1. The document summarizes key concepts from a lecture on alkenes, including their structure, bonding properties, and reactions. It discusses molecular orbital theory and frontier orbital theory as applied to alkenes. 2. Acid-base chemistry concepts are also covered, including Brønsted-Lowry definitions of acids and bases, proton transfer reactions, and factors that influence acidity such as electronegativity and resonance effects. Common acids and their pKa values are listed. 3. Frontier molecular orbital theory is used to predict reactions between reactants by identifying their HOMO and LUMO orbitals and showing curved arrow mechanisms. The roles of acids/bases and electrophiles/nucleophiles

1. Chem E-2a Lecture 2
Chapter 3

2. Introduction to Alkenes
• Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond.
General formula: CnH2n where n = 2, 3, 4…
Terpene Natural Products
β-Pinene Thujene Camphene Caryophyllene
b-Carotene
Lycopene
1

3. Bonding in Alkenes
• Construct an MO diagram for the C=C double bond in ethene, H2C=CH2.
H H
C C
H H
s * C- C
Energy
p* C=C
C p C p
pC=C
C sp2 C sp2
σC- C
Reading: Section 3.2
2

4. Energies of Molecular Orbitals
• Many organic molecules contain carbon, hydrogen, and several more electronegative elements
such as O, N. Cl, Br, etc. In general, the energies of the molecular orbitals in such a molecule will
have the following pattern: (X represents an electronegative atom)
s * CÐH
s * CÐC
s * CÐX
p* C=C
p* C=X
C nonbonding orbital
(lone pair OR carbocation)
Energy X nonbonding orbital
(lone pair)
π C=C
p C=X
s CÐH
s CÐC
s CÐX
Reading: Supplemental Handout, Section 2.3
3

5. Energy-Level Diagrams for Simple Molecules
• Construct an approximate energy-level diagram for acetonitrile, CH3CN, using the following steps:
Step 1 Draw a complete Lewis structure for the molecule,
including all lone pairs
Step 2 Make a list of all the molecular orbitals in the molecule. Count the orbitals to make
sure that you haven’t forgotten any! (How many molecular orbitals must there be?)
Step 3 Arrange the molecular orbitals in order
using the general order on the previous page.
Step 4 Fill the molecular orbitals with the correct number of electrons.
Step 5 Check your energy-level diagram:
- Does it have the correct number of orbitals?
- Does it have the correct number of electrons?
- Are all the bonding orbitals filled with electrons? (They should be!)
- Are all the antibonding orbitals vacant? (They should be!)
- Count the total number of filled bonding orbitals: is that equal to the number of
bonds in the Lewis structure?
Reading: Supplemental Handout, Section 2.4
4

6. Introduction to Reactions of Alkenes: An Overview
• Addition of H–X to alkenes
H
+ H X H
H
H X
• Hydration of alkenes
H
H+ cat.
+ H OH H
H
H OH
5

7. What Happens When Two Molecules React?
• Consider a general reaction between molecules A and B to yield products C and D:
A+B → C+D
• Each of the reacting species (A and B) has many molecular orbitals, filled, and unfilled. What
happens when these two molecules interact (when they come close together)?
Molecule A Molecule B
• There are three basic types of interactions between the orbitals of A and the orbitals of B,
depending on whether the orbitals are filled or unfilled. What can we say about these three types
of interactions?
filled− filled unfilled− unfilled filled− unfilled
Reading: Supplemental Handout, Section 2.5
6

8. Frontier Orbitals:
The Importance of the HOMO and LUMO
• It turns out that the interaction between orbitals that are close in energy is more important than the
interaction between orbitals that are far apart in energy. Why is this the case?
• Because of the above observation, we can understand most of the reactivity of organic molecules
by examining only a small number of orbitals, known as the frontier orbitals. These are:
HOMO (Highest Occupied Molecular Orbital)
LUMO (Lowest Unoccupied Molecular Orbital)
Molecule A Molecule B
Reading: Supplemental Handout, Section 2.5
7

9. Identifying the HOMO and LUMO
• Given the order of the energies of molecular orbitals of organic compounds, we can make some
simple generalizations that will help us locate quickly the HOMO and LUMO for a given
molecule. What are those generalizations? What should we look for?
s * CÐH
s * CÐC
s * CÐX
p* C=C
p* C=X
C nonbonding orbital
(lone pair OR carbocation)
Energy X nonbonding orbital
(lone pair)
π C=C
p C=X
s CÐH
s CÐC
s CÐX
• Using those guidelines, find the HOMO and LUMO for the following species:
O
Br
Reading: Supplemental Handout, Section 2.5
8

10. The Shapes of Frontier Orbitals
• Draw “cartoon orbitals” to represent the shapes of the specified orbitals in the following molecules:
The HOMO of CH3OH
The HOMO of ethylene (H2C=CH2)
The LUMO of tert-butyl carbocation ((CH3)3C+)
The LUMO of methyl bromide (CH3Br)
The LUMO of formaldehyde (H2C=O)
Reading: Supplemental Handout, Section 2.5
9

11. Arrows: Non-Bonding HOMO + Non-Bonding LUMO
• Curved arrows:
Must start on a pair of electrons (filled orbital -- HOMO).
Must point at a place where electrons can go (vacant orbital -- LUMO).
• For the following reaction:
1) Identify the possible HOMO’s and LUMO’s
2) Select the likely nucleophile and electrophile from these two species
3) Show how these species will react using curved arrows
4) Predict the immediate product of that reaction
CH3
Cl +
H3 C CH3
Reading: Sections 1.4, 3.17
Supplemental Handout, Section 2.6
10

12. Arrows: Non-Bonding HOMO + Antibonding LUMO
• For the following reaction:
1) Identify the possible HOMO’s and LUMO’s
2) Select the likely nucleophile and electrophile from these two species
3) Show how these species will react using curved arrows
4) Predict the immediate product of that reaction
H
HO + C Br
H
H
Reading: Sections 1.4, 3.17
Supplemental Handout, Section 2.6
11

13. Predicting Reactions Using Frontier Orbitals
• Using Frontier Molecular Orbital Theory (FMO Theory), predict the reaction between these two
species. Draw the curved-arrow mechanism that shows how they react and show the product that
would result.
O
H C C +
Reading: Sections 1.4, 3.17
Supplemental Handout, Section 2.6
12

14. Brønsted-Lowry Acids and Bases
• One of the most useful theories of acidity and basicity is the Brønsted-Lowry Theory. In this
theory, acids and bases are defined as follows:
A Brønsted Acid is a species that can donate a proton (H+).
A Brønsted Base is a species that can accept a proton.
• Every Brønsted acid, therefore, must have a conjugate base. What are some examples of
Brønsted acids? For each one, identify its conjugate base.
Brønsted Acid Conjugate Base
• When a Brønsted acid, reacts with a Brønsted base, a proton is transferred from the acid to the
base. These reactions are called proton-transfer reactions. Give some examples of proton–
transfer reactions using the above list of acids and bases.
13 Reading: Section 6.4

15. Strengths of Acids and Bases: Ka and pKa
• Because so much chemistry takes place in water, we typically use water as a “standard
reference” for the strengths of acids and bases. For any Brønsted acid HA, the reaction with water
will reach some equilibrium:
The equilibrium constant of this reaction is
• Just as “pH” means “the negative log of the H+ concentration,” we use the term “pKa” to refer
to “the negative log of the Ka.” How can we express that fact mathematically?
A strong acid will have a small (negative) pKa.
A strong base will have a conjugate acid with a large (positive) pKa.
14 Reading: Section 6.4

16. pKa Values for Common Acids
• Here are some important pKa values. You should memorize these values, at least to the
nearest 5 pKa units. For each acid, fill in the conjugate base.
Conjugate Acid pKa Conjugate Base
CH4 48
NH3 35
HC CH 24
H3C OH 16
H2O 16
NH4+ 9
HCN 9
O
H3C 5
OH
H3O+ −2
HCl −7
• We typically define a strong acid as an acid that is at least as strong as H3O+, and a strong base
as a base that is at least as strong as OH–. Identify the strong acids and strong bases in the above
list.
15 Reading: Supplemental Handout, Section 2.7

17. Using pKa Values to Predict Acid-Base Equilibria
• All proton–transfer reactions are reversible (at least in principle). In general, though, one
direction of the reaction will prodominate over the other (that is, the proton transfer will tend to be
unequal). Consider the reaction between the ammonium ion and the hydroxide ion. What will be
the predominant species present in this system at equilibrium? Do you have a sense of whether this
reaction proceeds mainly to the right or mainly to the left?
NH4+ + OH NH3 + H2O
• Can you calculate the equilibrium constant for this reaction? How?
• What does the value of the equilibrium constant tell you about the concentrations of reactants
and products at equilibrium?
16

18. Frontier Orbitals of Proton–Transfer Reactions
• We can, of course, look at proton–transfer reactions in terms of donor and acceptor involved
in the reaction. Can you find the donor and acceptor of the following proton–transfer reactions?
H
O OH2
+ H Cl + Cl
H3C CH3 H3C CH3
H
O O
+ H3C C C + H3C C CH
H3C CH3 H3C CH3
• What generalization can we make about the acceptor in any proton–transfer reaction?
17 Reading: Supplemental Handout, Section 2.8

19. What’s Your Role: Acid or Electrophile?
• In one of the following reactions, acetone plays the role of an acid; in the other it plays the
role of an electrophile. Which is which, and why? Can you draw the curved arrows and identify
the donor and acceptor of each reaction?
O O OH
+ HO
H3C CH3 H3C CH3
O O
+ HO + H2O
H3C CH3 H3C CH2
18 Reading: Supplemental Handout, Section 2.8

20. What’s Your Role: Base or Nucleophile?
• In one of the following reactions, acetone plays the role of a base; in the other it plays the role
of a nucleophile. Which is which, and why? Can you draw the curved arrows and identify the
donor and acceptor of each reaction?
O OH
+ H3O + H2O
H3C CH3 H3C CH3
CH3
O CH3 O
+ O
+ O H3C CH3
H3C CH3 H3C CH3 H3C CH3
19 Reading: Supplemental Handout, Section 2.8

21. Factors That Influence Acidity: The Main Atom
• Examine the pKa’s for each of the following pairs of acids and explain why one acid is
stronger than the other.
H OH H F
+ 15.7 + 3.2
H OH H OH2
+ 15.7 − 1.7
H3C H
H2C N H3C C N H
H
H
+ 10 − 10
• Next examine the four acids from the upper-right corner of the periodic table. Do the
strengths of these acids follow the trend you would expect? Why or why not?
H OH H F
+ 15.7 + 3.2
H SH H Cl
+ 7.0 −7
20

22. Factors That Influence Acidity: The Adjacent Groups
• The acidity of a particular proton can be influenced by adjacent or nearby groups in the
molecule. Can you explain the difference in the following pKa’s?
O
H3C
O H
+ 4.76
O
FH2C
O H
+ 2.66
• Whenever a molecule exhibits resonance, and the resonance allows charge to be delocalized,
then the charged structure will be more stable than a comparable structure that does not have the
delocalized charge. Thus, resonance can stabilize either the conjugate acid or the conjugate base,
whichever is charged. Let’s look at some examples:
CH3
H3C N H
O H
+ 5.25
+ 16.5
O H3C
H3C N N H
O H H3C
+ 4.76 + 9.2
21