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Radical Reactions. Chapter 15 Smith. 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•. 1 o , 2 o and 3 o Radicals.

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Radical reactions

Radical Reactions

Chapter 15

Smith


Introduction
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•


1 o 2 o and 3 o radicals
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


Radical reactions1
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.


Two radicals may react with each other
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.


Monohalogenation of alkanes replacement of one h with br or cl
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.


Show the monohalogenation products for the following reaction
Show the monohalogenation products for the following reaction.

Rule: We only make monohalogenation products in this course.


Example of a radical reaction
Example of a Radical Reaction reaction.

  • Halogenation (bromination or chlorination) of alkanes.

  • CH4 + Br2+ heat or UV light  CH3Br + HBr

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


How do halogenation reactions occur
How do halogenation reactions occur? reaction.

  • Three essential steps :

  • 1. Radical Initiation (heat or UV light)

  • 2. Radical Propagation (two steps)

  • 3. Radical Termination (three ways)


Radical initiation
Radical Initiation reaction.

  • Halogen + heat or UV light  radical

Initiation: Two radicals are formed by homolysis of a s bond, starting the rxn.


Radical propagation
Radical Propagation reaction.

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.


Radical termination
Radical Termination reaction.

Termination: Two radicals combine to form a stable bond.


Energy profile of propagation
Energy Profile of Propagation reaction.

Two propagation steps; the first is rate determining.


The weaker a c h bond the easier it is to remove h
The weaker a C-H bond, the easier it is to remove H reaction.

Use this information to predict the product distribution when more

than one kind of H is present in the substrate.


Predict which h is easiest to abstract in each compound
Predict which H is easiest to abstract in each compound. reaction.

Tertiary H is easier to remove than secondary, and secondary is

easier to remove than primary H.


Bromination vs chlorination
Bromination vs Chlorination reaction.

  • Bromination is slower and more selective than chlorination.

  • Selectivity is in the order IIIo > IIo > Io, the order of radical stability.

  • The selectivity of bromination can be explained by Hammond’s postulate, because alkyl radical formation in bromination is endothermic and in chlorination is exothermic.


Halogenation in synthesis
Halogenation in Synthesis reaction.

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


Make trans 1 2 dibromocyclohexane from cyclohexene
Make reaction.trans-1,2-dibromocyclohexane from cyclohexene

Classroom Activity



Halogenation of an achiral compound
Halogenation of an Achiral Compound reaction.

Halogenation of a Chiral Compound

Halogenation at Io carbon away from * retains the R configuration.


Problems
Problems reaction.

  • Work problems 15.1 through 15.18 in Smith.


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