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 what is Sn1 and Sn2 reactions?

Asked by Baljeet Kaur Talwar(Swami Vivekanand Public School, Rajgarh) , on 23/11/10


Hi Neha,
First let us discuss the SN1 reactions.
These  are basically unimolecular nucleophilic substitution recations. The mechanism of SN1 reactions is as follows.

These reactions are carried out in polar protic solvents such as water, alcohol, acetic acid, etc.
The increasing order of reactivity is
1° halide < 2° halide < 3° halide
The reason behind the above trend is as follows.
Greater the stability of a carbocation, more easily the alkyl halide is formed and hence, faster is the reaction rate. The increasing order of stability of carbocation is 1° < 2° < 3°. Since 1° halide forms 1° carbocation, 2° halide forms 2° carbocation, and 3° halide forms 3° carbocation. Therefore, the increasing order of reactivity is 1° halide < 2° halide < 3° halide.
Now, let’s come to the SN2 reactions.
These reactions are known as bimolecular nucleophilic substitution reactions. The mechanism for these types of recation is as follows.
In these reactions, inversion of configuration takes place. The increasing order of reactivity is
3° halide < 2° halide < 1° halide
The reason behind the above trend is as follows.
Due to the presence of bulky substituents in 3° and 2° halides, nucleophile cannot approach.
For both SN1 and SN2 reaction, the order of reactivity of halides is
R−F << R−Cl < R−Br < R−I
The stereochemical aspects of nucleophilic substitution recation can be summarised as
·  In SN2 reaction, complete stereochemical inversion takes place.
·  In SN1 reaction, racemisation takes place.
Hope, the concept of SN1 and SN2 recations is quite clear now.

Posted by Shukla Paladhion 24/11/10

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SN1 stands for unimolecular nucleophilic substitution.this mechanism involves two step.e.g.hydrolysis of tert-butyl bromid ewith aqueous NaOH.on theother hand SN2 stands for biomolecule nucleophilic substitution.this mechnism involves one step .e.g hydrolysis of methyle bromide with aqueous NaOH.


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