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Q8.16(i):
Describe the following: (i) Acetylation

Solution :

Definition and Theoretical Foundation

Acetylation is a specific type of acylation reaction in organic chemistry where an acetyl group ($\text{CH}_3\text{CO}-$) is introduced into a molecule by substituting a hydrogen atom, typically attached to an oxygen ($-\text{OH}$) or nitrogen ($-\text{NH}_2$ or $-\text{NHR}$) atom. This process replaces the active hydrogen with the acetyl moiety, forming an ester or an amide, respectively [Per the Nucleophilic Acyl Substitution mechanism].

Chemical Equation and Mechanism

In the context of aldehydes and ketones, or related functional groups, acetylation often involves nucleophilic attack on an acetylating agent such as acetic anhydride $(\text{CH}_3\text{CO})_2\text{O}$ or acetyl chloride $\text{CH}_3\text{COCl}$.

The general reaction for an alcohol (which acts as a nucleophile) to form an acetate ester is represented as:

$\text{R-OH} + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{Pyridine}} \text{R-O-COCH}_3 + \text{CH}_3\text{COOH}$

Visual Representation of Acetylation (Alcohol to Ester)

R-OH + CH₃CO-X R-O-COCH₃ + HX (X = OCOCH₃ or Cl)

Step-by-Step Analytical Process

  • Step 1: Nucleophilic Attack: The oxygen atom of the hydroxyl group (or nitrogen of the amine) acts as a nucleophile, possessing a lone pair of electrons. It attacks the electrophilic carbonyl carbon of the acetylating agent [By Lewis Base theory].
  • Step 2: Tetrahedral Intermediate Formation: The $\pi$-bond of the carbonyl group of the acetylating agent breaks, shifting electrons to the oxygen atom, creating a transient tetrahedral intermediate.
  • Step 3: Elimination of Leaving Group: The electrons from the negatively charged oxygen collapse back to reform the $\text{C=O}$ double bond, resulting in the expulsion of a leaving group (e.g., acetate ion or chloride ion) [Per the Addition-Elimination Mechanism].
  • Step 4: Deprotonation: A base, commonly pyridine, removes the proton from the intermediate to restore the final ester or amide structure.

Comparative Analysis Table

Feature Acetylation (O-Acetylation) Acetylation (N-Acetylation)
Substrate Alcohol/Phenol Amine/Ammonia
Product Ester Amide
Reagent $\text{CH}_3\text{COCl}$ or $\text{Ac}_2\text{O}$ $\text{CH}_3\text{COCl}$ or $\text{Ac}_2\text{O}$

Final Solution: Acetylation is the process of introducing an acetyl group $(\text{CH}_3\text{CO}-)$ into a chemical compound via nucleophilic acyl substitution, resulting in the formation of an ester or amide depending on the nucleophilic starting material.


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