Blanc reaction

What is Blanc reaction?

The Blanc reaction, also known as chloromethylation reaction, is a chemical process that involves the reaction of an organic compound with formaldehyde and hydrochloric acid to form a chloromethylated compound. The reaction is named after the French chemist Charles Blanc, who first described it in 1864..

Blanc reaction chloromethylation
Blanc reaction

The Blanc reaction is a versatile process that can be used to chloromethylate a wide range of organic compounds, including alkenes, alkynes, and aromatics. The reaction typically proceeds via an electrophilic substitution mechanism, in which the electrophile is the chloromethyl cation and the nucleophile is the organic compound..

One of the key advantages of the Blanc reaction is its high selectivity. It can produce a wide range of different chloromethylated compounds from a single starting material, and it typically provides high yields of the desired product. The reaction also allows for the synthesis of a wide.range of different functional groups, including ethers, esters, and amides..

The Blanc reaction is widely used in organic synthesis and is particularly useful for the preparation of chloromethylated compounds, which are important building blocks for many organic compounds, such as pharmaceuticals and agrochemical compounds..

The reaction is typically performed in the presence of an acid catalyst, such as sulfuric acid or hydrochloric acid. The reaction is highly exothermic, so the reaction must be cooled to prevent overheating (typically carried out at a temperature between 50-80°C for several hours). The reaction conditions are relatively mild, and the reaction can be performed at room temperature..

Blanc reaction remains an important tool due to its high selectivity and efficiency. Thus, Blanc reaction has played an important role in the development of many new compounds and pharmaceuticals..

In recent years, alternative chloromethylation methods have been developed, such as the use of siloxanes or the use of formaldehyde alternatives, such as paraformaldehyde. However, the Blanc reaction still remains an attractive method due to its versatility, mild reaction conditions and high selectivity..

The reaction has some limitations, as it can only be used with aldehydes and ketones, and the formaldehyde must be used in high excess and the reaction is highly exothermic, so the reaction must be cooled to prevent overheating..


Blanc reaction, also known as chloromethylation, is a chemical reaction used to introduce a chloromethyl group (-CH2Cl) onto an organic compound. It is named after the French chemist Charles Blanc, who first described it in 1851. It has many applications in organic synthesis and pharmaceuticals, but it has some limitations as well..


One example of the Blanc reaction is the chloromethylation of benzene to form chloromethylbenzene. The reaction is typically performed using formaldehyde and hydrochloric acid as the reagents..

The reaction proceeds through the formation of paraformaldehyde and methanol from the reaction between formaldehyde and hydrochloric acid, which in turn reacts with benzene to form chloromethylbenzene:

H2SO4 + HCHO → CH2O + H2O + H2SO4

CH2O + C6H6 → C6H5CH2Cl + H2O

The resulting product, chloromethylbenzene, can be isolated and purified by distillation..

Mechanism of reaction

The mechanism of the Blanc reaction, also known as chloromethylation, can be divided into several steps:

Step 1: The first step is the formation of paraformaldehyde from formaldehyde and hydrochloric acid. This is an acid-catalyzed condensation reaction, where formaldehyde reacts with itself in the presence of an acid catalyst to form paraformaldehyde:

H2SO4 + HCHO → CH2O + H2O + H2SO4

Step 2: The second step is the addition of paraformaldehyde to the organic compound to form an intermediate adduct. This step is a cycloaddition reaction, where the electrophilic carbon atom of paraformaldehyde attacks the electron-rich double bond of the organic compound:

CH2O + C6H6 → C6H5CH2O

Step 3: The final step is the deprotonation of the intermediate adduct to form the chloromethylated product. This step is acid-catalyzed, where the intermediate adduct loses a proton from the oxygen atom to form the chloromethylated product and water:

C6H5CH2O → C6H5CH2Cl + H2O