Important things from Ch. 3 Flashcards
List 2 ways to break a bond.
1.) Heterolytic Cleavage
2.) Homolytic Cleavage
Describe Homolytic Cleavage.
Electrons in the bond split evenly to the two atoms; results in the formation of 2 radicals
Describe Heterolytic Cleavage.
Both electrons in the bond travel together; results in the formation of ions
What is bond dissociation energy?
Energy required to break a covalent bond via hemolytic cleavage
List the bond strength factors.
- Stronger bonds form between overlapping orbitals of same size
- Stronger bonds create less stable radicals
- Stronger bonds are harder to break
What implications does bond strength have for reactivity?
It is easier to break bonds that result in stable intermediates
General carbon radical stability trend: ______.
Methyl<primary<secondary<tertiary
What is hyperconjugation?
A stabilizing interaction from aligned sp3 C-H orbitals (opposite of torsional strain)
Hyperconjugation is a type of ____.
Induction
How is resonance possible with radicals?
Electron is in p atomic orbital
More stable radicals are ______ to create and come from _________.
Easier to create, weaker bonds
What is an important to remember about heat in reactions?
Not all reactions requiring heat are exothermic
Radical chain mechanism repeats until it ________.
Runs out of starting material
List the three steps of the radical chain mechanism.
- Initiation
- Propagation
- Termination
What is initiation?
Forming the first radical by breaking the weakest bond (homolytic cleavage)
What is propagation?
Use the radical just created to form products (typically needs 2 steps)
What is termination?
Step in which any 2 radicals combine
What is the difference between resonance and hyperconjugation?
-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
What are the two steps of propagation?
- Abstracts H atom (endothermic)
- Methyl radical abstracts halogen atom from starting material (exothermic)
What types of radicals typically combine in termination?
-starting material
-anything that will make more of the desired product
Describe the transition states of endothermic and exothermic reactions.
-Endothermic = late transition state
-Exothermic = early transition state
What is Hammond’s postulate?
the structure of the transition state resembles the species it is closest in energy to
List the delta H values of the halogens.
F = -103
Cl = -25
Br = -6
I = +13
Which halogen is the most stable? Which is the least stable?
Fluorine is the most stable and Iodine is the least stable
List the selectivity of Cl from tertiary, to secondary, to primary.
5 : 4 : 1
Why is Cl 4x more selective for a secondary proton over a primary proton?
Abstraction of a secondary hydrogen is more exothermic with a smaller activation energy, and thus reacts quicker
Why is Cl 5x more selective for a tertiary proton over a primary proton?
Radical stability
Describe the selectivity of F.
-extremely exothermic; most reactive halogen
-early transition state
-not selective; reacts too quickly to be controlled
Describe the selectivity of Br.
-highly selective
-late transition state; highly stable radical
Increased reactivity = reduced _____.
selectivity
Chlorination is only useful if …
one product can be formed
What can be used as a substrate with Chlorination to yield one product?
alkanes with a single type of H (i.e. cyclopentane)
Bromination only occurs on _____.
one carbon
Stereochemistry is destroyed at …
reactive center (b/c radical intermediate is planar)
What is a stereoselective reaction?
reaction leads to formation of one possible stereoisomer over the others
Reactions with no chiral center present result in …
a racemic mixture
