Chemistry 211
Equilibrium Controlled Reactions:
Acid Base Reactions and Electron Energies - 7
Conjugate Acid
CH3
CH2
CH2
CH
CH2
CH2 2 CH3 CH3
CH2:
CH2
CH
CH
CH3
CH3
CH2
CH
:O:
O
CH3 C NH2
H
H
:-
16
H
-
O:
CH3 C O : -
OH
O
H
N
O
:O:
CH3 C OH
O
N
-
O
10
9.6
:O:
: :
~ 27
NH
16
CH3 C NH
: :
: :
:
-
2
~33
-
16
: :
CH
O:
: :
CH2
16
-
OH
~ 37
CH :
19
: :
: :
CH3
CH
-
C CH2:
~43
~ 37
-
CH3
CH3 CH2 O :
:
: :
:
:
NH2
-
:O:
CH3 CH2 OH
40
NH
CH
CH3 C CH3
pKa
Conjugate Base
~ 50
41
CH3 CH2 NH
NH2
CH
-
H
CH3 CH2 NH2
CH
-
:-
H
H
CH
CH2:
CH2:
CH3
CH2
CH
: O:
: :: :
CH2
~50
Conjugate Acid
: : : :
-
CH2:
CH3
CH3
pKa
Conjugate Base
: :
: :
: :
Table E
: :
:
: :
A.
Fall 2008
4.8
B. Exploration:
1. How do the structures below relate to the structures in Table E? Provide specific reactions in Table E that relate to each struture.
S
O
CH3
CH
CH
CH
S
CH2
S
CH3 CH2 C S
CH
CH3
For number one, the molecule relates the most to the reaction with the pKa at about 33 because they both have the same hydrocarbon backbone with the
two double bonds and they only differ by the sulfur on the end.
: :
:
: :
:
: :
: :
:
:
: :
For number two, the molecule most relates most to the reaction with the pKa of 4.8 because it had a carbon chain with a double bonded oxygen on it
and they differ by the number of carbons and then there is an oxygen in the place of sulfur in table E.
For number 3, the molecule relates most to the reaction with the pKa of 10 and about 27 and 40, this is because they both have a benzene ring and differ
by the atom connected to the ring.
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Acid-Base & Electron Energy- 7
For number 4, the molecule relates most to the reaction of the first pKa with 16. For this one it was a lot harder to relate this structure to the table. But
the first number 16 has a two carbon chain bonded to a heteroatom with all single bonds and they differ in the number of carbons in the carbon chain.
2. Based upon the data in Table E: List the ions in 1. In order of decreasing energy of their highest energy electrons: (i.e. Highest energy first and lowest
energy last) Explain how you determined your order of energies citing as many supporting examples from the data as possible.
We identified four characteristics, single bonded, double bonded straight chain, double bonded ring and structures with a double bonded O.
We then look in table E at structures which had the same heteroatom between them and see the trends between the groups of them.
With C as the constant atom holding HEE’s, we used acids with pKa ‘s of 50 (single bonded), 47 (double bonded ring), 33 (double bonded
straight chain) and 19 (structure with double bonded O).
Other data partially supports this trend. Keeping N a constant heteroatom with the HEE’s, data which supports this trend are pKa’s of 37
(single bonded), 27 (double bonded ring), and 16 (structure with double bonded O).
Also, when O is kept as a constant heteroatom holding HEE’s, pKa’s that support the trend are 16 (single bonded), 10 (double bonded ring),
and 4.8 (structure with double bonded O).
3. Resonance and “Curved Arrows”: (See Review of General Chemistry and Math # 25)
Curved arrows can be used to assist in converting one structure into another. See the example below:
Acid-Base & Electron Energy-7
H
H
:
:
:
S
H
S
:
H
H
:
b
H
3
H
:
H
a
a. Where does arrow a begin (base of the arrow)?
H
H
On unbounded electrons on S.
b. Where does arrow a end (point of the arrow)?
C-S single bond
c. Compare the two structures and determine the result of the change indicated by arrow a?
Unpaired electrons on S are being shared with C, forming another C-S bond. Therefore, electrons are being moved.
d. Where does arrow b begin (base of the arrow)?
1 of the bonds of the C-C double bond.
e. Where does arrow b end (point of the arrow)?
Points to C #2.
f. Compare the two structures and determine the result of the change indicated by arrow b?
1 pair of electrons in a C-C double bond is no longer being shared, causing 1 C to hold both now unpaired electrons and gaining a
neutral charge. Therefore, electrons are being moved.
g. The total effect of the operations indicate by arrows a & b together illustrate the movement of two types of structural elements. What are those
structural elements? Explain how you reached your conclusion.
These are unpaired electrons and single bonds. This is because
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Acid-Base & Electron Energy- 7
h. What is the difference between the two original structures? Be as specific as you can. Explain how you reached your conclusion.
i. Note that the arrow that connects the two original structures is different from those used before. This arrow is used to relate two resonance
structures. Resonance structures are similar, but are not isomers. Based on your experience in this activity, define the relationship between
resonance structures.
Resonance structures are structures with the same # of atoms of each element but different bonding sequence. They also have a change
in HEE’s from 1 atom to another.
j. From your experience in this activity, use “curved arrows” to find new resonance structures of the original structures. Explain how you devised
your structures.
k. Now use curved arrows to find all resonance structures of the other structures in question B.1.
l. Structures that have two or more possible resonance structures that place the HEE’s on different atoms are said to have “a delocalized HEE
system.” Based on the resonance structures you have drawn, how far can the HEE’s be delocalized in each structure in question B.1.? List the
Acid-Base & Electron Energy-7
5
original structures in order of the length of the structure over which the HEE’s are delocalized (length of the delocalized HEE system) based on
resonance structures.
C. Considering your experience with resonance structures in B.2.:
1. Suggest a correlation between the order of energies proposed in B. 2 and the length of the delocalized HEE systems that differ only in the number of
-bonds.
As the C chain increases the number of pi bonds increases
2. What effect does the presence of an additional higher effective nuclear charge atom have on the energy of a delocalized HEE system? Explain how
the data support your conclusion.
D. Based upon your theory developed in C. above, predict which proton(s) in the molecule below should be most acidic.
CH3
:
N
CH2
O
CH2
:
:
CH2
CH3
CH3
:O:
E. Note the differences in the pKa's of the following compounds from Table E.
6
Acid-Base & Electron Energy- 7
Conjugate Acid
CH3
CH3
CH2
CH2
CH2
CH
CH2
Conjugate Base
CH3
CH
CH3
CH3
CH3
CH2
CH 2
CH2
CH
CH
CH
CH
H
H
CH
CH2 :
-
~50
-
~43
CH2 :
:-
H
H
CH2
CH2
pKa
40
H
CH3
CH 2
CH
CH
CH
CH2 :
-
~33
:16
H
Are these differences consistent with the theory developed in C. above? Note specifically the pKa of the last compound.
Why or Why not?
They are all consistent with 5 pi bonds except for the first one has one pi bond while the second and the last one have three pi bonds. The first
four are consistent with the theory because as the length increases the pKa decreases. For acid 16 we would predict that it should be the second
highest pKa but it isn’t so it is not consistent.
Acid-Base & Electron Energy-7
7
D. Out of Class Applications for acid-Base-6
1. For each of the following compounds, circle the most acidic proton(s) and explain your choice using electron energy arguments.
O
O
2. Which of the following compounds are aromatic according to the Hückel theory?
N
H
:
H
:
H
CH3
+
N
H H
3. For each of the following acid-base reactions, predict whether the equilibrium constant should be > or < 1. Explain your conclusion using electron
energy arguments.
a.
O
O
OH
O +
+
OH
O
b.
O
C
NH
O +
O
C OH
C OH
+
NH
C O
-
4. Which of the following equilibrium controlled reactions should produce more product at equilibrium. Explain your conclusion using electron
energy arguments.
O
CH3
CH3
CH2 C
O
+
CH2 C
O
O
CH3
O
CH2 C
CH3
O CH2
+
O
-
O
CH3
CH2
-
C
+
O
O
CH3
CH2
C
O
O
CH3
CH2
C
O
+
CH3 CH2 O
-
-
C. Nomenclature of Amines
1. CGWW: Ch. 2.0 p. 33
2. Tutorials:
a.
8
Acid-Base & Electron Energy- 7
References:
http://chemistry.boisestate.edu/people/richardbanks/organic/nomenclature/organicnomenclature1.htm
Developed by Richard C. Banks, Professor of Chemistry, Boise State University
Provides questions with answers
Sections
b.
Amines
http://www.sci.ouc.bc.ca/chem/nomenclature/index-2.htm
Developed by Professor Dave Woodcock, Okanagan University College,
British Columbia, Canada
(Contains many examples.)
Sections:
5. Functional Groups with Suffix and Prefix
I. Introduction: Priority Rules
II. Alkanamines (Amines)
Note: Your browser must have the chemscape chime plug-in for these pages to work. Select "1. Introduction to these pages", then click
on "Nomenclature Index - Chemscape Chime" and begin with the "How and Why". If you have problems, go to the
http://www.sci.ouc.bc.ca/chem/nomenclature/nom1.htm and click on "chemscape chime" and follow directions to download the
"chemscape chime plug-in." If you have problems contact me.
c. http://www.acdlabs.com/iupac/nomenclature
Developed by Advanced Chemistry Development Laboratories
(Gives detailed rules for nomenclature.)
Recommendations 1993
R-5 Applications to Specific Classes of Compounds
R-5.4 Amines and Imines
R-5.4.1 Primary amines
R-5.4.2 Secondary and tertiary amines
3. Applications
a. Name the following:
Acid-Base & Electron Energy-7
9
N
NH2
N
Br
NH2
H
b. Draw structural formulas for the following compounds:
N-propyl-2,3-dimethylbutanamine
N-cyclopropyl-N-methyl-2-hexanamine
N,N-diethyl-4-methoxycyclohexanamine
3-octanamine