CH331 Questions

Chemistry 331 Lecture Notes, Bruice Readings and Assigned Questions
Unit 2 Acids and bases
This lecture notes document provides a list of readings from the seventh edition of Bruice. It concludes with a
list of assigned questions from the seventh editions of Bruice.
Introduction
These lecture notes review and expand upon Bronsted/Lowry acid-base chemistry and Lewis acid-base
chemistry from General Chemistry.
Overview
Sections in the 7th edition of Bruice*
Study
1
2
3
4
5
6
7
A few important terms
Assigning and interpreting pKa values
Determining conjugate acids and bases
Determining products and drawing the reaction mechanism
Calculating the equilibrium constant and determining the
position of equilibrium
Factors that affect acidity
Lewis acids and bases
2.1, 2.2
2.3
2.3
2.4
2.5
2.6
2.12
* the custom 7th edition is just the regular 7th edition minus a few chapters not covered in CH 331/CH 332/CH 337
Comprehensive list of learning objectives
(Learning objectives form the basis for quiz and exam questions)
So that you may assess your progress through the material of these lecture notes I provide the following
“checklist” of learning objectives. As we move through the various Studies of these lecture notes we’ll see a
re-listing of the corresponding learning objectives.
At the conclusion of these lecture notes one should be able to,
¨ define acids and bases according to the Bronsted-Lowry definition
¨ select/identify acids and bases according to the Bronsted-Lowry definition
¨ convert Ka to pKa
¨ convert pKa to Ka
¨ assign approximate pKa values to the various hydrogen atoms of a given compound
¨ rank a selection of acids in order of acid strength
¨ rank a selection of bases in order of base strength
¨ draw/identify the conjugate base of any acid
¨ draw/identify the conjugate acid of any base
¨ predict the products of any Bronsted/Lowry acid-base reaction
¨ provide the mechanism of any Bronsted/Lowry acid-base reaction
¨ calculate the equilibrium constant (Keq) of any Bronsted/Lowry acid-base reaction
¨ determine/indicate whether a given Bronsted-Lowry acid-base equilibrium lies to the left or to the right
¨ to rank a selection of acids in order of acid strength
¨ define acids and bases according to the Lewis definition
¨ select/identify acids and bases according to the Lewis definition
¨ predict the products of any Lewis acid-base reaction
¨ provide the mechanism of any Lewis acid-base reaction
Unit 2 Lecture notes and assigned questions
Study 1
A few important terms
Bruice 7th section(s) 2.1, 2.2
Learning objective(s) At the conclusion of this Study one should be able to,
a)
b)
c)
d)
define acids and bases according to the Bronsted-Lowry definition
select/identify acids and bases according to the Bronsted-Lowry definition
convert Ka to pKa
convert pKa to Ka
Bronsted/Lowry acid
Bronsted/Lowry base
Ka
pKa
Let’s take a look at a few worked examples
Page 2 of 14
Unit 2 Lecture notes and assigned questions
Page 3 of 14
Tables of pKa values
Where appropriate the most acidic hydrogen is underlined
You will be provided with relevant segments of following tables on the exams
Acid
~ pKa
HI
-10
I
HBr
-9
Br
HCl
-7
Cl
protonated aldehydes
and ketones
OH
R
protonated hydrogen
sulfide, thiols and
thioethers
H
S
OH
H
C
H R
R
H
H
R
R
S
H
R
OH
H
S
H
C
H
R
HO
H
O
H
C
R
OR
OH
HO
H
R
O
H
-3
H
O
H
S
R
H
N
R
O
O
O
O
OH
N
C
(R or H)
0
(R or H)
N
C
O
P
OH
(R or H)
(R or H)
(R or H)
HO
O
O
-1
OH
O
phosphoric acid
(H3PO4)
OR
O
N
(R or H)
C
R
O
protonated amides
R
O
H
nitric acid
(HNO3)
R
O
C
-5
OH
O
H
S
O
S
R
R
-6
O
protonated water,
alcohols and ethers
R
C
H
S
O
sulfuric acid
(H2SO4)
R
O
OH
C
-7
O
R
OH
protonated carboxylic
acids and esters
O
-7
R
C
H R
S
Conjugate Base
O
OH
2
HO
P
OH
O
R
Unit 2 Lecture notes and assigned questions
Page 4 of 14
Acid
~ pKa
HF
Conjugate Base
3
F
O
O
carboxylic acids
5
(R or H)
C
OH
(R or H)
H
C
(R or H)
N
N
5
(R or H)
(R or H)
(R or H)
O
H
O
C
C
C
(R or H)
O
9
(R or H)
(R or H)
O
C
C
(R or H)
H
S
R
(R or H)
C
(R or H)
protonated ammonia and amines
NH4+, RNH3+, R2NH2+, R3NH+
H
O
S
10
10
H
NH3, RNH2, R2NH, R3N
H
S
R
OH
S
O
10
(R or H)
O
H
O
C
C
C
O
11
OR
(R or H)
C
(R or H)
water and alcohols
H
O
C
C
OR
(R or H)
H
O
R
O
H
15
H
O
R
O
O
O
amides
(R or H)
C
N
(R or H)
16
(R or H)
C
N
(R or H)
C
(R or H)
H
aldehydes and ketones
(R or H)
O
H
C
C
O
(R or H)
(R or H)
20
(R or H)
C
(R or H)
Unit 2 Lecture notes and assigned questions
Page 5 of 14
~
pKa
Acid
esters
RO
O
H
C
C
Conjugate Base
O
(R or H)
25
RO
C
(R or H)
Terminal
alkynes
R
C
(R or H)
25
H
C
(R or H)
C
R
C
C
H
N
N
(R or H)
30
(R or H)
Ammonia and amines
NH3, RNH2, R2NH
35
NH2
NHR2
NR2
arenes
alkenes
(R or H)
(R or H)
H
H
45
(R or H)
(R or H)
(R or H)
(R or H)
H
alkanes
(R or H)
C
(R or H)
(R or H)
50
(R or H)
C
(R or H)
(R or H)
Unit 2 Lecture notes and assigned questions
Study 2
Page 6 of 14
Assigning and interpreting pKa values
Bruice 7th section(s) 2.3
Learning objective(s) At the conclusion of this Study one should be able to,
a) assign approximate pKa values to the various hydrogen atoms of a given compound
b) rank a selection of acids in order of acid strength
c) rank a selection of bases in order of base strength
Let’s take a look at a few worked examples
a) Levodopa has been used for the treatment of Parkinson’s disease since the early 1960’s.
What is the pKa of hydrogen A in levodopa?
MAKING CONNECTIONS
HO
H
What is the pKa of hydrogen B in levodopa?
What is the pKa of hydrogen C in levodopa?
What is the pKa of hydrogen D in levodopa?
HA
O
CH2
H
HB
H
O
C
C
N
HC
OHD
H
Levodopa
b) Gephyrotoxin has been isolated from the skin secretions of a Columbian poison-dart frog. Circle
gephyrotoxin’s most acidic hydrogen atom.
MAKING CONNECTIONS
HO
H
N
C
C
Gephyrotoxin
Unit 2 Lecture notes and assigned questions
Study 3
Page 7 of 14
Determining conjugate acids and bases
Bruice 7th section(s) 2.1, 2.2
Learning objective(s) At the conclusion of this Study one should be able to,
a) draw/identify the conjugate base of any acid
b) draw/identify the conjugate acid of any base
Let’s take a look at a few worked examples
a) Draw the conjugate acids of the following compounds
O
O
O
N
H
conjugate acid
conjugate acid
b) Draw the conjugate bases of the following compounds
NH2
O
C
conjugate base
conjugate base
H
Unit 2 Lecture notes and assigned questions
Study 4
Determining products and drawing the reaction mechanism
Bruice 7th section(s) 2.4
Learning objective(s) At the conclusion of this Study one should be able to,
a) predict the products of any Bronsted/Lowry acid-base reaction
b) provide the mechanism of any Bronsted/Lowry acid-base reaction
Let’s take a look at a few worked examples
a) Draw the products and mechanism of the following acid-base reaction
H
H
O
H
C
C
C
H
H
H
Cl
H
b) Draw the products and mechanism of the following acid-base reaction
H
Li
H2N
Page 8 of 14
H
H
CH2CH2CH3
Unit 2 Lecture notes and assigned questions
Study 5
Page 9 of 14
Calculating the equilibrium constant and determining the position
of the equilibrium
Bruice 7th section(s) 2.5
Learning objective(s) At the conclusion of this Study one should be able to,
a) calculate the equilibrium constant (Keq) of any Bronsted/Lowry acid-base reaction
b) determine/indicate whether a given Bronsted-Lowry acid-base equilibrium lies to the
left or to the right
An important rule from General Chemistry
A Bronsted/Lowry acid-base equilibrium lies to the side with the
acid and
base.
Let’s take a look at a worked example
What is the equilibrium constant of the following acid-base reaction and what arrow notation reflects the
position of the equilibrium?
O
OH
H
O
S
O
OH
O
O
K eq
æ K of the reac tan t acid ö æç 10 -pKa
÷=
= çç a
÷
-pKa
è K a of the product acid ø çè 10
ö æ 10 -( -5 )
÷=ç
÷ ç 10 -( -3 )
ø è
H
ö æ 10 +5
÷=ç
÷ ç 10 +3
ø è
ö
÷ = 10 + 2 = 100
÷
ø
H
O
S
O
If Keq < 1 then we use If Keq = 1 then we use If Keq > 1 then we use
OH
Unit 2 Lecture notes and assigned questions
Page 10 of 14
Let’s take a look at a few more examples
a) For the following acid-base reaction calculate the equilibrium constant and determine the position of the
equilibrium
C
C
H
Li
NH2
C
C Li
NH3
b) For the following acid-base reaction calculate the equilibrium constant and determine the position of the
equilibrium
O
H3C
C
O
O
H
Na
OH
H3C
C
O Na
H2O
Unit 2 Lecture notes and assigned questions
Study 6
Page 11 of 14
Factors that affect acidity
Bruice 7th section(s) 2.6
Learning objective(s) At the conclusion of this Study one should be able to rank a selection of acids in order of
acid strength
Here are the trends we will consider
Trend #1
When comparing acids in which the atoms to which the hydrogens are attached have very
different sizes (i.e. taken from the same group of the periodic table)
pKa
H2O
H2S
H2Se
H2Te
Trend #2
15
7
4
3
When comparing acids in which the atoms to which the hydrogens are attached have
comparable sizes (i.e. taken from the same row of the periodic table)
pKa
NH3
H2O
HF
+35
+15
3
Unit 2 Lecture notes and assigned questions
Trend #3
Page 12 of 14
When comparing acids in which the atoms to which the hydrogens are attached are the same
element
H
CH3
CH2
H
C
NH3
N
CH3
C
N
H
H
pKa = 6
pKa = 10
Study 7
CH3
pKa = -10
Lewis acids and bases
Bruice 7th section(s) 2.12
Learning objective(s) At the conclusion of this Study one should be able to,
a)
b)
c)
d)
define acids and bases according to the Lewis definition
select/identify acids and bases according to the Lewis definition
predict the products of any Lewis acid-base reaction
provide the mechanism of any Lewis acid-base reaction
Lewis acids (e.g. carbocations, BF3, AlCl3, FeBr3, FeCl3, ZnCl2)
Lewis bases
Predicting the products of a Lewis acid-base reaction; providing the mechanism of a Lewis acid-base reaction
-let’s take a close look at the following examples
Unit 2 Lecture notes and assigned questions
Page 13 of 14
Although the following assigned questions are not turned in they provide an excellent opportunity for you to
assess your progress through the course material.
Assigned questions from the seventh edition of Bruice
(the custom 7th edition is just the regular 7th edition minus a few chapters not covered in CH 331/CH 332/CH
337)
2.3, 2.4, 2.8, 2.9, 2.10, 2.24, 2.25, 2.27, 2.45, 2.46, 2.49, 2.51, 2.55
Additional assigned questions
Solutions are available on Canvas
1. Rank the following compounds from strongest acid to weakest acid (strongest acid first).
NH3
2. Rank the following compounds from strongest base to weakest base (strongest base first).
3. Rank the following compounds from strongest acid to weakest acid (strongest acid first).
O
H
N
OH
CH 3
O
Compound G
Compound H
Compound I
Unit 2 Lecture notes and assigned questions
Page 14 of 14
4. What is the equilibrium constant of the following equilibrium? Which set of arrows correctly indicates the
position of the equilibrium?
5. What is the equilibrium constant of the following equilibrium? Which set of arrows correctly indicates the
position of the equilibrium?
Set 1
O
O
Set 2
SH
S
Na
Set 3
6. Draw the product(s) of the following reaction.
H3C
CH3
C
H2O
CH3
7. Using curved arrow notation draw the mechanism of the following reaction.
O
H
C
H
CH3
CH2CH3
H
N
H
OH
H
H
C
CH3
CH2CH3
NH3
Na
Chemistry 331 Lecture Notes, Bruice Readings and Assigned Questions
Unit 3 Alkanes and cycloalkanes
This lecture notes document provides a list of readings from the seventh edition of Bruice. It concludes
with a list of assigned questions from the seventh editions of Bruice.
Introduction
This part of the course examines the chemical and physical properties of alkanes and cycloalkanes. Our
topic list includes combustion of alkanes and cycloalkanes, a review of structural isomerism and
stereoisomerism from General Chemistry and conformational analysis of alkanes and cycloalkanes (e.g.
ethane, propane, butane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, substituted
cyclohexanes).
Overview
Sections in the 7th edition of Bruice*
Study
1
2
3
4
5
6
7
Alkane and cycloalkane nomenclature
Combustion of alkanes and cycloalkanes
How to interpret skeletal structures
A few important terms
Rotation about carbon-carbon single bonds
Strained cycloalkanes
Conformations of cyclohexane and
substituted cyclohexanes
3.intro, 3.1, 3.2, 3.3, 3.9 (alkane segments only)
3.3
3.10, 3.11 (see also 4.intro)
3.10
3.11
3.12, 3.13, 3.14
* the custom 7th edition is just the regular 7th edition minus a few chapters not covered in CH 331/CH 332/CH 337
Comprehensive list of learning objectives
(Learning objectives form the basis for quiz and exam questions)
So that you may assess your progress through the material of these lecture notes I provide the following
“checklist” of learning objectives. As we move through the various Studies of these lecture notes we’ll see
a re-listing of the corresponding learning objectives.
At the conclusion of these lecture notes one should be able to,
¨ give the IUPAC names of alkanes and cycloalkanes
¨ give the products arising from the combustion of alkanes and cycloalkanes
¨ give the balanced chemical equation for the combustion of alkanes and cycloalkanes
¨ interpret skeletal structures
¨ define and use these terms – conformation, torsional strain, steric strain, angle strain, structural (or
constitutional) isomers, stereoisomers
¨ use the terms eclipsed conformation, staggered conformation
¨ use the terms eclipsed interaction and gauche interaction
¨ draw eclipsed conformations using wedge/dash notation and Newman projections
¨ draw staggered conformations using wedge/dash notation and Newman projections
¨ give the strain energy (in kcal/mole) associated with a H: H eclipsed interaction
¨ give the strain energy (in kcal/mole) associated with a CH3:H eclipsed interaction
¨ give the strain energy (in kcal/mole) associated with a CH3:CH3 eclipsed interaction
¨ give the strain energy (in kcal/mole) associated with a CH3:CH3 gauche interaction
¨ deduce the strain energy for a particular eclipsed interaction in a given conformation
¨ deduce the strain energy for a particular gauche interaction in a given conformation
¨ rank conformations according to their relative stabilities and outline the underlying reasoning
¨ deduce the approximate strain energy for a given conformation
¨ provide the heat of formation of a “strainless” CH2 group
¨ calculate the strain energy of a cycloalkane when provided with the necessary experimental data
¨ apportion cyclopropane’s strain energy into its torsional strain and angle strain components
Unit 3 Lecture notes and assigned questions
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
Page 2 of 33
apportion planar cyclobutane’s strain energy into its torsional strain and angle strain components
use the terms chair conformation and 1,3-diaxial interaction
use the terms axial position and equatorial position as they relate to cyclohexane and substituted
cyclohexanes
draw chair conformations of cyclohexane and substituted cyclohexanes
give the strain energy (in kcal/mole) associated with a methyl:hydrogen 1,3-diaxial interaction
give the strain energy (in kcal/mole) associated with a ethyl:hydrogen 1,3-diaxial interaction
give the strain energy (in kcal/mole) associated with a tert-butyl:hydrogen 1,3-diaxial interaction
give the strain energy (in kcal/mole) associated with a CH3:CH3 gauche interaction
deduce the strain energy for a given chair conformation
deduce the strain energy for a particular 1,3-diaxial interaction in a given chair conformation
deduce the strain energy for a particular gauche interaction in a given chair conformation
rank conformations according to their relative stabilities and outline the underlying reasoning
draw a substituted cyclohexane in its most (or least) stable chair conformation
calculate DG for a given equilibrium between chair conformations*
calculate the equilibrium constant for a given equilibrium between chair conformations*
calculate the composition at equilibrium when provided with DG or the equilibrium constant*
*when provided with the necessary experimental data
¨
Study 1
Independent study of alkane and cycloalkane nomenclature
Bruice 7th section(s) 3.intro, 3.1, 3.2, 3.3
Learning objective(s) At the conclusion of this independent Study one should be to give the IUPAC names
of alkanes and cycloalkanes
Study 2
Combustion of alkanes and cycloalkanes
Introduction
General Chemistry
This Study reviews the combustion of alkanes and cycloalkanes discussed in the
Bruice section(s)
N/A
Learning objective(s) At the conclusion of this Study one should be able to,
a) give the products arising from the combustion of alkanes and cycloalkanes
b) give the balanced chemical equation for the combustion of alkanes and
cycloalkanes
Let’s take a look at a few worked examples
11 O2
9 O2
8 H2O
6 H2O
6 CO2
7 CO2
Unit 3 Lecture notes and assigned questions
Study 3
Page 3 of 33
How to interpret skeletal structures
Bruice 7th section(s) 3.3
Learning objective(s) At the conclusion of this Study one should be able to interpret skeletal structures
Let’s take a look at a worked example
N
H
O
Since each carbon atom is neutral each carbon atom must be bonded to four atoms. In a skeletal
structure carbon atoms and CH bonds are usually omitted for clarity.
H
a
a
N
H
N
H
O
O
Carbon a in the above skeletal structure shows three bonds; the fourth bond is understood to be to
hydrogen.
H
b
b
N
H
O
N
H
O
Carbon b in the above skeletal structure shows three bonds; the fourth bond is understood to be to
hydrogen.
H
c
N
H
O
H
c
N
H
O
H
H
Carbon c in the above skeletal structure shows one bond; the remaining three bonds are understood to
be to hydrogens.
Unit 3 Lecture notes and assigned questions
Page 4 of 33
H
d
H
d
H
N
H
O
N
H
O
Carbon d in the above skeletal structure shows two bonds; the remaining two bonds are understood to be
to hydrogens.
Now with all carbon atoms shown and all carbon-hydrogen bonds shown,
H
H
C
C
H
H
H
H
H
H
C
C
aC
H
dC
bC
C
C
H
H
C
C
C
H
H
C
N
C
O
H
H
H
C
H
H
H
H
C
H
H
c
H
H
H
H
H
Unit 3 Lecture notes and assigned questions
Study 4
Page 5 of 33
A few important terms
Bruice 7th section(s) 3.10, 3.11, 4.intro
Learning objective(s) At the conclusion of this Study one should be able to define and use the following
terms
Conformation (aka conformer)
Different spatial arrangements of the atoms of a molecule that arise from rotation about single bonds are
called conformations.
e.g.
H
H
H
H
H
H
H
H
H
H
H
H
Torsional strain
The destabilizing interaction between eclipsed bonds is called torsional strain.
e.g.
H
H
H
H
H
H
a H:H eclipsed interaction
(a source of torsional strain)
Steric strain
The destabilizing interaction between groups in a molecule trying to occupy the same region of space and
thus repelling each other is called steric strain.
e.g.
H3C
CH2CH3
a CH3:CH2CH3 repulsive interaction
(a source of steric strain)
H
H
Angle strain
The destabilizing factor in some cyclic molecules resulting from bond angles within the ring which are
necessarily different from the angles required by the hybridizations of the atoms making up the ring is
called angle strain.
e.g.
Unit 3 Lecture notes and assigned questions
Page 6 of 33
Structural (or constitutional) isomerism
Structures that have the same molecular formula but differ in their connectivity of atoms are said to be
related as structural (or constitutional) isomers.
e.g.
Structures A and B are related as structural (or constitutional) isomers.
Stereoisomerism
Structures that have the same molecular formula, the same connectivity of atoms, but differ in the
placement of atoms in three dimensional space are said to be related as stereoisomers. There are
various sub-classes of stereoisomerism which we will see later in the course.
Unit 3 Lecture notes and assigned questions
Study 5
Page 7 of 33
Rotation about carbon-carbon single bonds
Bruice 7th section(s) 3.10
Learning objective(s) At the conclusion of this Study one should be able to,
a) use the terms eclipsed conformation, staggered conformation
b) use the terms eclipsed interaction and gauche interaction
c) draw eclipsed conformations using wedge/dash notation and Newman
projections
d) draw staggered conformations using wedge/dash notation and Newman
projections
e) give the strain energy (in kcal/mole) associated with a H: H eclipsed interaction
f) give the strain energy (in kcal/mole) associated with a CH3:H eclipsed interaction
g) give the strain energy (in kcal/mole) associated with a CH3:CH3 eclipsed
interaction
h) give the strain energy (in kcal/mole) associated with a CH3:CH3 gauche
interaction
i) deduce the strain energy for a particular eclipsed interaction in a given
conformation
j) deduce the strain energy for a particular gauche interaction in a given
conformation
k) rank conformations according to their relative stabilities and outline the
underlying reasoning
l) deduce the approximate strain energy for a given conformation
Let’s take a look at a few worked examples
Ethane (CH3CH3)
Staggered conformation
Side view
End view
-looking along the C(2) C(1) bond
Unit 3 Lecture notes and assigned questions
Page 8 of 33
Eclipsed conformation
Side view
End view
-looking along the C(2) C(1) bond
Relative stabilities (least stable conformation at the top of the graph)
Destabilizing interactions?
-a H:H eclipsed interaction between H(3) and H(4)
-a H:H eclipsed interaction between H(5) and H(7)
-a H:H eclipsed interaction between H(6) and H(8)
free energy →
~3 kcal/mole
Total strain energy?
3.0 kcal/mole (determined experimentally)
Strain energy associated with a single H:H
eclipsed interaction?
1.0 kcal/mole (1/3 of 3.0 kcal/mole)
Unit 3 Lecture notes and assigned questions
Page 9 of 33
Propane (CH3CH2CH3)
Staggered conformation
Side view
End view
(4) is a methyl group
-looking along the C(2) C(1) bond
Eclipsed conformation
Side view
End view
(4) is a methyl group
-looking along the C(2) C(1) bond
Unit 3 Lecture notes and assigned questions
Page 10 of 33
free energy →
Relative stabilities (least stable conformation at the top of the graph)
Destabilizing interactions?
-a H:H eclipsed interaction between H(5) and H(7)
-a H:H eclipsed interaction between H(6) and H(8)
-a CH3:H eclipsed interaction between methyl
group (4) and H(3)
~3.4 kcal/mole
Total strain energy?
3.4 kcal/mole (determined experimentally)
Strain energy associated with a single CH3:H
eclipsed interaction?
1.4* kcal/mole
* 3.4 (total strain energy)
– 1 (H:H eclipsed interaction)
– 1 (H:H eclipsed interaction)
= 1.4 (CH3:H eclipsed interaction)
Unit 3 Lecture notes and assigned questions
Page 11 of 33
Butane (CH3CH2CH2CH3)
Staggered conformations
Side view of anti conformation
End view of anti conformation
(3) and (4) are methyl groups
-looking along the C(2) C(1) bond
Side view of a gauche conformation
End view of a gauche conformation
(3) and (4) are methyl groups
-looking along the C(2) C(1) bond
Unit 3 Lecture notes and assigned questions
Page 12 of 33
Eclipsed conformations
Side view
End view
(3) and (4) are methyl groups
-looking along the C(2) C(1) bond
Methyl group (4) is partially obscured by H(8)
Side view
End view
(3) and (4) are methyl groups
-looking along the C(2) C(1) bond
Unit 3 Lecture notes and assigned questions
Page 13 of 33
Relative stabilities (least stable conformation at the top of the graph)
Destabilizing interactions?
-a H:H eclipsed interaction between H(5) and H(7)
-a H:H eclipsed interaction between H(6) and H(8)
-a CH3:CH3 eclipsed interaction between methyl groups (3) and (4)
Total strain energy?
4.5 kcal/mole (determined experimentally)
Strain energy associated with a single CH3:CH3 eclipsed
interaction?
2.5* kcal/mole
* 4.5 (total strain energy)
– 1 (H:H eclipsed interaction)
– 1 (H:H eclipsed interaction)
= 2.5 (CH3:CH3 eclipsed interaction)
free energy →
Destabilizing interactions?
-a H:H eclipsed interaction between H(6) and H(7)
-a CH3:H eclipsed interaction between methyl group (3) and H(5)
-a CH3:H eclipsed interaction between methyl group (4) and H(8)
Total strain energy?
3.8** kcal/mole (determined experimentally)
**1.4 (CH3:H eclipsed interaction) +1.4 (CH3:H eclipsed interaction)
+1 (H:H eclipsed interaction)
Destabilizing interactions?
-a CH3:CH3 gauche interaction between methyl groups (3) and (4)
Total strain energy?
0.87*** kcal/mole (determined experimentally)
Strain energy associated with a single CH3:CH3 gauche interaction?
0.87*** kcal/mole
*** NOTE Bruice reports 0.9 kcal/mole. But it’s actually 0.87
kcal/mole, so we’ll use 0.87 kcal/mole.
This staggered conformation of butane is strain free
Unit 3 Lecture notes and assigned questions
Study 6
Page 14 of 33
Strained cycloalkanes
Bruice 7th section(s) 3.11
Learning objective(s) At the conclusion of this Study one should be able to,
a) provide the heat of formation of a “strainless” CH2 group
b) calculate the strain energy of a cycloalkane when provided with the necessary
experimental data
c) apportion cyclopropane’s strain energy into its torsional strain and angle strain
components
d) apportion planar cyclobutane’s strain energy into its torsional strain and angle
strain components
How to calculate the strain energy of a cycloalkane
i)
Strain energy of a cycloalkane
= actual heat of formation – calculated “strain-free” heat of formation
ii)
Calculated “strain-free” heat of formation
= (heat of formation of a “strain-free” CH2 group)(number of CH2 groups)
iii)
Heat of formation of a “strain-free” CH2 group
=
Compound
Actual heat of
formation*
Cyclohexane
(strain-free)
-29.5 kcal/mole
Calculated “strain-free” heat of formation
(
kcal/mole)(
)=
kcal/mole
Strain energy
kcal/mole
e.g.
Compound
Actual heat of
formation*
Cyclopropane
+12.7 kcal/mole
(
kcal/mole)(
)=
kcal/mole
kcal/mole
Cyclobutane
+6.8 kcal/mole
(
kcal/mole)(
)=
kcal/mole
kcal/mole
Cyclopentane
-18.4 kcal/mole
(
kcal/mole)(
)=
kcal/mole
kcal/mole
* determined experimentally
Calculated “strain-free” heat of formation
Strain energy
Unit 3 Lecture notes and assigned questions
Page 15 of 33
Cyclopropane
Total strain energy? 27.5 kcal/mole (from previous page)
Sources of torsional strain?
-a H:H eclipsed interaction between H(4) and H(7)
-a H:H eclipsed interaction between H(4) and H(8)
-a H:H eclipsed interaction between H(7) and H(8)
-a H:H eclipsed interaction between H(5) and H(6)
-a H:H eclipsed interaction between H(5) and H(9)
-a H:H eclipsed interaction between H(6) and H(9)
Estimated torsional strain?
kcal/mole
Estimated angle strain?
kcal/mole
Cyclobutane
Total strain energy? 26.5 kcal/mole (from previous page)
Sources of torsional strain (assuming ring is planar)?
-a H:H eclipsed interaction between H(13) and H(16)
-a H:H eclipsed interaction between H(10) and H(13)
-a H:H eclipsed interaction between H(6) and H(16)
-a H:H eclipsed interaction between H(6) and H(10)
-a H:H eclipsed interaction between H(9) and H(12)
-a H:H eclipsed interaction between H(5) and H(9)
-a H:H eclipsed interaction between H(12) and H(15)
-a H:H eclipsed interaction between H(5) and H(15)
Estimated torsional strain (assuming ring is planar)?
kcal/mole
Estimated angle strain?
kcal/mole
Unit 3 Lecture notes and assigned questions
Study 7
Page 16 of 33
Conformations of cyclohexane and substituted cyclohexanes
Bruice 7th section(s) 3.12, 3.13, 3.14
Learning objective(s) At the conclusion of this Study one should be able to,
a) use the terms chair conformation and 1,3-diaxial interaction
b) use the terms axial position and equatorial position as they relate to
cyclohexane and substituted cyclohexanes
c) draw chair conformations of cyclohexane and substituted cyclohexanes
d) give the strain energy (in kcal/mole) associated with a methyl:hydrogen 1,3diaxial interaction
e) give the strain energy (in kcal/mole) associated with a ethyl:hydrogen 1,3-diaxial
interaction
f) give the strain energy (in kcal/mole) associated with a tert-butyl:hydrogen 1,3diaxial interaction
g) give the strain energy (in kcal/mole) associated with a CH3:CH3 gauche
interaction
h) deduce the strain energy for a given chair conformation
i) deduce the strain energy for a particular 1,3-diaxial interaction in a given chair
conformation
j) deduce the strain energy for a particular gauche interaction in a given chair
conformation
k) rank conformations according to their relative stabilities and outline the
underlying reasoning
l) draw a substituted cyclohexane in its most (or least) stable chair conformation
m) calculate DG for a given equilibrium between chair conformations*
n) calculate the equilibrium constant for a given equilibrium between chair
conformations*
o) calculate the composition at equilibrium when provided with DG or the
equilibrium constant*
*when provided with the necessary experimental data
Cyclohexane
Unit 3 Lecture notes and assigned questions
Page 17 of 33
Methylcyclohexane
CH3
methylcyclohexane
Relative stabilities (least stable conformation at the top of the graph)
3
2
Destabilizing interactions?
-a CH3:H 1,3-diaxial interaction between methyl
group (1) and H(2)
-a CH3:H 1,3-diaxial interaction between methyl
group (1) and H(3)
1
free energy →
Total strain energy?
1.74 kcal/mole (determined experimentally)
Strain energy associated with a single CH3:H 1,3diaxial interaction?
(1) is a methyl group
kcal/mole
1
(1) is a methyl group
Unit 3 Lecture notes and assigned questions
Page 18 of 33
Ethylcyclohexane
CH2CH3
ethylcyclohexane
Relative stabilities (least stable conformation at the top of the graph)
3
2
Destabilizing interactions?
-a CH3CH2:H 1,3-diaxial interaction between ethyl
group (1) and H(2)
-a CH3CH2:H 1,3-diaxial interaction between ethyl
group (1) and H(3)
1
free energy →
Total strain energy?
1.81 kcal/mole (determined experimentally)
Strain energy associated with a single CH3CH2:H
1,3-diaxial interaction?
(1) is an ethyl group
kcal/mole
1
(1) is an ethyl group
Unit 3 Lecture notes and assigned questions
Page 19 of 33
Tert-butylcyclohexane
C(CH3)3
tert-butylcyclohexane
Relative stabilities (least stable conformation at the top of the graph)
3
2
Destabilizing interactions?
-a C(CH3)3:H 1,3-diaxial interaction between tertbutyl group (1) and H(2)
-a C(CH3)3:H 1,3-diaxial interaction between tertbutyl group (1) and H(3)
1
free energy →
Total strain energy?
5.05 kcal/mole (determined experimentally)
Strain energy associated with a single C(CH3)3:H
1,3-diaxial interaction?
(1) is a tert-butyl group
kcal/mole
1
(1) is a tert-butyl group
Unit 3 Lecture notes and assigned questions
Page 20 of 33
Cis-1,2-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Trans-1,2-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Unit 3 Lecture notes and assigned questions
Page 21 of 33
Cis-1,3-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Trans-1,3-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Unit 3 Lecture notes and assigned questions
Page 22 of 33
Cis-1,4-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Trans-1,4-dimethylcyclohexane
“Reactant”
Repulsive Interactions
“Product”
Repulsive Interactions
Unit 3 Lecture notes and assigned questions
Page 23 of 33
Calculating the equilibrium constant and composition at equilibrium
Methylcyclohexane
-with axial methylcyclohexane on the “reactant” side and equatorial methylcyclohexane on the “product” side the
analysis yields
free energy →
DG° = -1.74 kcal/mole (sign is negative since the
“product” side of the equilibrium is more stable than
the “reactant” side of the equilibrium)
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(-1.74)/(1.364) = 18.9
é conformati on on the ” product” side ù 18.9
ú= 1
ë conformati on on the ” reactant” side û
1.74 kcal/mole
K eq = ê
% conformation with equatorial R group?
é 18.9 ù
ê
ú x 100 = 95%
ë1+ 18.9 û
% conformation with axial R group?
é
1 ù
ê
ú x 100 = 5%
ë1+ 18.9 û
“reactant”
“product”
-if axial methylcyclohexane is on the “product” side and equatorial methylcyclohexane is on the “reactant” side then
the analysis yields
DG° = +1.74 kcal/mole (sign is positive since the
“product” side of the equilibrium is less stable than the
“reactant” side of the equilibrium)
free energy →
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(+1.74)/(1.364) = 0.0529
é conformati on on the ” product” side ù 0.0529
ú=
1
ë conformati on on the ” reactant” side û
K eq = ê
1.74 kcal/mole
% conformation with axial R group?
é 0.0529 ù
ê1+ 0.0529 ú x 100 = 5%
ë
û
% conformation with equatorial R group?
1
é
ù
ê1+ 0.0529 ú x 100 = 95%
ë
û
“reactant”
“product”
Unit 3 Lecture notes and assigned questions
Page 24 of 33
Ethylcyclohexane
-with axial ethylcyclohexane on the “reactant” side and equatorial ethylcyclohexane on the “product” side the analysis
yields
free energy →
DG° = -1.81 kcal/mole (sign is negative since the
“product” side of the equilibrium is more stable than
the “reactant” side of the equilibrium)
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(-1.81)/(1.364) = 21.2
é conformati on on the ” product” side ù 21.2
ú= 1
ë conformati on on the ” reactant” side û
K eq = ê
1.81 kcal/mole
% conformation with equatorial R group?
é 21.2 ù
ê
ú x 100 = 95.5%
ë1+ 21.2 û
% conformation with axial R group?
é
1 ù
ê
ú x 100 = 4.5%
1
+
21.2
ë
û
“reactant”
“product”
-if axial ethylcyclohexane is on the “product” side and equatorial ethylcyclohexane is on the “reactant” side then the
analysis yields
DG° = +1.81 kcal/mole (sign is positive since the
“product” side of the equilibrium is less stable than the
“reactant” side of the equilibrium)
free energy →
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(+1.81)/(1.364) = 0.0472
1.81 kcal/mole
é conformati on on the ” product” side ù 0.0472
ú=
1
ë conformati on on the ” reactant” side û
K eq = ê
% conformation with axial R group?
é 0.0472 ù
ê1+ 0.0472 ú x 100 = 4.5%
ë
û
% conformation with equatorial R group?
1
é
ù
ê1+ 0.0472 ú x 100 = 95.5%
ë
û
“reactant”
“product”
Unit 3 Lecture notes and assigned questions
Page 25 of 33
Tert-butylcyclohexane
-with axial tert-butylcyclohexane on the “reactant” side and equatorial tert-butylcyclohexane on the “product” side
then the analysis yields
free energy →
DG° = -5.05 kcal/mole (sign is negative since the
“product” side of the equilibrium is more stable than
the “reactant” side of the equilibrium)
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(-5.05)/(1.364) = 5039
é conformati on on the ” product” side ù 5039
K eq = ê
ú= 1
ë conformati on on the ” reactant” side û
5.05 kcal/mole
% conformation with equatorial R group?
é 5039 ù
ê
ú x 100 = 99.98%
ë1+ 5039 û
% conformation with axial R group?
é
ù
1
ê
ú x 100 = 0.02%
1
+
5039
ë
û
“reactant”
“product”
-if axial tert-butylcyclohexane is on the “product” side and equatorial tert-butylcyclohexane is on the “reactant” side
then the analysis yields
DG° = +5.05 kcal/mole (sign is positive since the
“product” side of the equilibrium is less stable than the
“reactant” side of the equilibrium)
free energy →
Keq?
Keq = 10-DG°/2.3RT
Keq = 10-(+5.05)/(1.364) = 0.000198
é conformati on on the ” product” side ù 0.000198
K eq = ê
ú=
1
ë conformati on on the ” reactant” side û
5.05 kcal/mole
% conformation with axial R group?
é 0.000198 ù
ê1+ 0.000198 ú x 100 = 0.02%
ë
û
% conformation with equatorial R group?
1
é
ù
ê1+ 0.000198 ú x 100 = 99.98%
ë
û
“reactant”
“product”
You will be provided with the following items on the exams
DG° = -RTlnK = -2.3RTlogK
o
K = e−ΔG
/RT
o
= 10−ΔG /2.3RT
2.3RT = 1.364 kcal/mol (T = 25°C = 298K)
Unit 3 Lecture notes and assigned questions
Page 26 of 33
Summary of repulsive interactions and their associated strain energies
Repulsive
interaction
Strain
energy
Comments
Examples
H:H
1.0
eclipsed
kcal/mole
interaction
H
CH3:H
1.4
eclipsed
kcal/mole
interaction
H
CH3:CH3
2.5
eclipsed
kcal/mole
interaction
H
H
H
CH3
H
H
H
CH3
H
H
H
H
H
H
CH3
CH3
H
H
H
H:C(CH3)3
2.53
1,3-diaxial
kcal/mole
interaction
H
H
H
H
H:CH2CH3
0.91
1,3-diaxial
kcal/mole
interaction
H
H3C
CH3:CH3
0.87
gauche
kcal/mole
interaction
H:CH3 1,30.87
diaxial
kcal/mole
interaction
H
Limited to substituted
cyclohexanes in the chair
conformation
Side views –therefore
wedge/dash notation not
required
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH2CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C(CH3)3
H
H
C(CH3)3
H
H
H
H
CH2CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
Limited to substituted
cyclohexanes in the chair
conformation
Side views –therefore
wedge/dash notation not
required
H
CH3
H
H
Limited to substituted
cyclohexanes in the chair
conformation
Side views –therefore
wedge/dash notation not
required
H
H
H
H
H
H
H
Unit 3 Lecture notes and assigned questions
Page 27 of 33
Gauche Interactions in General
e.g.
H
H
H
H
X
H
H
CH3
H
Br
H
CH3
H
Cl
H
H
H
H
H
Y
H
X
H
Y
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
H
H
H
H
X
H
H
H
Y
H
H
CH3
H
CH3
H
H
H
CH3
H
H
H
H
H
H
provided neither X nor Y (they need not be the same) are hydrogen atoms then there is an
X:Y gauche interaction and it will have an associated strain energy
1,3-Diaxial Interactions in General
e.g.
H
H
Y
H
Y
H
X
H
X
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Y
H
X
H
H
H
H
Br
H
Cl
Cl
H
H
H
H
H
H
Br
H
H
H
H
HH
H
H
H
H
H
H
H
CH3
H
CH3
CH3
H
H
H
CH3
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
provided X and Y (they need not be the same) are not both hydrogen atoms then there is an
X:Y 1,3-diaxial interaction and it will have an associated strain energy
Unit 3 Lecture notes and assigned questions
Page 28 of 33
Let’s take a look at a few worked examples
H
Question
What is the strain energy (in kcal/mole) of a single R:R
gauche interaction, where each R represents the same
unspecified alkyl group?
Answer
H
R
H
R
R
H
H
H
kcal/mole
H
H
C(CH3)3
Total strain energy is 11.7 kcal/mole
Destabilizing interaction
Associated strain energy
Total strain energy
11.7
Question
kcal/mole
What is the total strain energy (in kcal/mole) of the conformation
shown at right?
Answer
10.99
kcal/mole
Destabilizing interaction
Associated strain energy
Four C(CH3)3:H 1,3 diaxial interactions
4 x 2.53 kcal/mole = 10.12 kcal/mole
One CH3:CH3 gauche interaction
0.87 kcal/mole
Total strain energy
10.99
kcal/mole
Unit 3 Lecture notes and assigned questions
Page 29 of 33
Although the following assigned questions are not turned in they provide an excellent opportunity for you
to assess your progress through the course material.
Assigned questions from the seventh edition of Bruice
(the custom 7th edition is just the regular 7th edition minus a few chapters not covered in CH 331/CH
332/CH 337)
3.1, 3.11 (a to d), 3.13, 3.14, 3.15, 3.16, 3.37, 3.39, 3.41, 3.43, 3.44, 3.45, 3.46, 3.48, 3.49, 3.51, 3.55,
3.67, 3.68, 3.71, 3.76, 3.79, 3.82
Additional assigned questions
Solutions are available on Canvas
1. What is the total strain energy (in kcal/mole) of the following conformation? Outline your reasoning.
CH3
H
CH3
CH3
CH3
H
2. Given that the ring carbon atoms of compound A are co-planar and assuming the angle strain of
compound A is 18.5 kcal/mole what is the total strain energy (in kcal/mole) of compound A? Carry
maximum number of significant figures to final answer then enter just the numerical value rounded to
one decimal place.
3. Consider the following equilibrium (T = 25°C)? R represents an unspecified alkyl group.
At equilibrium what percent of the molecules have an axial R group?
Unit 3 Lecture notes and assigned questions
Page 30 of 33
4. Which one of the following statements is correct?
¨
¨
¨
Conformation B is more stable than conformation C
Conformation B is less stable than conformation C
None of the above
5. Best to build a model first.
What atom or group must be placed in position W to complete the Newman projection in Figure 1?
What atom or group must be placed in position X to complete the Newman projection in Figure 1?
What atom or group must be placed in position Y to complete the Newman projection in Figure 1?
What atom or group must be placed in position Z to complete the Newman projection in Figure 1?
6. What strain energy (in kcal/mole) is associated with an R:R gauche interaction? R represents the
same unspecified alkyl group.
Unit 3 Lecture notes and assigned questions
Page 31 of 33
7. What is the strain energy (in kcal/mole) of a single R:R gauche interaction, where each R represents
the same unspecified alkyl group?
H
H
R
H
R
R
H
H
H
H
H
C(CH3)3
Total strain energy is 11.7 kcal/mole
8.
Which one of the following statements about conformations A and B are correct?
¨
¨
¨
Conformation A is more stable than conformation B
Conformation A is less stable than conformation B
None of the above
H
H
CH3CH2
CH3
H
CH2CH3
H
H
CH3
H
Conformation A
9.
Conformation B
Which one of the following statements about conformations C and D are correct?
¨
¨
¨
Conformation C is more stable than conformation D
Conformation C is less stable than conformation D
None of the above
CH2CH3 H
H
H
H
H
H
H
H
H
H
H
CH3
CH2CH3
H
H
H
H
H
H
H
H
Conformation C
CH3
H
Conformation D
Unit 3 Lecture notes and assigned questions
Page 32 of 33
10. Based on the information provided in Figure #1 what strain energy (in kcal/mole) is associated with a
single R:R gauche interaction (where R represents the same unspecified alkyl group)?
Figure #1
H
H
R
H
R
H
R
H
H
H
CH2CH3 H
Total strain energy is 8.6 kcal/mole
11. Which one of the following statements is correct?
Br
Cl
Br
Cl
Structure E
¨
¨
¨
Structure F
Structures E and F are related as stereoisomers
Structures E and F are related as structural isomers
None of the above
12. Assuming that the ring carbon atoms of compound G (depicted in Figure #2) are co-planar and
assuming the angle strain of compound G is 9.0 kcal/mole what is the total strain energy (in kcal/mole)
of compound G?
Figure #2
CH3
CH3
CH3
CH3
H
H
H
H
H
H
H
H
Compound G
Unit 3 Lecture notes and assigned questions
Page 33 of 33
13. Which set of arrows correctly indicates the position of the equilibrium depicted in Figure #3?
Figure #3
Set 1
Set 2
Set 3