What is SN1&SN2 mechanism?Why does the rate depends only upon the concentration of alkyl halide while in SN2 it depends upon the concentration of alkyl halide as well as nucleophile

Asked by  | 21st Sep, 2011, 12:00: AM

Expert Answer:

SN2 reaction :

The breaking of the C-X bond and the formation of the new C-Nu bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180° to the leaving group,  The leaving group is then pushed off the opposite side and the product is formed.

ex:

SN1 reaction:

This SN1 reaction takes place in three steps:

  • Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion in this example) from the carbon atom: this step is slow and reversible. 
  • Nucleophilic attack: the carbocation reacts with the nucleophile. If the nucleophile is a neutral molecule (i.e. a solvent) a third step is required to complete the reaction. When the solvent is water, the intermediate is an oxonium ion. This reaction step is fast.
  • Deprotonation: Removal of a proton on the protonated nucleophile by water acting as a base forming the alcohol and a hydronium ion. This reaction step is fast.

 

The rate of an SN2 reaction is second order, as the rate-determining step depends on the nucleophile concentration, [Nu?] as well as the concentration of substrate, [RX].

r = k[RX][Nu?]
This is a key difference between the SN1 and SN2 mechanisms. In the SN1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in SN2 the nucleophile forces off the leaving group in the limiting step. In other words, the rate of SN1 reactions depend only on the concentration of the substrate while the SN2 reaction rate depends on the concentration of both the substrate and nucleophile.

Answered by  | 21st Sep, 2011, 09:57: AM

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