Arenium ions are not obligatory intermediates in electrophilic aromatic substitution

Abstract

Our computational and experimental investigation of the reaction of anisole with Cl 2 in nonpolar CCl 4 solution challenges two fundamental tenets of the traditional S E Ar (arenium ion) mechanism of aromatic electrophilic substitution. Instead of this direct substitution process, the alternative addition-elimination (AE) pathway is favored energetically. This AE mechanism rationalizes the preferred ortho and para substitution orientation of anisole easily. Moreover, neither the S E Ar nor the AE mechanisms involve the formation of a σ-complex (Wheland-type) intermediate in the rate-controlling stage. Contrary to the conventional interpretations, the substitution (S E Ar) mechanism proceeds concertedly via a single transition state. Experimental NMR investigations of the anisole chlorination reaction course at various temperatures reveal the formation of tetrachloro addition by-products and thus support the computed addition-elimination mechanism of anisole chlorination in nonpolar media. The important autocatalytic effect of the HCl reaction product was confirmed by spectroscopic (UV-visible) investigations and by HCl-augmented computational modeling.organic chemistry | reaction mechanisms | alternative computed routes | autocatalysis

Noncatalytic bromination of benzene: A combined computational and experimental study

“… Catalysis by HBr produced by the reaction. (The autocatalytic effect of HCl formed as one of the reaction products was observed experimentally for anisole chlorination and studied theoretically for benzene chlorination: However, the presence of formed HBr does not influence the benzene bromination rate during the whole time course of the reaction [see e.g., Supporting Information, pp. S6–S7]). Catalysis by light. Catalysis by triplet oxygen. …”

“…Electrophilic substitution of aromatic compounds, including halogenation, is typically described by a two‐stage mechanism with formation of a Wheland (arenium cation) intermediate at the first stage of the reaction . However, in a recent paper, an alternative addition–elimination (A E ) mechanism for electrophilic substitution, first proposed about a 100 years ago, has been firmly established for the chlorination of anisole by Cl 2 in CCl 4 . It is of interest to reveal if such a mechanism takes place in the case of noncatalytic bromination of benzene itself.…”

Solvent and Autocatalytic Effects on the Stabilisation of the σ‐Complex during Electrophilic Aromatic Chlorination

“…However, this was only possible when AlCl 3 was included as a Lewis acid catalyst in the static calculations . Nevertheless, multiple researchers have contested this classic mechanism and an alternative mechanism has been proposed, which does not include the formation of a Wheland‐type intermediate …”

“…The direct substitution mechanism consists of a single concerted reaction step, in which the hydrogen transfer from HCl to Cl 2 and from the aromatic compound to HCl takes place. The addition–elimination mechanism, as proposed by Galabov, Schleyer and co‐workers, involves three steps. The first step is an addition of Cl 2 to the aromatic ring.…”

Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates

“…Furthermore, this reaction is also of academic interest, as the underlying mechanistic principles of S E Ar are currently the subject of debate. According to a few recent studies, the classical Wheland intermediate is not always considered to be an obligatory intermediate during S E Ar [10][11][12][13] . Thus, the objectives of this work are to investigate the S E Ar mechanism at the solid/gas interface of a heterogeneous catalyst, and to resolve the ambiguity about the involvement of Wheland intermediates in zeolite-catalysed S E Ar reactions.…”