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:

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 organic 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:

• Step 1: The nucleophile attacks the δ⁺ carbon atom in the acyl group.
• Step 2: The attacking nucleophile forms a new bond to the δ⁺ carbon atom in the carbonyl group creating a tetrahedral intermediate with a negatively charged oxide ion.
• A lone pair of electrons on the oxide ion then reforms the carbonyl bond (C=O) and a leaving group is eliminated.

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).

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

• The acyl group is a very reactive and versatile group in organic chemistry simply because it can be easily converted into a wide range of other functional groups which also contain an acyl group, these include:
• carboxylic acids
• acid halides
• acid anhydrides
• esters
• amides

Nucleophilic addition-elimination involves:

• The attack of a nucleophile on the δ⁺ carbon atom in the acyl group followed by the elimination of a leaving group.
• The leaving group is one that can stabilise a negative charge; that is it can readily form a stable anion.

Check your understanding - Questions on nucleophilic addition-elimination reactions.

Next