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Mechanism of Chain Reactions

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Mechanism of Chain Reactions

Chain reactions usually consist of many repeating elementary steps, each of which has a chain carrier. Once started chain reactions continue until the reactants are exhausted. Fire and explosion are some of the phenomena associated with chain reactions. The chain carriers are some intermediates appear in the repeating elementary steps. These are usually free radicals.

Once initiated, repeating elementary steps continue until the reactants are exhausted. When repeating steps generate more chain carriers, they are called chain branching reactions, which leads to explosions. If the repeating elementary steps do not lead to the formation of new product, they are called chain inhibition reactions. Addition of other materials in the reaction mixture can lead to the inhibition reaction to prevent the chain propagation reaction. When chain carriers react with one another forming stable product, the elementary steps are called chain termination reactions.

Explosions, polymerizations, and food spoilage often involve chain reactions. The chain reaction mechanism is involved in nuclear reactors, in this case the chain carriers are neutrons.

The mechanisms describing chain reactions are useful models for describing chemical reactions.

Free Radicals

Most chemical chain reactions have very reactive intermediates called free radicals. The intermediate that maintains the chain reaction is called a chain carrier. These atoms or fragments are usually derived from stable molecules due to photo- or heat-dissociation.

Usually, a free radical is marked by a dot beside the symbol. A dot represents an odd electron on the species. The odd electron on the species makes it very reactive. We represent the odd electron with *, because a period is too confusing. For example, the oxygen, chlorine and ethyl radicals are represented by O*, Cl*, and C2H5* respectively.

The Cl* radicals can be formed by the photo dissociation reaction,

Cl2 + photon -> Cl* + *Cl

Mechanism of Chain Reactions

The elementary steps used for mechanisms of chain reactions can be grouped into the following categories: initiation step
chain propagation steps
chain branching steps
chain inhibition steps
chain termination steps
For example, the chlorination of ethane is a chain reaction, and its mechanism is explained in the following way.

If we mix chlorine, Cl2, and ethane, CH3CH3, together at room temperature, there is no detectable reaction. However, when the mixture is exposed to light, the reaction suddenly initiates, and explodes. To explain this, the following mechanism is proposed.

The light initiates the reaction by providing free radicals. The light causes a photodissociation reaction, and the initiation step, can be written as:

Cl2 + photon -> Cl* + *Cl

Elementary steps in which the number of free radicals consumed is equal to the number of free radicals generated are called chain propagation steps. Once initiated, the following chain propagation steps repeat indefinitely or until the reactants are exhausted:

Cl* + H3CCH3 -> ClH2CCH3 + H*
Cl* + H3CCH3 -> H3CCH2* + HCl
H* + Cl2 -> HCl + Cl*
and many other possibilities.
In each of these steps, a radical is consumed, and another radical is generated. Thus, the chain reactions continue, releasing heat and light. The heat and light causes more radicals to form. Thus, the chain propagation steps cause chain branching reactions. Branching reactions are elementary steps that generate more free radicals than they consume. Branching reactions result in an explosion.

For example, in the reaction between hydrogen and oxygen, the following reaction may take place:

H* + O2 -> HO* + *O*

where *O* is a di-radical, because the O atom has an electronic configuration 2s2 2px2 2py1 2pz1. In this elementary step, three radicals are generated, whereas only one is consumed.

The di-radical may react with a H2 molecule to form two radicals.

*O* + H2 -> HO* + H*

Thus, together chain branching reactions increase the number of chain carriers. Branching reactions contribute to the rapid explosion of hydrogen-oxygen mixtures, especially if the mixtures have proper portion.

Chain Inhibition Reactions

The steps not leading to the formation of products are called inhibition reactions or steps. For example, the following steps are inhibition reactions. Cl* + ClH2CCH3 -> H3CCH2* + Cl2
Cl* + HCl -> H* + Cl2
H* + ClH2CCH3 -> H3CCH3 + Cl*.

Furthermore, sometimes another reactive substance *A may be added to the system to reduce the chain carriers to inhibit the chain reactions.

Cl* + *A -> ClA (not reactive)

The species A* is often called radical scavengers. In food industry, radical scavengers are added to prevent spoilage due to oxidation.

The mechanism in chain reactions is very complicated. When intermediates are detected, a reasonable mechanism can be proposed. Adding radical scavenger to prevent food spoilage is an important application in food chemistry. This application came from the application of the chain reaction model to natural phenomena.

Chain Termination Steps

Chain termination steps are elementary steps that consume radicals. When reactants are exhausted, free radicals combine with one another to give stable molecules. These elementary steps are responsible for the chain reactions to terminate: Cl* + *Cl -> Cl-Cl
H* + *H -> H-H
H* + *Cl -> H-Cl
H3CCH2* + *H2CCH3 -> CH3CH2-CH2CH3 (forming a dimmer)
and other possibilities

In chain reactions, many products are produced.

Confidence Building Questions

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