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Fischer esterification mechanism

Overview:

The general form of Fischer esterification mechanism is as follows:

The first step involves protonation of the carbonyl oxygen, followed by the nucleophillic attack of the alcohol.

Then a loss and regain of a proton,

followed by loss of water as electrons from the alcohol oxygen kick down to form the double bond. Loss of a proton yields the ester.

Example of fischer esterification:

Williamson ether synthesis

Overview:

The Williamson ether synthesis is a reaction that converts alcohols (R-OH) into ethers (R-O-R). The first step in this reaction is forming the conjugate base of the alcohol (called an alcoxide) by reacting the alcohol with sodium metal. This reaction forms hydrogen gas (H₂) as a biproduct, so if you perform this reaction take caution to keep all flame sources away during sodium addition.

The alcoxide can then be added to a suitable alkyl halide (typically a primary halide) to form the ether via an S_N2 mechanism.

Example:

An example of the Williamson ether synthesis to make diethyl ether.

Friedel-Crafts alkylation

Overview:

The general form of the Friedel-Crafts alkylation mechanism is as follows:

Adding an alkyl halide to the Lewis acid aluminum trichloride results in the formation of an organo-metallic complex. In this complex the carbon attached to the chlorine has a great deal of positive charge character (in fact, for practical purposes when dealing with this reaction, you can think of the partially positive charge as a carbocation).

The pi electrons in a benzene ring are mildly electrophilic, and can attack the partially positive carbon to create a non-aromatic intermediate (note that this intermediate has several resonance structures, so that it is not as unstable as it might appear). Elimination of a proton re-establishes the aromaticity of the ring, and the aluminum trichloride catalyst is regenerated along with a molecule of hydrochloric acid.

A word of caution about this reaction: because the aluminum trichloride generates what can essentially be thought of as a carbocation, rearrangments can occur to produce a more highly-substituted carbocation.

For example: Addition of 1-Chloro-2-Methylpropane to benzene with aluminum trichloride results in the rearranged product, t-butyl benzene, and not the product that you might initially expect (work out the mechanism if you cannot see how that product is attained).

An example of a Friedel-Crafts alkylation:

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