What is the major product when the following compound undergoes Étard reaction?

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Is it acetophenone or a terminal aldehyde. The mechanism discussed in Mechanism for an Étard reaction (chromium complex) does not give any conclusive prediction.


A mechanism has been proposed herein for the Étard oxidation of ethylbenzene that parallels the proposed oxidation of toluene with chromyl chloride (CrO2Cl2) as a [2,3]-sigmatropic rearrangement that is found on Wikipedia.[1] While such a mechanism is a convenient way to rationalize the formation of benzaldehyde, it does not consider the “Étard complex” that has a composition, in most cases, of RH/2CrO2Cl2 or greater proportion of CrO2Cl2.[2] This mechanism is incompatible with the Étard oxidation of n-propylbenzene and ethylbenzene, both of which studies are cited on Wikipedia. The products derived from these higher homologs reveal both α- and β- carbon oxidation.
Nenitzescu and co-workers[3] demonstrated that the Étard oxidation of ethylbenzene affords acetophenone and phenylacetaldehyde, which answers the OP’s question. The following summary is from Chemical Abstracts:

"The Etard complex of ethylbenzene (I) was prepd. in CCl4 at 0°. Decompn. of the Etard complex of I by water yielded phenylacetaldehyde (II) and acetophenone (III) in equal proportions, along with styrene and BzH.”[3]

That n-propylbenzene 1a affords benzyl methyl ketone 2a and propiophenone 3 has been known since 1890.[4] Wiberg, et al.[5] explored the mechanism of this reaction using n-propyl-β-d2-benzene 1b. Oxidation of 1b provided benzyl methyl ketone 2b containing 60% deuterium at the α-position, which indicated a rearrangement was occurring. The presence of α-chloro-n-propylbenzene 5 was also observed. These investigators demonstrated that oxidation of propenylbenzene 4 gave rise to benzyl methyl ketone 2a.
The evidence points to either a radical process[5,6] or one that invokes hydride transfer,[2] i.e., cationic, at the benzylic carbon. There is little evidence, if any, for a [2,3]-sigmatropic rearrangement in this reaction. If there is, please apprise me.

  1. https://en.wikipedia.org/wiki/%C3%89tard_reaction
  2. I. Necsoiu, A. T. Balaban, I. Pascaru, E. Sliam, M. Eliam, C. D. Nenitzescu, Tetrahedron, 1963, 19, 1133.
  3. C. N. Rentea, M. Rentea, I. Necsoiu and D. Nenitzescu, Revue Roumaine de Chimie, 1967, 12, 1495.
  4. W. v. Miller and G. Rhode, Chemische Berichte, 1890, 23, 1070.
  5. K. B. Wiberg, B. Marshall and G. Foster, Tetrahedron Letters, 1962, 345.
  6. M. Drees and T. Strassner, Inorg. Chem., 2011, 50, 5833.


Ethylbenzene yields acetophenone when it undergoes Étard reaction.

The mechanism is as follows:

Étard reaction Étard reaction

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