Aromatic nucleophilic substitution reactions of ambident nucleophiles. III. Reactivity of the nitrite ion

Abstract

The reactivity patterns in the SNAr reactions of the ambident nitrite ion with nitrohalobenzenes depend on the solvent, the leaving group, and the position of substituents. The rate-determining step can be either the formation or decomposition of the intermediate complex, depending on the leaving group and mode of attack by the nitrite ion. The activating effect of a para relative to the same ortho substituent depends on the leaving group and on whether nitrite is attacking via its nitrogen or oxygen atom. The nitrite ion is of comparable nucleophilicity to azide ion, but is a much weaker carbon base than azide when attacking through its nitrogen atom, although it is a strong carbon base when attacking through oxygen. The reactivity patterns of nitrite ion in SNAr reactions are compared with those of other nucleophiles, such as N3-, SCN-, the halides, RS-and RO-.In Part 11' we gave further evidence for a mechanism (Scheme I) first proposed by Rosenblatt, Dennis, and Goodin2 for the reactions of nitrite ion with aromatic compounds, ArX, which are suitably activated for aromatic nucleophilic substitution. A number of rate constants were reported and used to support the mechanism. This paper takes those rate constants and some new ones and discusses reactivity patterns,3 i.e., nucleophilicities, leaving group tendencies, solvent effects, and substituent effects in these reactions and in reactions involving related nucleophiles, such as azide, thiocyanate, and the halides.The ambident nitrite i 0 n~3~ bonds to aromatic carbon via its nitrogen or its oxygen atom (N-attack or 0-attack), but the end product of reaction with ArX is always the phenoxide, ArO-, owing to the reactivity of the nitro intermediate ArN02, which can be isolated in certainThe reactions of nitrite ion have similarities to the SNAr reactions of the ambident thiocyanate ion, which were discussed in Part I.5 Thus the compounds ArSCN and ArN02 are reactive intermediates, and SCN-and NO2-are weak carbon bases. The nitrite ion has a soft6 nitrogen atom and two harder6 oxygen atoms, whereas SCN-has soft sulfur and harder nitrogen. These properties cause interesting variations in leaving group tendencies? in the activating effect of substituents,3 and even in the effect of solvents on rates and mechanism, as discussed below. Results and DiscussionRate constants were measured and processed as described in Parts I5 and 11.' Leaving Group Tendencies? Table I compares the rates of reaction of three sets of fluoro-, chloro-, and iodonitro-substituted benzenes, ArX, with nitrite ion, via Nattack as log NkX, and O-attack as log Ohx. Three comparisons are made, NkX/OkX (the ratio of N-to 0-attack), log NkF/NkC1, and log OkF/OkC1. The latter two comparisons show leaving group tendencies of fluorine relative to chlorine for N-and 0-attack, respectively. In some cases, only upper or lower limits to these ratios have been recorded because the other rate constant was too slow to measure in competition with other reactions. However, even these ratios give useful inform...

Nucleophilic substitution of the nitro group, fluorine and chlorine in aromatic compounds

“…The activities of the substituted groups in 1-X-4-nitrobenzenes 1, 31 and 32 in their reactions with the nitrite ion decrease in the following order: F > Cl & NO 2 , { while those in 1-X-2,4-dinitrobenzenes 57, 58 and 60 change as follows: NO 2 > Cl > F (Table 5). 77,78 The reason for the change in the activities of substituents is that the limiting step of the reaction of low-reactive substrates with the nitrite ion is the formation of the s-complex, in the reaction with highly reactive substrates it is its decomposition. 78 Thus, it is reasonable to use chlorine as a leaving group (especially in the case of highly reactive arenes) for the introduction of the nitro group into arenes according to the S N Ar mechanism, This has recently been demonstrated for the synthesis of 1,2,4-trinitrobenzene (60) from 2,4-dinitrochlorobenzene (58) and KNO 2 under the conditions of phase-transfer catalysis (yield 71%).…”

“…77,78 The reason for the change in the activities of substituents is that the limiting step of the reaction of low-reactive substrates with the nitrite ion is the formation of the s-complex, in the reaction with highly reactive substrates it is its decomposition. 78 Thus, it is reasonable to use chlorine as a leaving group (especially in the case of highly reactive arenes) for the introduction of the nitro group into arenes according to the S N Ar mechanism, This has recently been demonstrated for the synthesis of 1,2,4-trinitrobenzene (60) from 2,4-dinitrochlorobenzene (58) and KNO 2 under the conditions of phase-transfer catalysis (yield 71%). 79 Substitution of the nitro group for fluorine is especially efficient in the case of low-reactive arenes, e.g., penta-or hexafluorobenzenes.…”

“…Rate constants for the reactions of 1-X-4-nitrobenzenes 1, 31 and 32 and 1-X-2,4-dinitrobenzenes 57, 58 and 60 with the nitrite ion (DMSO). 77,78 Compound X Temper- . i The value for fluorine calculated relative to chlorine.…”

Synthesis and Properties of Poly(imides) and Poly(imides)/Ionic Liquid Composites Bearing a Benzimidazole Moiety

“…Firstly, an aromatic nucleophilic substitution reaction between diphenol ( 1 – 3 ) and 4-nitrophthalonitrile in presence of potassium carbonate gave the tetranitrile derivatives. An important key in this step is the temperature of reaction, which must does not exceed 60 °C because the reaction changes from a red to an intense black color, indicating the presence of collateral reactions [35], which have been explained and studied by Parker et al [36,37]. The second step consists in the transformation of the tetranitrile compounds ( 4 – 6 ) into the corresponding tetracarboxylic acids by alkaline hydrolysis, using potassium hydroxide in a mixture of ethanol-water.…”

Novel thermooxidatively stable poly(ether‐imide‐benzoxazole) and poly(ester‐imide‐benzoxazole)

“…21,22 The inertness of the imide carbonyls to sodium phenoxides during the NAS polymerization was a critical consideration in this polymerization for the maintenance of polymer backbone integrity. [33][34][35] The advantage of the AB monomer's inherent stoichiometry combined with the formation of an ether linkage to impart backbone flexibility makes this a n attractive approach to a novel polymer structure. Thermally stable polymers were obtained under polymerization conditions employing a variety of solvents and temperatures ( Table 11).…”