Mechanism of Chain Reactions

Skills to develop

Explain the mechanisms of chain reactions in terms of elementary steps.

Define these terms: radical, chain carrier.

Classify elementary steps as initiation, chain propagation, chain branching, chain inhibition, and chain termination.

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 2p_x² 2p_y¹ 2p_z¹. 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

Is argon atom Ar a free radical? (yes/no)

Discussion -
Argon exists as a mono-atomic gas. All noble gases have mono-atomic molecules.
In the chlorination of ethane, what would you call this elementary reaction? Cl* + ClH₂CCH₃ -> H₃CCH₂* + Cl₂ a. initiation step
b. chain propagation steps
c. chain branching steps
d. chain inhibition reactions
e. chain termination steps

Skill -
Identify steps for the names in the multiple choices.
A 1:1 mole ratio of H₂ and Br₂ mixture was a stable until it was exposed to light, at which time it exploded. A chain reaction mechanism is suggested for this reaction. What is the chain carrier? Give the formula

Skill -
Predicting the intermediate from the nature of the reactants.
Which one of the following is not a chain propagation reaction in the chlorination of ethane?
a. Cl* + H₃CCH₃ -> ClH₂CCH₃ + H*
b. Cl* + H₃CCH₃ -> H₃CCH₂* + HCl
c. H* + Cl₂ -> HCl + Cl*
d. Cl* + HCl -> H* + Cl₂

Discussion -
The reactant HCl in the step is a product in overall reaction. When HCl reacts with Cl*, the reaction is retarted. Cl* attacked one of the product molecule HCl causing a reversal of the reaction.

For your pleasure, the following is a commercial site that used the phrases: free radicals, chain reactions. The link is NOT an endorsement of the product they sell, but you may find the reading interesting. http://www.inetme.com/iom/team/cellular.html