CH3
C O:
Delocalized O
4.8 alpha to C=O
electrons
For each of the following reactions:
 Circle the HEE in the reactants and products.
 Use the change in energy of the highest energy e-'s (HEE) to predict whether the equilibrium constant should be > or < 1.
 Provide a warrant indicating how you developed your claim.
 Answer the reflection question for each.
1. CH3 NH3 +
H2O
OH
+
CH3 NH2
Warrant:
Reflection: What conclusions can you draw about the relative energies of a lone pair of electrons isolated on a neutral nitrogen atom vs. one
isolated on a negative oxygen atom?
+ CH3 NH2
: :
: :
O
O
:
: :
2. CH3 C
Warrant:
H
: :
O
CH3 C O :
-
+
+
CH3 NH3
4
Acid-Base & Electron Energy- 8
Reflection: What conclusions can you draw about the relative energies of a lone pair of electrons isolated on a neutral nitrogen atom vs. one
delocalized over two carboxylate oxygen atoms?
: :
: :
3.
O
CH3 C
Warrant:
O
CH3
+
H2O
OH
+
CH2
C CH2
Reflection: What conclusions can you draw about the relative energies of a lone pair of electrons isolated on a negative oxygen atom
vs. one delocalized over an -carbanion system?
C. General applications of electron energies to organic reactions.
For the following pair of equilibrium controlled reactions use the change in energy of the highest energy e-'s to predict which reaction should
produce more products at equilibrium. Provide a warrant indicating how you developed your claim, being sure to identify the HEE of the
reactants and products of each reaction. Illustrate the relative energy changes using the Rxn. Coordinate Diagram provided.
O
CH3 CH2
C
+
O
NH
CH3 CH2
+
C
G
OH
O
CH3 CH2
C
O
N
N
+
CH3 CH2
NH
+
OH
C
Reactants
Rxn Coord
Products
Acid-Base & Electron Energy-8
5
Out of Class Applications for Acid-Base-8
A. Reading Assignment: CGW Chapter 8 pp. 161-181
B. Problems Claden, Greeves & Warren Website (http://www.oup.com/uk/orc/bin/9780199270293/) End of chapter questions:
Chapter 8 questions 1-8
C. Assignment:
1. On p. 167 CGW discusses the preferred form of amino acids (the building blocks of proteins) in water. They indicate that the predominant
form of amino acids is the zwitterion form rather than the amino-carboxylic acid form. Provide a warrant based on the HEE of each amino
acid form and the benchmark pKas introduced in Acid-Base-8 that accounts for the results stated in CGW.
2. Analyze the structures of each of the following compounds, then circle the most acidic proton(s) and provide a warrant indicating how you
developed your claim. Your warrant must be based on the theories of the effects of structure on e- energies developed in class.
H
H
a.
H
H
H
H
b.
H
C
O
C
c. H2N
H
OH
d.
H2N
NH3
H
C
O
C
C
C
CH2
CH3
H
C
H
H
H
3. For each of the following acid-base reactions use the change in energy of the HEE to predict whether the equilibrium constant should be > or
< 1. Provide a warrant indicating how you developed your claim being sure to identify the highest energy e-'s of the reactants and products.
O
a.
O
C OH
+
O
O
C OH
b.
O
+
O
C O
O
O
C O
C O
O
+
OH
O
+
C OH
O
6
Acid-Base & Electron Energy- 8
4. For the following pair of equilibrium controlled reactions use the change in energy of the highest energy e-'s to predict which reaction should
produce the more product at equilibrium. Provide a warrant indicating how you developed your claim being sure to identify the highest
energy e-'s of the reactants and products of each reaction.
O
CH3 CH2
A
O
O
C
O
CH3 CH2
CH3
C
+
O
O
+
O
CH2
C
O
CH3
CH3 CH2
CH2 C
C
B
+
O
CH3
O CH2
O
CH3
CH2
C
O
+
CH3 CH2
O
O
D. Nomenclature of Aldehydes and Ketones
References:
1. CGW: Ch. 2 p. 30-31
2. Tutorials:
a. https://chemistry.boisestate.edu/richardbanks/organic/nomenclature/organicnomenclature1.htm
Developed by Richard C. Banks, Professor of Chemistry, Boise State University
Provides questions with answers
Sections
Aldehydes
Ketones
b. http://elchem.kaist.ac.kr/jhkwak/OkanaganPdb97/nomenclature/index.htm
Developed by Professor emeritus Dave Woodcock at Okanagan College of the University of British Columbia, Canada
(Contains many examples.)
Sections:
6. Functional Groups with Suffix and Prefix
IV. Alkanones (Ketones)
V. Alkanals (Aldehydes)
Acid-Base & Electron Energy-8
7
c. http://www.acdlabs.com/iupac/nomenclature
Developed by Advanced Chemistry Development Laboratories
(Gives detailed IUPAC rules for nomenclature.)
Recommendations 1993
R-5 Applications to Specific Classes of Compounds
R-5.6 Aldehydes, Ketones, their Derivatives and Analogues
R-5.6.1 Aldehydes, thioaldehydes, and their analogues
R-5.6.2 Ketones, thioketones, and their analogues.
3. Applications
a. Name the following:
O
O
O
O
O
b. Draw structural formulas for the following compounds:
3-phenyl-2-pentanone
4-bromo-3-ethyl-5-heptenal
3-cyclopentenone
4-methoxybenzaldehyde