Acyl Groups
Acyl groups
- Acyl groups 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
- Acyl chlorides are named by identifying the parent hydrocarbon chain and adding the suffix -oyl chloride
- They can also be named by removing the -oic acid from the carboxylic acid and adding -oyl chloride
- Acid anhydrides are also derivatives of carboxylic acids formed by substitution of the -OH group by an alkanoate
- Acid anhydrides are named by identifying the parent hydrocarbon chain and adding the suffix -oic anhydride
- They can also be named by removing the -oic acid from the carboxylic acid and adding -oic anyhydride
Ethanoic acid derivatives
Worked Example
Draw the displayed formula for the following:a) butanoyl chlorideb) butanoic anhydride
Answer:
Nucleophilic Addition–Elimination
- Acyl chlorides are reactive organic compounds that undergo many reactions such as nucleophilic addition-elimination reactions
- In nucleophilic addition-elimination reactions, the nucleophilic addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule
- Examples of these nucleophilic addition-elimination reactions include:
- Hydrolysis
- Reaction with alcohols to form esters
- Reaction with ammonia and primary amines to form amides
Hydrolysis
- The hydrolysis of acyl chlorides results in the formation of a carboxylic acid and HCl molecule
- This is a nucleophilic addition-elimination reaction
- A water molecule adds across the C=O bond
- A hydrochloric acid (HCl) molecule is eliminated
- An example is the hydrolysis of propanoyl chloride to form propanoic acid and HCl
Acyl chlorides are hydrolysed to carboxylic acids
Formation of esters
- Acyl chlorides can react with alcohols to form esters
- The esterification of acyl chlorides is also a nucleophilic addition-elimination reaction
- The alcohol adds across the C=O bond
- A HCl molecule is eliminated
Acyl chlorides undergo esterification with alcohols to form esters
Formation of amides
- Acyl chlorides can 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 as
- The amine or ammonia molecule adds across the C=O bond
- A HCl molecule is eliminated
Acyl chlorides undergo reactions with ammonia and primary amines to form amides
Nucleophilic Addition–Elimination Mechanism
- Acyl chlorides undergo nucleophilic addition-elimination reactions such as hydrolysis, esterification reactions to form esters, and condensation reactions to form amides
- The general mechanism of these nucleophilic addition-elimination reactions involve two steps:
- Step 1 - Addition of a nucleophile across the C=O bond
- Step 2 - Elimination of a small molecule such as HCl or H2O
Mechanism of hydrolysis of acyl chlorides
- In the hydrolysis of acyl chlorides, the water molecule acts as a nucleophile
- Step 1 - Nucleophilic addition; the lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
- Step 2 - Elimination; this is followed by the elimination of a hydrochloric acid (HCl) molecule
Reaction mechanism of the hydrolysis of acyl chlorides
Formation of esters: reaction mechanism
- In the esterification reaction of acyl chlorides, the alcohols act as a nucleophile
- Step 1 - Nucleophilic addition; the lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
- Step 2 - Elimination; this is again followed by the elimination of an HCl molecule
Reaction mechanism of the esterification of acyl chlorides with alcohols
Formation of amides: reaction mechanism
- The nitrogen atom in ammonia and primary amines act as a nucleophile
- Step 1 - Nucleophilic addition; the lone pair on the nitrogen atoms carry out an initial attack on the carbonyl carbon
- Step 2 - Elimination; this is followed by the elimination of an HCl molecule
- Step 3 - Acid-Base reaction; the HCl formed would immediately react with excess ammonia to give ammonium chloride in an acid-base reaction
Reaction mechanism of the formation of amides from acyl chlorides with ammonia
Reaction mechanism of the formation of amides from acyl chlorides with primary amines
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