Nucleophilic Substitution Reactions

Substitution and nucleophile reactions in chemistry can be slightly intimidating topics that you feel would need more than a little effort to understand. However, in reality, these are common reactions and understanding the fundamentals can benefit you greatly in understanding the core of such Nucleophilic Substitution Reactions effectively.

Here, we get into the basics of Sn1 and Sn2 reactions along with the similarities and differences between the two Nucleophilic Substitution Reactions.

What is Sn1 Nucleophilic Substitution Reactions?

A Nucleophilic Substitution in which the Rate Determining Step involves 1 component. Sn1 is a unimolecular reaction. Sn1 proceeds through an intermediate carbocation in the rate-determining step. The most stable carbocation can produce the fastest reaction. In Sn1, the reaction depends only on the substrate molecule. 

What is Sn2 Nucleophilic Substitution Reactions?

Sn2 reactions are very common in organic chemistry. In the Sn2 reaction, one bond is broken while one bond is formed simultaneously in a single step. Sn2 is a bimolecular reaction. 

Sn2 reactions never proceed through an intermediate. It involves two reacting species in the rate-determining step of the reaction. Sn2 is incredibly powerful. In Sn2, the reaction depends both on the substrate and the nucleophile.

What is Nucleophile?

A nucleophile is a chemical species that forms bonds with electrophiles by donating an electron pair in chemistry. It seeks a positive centre. It makes a covalent bond. It is also known as Lewis Base because it donates electrons. For example, water, ammonia etc.  

What is Substitution Reaction?

A substitution reaction involves the direct replacement of an atom or a group of atoms in an organic molecule by another atom or group of atoms without any change in the remaining part of the molecule. 

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Substitution reaction is very important in synthetic organic chemistry. It is also known as a single displacement reaction, a single replacement reaction or a single substitution reaction.  

What is Nucleophile Substitution Reaction?

Nucleophile substitutions are a class of reactions where one electron rich nucleophile replaces another positively charged electrophile. It replaces one atom or group with another. It is the same as the normal displacement reaction we see in chemistry. 


A few examples of nucleophile substitution reaction are,

Nuc: + R-LG → R-Nuc + LG:

R-Br + OH → R-OH + Br

Types of Nucleophile Substitution Reaction

There are two types of nucleophile substitution reactions:

  • Sn1 reaction
  • Sn2 reaction

Here, S stands for substitution, N stands for nucleophilic, and the number represents the kinetic order of the reaction. 

Similarities between Sn1 and Sn2

Similarities between Sn1 and Sn2 are as below:

  • Transition states are present in Sn1 and Sn2 both
  • Sn1 and Sn2 both are nucleophilic substitutions
  • In both Sn1 and Sn2 reactions, one bond will break
  • One new bond will be formed in both Sn1 and Sn2 reaction
  • Alkyl halide show both Sn1 and Sn2 mechanisms
  • In both Sn1 and Sn2, hybridisation of carbon is not changed
  • Both Sn1 and Sn2 reactions do not require high temperatures
  • Both Sn1 and Sn2 happen for second R-X reactions
  • Both Sn1 and Sn2 reactions are exothermic 

Differences between Sn1 and Sn2

Differences between Sn1 and Sn2 are as below:

  • Sn1 involves one molecule ( unimolecular ), while Sn2 involves two molecules ( bimolecular ) in reaction.
  • Sn1 has a weak nucleophile, while Sn2 has a very strong nucleophile.
  • Sn1 is dependent on the concentration of the substrate, while Sn2 is dependent on the concentration of the substrate and the nucleophile.
  • Sn1 forms carbocation intermediate, while Sn2 is a single-step process.
  • Sn1 follows a first-order kinetic mechanism, while Sn2 follows a second-order kinetic mechanism.
  • Sn1 is a two-step process, while Sn2 is a single-step process.
  • Sn1 has tertiary carbon stability while Sn2 has primary carbon unstable.
  • In Sn1, a positive inductive effect will be there, while in Sn2, there will be no positive inductive effect.
  • Sn1 is a non-concerted reaction, while Sn2 is a concerted reaction.
  • Sn1 attacks from either side in step one, while in Sn2, backside attacks in step one.
  • Sn1 occurs in the warm and polar solvent, while Sn2 occurs in the cold, dilute and alkali solvent.
  • Sn1 has no mirror images, while Sn2 has Walden inversion.
  •  Sn1 is optically inactive while Sn2 possesses optical activity
  • In Sn1, there are two activated complexes, while in Sn2, there is one activated complex.
  • Sn1 forms a racemic mixture while Sn2 forms no mixture as it is one type of product only.
  • A solvent of Sn1 is polar protic ( for example, alcohol ), while solvent of Sn2 is polar aprotic ( for example, DMSO, acetone).
  • In Sn1, it is partial racemisation stereochemistry, while in Sn2, it is 100% inversion.
  • Intermediates are present in Sn1 and absent in Sn2.
  • Carbocation intermediates are present in Sn1 and absent in Sn2.
  • Rearrangements and shifts are possible in Sn1 but not possible in Sn2.
  • If we use more nucleophilic reagents, Sn1 will show no effect, while Sn2 will increase the reaction rate.
  • Sn1 will show no effect if we use increased nucleophiles, while Sn2 will increase the reaction rate.
  • Using polar protic, Sn1 will show a faster reaction rate while Sn2 will show a slower reaction rate.
  • If we use polar aprotic, Sn1 will show a slower reaction rate while Sn2 will be faster.
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We can conclude this topic by saying that Sn1 and Sn2 are two types of nucleophile substitution reactions discussed here with all important information. We hope you enjoyed this article and found it helpful.



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