Ankan

Chapter 9

Reactions of Alkanes

Radicals

Irene Lee
Case Western Reserve University
Cleveland, 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 s bonds and atoms with no partial charges
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 hydrogen
is abstracted
The Reactivity–Selectivity Principle
A 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 not
iodination

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 present
cannot be added to the double bond
The allylic radical has two resonance contributors
Stereochemistry of Radical Substitution Reactions
Why are both enantiomers formed?
Consider the first propagation step
What happens if the reactant already has an asymmetric
carbon and the radical substitution reaction creates a
second asymmetric carbon?
Reactions of Cyclic Compounds
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
Chlorofluorocarbons remain very stable in the
atmosphere until they reach the stratosphere