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When one functional group in a chemical molecule is replaced by another functional group, it is called a substitution reaction (also known as a single displacement reaction or a single substitution reaction). Substitution reactions are particularly essential in organic chemistry. Substitution reactions are classified as electrophilic or nucleophilic depending on the reagent employed, whether a reactive intermediate in the reaction is a carbocation, a carbanion, or a free radical, and if the substrate is aliphatic or aromatic. The ability to forecast the reaction’s product outcome is aided by a thorough understanding of the reaction type. It may also be used to optimise a temperature and solvent selection reaction.
A good example of a substitution reaction is halogenation. When chlorine gas (Cl2) is exposed to radiation, it breaks into two chlorine radicals (Cl•) with exceptionally nucleophilic free electrons. When one of them breaks a C–H covalent connection in CH4 and takes the hydrogen atom, the electrically neutral HCl is created. By creating a covalent bond with the CH3• The other radical produces CH3Cl (methyl chloride).
Types of Substitution Reaction:
Nucleophilic Substitution Reaction
Electrophilic Substitution Reaction
Radical substitution.
Organometallic substitution.
Nucleophilic Substitution Reaction
Nucleophilic substitution is a fundamental class of reactions in organic (and inorganic) chemistry in which a nucleophile preferentially interacts with or assaults the positive or partly positive charge on an atom or a group of atoms. It accomplishes so by displacing a weaker nucleophile, which becomes a leaving group, and the remaining positive or partly positive atom becomes an electrophile. The substrate refers to the whole molecular unit, including the electrophile and the leaving group.
The reaction may be written as
Nuc: + R-LG → R-Nuc + LG
in its most generic form.
The substrate is denoted by R-LG. The nucleophile’s (Nuc:) electron pair (:) attacks the substrate (R-LG), forming Nuc-R-LG, a new covalent bond. When the leaving group (LG) departs with an electron pair, the previous charge state is restored. R-Nuc is the main product in this scenario. The nucleophile is normally electrically neutral or negatively charged in these reactions, whereas the substrate is usually neutral or positively charged.
The hydrolysis of an alkyl bromide, R-Br, under basic circumstances, where the attacking nucleophile is the base, OH and the leaving group is Br, is an example of nucleophilic substitution:
R-Br + OH− → R-OH + Br−
In organic chemistry, nucleophilic substitution reactions are prevalent, and they can be classified as occurring at a carbon of a saturated aliphatic molecule or (less frequently) at an aromatic or another unsaturated carbon centre.
Mechanism
Unimolecular nucleophilic substitution (SN1) and bimolecular nucleophilic substitution (SN2) are two processes for nucleophilic substitution on aliphatic carbon atoms (SN2). There are two phases in the SN1 process. The departing group leaves in the first step, creating a carbocation C+. The nucleophilic reagent (Nuc:) binds to the carbocation and creates a covalent sigma bond in the second phase. If the substrate contains chiral carbon, this process might result in stereochemistry inversion or configuration retention. Both are common, yet neither has a preference. The racemization is the end consequence.
A good example of a substitution reaction is halogenation. When chlorine gas (Cl2) is exposed to radiation, it breaks into two chlorine radicals (Cl•) with exceptionally nucleophilic free electrons. When one of them breaks a C–H covalent connection in CH4 and takes the hydrogen atom, the electrically neutral HCl is created. By creating a covalent bond with the CH3•The other radical produces CH3Cl (methyl chloride).
If the backside route of the attack is not sterically restricted by substituents on the substrate, an SN2 attack may occur. As a result, this process is most likely to occur at an undisturbed main carbon core. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, an SN1 rather than an SN2 mechanism will be used; an SN1 is also more plausible in this situation since a sufficiently stable carbocation intermediate might be generated.
The reaction type is nucleophilic aromatic substitution when the substrate is an aromatic chemical, which can occur through a variety of methods. In nucleophilic acyl substitution, carboxylic acid derivatives react with nucleophiles. This type of reaction is useful in the creation of chemicals.
Electrophilic Substitution Reaction:
Electrophilic substitution processes, particularly electrophilic aromatic replacements, need electrophiles.
The electron resonance structure of the benzene ring, for example, is attacked by the electrophile E+. A carbocation resonating structure is formed when the resonating connection is disrupted. Finally, a proton is ejected, resulting in the formation of a new aromatic molecule.
Electrophilic addition rather than substitution occurs in electrophilic reactions with additional unsaturated molecules than arenes.
Mechanism
A two-step mechanism has been proposed for these electrophilic substitution reactions. In the initial, slow or rate-determining phase, the electrophile forms a sigma bond with the benzene ring, resulting in a positively charged intermediate. In the second, fast step, a proton is removed from this intermediate, resulting in a substituted benzene ring. Other carbocation intermediate-based approaches should be considered when considering this method for electrophilic aromatic substitution. Examples include alkyl halide SN1 and E1 reactions, as well as alkene Bronsted acid addition reactions.
Conclusion
Substitution reactions are chemical processes in which a functional group in a molecule or ion is replaced by a functional group from another molecule or ion. The link between the functional group (or ligand) and the reactive centre is broken during the substitution, and a new bond is created between the reactive centre and the new functional group (or ligand). Substitution reactions are one of the most common types of organic chemistry reactions.
