nucleophilic addition-elimination reactions

The acyl group

The acyl group (R-C=O) is perhaps one of the most versatile groups in organic chemistry, in that it can be readily and easily converted into a large variety of other function groups to produce a wide variety of useful organic molecules. Some of these molecules are shown below:

acyl derivatives

Nucleophilic addition-elimination mechanism

The acyl group contains a polar carbonyl group (C=O), with a δ+ carbon atom and a δ- oxygen atom. The δ+ carbon atom will therefore be susceptible to attack by a wide range of nucleophiles to produce the range of compounds shown above. The mechanism for the reactions which convert the acyl group into the compounds above is called nucleophilic addition- elimination (though the overall result of addition followed by elimination can be described as substitution), it is outlined below but it can be thought of as occurring in a number of steps:

We can summarise this as addition of a nucleophile followed by the elimination of a leaving group. This addition step followed by an elimination step is simply substitution overall, where the nucleophile basically takes the place or substitutes for the leaving group. A good leaving group is one that is able to form a stable anion (ion with a negative charge).

mechanism of nucleophilic acyl substitution

By simply changing the nucleophile (Nu) we can produce a wide range of organic compounds containing the reactive acyl group. These include acid halides, acid anhydrides, ester and amides.

Key Points