Two molecules combine in an addition reaction to form a single addition product (an adduct). The addition occurs in unsaturated molecules of alkenes (C=C), alkynes (C≡C), carbonyls (C=O), and imines (C=N).
There are four mechanisms in addition type of reactions-
a) Electrophilic addition (heterolytic, two electron movement)
b) Nucleophilic addition (heterolytic, two electron movement)
c) Free radical addition (homolytic, one electron movement)
d) Simultaneous addition (Pericyclic)
a) Electrophilic addition (heterolytic, two electron movement)
In an electrophilic addition reaction mechanism, the incoming reagent is an electrophile. The substrate is an alkene (C=C) molecule that is electron rich due to the extra pie bond. And the net outcome is an addition reaction where the alkene donates its two electrons to the electrophile and forms two new single covalent bonds.
The electrophilic reagent can be homo atoms like H2, Cl2, and Br2 that share the electrons equally between them, or it could be of different atoms like HCl, HBr, and H-OH. Such a heteroatomic reagent has two ends- electron-deficient and electron-rich, due to the different nature of the two atoms in pulling the bond electrons (known as the electronegativity).
The electron-deficient electrophilic end is the first to attract the electron-rich alkene's pie bond. In the case of the homo atoms, the presence of alkene in the environment induces polarity, urging them to react further.
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It's a win-win situation for the substrate molecule to form two single covalent bonds by losing one pie bond and undergoing an addition reaction.
Mechanism
The addition reaction occurs in two steps; the first step is the loss of the pie bond to form the first covalent bond with the electrophilic end of the reagent.
Since the pie bond is held between two atoms and only one of the two atoms of the pie bond forms a covalent bond with the reagent, this creates an electron deficiency in the other atom, seen as a positively charged ionic intermediate (known as a carbocation). This positive end of the alkene substrate then picks up the remaining negative end of the reagent and forms the second covalent bond in the second step.
For example, the addition of reagent HBr to 2-butene gives 2-bromobutane.
Symmetrical versus unsymmetrical alkenes in electrophilic addition reaction
In alkenes like 2-butene, there is an equal distribution of the same carbon groups on either side of the alkene, forming a symmetrical alkene. However, if the groups on either side of the double bond are different, the alkene is unsymmetrical. In that case, there is an equal probability that the reagent can add in two different ways forming two different products.
However, the formation of the product is based on the substrate's preference for stability. In one of the two probable addition mechanisms, the positive ionic intermediate formed after the first step is more stable. The reagent always chooses to add in a manner that amplifies the substrate's stability, therefore forming one major product over two products. The major product is the one in which the negative part of the reagent attaches to the carbon with the least hydrogens (most substituted) across the double bond, a rule proposed by Markovnikov.
The factors controlling the stability and influencing the reaction outcome are covered under Alkene addition reactions (most and least substituted alkenes, which side of alkene reagent attacks, Markovnikov's and Anti-Markovnikov's rules, stability order of substituted alkenes).
Alkynes also undergo an addition reaction by this method.
b) Nucleophilic addition (heterolytic, two electron movement)
The nucleophilic addition reactions are primarily seen in molecules with a dipolar nature due to the nature of the attached atoms. In carbonyls, specifically in aldehydes (RCHO) and ketones (RCOR), the C=O bond is polarised. The electron density is more towards the Oxygen atom, so the carbon is electron deficient, creating electrophilic centres. Therefore, the carbon of the carbonyl attracts the electron-richness from the nucleophile and undergoes an addition reaction.
In the above example, HCN adds across the carbonyl double bond of the ketone ((CH3)2CO).
In addition to carbonyls, the imine (C=N) is also a polarised bond and undergoes an addition reaction to form amines (C-NH2).
c) Free radical addition (homolytic, one electron movement)
Free radical addition involves the addition between two radicals or a radical (formed from a precursor) and a non-radical, like alkene.
The addition reaction on the alkene proceeds through three steps- radical initiation, chain propagation, and termination. The first step is the formation of a radical from a precursor. In the second step, the weak bond of the alkene allows homolytic fission and addition across the double bond. The end step is also an addition that terminates the reaction by binding the reactive radical species into covalent bonds, therefore quenching them.
Though the biggest benefit of the radical addition reaction is chain propagation, such radical reactions are not selective.
The free radical addition method forms most polymers like Teflon and PVC found plenty in everyday life.
d) Simultaneous addition (Pericyclic)
Two molecules add in a coordinated fashion where the bond-making and breaking occur in a single step without forming any ions or radicals as an intermediate.
The X-Y molecule breaks and adds to the AB molecule in one step without any prior cleavage.
In summary, the electrophilic and nucleophilic additions involve ionic intermediates and are polar addition reactions. The free radical and simultaneous addition are nonpolar addition reactions.
