Important things from Ch. 3
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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