Have you ever observed how the application of anaesthetic agents into your body reduces the
pain? Did you know that these agents are produced by the process of substitution reaction?
What is Substitution Reaction?
The substitution reaction is defined as a reaction in which the functional group of one chemical
compound is substituted by another group or it is a reaction which involves the replacement of
one atom or a molecule of a compound with another atom or molecule.
Substitution Reaction Example
These type of reactions are said to possess primary importance in the field of organic chemistry.
For example, when CH3Cl is reacted with the hydroxyl ion (OH-), it will lead to the formation of
the original molecule called methanol with that hydroxyl ion. The following reaction is as shown
below-
CH3Cl + (OH−) → CH3OH( methanol) + Cl–
One more example would be the reaction of Ethanol with the hydrogen iodide which forms
iodoethane along with water. The reaction is as shown-
CH3CH2OH + HI → CH3CH2I + H2O
Substitution Reaction Conditions
In order to substitution reaction to occur there are certain conditions that have to be used. They
are-
Maintaining low temperatures such as room temperature
The strong base such as NaOH has to be in dilute form. Suppose if the base is of a higher
concentration, there are chances of dehydrohalogenation taking place
The solution needs to be in an aqueous state such as water
Substitution Reactions – Types
Substitution Reactions are of two types naming nucleophilic reaction and electrophilic reactions.
These two types of reactions mainly differ in the kind of atom which is attached to its original
molecule. In the nucleophilic reactions the atom is said to be electron-rich species, whereas, in
the electrophilic reaction, the atom is an electron-deficient species. A brief explanation of the
two types of reactions is as given below.
1. Nucleophilic Substitution Reaction:
(a) What are nucleophiles?
Nucleophiles are those species in the form of an ion or a molecule which are strongly attached to
the region of a positive charge. These are said to be fully charged or have negative ions present
on a molecule. The common examples of nucleophiles are cyanide ions, water, hydroxide ions,
and ammonia.
(b) What is Nucleophilic substitution reaction?
A Nucleophilic substitution reaction in organic chemistry is a type of reaction where a
nucleophile gets attached to the positive charged atoms or molecules of the other substance.
(c) Mechanism of Nucleophilic substitution reaction:
Two mechanisms of nucleophilic substitution reaction are discussed here. SN1 reaction and
the SN2 reaction, where S represents chemical substitution, N represents nucleophilic, and the
number stands for the kinetic order of a reaction.
SN2 Reaction – Mechanism of SN2 Reaction
In this reaction, the elimination of the leaving group and the addition of the nucleophile occur
simultaneously. SN2 takes place where the central carbon atom has easy access to the
nucleophile.
In SN2 reactions, the rate of the reaction is affected by a few conditions. They are discussed
below:
The numerical value 2 in S 2 states that there are two concentrations of substances which
N
affect the rate of reaction, viz. nucleophile and substrate.
The rate equation for the above reaction is written as Rate = k [Sub][Nuc].
An aprotic solvent such as acetone, DMSO, or DMF is suited best for the S 2 reaction as
N
they do not add the H ions in the solution.
+
In case if there are protons present, they react with the nucleophile to critically limit the rate
of reaction. It is a one-step reaction and the reaction speed is driven by the steric effects.
During the intermediate step, the position of the leaving group is inverted whereas the
nucleophile is 180°.
Nucleophilicity also affects the reaction rate.
SN1 Reaction – Mechanism of SN1 Reaction
There are certain factors that affect the SN1 reaction as well. A few are discussed below:
Instead of two concentrations only one i.e., the substrate affects the rate of reaction.
The rate equation for the above reaction is written as Rate = k[Sub].
The reaction rate is determined by its slowest step. Therefore, the leaving group leaves at a
particular rate which helps in determining the reaction speed.
It is considered that the weaker the conjugate base, the better is the leaving group.
S 1 reactions can be determined by bulky groups that are attached to the carbocations.
N
The reaction of the tertiary carbocation is faster than secondary carbocation which is faster
than primary carbocation.
The nucleophile is not required in the rate-determining step.
Learn more about the SN1 reaction mechanism in detail.
Example of Nucleophilic Substitution Reaction:
A good example of a nucleophilic substitution reaction is the hydrolysis of alkyl bromide (R-Br),
under the basic conditions, wherein the nucleophile is nothing but the base OH−, whereas the
leaving group is the Br−. The reaction for the following is as given below R-Br + OH− → R-OH
+ Br−
Nucleophilic reactions are as important in the field of organic chemistry, and these reactions are
broadly classified as to occur at the place of a carbon atom of a saturated aliphatic carbon
compound.
2. Electrophilic Substitution Reactions:
(a) What are electrophiles?
The electrophilic substitution reaction involves the electrophiles. Electrophiles are those which
donate a pair of electrons in the formation of a covalent bond. The Electrophilic reactions occur
mostly with the aromatic compounds. These compounds have about an excess of electrons that
can be shared throughout the system of reaction.
(b) What is Electrophilic substitution reaction?
The Electrophilic substitution reactions are basically defined as those chemical reactions where
the electrophile replaces the functional group in a compound but not the hydrogen atom. Some
examples of species of electrophiles include hydronium ion (H3O+), halides of hydrogen such as
HCl, HBr, HI, sulphur trioxide (SO3), the nitronium ion (NO2+), etc.
(c) Types of Electrophilic substitution reaction:
Two types of electrophilic substitution reactions are discussed here. Aromatic electrophilic
substitution reaction and aliphatic electrophilic substitution reaction.
Electrophilic Aromatic Substitution
In this type of electrophilic substitution, an atom attached to the aromatic ring which is mostly
hydrogen is substituted by an electrophile. The reactions that occur are aromatic halogenation,
alkylating Friedel-Crafts reactions, aromatic nitration, and aromatic sulfonation and acylation. It
further comprises acylation and alkylation.
Electrophilic Aliphatic Substitution
In this type of electrophilic substitution reaction, an electrophile dislocates one functional
group. The four electrophilic aliphatic substitution reactions which are similar to counterparts of
nucleophile SN1 and SN2 are as follows – SE1, SE2(front), SE2(back) and
SEi (Substitution Electrophilic). During the SE1 reaction, the substrate ionizes to
a carbanion briskly recombines with the electrophile. During the SE2 reaction, a single transition
state occurs where the old and newly formed bond are present.
The other types of substitution reactions include radical reactions and organometallic substitution
reaction. For more details on aromatic and aliphatic electrophilic substitution checkout
the Electrophilic substitution reaction mechanism.
Frequently Asked Questions
Q1
What is a Nucleophile substitution reaction?
Nucleophilic substitution is a fundamental class of reactions in organic and inorganic chemistry
in which an electron-rich nucleophile selectively binds or attacks the positive or partially positive
charge of an atom or group of atoms to replace a left group.
Q2
What occurs in a substitution reaction?
Substitution reaction is also known as single displacement reaction or single replacement
reaction is a chemical reaction during which one functional group is replaced by another
functional group in a chemical compound. Halogenation is a good example of a replacement
reaction.
Q3
What are the substitution reactions used for?
One of the most important reactions in synthetic organic chemistry is an electrophilic aromatic
substitution. These reactions are used to synthesize essential intermediates that can be used as
precursors for pharmaceutical, agrochemical, and industrial products manufacturing.
Q4
Is hydrolysis a substitution reaction?
Hydrolysis is just a heat substitute. This can be achieved by sn2 displacement of a primary
halogen by water, by the reaction of a water-stabilized trapped carbon nucleophile, or by adding
water through a double bond.
Q5
What is the difference between substitution and
elimination reactions?
The obvious difference between replacement and elimination reactions is that replacement
reactions substitute one replacement with another when elimination reactions simply remove the
replacement. The big difference between the groups is the number of steps over which the
reaction takes place.
