We can use experimental product distributions (Obs) to predict
the relative reactivities (Sel) of any given Alkane carbon
Sel A
=
Sel B
For 2-methyl
propane:
Sel A
Sel B
=
Obs A
Stat B
Obs B
Stat A
36
9
64
1
=
324
64
= 5
The relative reactivities of C-H bonds in chlorinations are roughly:
Tertiary:Secondary:Primary = 5:4:1
These numbers are for alkanes in which there are no other
structural influences at play (ie no resonance or inductive effects)
1
Relative Reactivities (Selectivities)
depends on Halogen
2
Allylic Halogenation
Free Radical Bromination of Alkanes is highly selective
for the allylic (most stable) position
Competing Reaction
3
N-Bromo Succinimide preferred over
Bromine for Allylic Halogenation
N-Bromo Succinimide
(NBS) is a source of low
concentrations of Br2
4
NBS Mechanism
5
RESONANCE in INTERMEDIATES MEANS POTENTIAL FOR MULTIPLE
PRODUCTS
6
AutoOxidation at allylic/ benzylic carbon
7
Mechanism Autooxidation
8
Copyright © 2015 John
Wiley &11
Sons,
Unnumbered
p527 Inc. All
Ethers are even more easily oxidized in air and
must be stored carefully
10
Radical Inhibitors
When an inhibitor reacts with the radical, it creates a
stable intermediate, and any further reactions will be
endothermic and slow.
11
BHA is a common food preservative.
Phenoxy radicals are stabilized through
resonance
Vitamin C is an Antioxidant
Anti-Markovnikov HBr addition
14
Anti-Markovnikov HBr addition
15
Free Radical Induced Alkene
Polymerization
Polystyrene
16
17
Why is pattern regular?
Problems
1. How many chlorination products form in the chlorination of
1,3-dimethylcyclopentane?
2. What is their expected distribution?
3. In the reaction profile shown below, label the rate
limiting transition state? Label the activation energy?
According to Hammonds postulate, which step would be
the least selective if it represented a hydrogen
abstraction step?
B
A
C
D
19
What do you estimate is the product distribution in
the following reaction?
20