Substitution Reaction

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?

Substitution reaction mechanism

Substitution Reaction mechanism

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———— CH3CH2OI + 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 are as given below-

1. Nucleophilic substitution reaction:

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.

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.

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 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 SN2 states that there are two concentrations of substances which 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 SN2 reaction as 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.

Learn more about SN2 reaction mechanism in detail.

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.
  • SN1 reactions can be determined by bulky groups that are attached to the carbocations.
  • 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 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:

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.

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 of the examples of species of electrophiles include hydronium ion (H3O+), halides of hydrogen such as HCl, HBr, HI, sulfur trioxide (SO3), the nitronium ion (NO2+), etc.

Types of Electrophilic substitution reaction:

Two types of electrophilic substitution reaction 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, SE2 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.

Read more:

Practise This Question

The ionic product of water does not change with the temperature - True or false?