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CBSE - XII Science - Chemistry - Haloalkanes and Haloarenes

All difference between Sn1 and Sn2 mechanism including its reactivity?

Asked by shrijeet 6th March 2013, 11:51 AM
Answered by Expert


Sn1 reaction is a substitution reaction in organic chemistry, where, Sn stands for nucleophilic substitution and 1 is the rate determining factor denoting that it is unimolecular.

Hydrolysis of tert-butyl bromide with water forming tert-butyl alcohol:

The SN1 reaction takes place in three steps:

1. Tert-butyl carbocation is formed by separating a leaving group (a bromide anion) from the carbon atom. This step is slow and reversible.                         

2. Nucleophilic attack: the carbocation reacts with the nucleophile. A third step is required to complete the reaction if the nucleophile is a neutral molecule (a solvent). If the solvent is water, the intermediate is an oxonium ion. This is a fast step in the reaction.     


3. Deprotonation: Water which acts as a base removes a proton on the protonated nucleophile to form alcohol and a hydronium ion. This is a fast step as well. 

Sn2 Reactions

   * The SN2 reaction is also known as bimolecular nucleophilic substitution.

   * It is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds with it, leaving behind what may be called as a leaving group. So, the leaving group is replaced by the incoming group in the very first step.

   * Such reactions are named as bimolecular nucleophilic substitution because two reacting species are involved in this process. It is a slow and rate-determining step.

   * The SN2 reaction is known as the interchange mechanism among inorganic chemists.


Here, in this reaction, OH (nucleophile) attacks on a bromoethane (electrophile) resulting in ethanol, with the ejection of bromide (leaving group):

Distinction between SN2 and SN1reactions 


SN2 Reactions

SN1 Reactions

1. Number of steps

                              Θ                          Θ

One: R : L + : Nu → R : Nu + L


Two: 1. R : L ————→ R+ + : L


2.      R+ + Nu ———→ R : Nu

3. Reaction rate and order

Second order:

Rate ∝ [Substrate] [Nucleophile] or Rate = K2[RL][:Nu]

First order:

Rate ∝ [Substrate] or Rate = K1[RL]

4. Molecularity



5. TS of slow step

    δ–                              δ

: Nu – – – C – – – : L

                     δ+             δ

: Nu – – – C – – – : L – – – Nu :

6. Reacting nucleophile

The carbon of the substrate is attacked by the nucleophile, exclusively from the back side.

The nucleophile can attack the carbon of the substrate from both sides. However, the attack from the back side predominates.

7. Stereochemistry

Complete inversion of configuration takes place.

Inversion and retention takes place.

8. Reactivity order of alkyl halides

Methy l>1°>2°>3° halides. (I>Br>Cl>F)

3°>2°>1° > methyl halides. (I>Br>Cl>F)

9. Rearrangement

No rearranged product is formed (except for allylic).

Rearranged products can be formed.

10. Nature of nucleophiles

Favoured by strong and high concentration of nucleophiles.

Favoured by mild and low concentration of nucleophiles.

11. Polarity

Favoured by solvents of low polarity.

Favoured by solvents of high polarity.

12. Reaction rate determining factor

By steric hindrance.


By electronic factor (stability of R).

13. Catalysis

Not catalysed by any catalyst (phase transfer).

Catalysed by Lewis and Bronsted acids, e.g.,

 Ag, AlCl3, ZnCl2 and strong HA.


Answered by Expert 7th March 2013, 2:40 PM

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