What are nucleophilic and electrophilic substitution reactions?

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

A typical general equation is

#underbrace("Y:"^"-")_color(red)("nucleophile") +underbrace("R"-"X")_color(red)("substrate") → "Y-R" + underbrace(":X"^"-")_color(red)("leaving group")#

where #"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 #"Y"# replaces a leaving group #"X"#.

Specific examples are

#":CN"^"-" + "CH"_3"Br" → "CH"_3"CN" + ":Br"^"-"#

and

An electrophilic substitution is a reaction in which an electrophile #"E"# substitutes for another atom or group #"Y"# in a compound #"R-Y"#

A typical general equation is

#"E"^+ + "R-Y" → "E-R" + "Y"^+#

where #"R"# can be either an alkyl or an aryl group.

An example is an electrophilic aromatic halogenation.

In the above reaction, a #"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:

#"R-M" + "H"^+ → "R-H" + "M"^+#

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

#"CH"_3"CH"_2"-MgBr" + "H"^+ → "CH"_3"CH"_2"-H" + ""^+"MgBr"#