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 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:

• A nucleophile attacks the δ⁺ carbon atom in the acyl group.
• 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 C=O bond 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.
  
  

  Practice questions

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

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