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Difference between SN1 and SN2 reaction




SN2 Reactions

SN1 Reactions

Number of steps

Θ                          Θ

One: R : L + : Nu → R : Nu + L


Two: (i) R : L ————→ R+ + : L


(ii) R+ + Nu ———→ R : Nu


Reaction rate and order

Second order:

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

First order:

Rate  [Substrate] or Rate = K1[RL]




TS of slow step

δ–                              δ

: Nu – – – C – – – : L

δ+             δ

: Nu – – – C – – – : L – – – Nu :

Reacting nucleophiles

The nucleophile attacks the carbon of the substrate exclusively from the back side.

The nucleophiles can attack the carbon of the substrate both from the back and front sides although the back side attack predominates.


Complete inversion of configuration takes place.

Inversion and retention takes place.

Reactivity order of alkyl halides

Methyl >1°>2°>3° halides. (I>Br>Cl>F)

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


No rearranged product is formed (except for allylic).

Rearranged products can be formed.

Nature of nucleophiles

Favoured by strong and high concentration of nucleophiles.

Favoured by mild and low concentration of nucleophiles.


Favoured by solvents of low polarity.

Favoured by solvents of high polarity.

Reaction rate determining factor

By steric hindrance.

By electronic factor (stability of R).


Not catalysed by any catalyst (phase transfer).

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

Ag, AlCl3, ZnCl2

and strong HA.






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