Radical Notes

8/2/2019 Radical Notes

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Chemistry 211 Clark College

Radical Reactions Notes Rev F07 NF Page 1 of 3

Notes on Radical Reactions

Although typically thought as unreactive, saturated alkanes readily undergo halogen substitutionthrough a radical process. These radical reactions proceed through a variety of single electron steps;

as such we will introduce the single-headed, or fish hook arrow to denote the movement of a single

electron. This reaction introduces functionality into a molecule and allows for a variety of substitutionand elimination products. The reaction requires initiation by heat or light, and is facilitated by the

addition of organic peroxides. We will focus our discussion to the substitution of chlorine or bromine, asfluorine can be explosively reactive and iodine is generally unreactive.

The general reaction:

CH4 + Cl2 CH3Cl + HCl + other halogenated productsh! or!

Mechanism

The mechanism for any radical reaction can be broken down into three parts: initiation, propagation and

termination. We will consider each step individually.

h! or!Initiation Cl Cl 2 Cl

Initiation of a radical reaction requires the creation of the initial radicals in solution, through thehomolytic (equal) splitting of a bond. Typically, the bond that cleaves is the halogen-halogen bond, as itis the weakest bond present. A single photon of light in the visible range will initiate thousands of

molecules of chlorine or bromine.

PropagationH Cl H Cl

Cl Cl Cl Cl

Propagation is a two-step process that involves H-atom abstraction by the halogen radical to form areactive carbon radical, followed by the splitting of the halogen-halogen bond and creation of the

halogenated product. In every step, one radical creates another, so the net concentration of radical

remains fairly constant. During the reaction, this concentration of radicals is small, so it is more likelythat a radical runs into a neutral compound than another radical. The rate-determining step of thereaction is the first propagation step the H-atom abstraction by the halogen radical. The energetics of

this step sets the regioselectivity of a radical reaction, which will be discussed in a later section.

Termination ClCl Cl Cl

ClCl

As the reaction nears an end, and the concentration of reactants decreases, the effective concentrationof radicals increases, causing radicals to couple and terminate the reaction. Termination results when

any two radicals couple to form a neutral molecule. The products of termination include the halogen


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molecule, which can be fed back into propagation, the desired halogenated product, and an unwanteddimer of the hydrocarbon.

Reaction Energetics and Regioselectivity

When propane is halogenated, the following results are obtained:

CH3CH2CH3 CH3CHCH3 + CH3CH2CH2Cl

2, !

ClCl

Br Br

CH3CH2CH3 CH3CHCH3 + CH3CH2CH2Br2, !

55% 45%

97% 3%

Neither of these results match the statistics of the reaction, since 75% of the hydrogens are 1 and 25%

are 2. What causes this distribution? And why is bromine so much more selective?

Overall, radical halogen substitution reactions are exothermic, however, not all individual steps in themechanism are exothermic. This fact provides us with an explanation of the results above and a

method of controlling the regioselectivity of the reaction. Lets consider the rate-determining step of thereaction, the H-atom abstraction by the halogen radical.

Cl H Cl

H

Br H Br

H

Bond dissociation energies (BDE) show that the C-H bond from more substituted carbons are easier tobreak (1 C-H ~100 kcal/mol, 2 C-H ~95 kcal/mol, 3 ~93 kcal/mol), explaining the deviation fromstatistics. The strength of the H-X bond formed explains the enhanced selectivity of bromine; the H-Cl

BDE is 103 kcal/mol, whereas the H-Br BDE is 88 kcal/mol. The single step reaction with chlorine is

exothermic, for bromine it is endothermic; endothermic reactions favor a more stable transition statewhich in this case is the more stable radical. The following table shows the ratio of products that form

upon radical halogenation of a molecule with 3, 2, and 1 centers, when either Br2 or Cl2 is used.3 : 2 : 1

Br2 1600 : 80 : 1

Cl2 5 : 4 : 1

Allylic (benzylic) Halogenation

Radical halogenation is not limited to alkanes, reactions can also be performed on alkenes, with somespecial considerations. Radical reactions on alkenes cannot be performed with Cl2 or Br2, as they will

add directly to the double bond. To eliminate this competing reaction, a new reagent is needed:N-bromosuccinimide (NBS) or N-chlorosuccinimide (NCS). These reagents, in conjunction with anorganic peroxide, will selectively substitute for an allylic hydrogen.

N N

O

Br

O

O

Cl

O

Allylic positions are rightnext to the double bond

NBS NCS

CH3

Benzylic positions are rightnext to a benzene ring.

Upon H-atom abstraction, the allylic radical can be resonance delocalized, resulting in up to two

products for each allylic hydrogen. A similar reaction occurs with benzylic radicals, however only thebenzylic position is halogenated, maintaining the benzene structure.


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Resonance delocalization of an allylic radical

Examples

Br2, !

Cl2, !

Br

Cl

Cl

Cl

Cl Cl

NBS, !

!

!

*

3 allylic positions, upto 6 different products

Br

BrBr

BrBr

Br

These twoproducts areequivalent

major

minor

ROOR

Stereochemistry and Radical Substitutions

So far, we have discussed the energetics and selectivity of radical halogenation, but not the

stereochemistry of a radical reaction. To do this, we must consider the carbon-radical intermediate,typically formed in the first propagation step. We will use 2-methylpropane as an example, which

results in a 3 radical:

When the radical forms, the carbon becomes sp2-hybridized. The single

electron occupies the unhybridized p orbital. When this radical reacts, the

new bond can form with the top part of the p orbital or the bottom, giving rise

to a pair of enantiomers, and a racemic mixture of halogenated products.

Br Br

The new bond can form

from the top or bottom

Br

Br

+ + Br

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