CBSE Class 12-science Answered
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 | 21 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?]
Answered by | 21 Sep, 2011, 09:57: AM
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