CHAP ELEVEN OUTLINE: KLEIN
SELECT FREE RADICAL REACTIONS
1) Homolytic Cleavage and the Structure of carbon radicals
a) sp2 hybridization state
b) Hyperconjugations and radical stability: 3°>2°>1°
2) Free Radical Halogenation (FRH) of Alkanes
a) Mechanism
b) Enthalpy of reaction from BDE’s
c) Construction and Interpretation of Energy Diagrams
i) Product Distribution in FRH: Bromination vs Chlorination
ii) Application of Hammonds Postulate to Energy profiles of Propagation
Step #1 :Exothermic reactions have late ‡, whereas Endothermic reactions
have “early” ‡(in terms of structural change, not time)
iii) More reactive is generally less selective; (Br·>Cl·>F·)
iv) Observed Product Distribution in FRH
ObsA StatA SelA
=
X
ObsB StatB SelB
Where ObsA is the observed (actual) distribution of product A, StatA is the
Statistical likelihood of abstracting a hydrogen to form product A, and SelA is
the relative rate with which a single hydrogen is abstracted from position A.
3) Allylic Halogenation using NBS
4) Free Radical Anti-Markovnikov addition of HBr to alkenes
5) Free Radical Induced alkene polymerization: Mechanism and structural
variability
6) Radical Inhibitors
7) Synthesis incorporating Free Radical reactions
LEARNING OUTCOMES:
1) Recognize significance of non-polar C-H bond in limiting primary reactivity of
alkanes to Free Radical reactions
2) Understand structural features relevant to stability of carbon and other radicals
3) Predict products of Free Radical Halogenation (FRH)
4) Understand the mechanism of a Free Radical Halogenation reaction and the Free
Energy Reaction Diagram
5) Understand and apply Hammond’s Postulate
6) Predict the Statistical Distribution of a FRH reaction and determine selectivity given
the observed product distribution
7) Write balanced chemical equations for combustion reactions and relate their enthalpy
changes to relative energies when appropriate
EXAMPLE EXAM PROBLEMS:
1) Draw the Free Radical Chlorination products for 1,3-dimethylcyclopentane and
calculate the expected relative (Observed) distribution of each. Report the
distribution as a percent of the total.
2) Draw on your understanding of the Free Energy Diagram for Halogenation of an
alkane to explain the following:
CHAP ELEVEN OUTLINE: KLEIN
SELECT FREE RADICAL REACTIONS
When a mixture of both bromine and chlorine are reacted with an alkane, both
alkyl bromides and alkyl chlorides are formed, but the product distribution of the
alkyl bromides formed are the same as for chlorination (ie selectivity follows
chlorination trend of approx. 5:4:1 for 3°:2°:1°).