E1, E2 and SN1,SN2 Mechanisms.

This power point file is about nucleophilic substitution and elimination reaction.

1. E1 REACTION The general form of the E1 mechanism is as follows B : = base X = leaving group (usually halide or tosylate) In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation. The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond. Since this mechanism involves the formation of a carbocation, rearangements can occur An example of the E1 reaction:

2. E1 REACTION (Cont. ) Base Strength : A strong base not required, since it is not involved in the rate-determining step Leaving groups : A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step. Rearangements : Since the mechanism goes through a carbocation intermediate, rearangements can occur.

3. E2 REACTION The term E2 stands for "elimination bimolecular." Like any elimination reaction, the product of an E2 elimination reaction has one more degree of unsaturation than the starting materials did. For instance, the base-induced elimination of "HX" of an alkyl halide gives rise to an alkene .

4. E2 REACTION

5. S N 1 reaction The S N 1 reaction is a substitution reaction in organic chemistry. "S N " stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular . Mechanism An example of a reaction taking place with an S N 1 reaction mechanism is the hydrolysis of tert -butyl bromide with water forming tert -butanol .

6. S N 1 reaction This S N 1 reaction takes place in three steps: Formation of a tert -butyl carbocation by separation of a leaving group (a bromide anion) 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 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.

7. S N 1 reaction 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.

8. S N 2 reaction The is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i.e., in one step. S N 2 is a kind of nucleophilic substitution reaction mechanism.

9. S N 2 reaction The term S N 2 means that two molecules are involved in the actual transition state. The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile. The S N 2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion (reversal of the configuration). In the S N 1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization