Addition Reaction - Definition, Types and Examples

In this article we will discuss about Addition Reaction. In chemistry, Addition Reaction is a chemical reaction where two or more molecules combine together to form adduct. Here we will learn definition, types and examples of Addition Reaction.

Key words: Addition Reaction, Electrophilic Addition Reactions, Nucleophilic Addition Reactions, Free Radical Addition, Polymerization and Cycloaddition Reactions.

What is Addition Reaction?

An Addition Reaction is a chemical reaction where two or more molecules combine together to form adduct. The addition reactions are possible due to multiple bonds in the compound, these could be carbon-carbon double (C=C, alkene) or triple bonds (C≡C, alkyne), carbon-hetero atom multiple bonds like carbonyl groups (C=O), imine groups (C=N) and nitrile groups (C≡N). In this reaction double or triple bond breaks to form new sigma bond.

Figure 1: Typical Representation of Addition Reaction

In general there are two main types of addition reaction are possible, that is Polar Addition Reactions and Non-polar Addition Reactions. In polar addition reactions electrophile or nucleophile add on to the multiple bonds to form adduct. Hence polar addition reactions are classified as Electrophilic Addition Reactions and Nucleophilic Addition Reactions. In Polar Addition Reactions there is formation of anionic or cationic intermediates. Whereas in non-polar addition reactions the intermediates are free radicals or the reaction has concerted mechanisms. The non-polar addition reactions can be further classified as Free Radical Addition, Addition Polymerization and Cycloaddition Reactions.

Figure 2: Classification of Addition reactions

1.    Polar Addition Reactions

1.1 Electrophilic Addition Reactions

The electrophile is an electron deficient molecule which accepts pair of electrons from nucleophile. In electrophilic addition reactions an electrophile accepts electron pair from double bond to form carbocation intermediate. Then in second step a nucleophile attacks on positive species to form addition product. 

Figure 3: Electrophilic Addition Reaction

For example, hydrohalogenation of alkene is a case of electrophilic addition reaction. In this reaction addition of alkene and hydrogen halides takes place to form alkyl halide. In the first step of this reaction carbon-carbon double bond donates pair of electrons to the hydrogen to form new carbon-hydrogen bond. This results in formation of intermediate carbocation and bromide ion. Then in second step, bromide ion attacks on carbocation to form new carbon-bromine bond.

Figure 4: Hydrohalogenation of Alkenes

1.2 Nucleophilic Addition Reactions

In nucleophilic addition reaction a nucleophile attacks on the electron deficient double bond to form anion intermediate. Then second step is protonation of anion to form addition product. 

Figure 5: Nucleophilic Addition Reactions

For example, reaction of organolithium compound with carbonyl compound is an addition reaction to form an alcohol compound. In this reaction first step is attack of organolithium reagent on carbonyl carbon to form alkoxide ion. Then second step is protonation of anion to form alcohol compound.

Figure 6: Addition of Organolithium reagent on Carbonyl Compound

2.    Non-polar Addition Reactions

Non-polar addition reactions have radical intermediates or they have concerted mechanism that means no intermediates in the reaction. There are three types of non-polar addition reactions found in nature. They are Free Radical Addition, Addition Polymerization and Cycloaddition Reactions.

2.1 Free Radical Addition

Free radical addition reactions have a radical intermediate in the reaction mechanism. Here the carbon-carbon double bond acts as nucleophile and donate one electron to the halogen radical to form carbon-halogen bond. Also in this step there is formation of another radical as a result of breaking of double bond. Then in second step a newly generated carbon radical pluck hydrogen form hydrogen halide to for new carbon-hydrogen bond. Hence radical addition reaction takes place. 

Figure 7: Free Radical Addition Reaction

For example, hydrohalogenation of alkene in presence of hydrogen halide and peroxide is a free radical reaction. In this reaction HBr reacts with peroxide to form bromine radical. Then bromine radical add to the alkene to form carbon radical intermediate. In last step radical intermediate takes hydrogen atom to give addition product.

Figure 8: Hydrohalogenation of Alkene in presence of HBr/ Peroxide 

2.2 Addition Polymerization

Addition polymerization is basically free radical addition reaction but here large amount of monomer gets added in the chain to form polymer compounds. For example, synthesis of polyvinyl chloride (PVC) from vinyl chloride is an addition polymerization reaction. 

Figure 9: Addition Polymerization

2.3 Cycloaddition Reactions

Cycloaddition reactions are concerted reactions which does not have any intermediates in reaction mechanism. For example, Diels Alder reaction is a [4 + 2] cycloaddition reaction. Here 2,4-diene reacts with alkene (or dinophile) to form six member cyclic ring.

Figure 10: Diels Alder Reaction

To summarize this topic, addition reactions are mainly classified as Polar addition reactions and Non-polar addition reactions. The Polar addition reactions can be further classified as Electrophilic addition reactions and Nucleophilic addition reactions. The Non-polar addition reactions are of three types these are Free radical addition, Addition polymerization and Cycloaddition reactions.

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