Basic Understanding of Substitution Reaction

Substitution reaction is the basic types of organic reaction which possess primary importance in the field of organic chemistry.

What is substitution reaction?

This 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.

Example of substitution reaction

CH₃I + OH^– → CH₃OH + I^–

Types of Substitution Reaction

• Nucleophilic SR
• Electrophilic SR

What are conditions for substitution reaction to occur?

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.

What occurs in substitution reaction?

This 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.

Use of the reaction

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.

Nucleophilic Substitution Reaction

What is nucleophilic substitution reaction?

It is a type of reaction where a nucleophile gets attached to the positive charged atoms or molecules of the other substance.

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.

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.

Types of nucleophilic substitution reaction

• S_N1
• S_N2

where S represents chemical substitution, N represents nucleophilic, and the number stands for the kinetic order of a reaction

S_N1 Reaction

In this reaction, the elimination of the leaving group and the addition of the nucleophile occur in two steps.

There are certain factors that affect the S_N1 reaction that are discussed below:

1. Instead of two concentrations only one i.e., the substrate affects the rate of reaction.
2. The rate equation for the above reaction is written as Rate = k[Sub].
3. 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.
4. It is considered that the weaker the conjugate base, the better is the leaving group.
5. S_N1 reactions can be determined by bulky groups that are attached to the carbocations.
6. The reaction of the tertiary carbocation is faster than secondary carbocation which is faster than primary carbocation.
7. The nucleophile is not required in the rate-determining step.

S_N2 Reaction

In this reaction, the elimination of the leaving group and the addition of the nucleophile occur in a single step.

In S_N2 reactions, the rate of the reaction is affected by a few conditions that are discussed below:

1. The numerical value 2 in S_N2 states that there are two concentrations of substances which affect the rate of reaction, viz. nucleophile and substrate.
2. The rate equation for the above reaction is written as Rate = k [Sub][Nuc].
3. An aprotic solvent such as acetone, DMSO, or DMF is suited best for the S_N2 reaction as they do not add the H⁺ ions in the solution.
4. 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°.
5. Nucleophilicity also affects the reaction rate.

Example of the 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.

Electrophilic Substitution Reaction

What is electrophilic substitution reaction?

These 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.

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.

Types of electrophilic substitution reaction

• Aromatic electrophilic substitution reaction
• Aliphatic electrophilic substitution reaction

Aromatic ESR

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.

Aliphatic ESR

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 S_N1 and S_N2 are as follows – S_E1, S_E2(front), S_E2(back) and S_Ei (Substitution Electrophilic). During the S_E1 reaction, the substrate ionizes to a carbanion briskly recombines with the electrophile. During the S_E2 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.

What is the difference between substitution and elimination reaction?

The basic difference between substitution and elimination reactions is that substitution reactions substitute one species with another while elimination reaction simply remove the species. The big difference between the groups is the number of steps over which the reaction takes place.

Read More: How carbylamine reaction give foul smelling compound?

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