What is Acetylation?

Acetylation is a chemical reaction in which a hydrogen atom is substituted for an acetyl group (CH3C=O group) in a compound. The products formed in acetylation reactions typically have an acetoxy functional group. When the hydrogen atom belonging to an alcohol group replaced with an acetyl group in an acetylation reaction, an ester is formed as the product. For such reactions (where the reactant compound contains free hydroxyl groups), the most commonly used acetylating agent is acetic anhydride. An example of an acetylation reaction is illustrated below.


Acetylation of Salicylic Acid

Here, salicylic acid is subjected to acetylation with the help of acetic anhydride to yield acetylsalicylic acid (commonly known as aspirin) and acetic acid as the final product. It can be noted that acetic anhydride is also used as an acetylating agent in the synthesis of THC acetate ester and diacetylmorphine.

Mechanism of Acetylation of Salicylic Acid

The acetylation of salicylic acid occurs in an acidic medium. Here, the acidic medium (usually concentrated sulfuric acid) aids in the detachment of the acetate ion from the acetic anhydride. This acetate ion goes on to associate with the proton liberated by the phenolic OH group (hydroxy group) of the salicylic acid molecule, resulting in the formation of acetic acid (CH3COOH). The mechanism of the acetylation reaction undergone by salicylic acid to afford aspirin and acetic acid as the product is illustrated below.

Acetylation Mechanism

Mechanism of Acetylation of Salicylic Acid to Yield Aspirin

First, one oxygen atom of the acetic anhydride is protonated by the acid. The resulting positive charge is relayed to the adjacent carbonyl carbon, which binds itself to the phenolic hydroxy group. This intermediate is now deprotonated and one acetic acid molecule detaches itself from it. Thus, the required products (aspirin and acetic acid) are formed.

Other Notable Examples of Acetylation Reactions

  • Acetylation of aniline with acetic anhydride (in the presence of glacial acetic acid) to afford acetanilide.
  • N-terminal acetylation, which is a very common type of co-transitional covalent modification of proteins, especially in eukaryotes.
  • Histone acetylation and deacetylation (removal of an acetyl group), which the lysine residues that are present inside the N-terminal tail that protrudes from the histone core undergo acetylation or deacetylation in gene regulation activities.

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