Notes On Chemical Reactions Of Alkyl Halides: Nucleophilic Substitution - CBSE Class 12 Chemistry
Haloalkanes undergo three types of reactions. They are nucleophilic substitution reactions, elimination reactions and reaction with metals. The word 'nucleophile' means "nucleus-seeking". A nucleophile has an unshared pair of electrons and it can use these electrons to make a covalent bond. Nucleophiles can be thought of as Lewis bases (or) electron pair donors. Nucleophiles may be anions or neutral molecules. Substitution reactions can occur when a nucleophile substitutes for a halide substituent on carbon. Common nucleophiles are the hydroxide ion and water. Some nucleophiles can be described as "ambident nucleophiles". These contain two nucleophilic centres and so can link to the alkyl halide in two different ways. Depending upon the mechanism or the reaction conditions, nucleophilic substitution reactions can be classified as SN1 (or) SN2 reactions. In the SN2 mechanism, the reaction shows second order kinetics and the mechanism occurs in a single step with inversion of configuration. Only one product can form in an SN2 mechanism. In the SN2 mechanism, the nucleophile attacks from the back of the molecule, on the opposite side from the halogen atom, the leaving group in this reaction. The leaving group leaves simultaneously. In the SN1 mechanism, the reaction shows first order kinetics. The mechanism occurs in two steps. Due to the greater stability of tertiary carbocations, they readily undergo SN1 reactions. SN1 reactions are usually carried out in polar protic solvents such as water, alcohols and acetic acid. In the SN1 mechanism there are two directions from which the nucleophile can attack the carbocation. Thus forms two different products which are mirror images of each other.

Summary

Haloalkanes undergo three types of reactions. They are nucleophilic substitution reactions, elimination reactions and reaction with metals. The word 'nucleophile' means "nucleus-seeking". A nucleophile has an unshared pair of electrons and it can use these electrons to make a covalent bond. Nucleophiles can be thought of as Lewis bases (or) electron pair donors. Nucleophiles may be anions or neutral molecules. Substitution reactions can occur when a nucleophile substitutes for a halide substituent on carbon. Common nucleophiles are the hydroxide ion and water. Some nucleophiles can be described as "ambident nucleophiles". These contain two nucleophilic centres and so can link to the alkyl halide in two different ways. Depending upon the mechanism or the reaction conditions, nucleophilic substitution reactions can be classified as SN1 (or) SN2 reactions. In the SN2 mechanism, the reaction shows second order kinetics and the mechanism occurs in a single step with inversion of configuration. Only one product can form in an SN2 mechanism. In the SN2 mechanism, the nucleophile attacks from the back of the molecule, on the opposite side from the halogen atom, the leaving group in this reaction. The leaving group leaves simultaneously. In the SN1 mechanism, the reaction shows first order kinetics. The mechanism occurs in two steps. Due to the greater stability of tertiary carbocations, they readily undergo SN1 reactions. SN1 reactions are usually carried out in polar protic solvents such as water, alcohols and acetic acid. In the SN1 mechanism there are two directions from which the nucleophile can attack the carbocation. Thus forms two different products which are mirror images of each other.

Previous
Next