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.
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.
That's
all for this topic. If you have any questions please feel free to ask me in the
comment box. Thank you..!
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