In addition reaction, two molecules combine to create one bigger
molecule.
LEARNING OBJECTIVES [ edit ]
Define an addition reaction.
Recall the particulars of how an electrophilic addition reaction occurs.
Recall the particulars of how a nucleophilic addition reaction occurs.
KEY POINTS [ edit ]
An addition reaction is an organic reaction in which two or moremolecules recombine to form a
larger molecule. The reaction only occurs between chemical compounds that have multiplebonds.
An elimination reaction is the opposite of an addition reaction.
Polar addition reactions are electrophilic and nucleophilic additions: in electrophilic
additions, electrons from the double bond "attack" another compound to form bonds; in
nucleophilic additions, a pair of electrons from the new compound "attack" the double bond to
form new single bonds.
Non-polar addition reactions include free radical additions. Freeradicals attack the double bonds
so that new single bonds form and the radical will no longer have an unpaired electron.
In an addition-elimination reaction, elimination reaction takes place after addition reaction.
TERMS [ edit ]
addition reaction
an organic reaction in which two or more molecules combine to form a larger molecule
radical
a very reactive substance with an unpaired electron
electrophilic addition
a reaction in which a compound is added to an alkene by breaking the double bond to create new
single bonds to the additional compound
Give us feedback on this content: FULL TEXT [edit ]
In organicchemistry, anaddition
reaction is, in its simplest terms, an
organic reaction in which two or
moremolecules combine to form a larger
molecule. Addition reactions are limited to
chemical compounds that have
multiplebonds, such as molecules with
carbon-carbon double bonds(alkenes) or
with triple bonds (alkynes). Molecules
containing carbon based double bonds,
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like carbonyl (C=O) groups or imine (C=N) groups, can also undergo addition. An addition
reaction is the opposite of an elimination reaction. For instance, an alkene's hydration
reaction adds water to an alkene, and analcohol's dehydration removes water from the
alkene; these two reactions are opposites and are considered addition-elimination pairs.
There are two main types of polar addition reactions:electrophilic addition and nucleophilic
addition. Two non-polar addition reactions also exist: free radical addition and
cycloadditions. In the related addition-elimination reaction, an addition reaction is followed
by an elimination reaction; in most reactions, this involves addition to carbonyl compounds
in nucleophilic acyl substitution. The hydrolysis of nitriles tocarboxylic acids is also a form of
addition-elimination.
General outline of addition reactions
Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl, and free
radical addition of halide to alkene.
Electrophilic Addition
Most addition reactions to alkenes follow the mechanism of electrophilic addition. An
example is the Prins reaction, where the electrophile is a carbonyl group. In halogenation,
adding elementary bromine or chlorine to alkenes yields dibromo- and dichloro-alkanes,
respectively.
Adding chlorine to ethene
This shows an addition reaction, where chlorine and ethene form single molecule 1,2­dichloroethane.
Nucleophilic Addition Reactions
In nucleophilic addition reactions, the nucleophile donates anelectron pair to the electrophile
(one of the atoms in the double bond). In hydrohalogenation, the nucleophile is the halogen.
The halogen donates an electron pair to one of the carbons participating in a double bond.
The hydrogen is then added to the other carbon that was originally double bonded. Addition
of hydrohalic acids like HCl or HBr to alkenes yields the corresponding haloalkanes. An
example of this type of reaction is:
CH 3 CH = CH 2 + H Br
→
CH 3
−CH Br−CH
3
Free Radical Additions of Halogens to Alkanes
In free radical additions of halogens to alkanes (or alkenes), a radical halogen can attack an
alkane to produce another radical, in this case a radical version of the alkane. The radical
alkane can attack another compound, producing another radical that can continue on to
attack another compound.
Polymerization of alkenes is an economically important reaction that yields polymers of high
industrial value, such as the plastics polyethylene and polypropylene. Polymerization can
either proceed via a free-radical or an ionic mechanism.