Difference Between Alkylation and Acylation

Main Difference – Alkylation vs Acylation

Alkylation is a transfer of alkyl group from one molecule to another using an alkylating agent. These alkylating agents have the ability to add a desired aliphatic hydrocarbon chain to the starting material. Unlike alkylation, acylation is the process of adding an acyl group to a compound using an acylating agent. These acylating agents have the ability to add desired RCO- group to the starting material.  This is the main difference between Alkylation and Acylation.

What is an Alkylation

Alkylation is the process of introducing the hydrocarbon chain to the starting material. Hydrocarbons are the most common type of organic compounds, which consist of carbon and hydrogen atoms. The addition of one carbon atom (methyl group) to the starting material is known as methylation.

The alkyl group can be transferred as an alkyl carbocation, a free radical, a carbanion or carbine. Therefore, the alkylating agents can be mainly divided into two categories based on their electrophilic and nucleophilic character. Nucleophilic alkylating agents form an alkyl anion (carbanion) during the reaction and attack electron deficient carbon atom such as carbonyl group. (Ex: Grignard, organolithium, organocopper, and organosodium reagents). Electrophilic alkylating agents form an alkyl cation (carbocation) during the reaction (Ex: alkyl helides). The following reactions illustrate the mechanism for Friedel-crafts alkylation of benzene. 

Step 1: The alkyl halide reacts with Lewis acid to create more electrophilic carbon.Difference Between Alkylation and Acylation - image 1

Step 2: Removal of halide creates an alkyl carbocation.

Difference Between Alkylation and Acylation - image 2

Step 3: The π electrons in aromatic ring act as a nucleophile, and attacks to carbocation with losing the aromaticity.Difference Between Alkylation and Acylation

 

Step 4: Removal of proton regenerates the aromatic system

Difference Between Alkylation and Acylation

What is an Acylation

Acylation is the process of adding an acyl group to the starting material using an acylating agent. An acyl group is a functional group that has the molecular formula of the RCO.

The acylating agents form strong electrophiles when treated with a metal catalyst, and easily undergo electrophilic substitution. Acyl halides are the most commonly used acylating agents, and they produce ketones by electrophilic substitution. In addition, acyl halides and anhydrides of carboxylic acids are used as acylating agents to acylate amines and alcohols by nucleophilic substitution. Following reactions illustrate the mechanism for Friedel-crafts acylation of benzene.

Step 1: The acyl halide reacts with Lewis acid to create a complex.Difference Between Alkylation and Acylation - image 5

Step 2: The loss of halide from acyl halide creates electrophilic acylium ion.

Difference Between Alkylation and Acylation - image 6

Step 3: π electrons in benzene acts as a nucleophile and attacks to electrophilic acylium ion. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.Difference Between Alkylation and Acylation - image 7

Step 4: Removal of proton regenerate the aromatic system and active catalyst.Main Difference - Alkylation vs Acylation

Difference Between Alkylation and Acylation

Definition

Alkylation: Alkylation is the process of introducing the hydrocarbon chain to the starting material.

Acylation: Acylation is the process of adding an acyl group to the starting material using an acylating agent.

Overall Transformation

Alkylation: The overall transformation is R-H to R-R’.

Acylation: The overall transformation is R-H to R-COR’.

Reagents

Alkylation: Generally alkyl halides (i.e. R-Cl) and lewis catalyst such as aluminum trichloride (i.e. AlCl3) can act as reagents. Alternatively, organo-metal complexes i.e. R-MgBr can be used instead of alkyl halides. In addition, BF3, ZnCl2, FeCl3 can be used instead of AlCl3.

Acylation: Generally acyl halides (i.e. R-COCl) and lewis catalyst such as aluminum trichloride act as reagents. Alternatively, acid anhydrides i.e. (RCO)2O can be used instead of acyl halides.

Electrophilic Species

Alkylation: The carbocation (i.e. R +) is formed by the “removal” of the halide by the Lewis acid catalyst.

Acylation: The acyl cation or acylium ion (i.e. RCO ) is formed by the “removal” of the halide by the Lewis acid catalyst.

Rearrangement of Carbocation

Alkylation: The carbocation is prone to rearrange and form a very stable carbocation, which will undergo the alkylation reaction.

Acylation: The acylium ion is stabilized by the resonance structures. This extra stability prevents the rearrangement of carbocation.

Friedel-Craft Reactions

Alkylation: Vinyl or aryl halides do not undergo alkylation reaction because their intermediate carbocation is unstable.

Acylation: Acylation reactions always give ketones, because HCOCl decomposes to CO and HCl under the reaction conditions.Difference Between Alkylation and Acylation - infographic

About the Author: Nuwa

Nuwa has a B.Sc. special degree in Chemistry. She is currently a teaching assistant at Open University, where she teaches, researches, and writes chemistry content. She has worked previously as a Chemical Analyst.


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