Nucleophilic aromatic substitution kinetic features

The usual kinetic law for S/v Ar reactions is the second-order kinetic law, as required for a bimolecular process. This is generally the case where anionic or neutral nucleophiles react in usual polar solvents (methanol, DMSO, formamide and so on). When nucleophilic aromatic substitutions between nitrohalogenobenzenes (mainly 2,4-dinitrohalogenobenzenes) and neutral nucleophiles (amines) are carried out in poorly polar solvents (benzene, hexane, carbon tetrachloride etc.) anomalous kinetic behaviour may be observed263. Under pseudo-monomolecular experimental conditions (in the presence of large excess of nucleophile with respect to the substrate) each run follows a first-order kinetic law, but the rate constants (kQbs in s 1 ruol 1 dm3) were not independent of the initial concentration value of the used amine. In apolar solvents the most usual kinetic feature is the increase of the kabs value on increasing the [amine]o values [amine]o indicates the initial concentration value of the amine. 

Proposals for the mechanism of PPS formation include nucleophilic aromatic substitution (Sj Ar) (2radical-cation (27), and radical-anion processes (28,29). Some of the interesting features of the polymerization are that the initial reaction of the sodium sulfide-hydrate with NMP affords a soluble NaSH-sodium 4-(N-methylamino)butanoate mixture, and that polymers of higher molecular weight than pi edicted by the Caruthers equation are produced at low conversions. Mechanistic elucidation has been hampered by the harsh polymerization conditions and poor solubility of PPS in common organic solvents. A detailed mechanistic study of model compounds by Fahey provided strong evidence that the ionic S]s Ar mechanism predominates (30). Some of the evidence supporting the S s(Ar mechanism was the selective formation of phenylthiobenzenes, absence of disulfide production, kinetics behavior, the lack of influence of radical initiators and inhibitors, relative rate Hammet values, and activation parameters consistent with nucleophilic aromatic substitution. The radical-anion process was not completely discounted and may be a minor competing mechanism. 

All the reactions discussed in this review are aromatic nucleophilic substitutions in the ordinary sense. These reactions are briefiy described in the following sections with respect to their general kinetic features and mainly involve aza-activated six-membered ring systems, although a few studies of other heteroaromatic compounds are also available. 

B. Some Kinetic Features of Aromatic Nucleophilic Substitution Reactions... 

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

---