Acylation Reaction

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What is Acylation?

In Organic Chemistry, acylation (or alkanoylation) is the method of including an acyl group to a compound. The compound offering the acyl group is known as the acylating agent.

Acyl Halides are commonly used as acylating agents as they form a strong electrophile on reaction with some metal catalysts. For example, Friedel-crafts acylations make use of acetyl chloride (ethanoyl chloride), Aluminum chloride (AICI₃) as the catalyst, and CH₃COCI as an agent to include an ethanol (acetyl) group to benzene.

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The mechanism of the above reactions is considered as Electrophilic Aromatic Substitution.

Anhydrides and Acyl Halides of the carbolic acid are primarily used as acylating agents for acylating amines to form amides and alcohols to form esters through nucleophilic acyl substitution. 

Acylations can be used to avoid rearrangement reactions that generally appear in alkylation. To perform this an acylating reaction is executed, then the carbonyl group is separated by Clemmensen reduction or by other related processes.


Acylation Reaction Process

Electrophilic aromatic substitution is the process of the Acylation reaction. In Organic Chemistry, acylation is the process of including acyl groups (RCO) to a specific compound. This particular compound which provides an acyl group is known as the acylating agent.

As these form strong electrophiles when included with some of the metal catalysts, the acyl halides are primarily used as acylating agents. For example, the Friedel-Crafts acylation reaction uses acetyl chloride, CH3COCI acts as the acylating agent, and the compound known as aluminum chloride (AlCl3),  acts as the catalyst for including ethanol or the acetyl group to benzene. The below reactions show this process. 

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An acyl group is a part of the alkyl group which is linked to the carbon-oxygen double bond as shown in the figure given below. If the alphabet ‘R’ denotes the alkyl group, the acyl group will have the formula RCO-. The mechanism of Acylation is to place an acyl group into the reaction. The most commonly used acyl group is CH3CO. It is known as the ethanoyl group. 

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The anhydrides and the Acyl halides of the carboxylic acids are primarily used as the acylating reagents for acylating amines generating amides or also acylate alcohols which set up the esters. The alcohols and amines are nucleophilic acyl substitution reactions. Succinic acid is primarily used in different types of acylation reactions. Acylation forms when two or more than two salts of succinic acid are included to form a single compound. 

The acylation process is used in avoiding the rearrangement reactions which appears in the mechanism of alkylating. For attaining this, the acylation reaction primarily occurs, later the carbonyl group is separated by the Clemmensen reduction reaction or some related process. 


Acylation Reaction - Example

A simpler form of Acylation reaction example is discussed below.

The CH₃COOH or the acetic acid, vinegar which is the solution of 5% of the acetic acid in water are included in the form of acetyl chloride, having the chemical structure CH₃-COCI.

This mechanism of Acylating benzene with such substances, in the presence of catalyst aluminum chloride, obtains the below reaction.

CH₃-COCI + C₆H₆ + Catalyst? C₆H₅-CO-CH₃ + HCI 

The above reactions describe that one molecule of benzene and one molecule of acetyl chloride in the presence of catalyst aluminum chloride is said to obtain one molecule of methyl phenyl ketone and one molecule of hydrogen chloride gas. The ketones are of utmost importance as compounds providing a range of reaction paths for the further synthesis process


What is the Friedel-Craft Acylation?

The Friedel-Crafts acylation reaction additionally includes the acyl group to an aromatic ring. Generally, this is performed by utilizing an acid chloride (R-(C=O)-Cl) and a Lewis acid catalyst such as AlCl₃. In a Friedel-Crafts acylation reaction, the aromatic ring is transferred into a ketone. The reaction between benzene and an acyl chloride under these conditions is given below.

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An acid anhydride can be used as the substitution to the acyl halide in Friedel-Crafts acylations. The halogen related to the acyl halide forms a complex with the Lewis acid, forming a highly electrophilic acylium ion, which includes the general formula of RCO+ and is settled by resonance.


Friedel-Craft Acylation Process

The Friedel-Craft Acylation process includes four steps. These steps are discussed below.

1. A reaction takes place between the Lewis acid catalyst (AlCl3) and the acyl halide. A complex is formed and the acyl halide removes a halide ion, shaping an acylium ion that is secured by resonance.

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2. The acylium ion (RCO+) moves away to implement an electrophilic attack on the aromatic ring. The aromaticity of the ring is removed for a short period of time as a complex is formed.

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3. The transitional complex is now deprotonated, replacing the aromaticity to the ring. This proton joins itself to a chloride ion (through the complex Lewis acid), forming hydrogen chloride (HCl). The Aluminium Chloride (AlCl₃) catalyst is now reformed or regenerated.

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4. The regenerated catalyst can now strike the carbonyl oxygen. Hence, the ketone product must be released by including water to the products formed in step 3. This step can be represented as follows.

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Hence, the required acyl benzene product is formed through the Friedel-Crafts acylation reaction.