Radical Reactions

1
Radical Reactions

• Chapter 15
• Smith

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Introduction

• A radical is a chemical species with a single
  unpaired electron in an orbital.
• Two radicals arise when a bond is cleaved
  homolytically.

Cl-Cl heat ? Cl Cl
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1o, 2o and 3o Radicals
Order of stability is the same as for
carbocations a tertiary radical is more stable
than a secondary radical, and a secondary radical
is more stable than a primary radical
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Radical Reactions

• Radical reactions are initiated with heat (D) or
  light (hn) and often with a peroxide (RO-OR)
  initiator.
• Once formed, a radical reacts to form a new
  radical.
• A radical formed by an initiation reaction may
  abstract an H radical from a C-H bond or bond to
  a p electron of a p bond. A new s bond is formed
  in both cases.

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Two Radicals May React with Each Other
Compounds that prevent radical reactions are
called radical inhibitors or radical
scavengers. Vitamin E is a radical scavenger.
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Monohalogenation of Alkanes(Replacement of one H
with Br or Cl)
All Hydrogens are alike, replacement of any one
gives the same product.
Halogenation of an alkane is a substitution
reaction.
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Show the monohalogenation products for the
following reaction.
Rule We only make monohalogenation products in
this course.
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Example of a Radical Reaction

• Halogenation (bromination or chlorination) of
  alkanes.
• CH4 Br2 heat or UV light ? CH3Br HBr

• To do a halogenation, use a halogen (X2).

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How do halogenation reactions occur?

• Three essential steps
• 1. Radical Initiation (heat or UV light)
• 2. Radical Propagation (two steps)
• 3. Radical Termination (three ways)

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Radical Initiation

• Halogen heat or UV light ? radical

Initiation Two radicals are formed by homolysis
of a s bond, starting the rxn.
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Radical Propagation
sp2
sp3
Propagation A radical reacts with a reactant,
forming a new s bond and a radical. (A radical
makes a radical in propagation sub-steps.
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Radical Termination
Termination Two radicals combine to form a
stable bond.
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Energy Profile of Propagation
Two propagation steps the first is rate
determining.
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The weaker a C-H bond, the easier it is to remove
H
Use this information to predict the product
distribution when more than one kind of H is
present in the substrate.
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Predict which H is easiest to abstract in each
compound.
Tertiary H is easier to remove than secondary,
and secondary is easier to remove than primary H.
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Bromination vs Chlorination

• Bromination is slower and more selective than
  chlorination.
• Selectivity is in the order IIIo gt IIo gt Io, the
  order of radical stability.
• The selectivity of bromination can be explained
  by Hammonds postulate, because alkyl radical
  formation in bromination is endothermic and in
  chlorination is exothermic.

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Halogenation in Synthesis

• Convert alkanes (usually symmetrical) into alkyl
  halides, from which alcohols, ethers and alkenes
  can be formed in one step.

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Make trans-1,2-dibromocyclohexane from cyclohexene
Classroom Activity
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Stereochemistry of Halogenation
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Halogenation of an Achiral Compound
Halogenation of a Chiral Compound
Halogenation at Io carbon away from retains the
R configuration.
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Problems

• Work problems 15.1 through 15.18 in Smith.