Substitution Reaction
Substitution reactions are chemical reactions in which one atom or group of atoms in a molecule is replaced by another atom or group of atoms. The incoming group is bonded to the same carbon to which the leaving group was bonded. These reactions are generally carried out in a solvent, such as water or ethanol, and are catalyzed by a Lewis acid or base. The conditions for a substitution reaction to occur depend on the specific reactants and solvent used. However, some general conditions that often required are discussed below-
1. Leaving Group: The molecule must have a good leaving group so that they can readily depart with its electron pair.
2. Nucleophile: A strong nucleophile is needed to attack the electrophilic carbon atom where the leaving group is attached.
3. Solvent Effects: Polar protic solvents favor SN1 reactions by stabilizing the carbocation intermediate, while polar aprotic solvents are preferred for SN2 reactions as they do not interfere with the nucleophile.
4. Steric Hindrance: In SN2 reactions, a less sterically hindered carbon atom is more susceptible to nucleophilic attack.
Types of Substitution Reactions
There are three types of substitution reactions. They are-
1. Nucleophilic Substitution Reaction
2. Electrophilic Substitution Reaction
3. Free Radical Substitution Reaction
Nucleophilic substitution reactions occur when a nucleophile attacks an electrophile. The nucleophile donates its lone pair of electrons to the electrophile, forming a new bond and displacing the leaving group. Nucleophilic substitution reactions are of three types. They are-
1. Unimolecular Substitution (SN1) Reaction
2. Bimolecular Substitution (SN2) Reaction
3. Intermolecular Substitution (SNi) Reaction
Electrophilic substitution reactions occur when an electrophile attacks a nucleophile. The electrophile accepts a pair of electrons from the nucleophile, forming a new bond and displacing the leaving group.
Free radical substitution reactions are initiated by radicals in the gas phase or in non-polar solvents. For example, methane and chlorine react in the presence of sunlight or heat to give methyl chloride. Light energy or heat causes homolytic fission of chlorine producing chlorine radicals which attack methane to form methyl chloride.
Other Substitution Reactions
1. IPSO Substitution Reaction2. Cine Substitution Reaction
3. Tele Substitution Reaction
4. Vicarious Substitution Reaction
Factors Affecting the Rate of Substitution Reactions
The rate of a substitution reaction is affected by a number of factors. Some of which are discussed below-
1. Concentration: Higher the concentration of the reactants, faster the reaction will occur.
2. Temperature: Higher the temperature, faster the reaction will occur.
3. Nature of the Solvent: Solvent can affect the rate of reaction by changing the polarity of the reactants and by solvating the ions that are formed during the reaction.
4. Catalyst: A catalyst can change the rate of reaction. A positive catalyst speed up the reaction by lowering the activation energy while a negative catalyst slow down the speed of the reaction by increasing the activation energy.
