The acylation reaction, also known as acylation, is a key process in organic chemistry where hydrogen or other groups are replaced by acyl groups. The compound providing the acyl group is referred to as the acylating agent. The reaction can be expressed as:
RCOZ + SH → RCOS + HZ
In this formula:
- RCOZ represents the acylating agent, with Z often being OCOR, OH, ORˊ, etc.
- SH represents the acylate, with S denoting RO, R″NH, Ar, etc.
Reaction Mechanism
- Nucleophilic Mechanism:
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- Found in O-acylation, N-acylation, and C-acylation at the α-position of carbonyl groups.
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Electrophilic Mechanism:
- Seen in the C-acylation of aromatics and alkenes.
Acylation Reaction on Oxygen Atoms
Definition: Acylation on oxygen atoms occurs when the hydroxyl hydrogen atom in alcohol or phenol is replaced by an acyl group, forming an ester. This process is also known as esterification.
Reaction Rules:
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Primary alcohols react more easily, followed by secondary alcohols; tertiary alcohols are the hardest to acylate.
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Certain primary alcohols, such as benzyl and allyl alcohols, exhibit properties similar to tertiary alcohols due to their ease of forming stable carbocations.
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Phenolic hydroxyl groups are less reactive than alcohols due to the aromatic ring's influence.
Acylating Agents: Common agents include carboxylic acids, esters, anhydrides, acid chlorides, and ketenes.
Using Carboxylic Acid as an Acylating Agent
Carboxylic acids are weak acylating agents, and their reaction with alcohols is a reversible equilibrium process, catalyzed by acid.
Catalyst Options
1. Protonic Acids
Examples include concentrated sulfuric acid, perchloric acid, tetrafluoroboric acid, and p-toluenesulfonic acid.
- Concentrated sulfuric acid is widely used for its catalytic activity and water absorption.
- Organic acids like p-toluenesulfonic acid are ideal for alcohols sensitive to inorganic acids.
2. Lewis Acids
Common examples include BF₃, AlCl₃, and ZnCl₂. Lewis acids provide high yields, good product purity, and reduced side reactions.
3. Strong Acidic Ion Exchange Resins
These resins dissociate H⁺ ions, offering benefits like fast reactions, mild conditions, high yield, and recyclability.
4. Dicyclohexylcarbodiimide (DCC)
DCC and its analogs are dehydrating agents that enhance acylation activity, forming esters via intermediates with acid anhydrides.
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