Difference Between Acetylation and Methylation

The key difference between acetylation and methylation is that acetylation is a process of introducing an acetyl group to a molecule whereas methylation is the process of introducing a methyl group to a molecule. 

Acetylation and methylation are very important synthesis reactions that have many uses in industries. These reactions are helpful to form new compounds from molecules by introducing different functional groups. Acetylation and methylation can be found in biological systems as well.

CONTENTS

1. Overview and Key Difference
2. What is Acetylation
3. What is Methylation
4. Side by Side Comparison – Acetylation vs Methylation in Tabular Form
5. Summary

What is Acetylation?

Acetylation is the process of introducing an acetyl group to a molecule. Ac denotes an acetyl group, and it has the chemical formula –C(O)CH₃ in which an oxygen atom is bonded to the carbon atom via a double bond and a methyl group is attached to the carbon atom. This is a replacement reaction. It is also known as a substitution reaction because, in this reaction, the acetyl group replaces a functional that is already present in the molecule.

Most often, acetyl groups can replace reactive hydrogen atoms present in molecules. For example, hydrogen in –OH groups are reactive hydrogen. It is also possible to substitute this hydrogen atom with an acetyl group. This replacement causes the formation of an ester. That is because this replacement forms a –O-C(O)-O bond.

Figure 01: Acetylation of Salicylic Acid forms Acetylsalicylic Acid

Acetylation takes place in proteins commonly. And this process is known as the protein acetylation. Here, the N-terminal acetylation takes place via replacing a hydrogen atom of the –NH₂ group of the protein by an acetyl group. It is an enzymatic reaction because enzymes catalyze it.

What is Methylation?

Methylation is the process of introducing a methyl group to a molecule. Like in the process of acetylation, in methylation also a methyl group replaces a reactive atom. Therefore, it is a form of alkylation where alkylation is the substitution of an alkyl group.

Methylation occurs via two mechanisms;

1. Electrophilic methylation
2. Nucleophilic methylation

However, the electrophilic pathways are the most common way of doing methylation. But in Grignard reaction, aldehydes or ketones undergo methylation via nucleophilic addition. In these reactions, first, a metal ion combines with the methyl group. And it acts as the Grignard reagent.

Figure 02: Methylation of Cytosine

In biological systems, DNA methylation and protein methylation are common reactions. There a methyl group gets attached to a nitrogenous base of the DNA while in protein methylation, amino acids in the polypeptide chains attach with methyl groups.

What is the Difference Between Acetylation and Methylation?

Summary – Acetylation vs Methylation

Acetylation and methylation are very important reactions because they allow the formation of new compounds from the existing molecules via substitution (or sometimes via addition) of functional groups such as acetyl group and alkyl group. The key difference between acetylation and methylation is that the acetylation is a chemical reaction in which an acetyl group is introduced to a molecule whereas methylation is a chemical reaction in which a methyl group is introduced to a molecule.

Reference:

1. “Acetylation.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018. Available here  
2. “Grignard Reaction.” Wikipedia, Wikimedia Foundation, 18 Apr. 2018. Available here 

Image Courtesy:

1.’Acetylation of salicylic acid, mechanism’By Duldren – Composed in ChemDraw, (CC BY-SA 3.0) via Commons Wikimedia 
2.’DNA methylation’By Mariuswalter – Own work, (CC BY-SA 4.0) via Commons Wikimedia  

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