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Answer to Question #31988 in Organic Chemistry for naeim

Question #31988
What is the difference between SN2 reaction and E2 reaction?
Expert's answer
The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, this leads to the name bimolecular nucleophilic substitution, or SN2. The somewhat more transparently named analog to SN2 among inorganic chemists is the interchange mechanism.

The reaction most often occurs at an aliphatic sp3 carbon center. 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 pentavalent, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C-X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed.

E2 mechanism:

* It is a one-step process of elimination with a single transition state.
* Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
* The reaction rate both influenced by the alkyl halide and the base is second order.
* Because E2 mechanism results in the formation of a Pi bond, the two leaving groups (often a hydrogen and a halogen) need to be coplanar. An antiperiplanar transition state has staggered conformation with lower energy and a synperiplanar transition state is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions.
* Reaction often present with strong base.
* In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp3 to sp2.

If the carbon you're attacking is tertiary then the reaction is E2.
If the carbon is secondary, it's a mixture of both.
If the carbon is primary, it's MOST LIKELY to undergo SN2 reaction.

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