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S_N2 Mechanism

Overview:

The general form of the S_N2 mechanism is as follows:

nuc: = nucleophile
X = leaving group (usually halide or tosylate)

The S_N2 reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. This reaction works the best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides. In the following example, the hydroxide ion is acting as the nucleophile and bromine is the leaving group:

Because of the backside attack of the nucleophile, inversion of configuration occurs.

Solvents: Protic solvents such as water and alcohols stabilize the nucleophile so much that it won't react. Therefore, a good polar aprotic solvent is required such as ethers and ketones and halogenated hydrocarbons

.

Nucleophiles: A good nucleophile is required since it is 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.

S_N1 Mechanism

Overview:

The general form of the S_N1 mechanism is as follows:

Because the mechanism goes through a carbocation, the leaving group must be attached to either a tertiary or secondary carbon to stabilize the intermediate. A methyl or primary leaving group will not form a carbocation. Since it goes through a carbocation intermediate, there are possibilities for alkyl and hydrogen rearrangements (HINT: In mechanism problems if you see a change in the carbon skeleton between the reactant and the product, automatically suspect a carbocation intermediate (ie, E1, Sn1) stabilized by an alkyl or hydrogen rearangement). .

An example ofthe Sn1 Mechanism

Base Strength: Base strength is unimportant, since the base is not involved in the rate determining step (the formation of the carbocation). .

Leaving groups: A good leaving group is required, such as a halide or a tosylate, since the leaving group is involved in the rate-determining step.

Notes: Be wary of rearangements that can occur with the S_N1 reaction. Because it goes through a carbocation intermediate, both hydrogen shifts and alkyl shifts can occur!

E1 Mechanism

Overview:

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:

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.

E2 Mechanism

Overview:

The general form of the E2 mechanism is as follows:

B: = base
X = leaving group (usually halide or tosylate)

In the E2 mechanism, a base abstracts a proton neighboring the leaving group, forcing the electrons down to make a double bond, and, in so doing, forcing off the leaving group. When numerous things happen simultaneously in a mechanism, such as the E2 reaction, it is called a concerted step.

An example of the E2 reaction:

Base Strength: A strong base is required since the base is 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.

Stereochemistry requirements: Must occur with antiperiplanar stereochemistry

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