Free radical substitution reactions

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Free radical substitution reactions

Free radical substitution reactions involve the formation of reacting species that have unpaired electrons. Unpaired electrons are often very reactive and attack substances that would not otherwise react. Although they are generally very unreactive, many alkanes can undergo substitution reactions with halogens such as chlorine in the presence of ultraviolet light,

Example Chlorination of Methane

No reaction occurs between methane and chlorine in the dark, but in strong ultraviolet light, chloromethane and hydrogen chloride are formed. If excess chlorine is present chloromethane can be successively converted into dichloromethane, trichloromethane and carbon tetrachloride. These reactions occur through a free radical mechanism

CH

4

(g) + Cl

2

(g) UV CH

3

Cl(g) + HCl (g)

Chloromethane

For this reaction to happen both the Cl — Cl and C—H bonds must break.

Cl

2

à Cl

+ Cl

∆H° = +242 kJ mol

-1

CH

4

à CH

3

+ H

∆H° = +415 kJ mol

-1

When bonds are broken so as to leave a single unpaired electron with each fragment the process is known as hemolytic fission. Normal heating does not provide sufficient energy to break these bonds. Ultraviolet light does provide sufficient activation energy to split the chlorine molecule homolytically to form chlorine free radicals. This process is called is called

initiation

.

Because free radicals, such Cl

, as have unpaired electrons, they are very reactive. If a chlorine free radical collides with a hydrocarbon molecule such as methane, a hydrocarbon free radical and hydrogen chloride result. This production of a new free radical is called

propagation

.

Cl

H

3

C

--

H

à

H

3

C

+ H — Cl

The methyl free radical is also very reactive. It continues to undergo propagation by reacting with a chlorine molecule to form the methyl chloride product and to regenerate another chlorine free radical. Thus a chain reaction occurs as the process can repeats itself.

CH

3

+ Cl

2

à CH

3

— Cl + Cl

In theory, a single chlorine radical may cause up to 10,000 molecules of chloromethane to be formed.

When two radicals reaction together

termination

occurs. In this example termination can occur three ways.

Cl

+ Cl

CH

CH

3

3

à

+ Cl

à CH

+ CH

Cl

3

2

à C

3

Cl termination

2

H

6

The substituted free radical products can undergo further free radical reactions

Further substitution can occur when chlorine radicals react with the substituted products. For example: methyl chloride can be converted to dichloromethane

The overall mechanism is called

free radical substitution

.

Free Radical Substitution of Methylbenzene

Methylbenzene (toluene) behaves like a substituted alkane. It can react with chlorine in the presence of ultra violet light as follows

The mechanism is identical to the reaction of methane with chlorine. The initial product is chloromethylbenzene. In excess chlorine further substitution can also occur.

Reaction of Chloroalkanes with Ozone

The use of chlorinated organic compounds is being curtailed for two main reasons. Some of the compounds are potentially carcinogenic (cancer forming) and they also have the potential to lower the concentration of ozone in the ozone layer.

Ozone is continually being formed and depleted in the ozone layer between about 120 km and 50 km above the surface of the Earth. The strong double bond in oxygen is broken by high energy ultraviolet light from the Sun to form radicals. One oxygen radical can then react with an oxygen molecule to form ozone.

O=O(g)

UV (high energy)

2O

(g)

O

(g) + O

2

(g) à O

3

(g)

The bonds in ozone are weaker so UV light of less energy will break them. When they are broken, the reverse process happens and the ozone breaks down back to an oxygen molecule and an oxygen radical. The radical can then react with another ozone molecule to form two oxygen molecules.

O

3

(g)

UV (lower energy)

O

2

(g) + O

(g)

O

3

(g) + O

(g) à 2O

2

(g)

Overall, the rate of production of ozone is equal to the rate of ozone destruction – this is known as a steady state. However, compounds containing C—Cl bonds can alter this steady state.

Chlorinated hydrocarbons tend to be rather unreactive until they reach the stratosphere, where the ultraviolet light causes the C—Cl bond to break homolytically. The chlorine radicals can then react with ozone molecules by a chain reaction. This causes a substantial lowering of the ozone concentration, resulting in ‘holes’ in the ozone layer, particularly over the polar regions.

RCH

2

Cl(g) à RCH

2

(g) + Cl

(g) (radical initiation)

Cl

(g) + O

3

à

ClO

(g) + O

2

(g) (propagation of radicals)

ClO

(g) + O

3

(g) à Cl

(g) + 2O

2

(g)

Overall 2O

3

(g) à 3O

2

(g)

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