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Carboxylic acids & Derivatives
Outline the mechanism of nucleophilic addition–elimination reactions of acyl chlorides with water, alcohols, ammonia and primary amines.
The structures of:
The nucleophilic addition–elimination reactions of water, alcohols, ammonia and primary amines with acyl chlorides and acid anhydrides.
The industrial advantages of ethanoic anhydride over ethanoyl chloride in the manufacture of the drug aspirin.
Acyl groups are functional groups that can be built into many molecules using acyl chlorides or acid anhydrides (known as acylating agents). Acyl chlorides are derivatives of carboxylic acids by substitution of the -OH group by a chlorine atom. They can be named by identifying the parent hydrocarbon chain and adding the suffix -oyl chloride, or by removing the -oic acid from the carboxylic acid and adding -oyl chloride.
Acid anhydrides are also derivatives of carboxylic acids formed by the substitution of the -OH group for an alkanoate. They can be named by identifying the parent hydrocarbon chain and adding the suffix -oic anhydride, or by removing the -oic acid from the carboxylic acid and adding -oic anhydride.
Acyl chlorides are reactive organic compounds that undergo many reactions such as nucleophilic addition-elimination reactions. Nucleophilic addition-elimination reactions involve the nucleophilic addition of a small molecule, followed by the elimination of a small molecule. Examples of these reactions include hydrolysis, reaction with alcohols to form esters, and reaction with ammonia and primary amines to form amides.
In the hydrolysis of acyl chlorides, the water molecule acts as a nucleophile and the reaction results in the formation of a carboxylic acid and HCl molecule. This is a nucleophilic addition-elimination reaction, where the water molecule adds across the C=O bond and a hydrochloric acid (HCl) molecule is eliminated.
Acyl chlorides can also react with alcohols to form esters in a reaction called esterification. The esterification of acyl chlorides is also a nucleophilic addition-elimination reaction, where the alcohol adds across the C=O bond and an HCl molecule is eliminated.
Acyl chlorides can also form amides with primary amines and ammonia. The nitrogen atom in ammonia and primary amine has a lone pair of electrons which can be used to attack the carbonyl carbon atom in the acyl chlorides. The product is an amide (when reacted with ammonia) or N-substituted amide (when reacted with primary amines). This is also an example of a nucleophilic addition-elimination reaction, where the amine or ammonia molecule adds across the C=O bond and an HCl molecule is eliminated.
The general mechanism of these nucleophilic addition-elimination reactions involve two steps:
• Step 1: Addition of a nucleophile
• Step 2: Elimination of a small molecule such as HCl or H2O.
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