Chapter 9
Reactions of Alkanes
Radicals
Irene LeeCase Western Reserve
UniversityCleveland, OH
©2004, Prentice Hall
Organic Chemistry 4th Edition
Paula Yurkanis Bruice
Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation
Alkanes are very unreactive compounds because they have only strong bonds and atoms with no partial charges
However, alkanes do react with Cl2 and Br2
Reaction of Alkane with Cl2 or Br2
Factors that determine product distribution …
The rate-determining step of the overall reaction is hydrogen abstraction
Consider the relative stabilities of alkyl radicals,
The stable alkyl radical is formed faster, therefore 2-chlorobutane is formed faster
pg 341 middle
In determining the relative amounts of products obtained,both probability and reactivity should be considered
probability: the number of hydrogens that can be abstracted that will lead to the formation of the particular product
reactivity: the relative rate at which a particular hydrogenis abstracted
The Reactivity–Selectivity PrincipleA bromine radical is less reactive and more selective than a chlorine radical
Why are the relative rates of radical formation so different between the bromine versus the chlorine radical?
The more reactive a species is, the less selective it will be
What about fluorination and iodination?
Alkanes undergo chlorination and bromination, but notiodination Fluorination is too violent of a reaction to be useful
Consider the relative stabilities of radicals
The more stable radicals form faster
Radical Substitution of Benzylic Hydrogens
Because of the reactivity of allylic hydrogens,
a milder brominating reagent can be used
Advantage: the low concentration of Br2 and HBr presentcannot be added to the double bond
The allylic radical has two resonance contributors
CH3CH2CH CH2 CH3CHCH CH2 CH3CH
CH3CH2CH CH2 CH3CH2CH CH2
+
Br
+
Br
+
CHCH2+
HBr
Br2
Stereochemistry of Radical Substitution Reactions
Why are both enantiomers formed?
Consider the first propagation step
What happens if the reactant already has an asymmetriccarbon and the radical substitution reaction creates a second asymmetric carbon?
Reactions of Cyclic Compounds
+ Cl2 h
I
+ HCl
+ Br2h
Br
+ HBr
+ HBr
+ NBS
peroxide
Br
Br
Reactions of Cyclopropane
Radical Reactions in Biological Systems
Radicals and Stratospheric Ozone
• ozone is a major constituent of smog
• ozone shields the Earth from harmful radiation
O2
O3
hO + O
O + O2
Chlorofluorocarbons remain very stable in the atmosphere until they reach the stratosphere
C ClF
Cl
F
hCF
Cl
F
+ Cl
The chlorine radicals are ozone-removing agents
Cl + O3 ClO + O2
ClO + O3 Cl + 2 O2