Answer to Question #31988 in Organic Chemistry for naeim

Question

What is the difference between SN2 reaction and E2 reaction?

Answer 1

The S_N2 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 S_N2. The somewhat more transparently named analog to S_N2 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 S_N2 reaction.