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Kinetic studies electrophilicities

Kinetic studies of nitration using dilute solutions of dinitrogen pentoxide in organic solvents, chiefly carbon tetrachloride, have provided evidence for the operation, under certain circumstances of the molecular species as the electrophile. The reactions of benzene and toluene were inconveniently fast, and therefore a series of halogenobenzenes and aromatic esters was examined. [Pg.52]

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. [Pg.85]

From these results it appears that the 5-position of thiazole is two to three more reactive than the 4-position, that methylation in the 2-position enhances the rate of nitration by a factor of 15 in the 5-position and of 8 in the 4-position, that this last factor is 10 and 14 for 2-Et and 2-t-Bu groups, respectively. Asato (374) and Dou (375) arrived at the same figure for the orientation of the nitration of 2-methyl and 2-propylthiazole Asato used nitronium fluoroborate and the dinitrogen tetroxide-boron trifluoride complex at room temperature, and Dou used sulfonitric acid at 70°C (Table T54). About the same proportion of 4-and 5-isomers was obtained in the nitration of 2-methoxythiazole by Friedmann (376). Recently, Katritzky et al. (377) presented the first kinetic studies of electrophilic substitution in thiazoles the nitration of thiazoles and thiazolones (Table 1-55). The reaction was followed spec-trophotometrically and performed at different acidities by varying the... [Pg.104]

Furthermore kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third order overall and second order in hydrogen halide... [Pg.378]

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). [Pg.428]

At this point, attention can be given to specific electrophilic substitution reactions. The kinds of data that have been especially useful for determining mechanistic details include linear ffee-energy relationships, kinetic studies, isotope effects, and selectivity patterns. In general, the basic questions that need to be asked about each mechanism are (1) What is the active electrophile (2) Which step in the general mechanism for electrophilic aromatic substitution is rate-determining (3) What are the orientation and selectivity patterns ... [Pg.571]

Kinetic studies have been carried out on the displacement reactions of various chloroazanaphthalenes with ethoxide ions and piperi-dine. - 2-Chloroquinoxaline is even more reactive than 2-chloro-quinazoline, thus demonstrating the powerfully electrophilic nature of the -carbon atoms in the quinoxaline nucleus. The ease of displacement of a-chlorine in the quinoxaline series is of preparative value thus, 2-alkoxy-, 2-amino-, - 2-raethylamino-, 2-dimethyl-amino-,2-benzylamino-, 2-mercapto-quinoxalines are all readily prepared from 2-chloroquinoxaline. The anions derived from substituted acetonitriles have also been used to displace chloride ion from 2-chloroquinoxaline, ... [Pg.212]

In this chapter on electrophilic aromatic substitution, the literature published prior to 1969 is exhaustively surveyed. The reactions are presented in the same sequence used in an earlier and related review1, except that some newly discovered reactions are included, and others, for which no kinetic studies have been made, are omitted. [Pg.1]

Kinetic studies are of little value in attempting to determine the extent of complex formation in the reaction path of electrophilic substitution. The reasons for this have been adequately presented elsewhere29 and the conclusions are that, unless the formation of the complex is rate-determining, the kinetic form is independent of complex formation. Further, the influence of complex formation on reaction rates only comes from the factors which lead in the first place to complex formation, and substituent effects are inadequate for showing the extent of complex formation though when they indicate similar effects on substitution and complex formation they provide evidence that the latter is a pathway of the former. [Pg.8]

The outstanding problem is to decide how much, if any, association exists between N02 and X" in the generally rate-determining step of the reaction. Kinetic studies tend to indicate the presence of different electrophiles under different conditions whereas the derived partial rate factors are closely similar and therefore indicate one electrophile common to most, if not all, nitrating agents. The more electron-attracting is X , the more easily is N02 displaced from it and hence a reactivity sequence should be... [Pg.10]

A kinetic study of the electrophilic substitution of pyridine-N-oxides has also been carried out50b,c. Rate-acidity dependencies were unfortunately given in graphical form only and the rate parameters (determined mostly over a 30 °C range) are given in Table 4b. There is considerable confusion in Tables 3 and 5 of the original paper, where the rate coefficients are labelled as referring to the free base. In fact the rate coefficients for the first three substituted compounds in... [Pg.20]

Nitration by nitric acid in sulphuric acid has also been by Modro and Ridd52 in a kinetic study of the mechanism by which the substituent effects of positive poles are transmitted in electrophilic substitution. The rate coefficients for nitration of the compounds Pl CHi NMej (n = 0-3) given in Table 10 show that insertion of methylene groups causes a substantial decrease in deactivation by the NMej group as expected. Since analysis of this effect is complicated by the superimposed activation by the introduced alkyl group, the reactivities of the... [Pg.27]

Finally, it is customary to compare the partial rate factors obtained under different conditions to indicate the reactivity of the electrophile. Unfortunately, the medium, nitric acid in sulphuric acid, in which the nitronium ion is most clearly established as the electrophile, is such a poor solvent for aromatics that meaningful competitive nitrations are impossible and kinetic studies are hampered by the difficulties noted above. However, since the isomer distribution is a function of the rate factors, inspection of these distributions (Table 15) shows a very... [Pg.33]

Acetoxylation is found to accompany nitration of fairly reactive aromatics by nitric acid in acetic anhydride and gives rise to zeroth-order kinetics76. The electrophile is believed to be protonated acetyl nitrate the formation of which is rate-determining, hence the kinetic order (see p. 37). Acetoxylation can also accompany halogenation by positive halogenating agents in acetic acid solvent, especially in the presence of sodium acetate137, but no kinetic studies have been carried out. [Pg.56]

Use of kinetic studies of acylation as a measure of electrophilic reactivities... [Pg.182]

Electrophilic aromatic hydrogen exchange reactions fall into two classes, namely those reactions catalysed by acid and those reactions catalysed by base. Of these the former are by far the most common and have been subjected to the most extensive and intensive kinetic studies. [Pg.194]

Kinetic studies of substituent effects in electrophilic aromatic hydrogen exchange... [Pg.243]

As in the alkylation reaction, the reactive intermediate in Friedel-Crafts acylation can be a dissociated acylium ion or a complex of the acid chloride and Lewis acyl.49 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.50... [Pg.1019]

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... [Pg.175]

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... [Pg.499]

Kinetic Studies of C,C-Coupling Reactions of Bis- N,N-Oxyiminium Cations with C-Centered Nucleophiles To estimate the electrophilicity of cationic intermediates (349), the reaction kinetics was measured for several test cations with nucleophiles, which were characterized in the study (481) as reference compounds, (Table 3.22) (478). [Pg.629]

It is a good solvent for kinetic studies of aromatic electrophilic substitution. [Pg.308]

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... [Pg.322]

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). [Pg.182]

At present, we do not completely understand why only some of these very similar m-xylyl dicopper(I) complexes systems described above undergo ligand oxygenation reactions. However, based on the results outlined above, we can speculate on a number of aspects of this 02-activation process. Our studies implicate the presence of a copper-dioxygen (peroxo dicopper(II)) adduct as an intermediate in the oxygenation reaction and more recent kinetic studies (51) further support this conclusion. This adduct then either directly or via some further intermediate undergoes an electrophilic attack of the arene. The unique nature of this very fast reaction 2->3, and the observed inability to intercept the active... [Pg.90]


See other pages where Kinetic studies electrophilicities is mentioned: [Pg.199]    [Pg.199]    [Pg.91]    [Pg.428]    [Pg.60]    [Pg.123]    [Pg.554]    [Pg.123]    [Pg.1]    [Pg.3]    [Pg.56]    [Pg.98]    [Pg.102]    [Pg.107]    [Pg.123]    [Pg.194]    [Pg.280]    [Pg.355]    [Pg.168]    [Pg.344]    [Pg.57]    [Pg.18]    [Pg.600]    [Pg.280]   
See also in sourсe #XX -- [ Pg.2 ]




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