**2. Nucleophilic aromatic substitution reactions**

Nucleophilic substitution is an addition-elimination (ANDN) process that depending on the nature of the substrate, the attacking nucleophile, and the solvent effect may lead to a nucleophilic substitution (NS) product or a SNAr product or both [12–16]. A SNAr reaction occurs in activated aromatic compounds bearing good leaving groups (LG). In general, it is widely accepted that the mechanism of the SNAr reactions involves the formation of a σ-complex (also called Meisenheimer complex (MC)) that occurs after the nucleophilic attack step at the ipso atom of aromatic moiety. Next, the departure of LG with re-aromatization of the aromatic ring closes the set of steps to give the desired product. Commonly, the LG departure step is faster than the nucleophilic attack; therefore, the addition of the nucleophile to the ring moiety appears as the rate-limiting step in these processes [13, 17–24]. In the last time, a concerted reaction mechanism could be prevailing [25–27]. A lot of work has been carried out to clarify whether the concerted mechanism is an exception or the dominant pathway in these processes [28, 29].
