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