Important things from Ch. 3

Question 1

List 2 ways to break a bond.

Answer 1

1.) Heterolytic Cleavage
2.) Homolytic Cleavage

Question 2

Describe Homolytic Cleavage.

Answer 2

Electrons in the bond split evenly to the two atoms; results in the formation of 2 radicals

Question 3

Describe Heterolytic Cleavage.

Answer 3

Both electrons in the bond travel together; results in the formation of ions

Question 4

What is bond dissociation energy?

Answer 4

Energy required to break a covalent bond via hemolytic cleavage

Question 5

List the bond strength factors.

Answer 5

1. Stronger bonds form between overlapping orbitals of same size
2. Stronger bonds create less stable radicals
3. Stronger bonds are harder to break

Question 6

What implications does bond strength have for reactivity?

Answer 6

It is easier to break bonds that result in stable intermediates

Question 7

General carbon radical stability trend: ______.

Answer 7

Methyl<primary<secondary<tertiary

Question 8

What is hyperconjugation?

Answer 8

A stabilizing interaction from aligned sp3 C-H orbitals (opposite of torsional strain)

Question 9

Hyperconjugation is a type of ____.

Answer 9

Induction

Question 10

How is resonance possible with radicals?

Answer 10

Electron is in p atomic orbital

Question 11

More stable radicals are ______ to create and come from _________.

Answer 11

Easier to create, weaker bonds

Question 12

What is an important to remember about heat in reactions?

Answer 12

Not all reactions requiring heat are exothermic

Question 13

Radical chain mechanism repeats until it ________.

Answer 13

Runs out of starting material

Question 14

List the three steps of the radical chain mechanism.

Answer 14

1. Initiation
2. Propagation
3. Termination

Question 15

What is initiation?

Answer 15

Forming the first radical by breaking the weakest bond (homolytic cleavage)

Question 16

What is propagation?

Answer 16

Use the radical just created to form products (typically needs 2 steps)

Question 17

What is termination?

Answer 17

Step in which any 2 radicals combine

Question 18

What is the difference between resonance and hyperconjugation?

Answer 18

-Resonance deals with pi type overlap of p orbitals
-Hyperconjugation deals with a bonding pair of electrons in sigma orbital that can delocalize into the partly empty p orbital

Question 19

What are the two steps of propagation?

Answer 19

1. Abstracts H atom (endothermic)
2. Methyl radical abstracts halogen atom from starting material (exothermic)

Question 20

What types of radicals typically combine in termination?

Answer 20

-starting material
-anything that will make more of the desired product

Question 21

Describe the transition states of endothermic and exothermic reactions.

Answer 21

-Endothermic = late transition state
-Exothermic = early transition state

Question 22

What is Hammond’s postulate?

Answer 22

the structure of the transition state resembles the species it is closest in energy to

Question 23

List the delta H values of the halogens.

Answer 23

F = -103
Cl = -25
Br = -6
I = +13

Question 24

Which halogen is the most stable? Which is the least stable?

Answer 24

Fluorine is the most stable and Iodine is the least stable

Question 25

List the selectivity of Cl from tertiary, to secondary, to primary.

Answer 25

5 : 4 : 1

Question 26

Why is Cl 4x more selective for a secondary proton over a primary proton?

Answer 26

Abstraction of a secondary hydrogen is more exothermic with a smaller activation energy, and thus reacts quicker

Question 27

Why is Cl 5x more selective for a tertiary proton over a primary proton?

Answer 27

Radical stability

Question 28

Describe the selectivity of F.

Answer 28

-extremely exothermic; most reactive halogen
-early transition state
-not selective; reacts too quickly to be controlled

Question 29

Describe the selectivity of Br.

Answer 29

-highly selective
-late transition state; highly stable radical

Question 30

Increased reactivity = reduced _____.

Answer 30

selectivity

Question 31

Chlorination is only useful if …

Answer 31

one product can be formed

Question 32

What can be used as a substrate with Chlorination to yield one product?

Answer 32

alkanes with a single type of H (i.e. cyclopentane)

Question 33

Bromination only occurs on _____.

Answer 33

one carbon

Question 34

Stereochemistry is destroyed at …

Answer 34

reactive center (b/c radical intermediate is planar)

Question 35

What is a stereoselective reaction?

Answer 35

reaction leads to formation of one possible stereoisomer over the others

Question 36

Reactions with no chiral center present result in …

Answer 36

a racemic mixture