# What are nucleophilic and electrophilic substitution reactions?

Jul 6, 2016

Here's my explanation.

#### Explanation:

A nucleophilic substitution is a reaction in which an electron pair donor (a nucleophile, $\text{Y:}$) reacts with an electron pair acceptor (the substrate, $\text{R-X}$) and substitutes for the $\text{X}$ group (the leaving group).

A typical general equation is

$\underbrace{\text{Y:"^"-")_color(red)("nucleophile") +underbrace("R"-"X")_color(red)("substrate") → "Y-R" + underbrace(":X"^"-")_color(red)("leaving group}}$

where $\text{R}$ can be either an alkyl or an aryl group.

The mechanisms are different for aliphatic and aromatic nucleophilic substitutions, but the overall result is the same:

A nucleophile $\text{Y}$ replaces a leaving group $\text{X}$.

Specific examples are

$\text{:CN"^"-" + "CH"_3"Br" → "CH"_3"CN" + ":Br"^"-}$

and

An electrophilic substitution is a reaction in which an electrophile $\text{E}$ substitutes for another atom or group $\text{Y}$ in a compound $\text{R-Y}$

A typical general equation is

${\text{E"^+ + "R-Y" → "E-R" + "Y}}^{+}$

where $\text{R}$ can be either an alkyl or an aryl group.

An example is an electrophilic aromatic halogenation.

In the above reaction, a $\text{Br}$ atom substitutes for a hydrogen atom.

Electrophilic aliphatic substitutions are relatively uncommon.

One important example is the replacement of the metal atom in an organometallic compound by hydrogen:

${\text{R-M" + "H"^+ → "R-H" + "M}}^{+}$

The decomposition of a Grignard reagent by acid is an example.

$\text{CH"_3"CH"_2"-MgBr" + "H"^+ → "CH"_3"CH"_2"-H" + ""^+"MgBr}$