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Substitution Reaction


Substitution reaction is a reaction in which an ion or group of atoms displaces atoms or group of atoms in a molecule. Substitution reactions at an aliphatic carbon are generally nucleophilic. In aromatic systems the situation is reversed. Substitution reaction of chlorine with methane yields carbon tetra chloride. 

Substitution reactions at an aliphatic carbon are generally nucleophilic. In aromatic systems the situation is reversed. Three types of reagents can perform substitution in aromatic compounds, that is electrophiles, nucleophiles and free radicals. Substitution reactions are important reactions in organic chemistry. In these reactions, alkyl groups are transferred to the nucleophiles. Organic electrophiles of this type are referred to as alkylating agents. Through the use of these reactions, readily available alkyl halides can be converted into a wide variety of other compounds. 

Substitution Reaction Definition

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The exchange of one fuctional group, or atom for another is called a substitution reaction. The transesterification reaction above is an example of substituting a -OCH2CH3 group for a -OCH3 group. Another simple substitution reaction is the exchange of one halogen for another on a methyl group.

What is an electrophile?

Electrophiles are species that accept electrons (from a nucleophile) in a chemical reaction. Usually electrophiles have either a full or partial positive charge or in some cases have an unfilled octet of electron.

What a nucleophile?

A nucleophile is a molecule that donates electrons (to an electrophile) in a chemical reaction. These are typically groups with lone pairs of electrons, but can also be $\pi$ bonds or in some rare cases $\sigma$ bonds.

Electrophilic Substitution Reaction

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In an electrophilic substitution reaction, the attacking species is an electron deficient species. Therefore, the leaving group must also be electron deficient, an electrofuge in order to maintain the overall charge on the substrate. Write down the equation for the overall reaction of the electrophilic substitution of RX by an electrophile E+.

RX + E+ $\rightarrow$ RE + X+

It is obvious that the stability of the departing group as an electrofuge will be one of the principal factors that will affect how easy this pathway is. In a nucleophilic substitution reaction, it was found that either the leaving group left before the incoming nucleophile arrived, or else the arrival and departure were simultaneous. These different pathways were required so that the quota of electrons around the carbon undergoing substitution did not exceed eight.

In contrast in the case of electrophilic substitution, because the incoming electrophile is electron deficient, it may attack the substrate to form an intermediate adduct, from which an electrofuge can leave in order to generate the substituted product. Write down the general equation for this two step process.

RX + E+ $\rightarrow$ [RXE]+ $\rightarrow$ RE + X+

Nucleophilic Substitution Reaction

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In nucleophilic substitution reaction involving alkyl halides as the substrate, a lewis base (nucleophile) substitutes the halogen present in the alkyl halide. A general representation can be done as follows.

RX (Alkyl halide) $\rightarrow$ RNu (substitution product) + X-

During nucleophilic aromatic substitution reactions, a negatively charged intermediate is formed when the nucleophile attacks a halogen-bearing carbon atom. This intermediate in which the nucleophile and the halogen are bound to the same carbon atom is not aromatic but aromaticity is restored by elimination of halide. Nucleophilic substitution is feasible through processes that involve electron transfer (ET) steps. In these reactions a compound bearing an adequate leaving group is substituted at the ipso position by a nucleophile.

Nucleophilic substitutions that occur in solution and in the gas phase, involving electron pair mechanism, have been described to follow characteristics processes between those of an SN1 and SN2 mechanism. These two different pathways present typical properties from both a kinetic point of view and the stereochemical relation between the starting material and the product.