Etard reaction is a chemical reaction used to oxidize aromatic compounds containing methyl groups. An Etard reaction was given by a famous French chemist named Alexandre Léon Etard. Etard proposed the partial oxidation reaction of aromatic rings containing methyl groups. This organic reaction is carried out using chromyl chloride, which is also called Etard reagent.
The reactant involved in the Etard reaction is an aromatic ring consisting of a methyl group as a substituent. The saturated aqueous sodium sulfite solution is used to provide the reducing environment so that the etard complex can undergo decomposition. Chloroform and carbon disulfide is used as solvents in the etard reaction.
The alkene-allylic hydrogen of the aromatic ring reacts with the Etard reagent (chromyl chloride), resulting in the formation of the Etard complex precipitate. To prevent itself from getting oxidized into a carboxylic acid, the etard complex undergoes decomposition in the presence of a reducing environment. To obtain the aldehyde product in pure form, the formed etard complex must be purified before the decomposition reaction. The overall time frame required for the etard reaction may range from a few days to multiple weeks. However, high yields of the aldehyde product can be obtained.
To understand the mechanism of the etard reaction, let us consider the example of methyl benzene (toluene).
Toluene, the presence of chromyl chloride and carbon tetrachloride, undergoes a parietal oxidation reaction resulting in the formation of Benzaldehyde.
The mechanism of the etard reaction can be summarized into the following five steps.
Step 1: Homolytic cleavage of bonds in toluene
The reaction of Toluene with Chromyl chloride causes homolytic cleavage of 4 bonds. Two C-H (Carbon-Hydrogen) bonds in the methyl group attached to the benzene undergo a homolytic cleavage. Two Cr-O (Chromium-Oxygen) bonds present in the chromyl chloride also undergo a homolytic cleavage.
Step 2: Formation of etard complex intermediate
Once the cleavage of bonds takes place, the formation of the etard complex takes place. The Hydrogens released from the methyl group due to homolytic cleavage undergo bond formation with the oxygens of chromyl chloride. This bond formation results in the formation of two OCr(OH)Cl2 molecules. These OCr(OH)Cl2 attach themselves to the carbon atom of the methyl group present in toluene via second oxygen. It means that OCr(OH)Cl2 has substituted the hydrogens that have undergone the homolytic cleavage. Thus, the two hydrogens separated from the methyl group by homolytic cleavage are occupied by two OCr(OH)Cl2 molecules. This compound is unstable and is called the etard complex intermediate.
Step 3: Hydrolysis of the etard complex
The role of sodium sulfite used in the reaction is to prevent the further oxidation of the etard complex. The unstable etard complex undergoes hydrolysis resulting in the formation of two OCr(OH)2Cl2 groups.
Step 4: Formation of Benzaldehyde
The so formed OCr(OH)2Cl2 groups attached to the aromatic ring get separated, leaving behind the aldehyde product, i.e., benzaldehyde.
Step 5: Purification of Benzaldehyde
The etard complex is volatile. To protect itself from getting oxidized, it undergoes a decomposition reaction under the influence of a reducing environment. The etard complex gets decomposed with the help of a pericyclic reaction called [2,3] Sigmatropic Rearrangement. To protect the product from decomposition, it is purified using different solvents. The purification of this solution yields a high-quality end product, i.e., pure benzaldehyde.
Etard reaction is one of the most commonly used chemical reactions in organic chemistry. Etard reagent is again widely used for numerous organic reactions. Here are a few significant applications of the Etard reaction.
Etard reaction is one of the direct methods used to oxidize toluene into Benzaldehyde partially. However, like any other chemical reaction, the Etard reaction also comes with a few limitations, which are as follows.