Substitution Reaction

Introduction

Substitution Reaction is described as a chemical element's functional group, which is being replaced by another group. An atom or a molecule can be substituted by an extra atom or molecule in this type of reaction. The reaction works when two or more elements are connected in a definite situation. The chemical and physical properties of the element alter because of this reaction. The bonding of chemical changes for this reaction, it also affects molecules and atoms of the element.

What is Substitution Reaction?

Figure 1 − General substitution reaction

General substitution reaction, Creative Commons Attribution-Share Alike 4.0, via Wikimedia Common

Substitution Reaction is an isolated movement, an isolated replacement reaction, or an isolated substitution reaction. The molecule’s atom in the functional group is altered by another functional group. The amalgamation of two elements like oxygen (O2) and hydrogen (H2) makes one element, which is water (H2O). Hydrogen reacts with the existable oxygen and burns as fuel and all the procedure of ignition is assisted by oxygen. These two compounds can react chemically with each other and the result of the reaction comes out as water. Water (H2O) is one of the elements, which help to put off the fire. Many elements are available in nature, which helps to create new elements to break down the old. The reaction creates a new group of atoms in various types of elements and creates a new functional group.

Conditions for Substitution Reaction

Some conditions occur to make Substitution Reaction, the conditions are given below,

• Temperature control is a very important part to make the reaction successfully. The temperature should be moderate like room temperature.

• Some base, which is strong like NaOH, should be in the dilute state. During the time of the experiment, if the base state taken is concentrated then the reaction will not able to take its proper place. The concentrated base is appropriate for the dehalogenation process not for this reaction.

• The taken solution for this experiment should be in an aqueous form like water.

Substitution reaction types

Nucleophilic and Electrophonic are the two varieties of Substitution Reaction, these two types vary on the element’s molecule connected with atoms. The atom is in the state of electron rich species for nucleophilic reaction. The atom is in a state of electron- deficient species in an electrophonic reaction. An explanation of these two types is given below −

Nucleophilic reaction

Nucleophiles are powerfully attached with positive charge in a form of molecules or ions, in the molecule negative charge of ions can be excisable. Ammonia, cyanide and hydroxide ions are examples of Nucleophiles. Nucleophile is connected with other molecules or ions, which are positive in charge and can make Nucleophilic reaction. An example of this reaction is alkyl bromide(R-Br) hydrolysis reaction under general conditions, leaving the group as Br- and the nucleophile is OH-. The reaction is like R-Br + OH− → R-OH + Br−.

Electrophilic Reaction

Electrophiles are playing the main role in this reaction, electrophiles give two electrons as a covalent bond. The reaction mostly happened with atomic elements because they have many electrons to donate to the whole system. The reaction works as an electrophile substituted functional group in an element but the rule is not the same for hydrogen atom. Electrophiles examples are, HBr, Hl are two hydrogen halides, NO2+ (Nitronium ions) etc. Aliphatic electrophilic and aromatic electrophilic are the two types of reaction.

SN1 and SN2 Reaction mechanisms

SN1 and SN2 are the two mechanisms of nucleophilic reaction, s is written as a substitute for chemicals, N is as nucleophilic and the number represents kinetic order.

Figure2 − SN1 SN2 comparison

SN1 SN2 comparison, Public domain, via Wikimedia Common

SN1 mechanism reaction

Some factors give influence this reaction. These are described in detail.

• The time of reaction should be one concentration; the reaction rate is affected by the substrate.

• The reaction rate is written as Rate = k [Sub] of the reaction.

• The moderate step is defined as the reaction rate, for the reason the timing of leave of the leaving group is denoted properly in rate. So, this procedure helps to track the speed of the reaction.

• This reaction helps to connect the carbocations as a large group.

• The reaction gets slow in primary carbocation, secondary carbocation is quite faster than primary. Tertiary carbocation is the fastest process rather than the other two.

• The step of rate-determining nucleophiles is not essential.

SN2 mechanism reaction

The reaction works simultaneously between removal groups and the nucleophile. The reaction to getting successful on the time nucleophile gets connected with the main carbon atom. The conditions are given below for this reaction,

• The reaction is affected by the two concentrations of elements and 2 is the value of the SN2 state.

• The reaction’s equation rate is Rate = k[Sub].

• Acetone, DMSO these compounds are not attached to H+ ions in solution, these compounds is appropriate for this reaction.

• Reaction of nucleophiles in the presence of protons helps to understand the reaction’s limit; reaction rate is also affected by nucleophilicity.

Differences between SN1 and SN2 Reaction

The main differences between these two reactions are given below,

SN1	SN2
The reaction can be executed in two steps.	The reaction can be executed in one step
Carbocation is made in the middle time of the reaction.	Carbocation is not made in this type of reaction.
Unimolecular is the rate of this type of reaction.	Bimolecular is the rate of this type of reaction.
Carbon does not make any partial bond.	Carbon does make a partial bond with nucleophiles.
The reaction is happening with many steps, beginning with eliminating the groups.	The reaction is completed in one step, with an intermediate state.

Table 1 − Differences between SN1 and SN2 Reaction

Examples of Reaction

$\mathrm{CH_{3}Cl}$ reacting with the presence of hydroxyl ions (OH-), the reaction is known as the reaction of hydroxyl ion with methanol. The equation is given below,

$$\mathrm{CH_{3}Cl\:+\:(OH-)\:\rightarrow\:CH_{3}OH(methanol)\:+\:Cl-}$$

Another example is Ethanol with the hydrogen iodide which makes iodoethane and water. The reaction is -

$$\mathrm{CH_{2}CH_{2}OH\:+\:HI\:\rightarrow\:CH_{3}CH_{2}I\:+\:H_{2}O}$$

Conclusion

The substitution reaction is discussed briefly in the above article. The reaction is basically replaced from one functional group to another functional group. There are two types of reaction, nucleophilic and electrophilic, SN1 and SN2 is the under the type of nucleophilic reaction. The difference between SN1 and SN2 types is also discussed in the above article. Few examples are given to understand the Substitution Reaction.

FAQs

1. What are the purposes of using Substitutional Reaction?

The electrophilic reaction is crucial in organic chemistry; this reaction is used in important intermediates for synthesis action. The reactions are used in agrochemical and pharmaceutical industries.

2. What is the difference between elimination and substitution reactions?

The elimination reaction only removes the atoms but the substitution reaction replaces the atoms with one another. This is the basic difference between the two reactions.

3. Is hydrolysis a type of substitution reaction?

Hydrolysis is just a part of the SN2 reaction’s few parts, it is just used for the removal of heat.