Question:

A nucleophilic substitution reaction proceeds through $S _{ N } 1$ mechanism. So, the reaction is

Updated On: Jun 8, 2024
  • unimolecular
  • bimolecular
  • trimolecular
  • rate depends on concentration of incoming nucleophile
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Approach Solution - 1

A nucleophilic substitution reaction proceeds through $S_N1 $ mechanism, so the reaction is unimolecular.


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Approach Solution -2

Ans: A nucleophilic substitution will proceed through the SN1, it proceeds through a unimolecular reaction. This means that the rate-determining step of the reaction involves only one molecule (unimolecular). The SN1 is a two step process: 

  1. Formation  of carbocation: this step is the departure of leaving the group and forming a carbocation intermediate which is a positive charge. 
  2. Nucleophilic attack: in the second step, the nucleophile attacks the carbocation and replaces the leaving group, this results in the substitution product. 

The characteristics of nucleophilic substitution are as follows: 

  • Racemization: Because of the carbocation intermediate, the attack of the nucleophile can be from either side. Therefore there is a formation of a racemic mixture. 
  • Solvent effect: Solvents like water and alcohols make the carbocation and the transition state stable. 
  • Tertiary carbocation: SN1 reaction is more preferred on the compounds that form a stable tertiary carbocation as they are more stable than primary and secondary carbocation.
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Concepts Used:

Haloalkanes and Haloarenes

The hydrocarbons such as Haloalkanes and Haloarenes are the ones, in which one or more hydrogen atoms are replaced with halogen atoms. The main difference between Haloalkanes and Haloarenes is that Haloalkanes are derived from open chained hydrocarbons, also called alkanes, and Haloarenes are derived from aromatic hydrocarbons.

  • Haloalkanes have hydrocarbons made up of aliphatic alkanes and one or more hydrogen atoms replaced by halogens (elements such as Chlorine, Bromine, Fluorine, Iodine, etc.) whereas, haloarenes consist of aromatic ring or rings and one or more hydrogen atoms replaced by halogens.
  • In haloalkanes, the halogen atom is attached to the sp3 hybridized carbon atom of the alkyl group whereas, in haloarenes, the halogen atom is attached to the sp3 hybridized carbon atom of the alkyl group.
  • Haloalkanes are saturated organic compounds where all the chemical bonds are attached to the carbon atom with single bonds and a single carbon atom is attached to the Halogen atom, whereas, the haloarenes differ from Haloalkanes by their method of preparation and properties.
  • Haloalkanes are made by aliphatic alkanes by the process of free radical halogenation, whereas, haloarenes are made by direct halogenation of aromatic rings.
  • Haloalkanes are odorless compounds, whereas, haloarenes have a sweet odor.
  • Haloalkanes precipitate in SN2 substitution reactions, whereas, haloarenes do not precipitate in SN2 substitution reactions.
  • Example of haloalkanes is CH3Cl (Methyl Chloride) and CH3CH2Br (Ethyl Bromide) and the example of haloarenes is Chlorobenzene, Bromobenzene.