Nucleophilic constant aromatic

The polymers used in this study were prepared by a nucleophilic activated aromatic substitution reaction of a bisphenate and dihalo diphenyl sulfone ( ). The reaction was carried out in an aprotic dipolar solvent (NMP) at 170°C in the presence of potassium carbonate (Scheme 1) (5,6). The polymers were purified by repeated precipitation into methanol/water, followed by drying to constant weight. The bisphenols used were bisphenol-A (Bis-A), hydroquinone (Hq) and biphenol (Bp). Thus, the aliphatic character of Bis-A could be removed while retaining a similar aromatic content and structure. The use of biphenol allows an investigation of the possible effect of extended conjugation on the radiation degradation. 

If desolvation of the anionic nucleophile is to be the prominent factor in the molecular catalysis one would expect the most heavily solvated guest to display the largest rate enhancement factors at saturation in a given type of reaction. Heavy solvation of the anion may, however, impair host-guest complex formation so that the complete kinetic analysis with evaluation of every rate- or complex stability constant might not be possible. This turned out to be the case when the rate effects of different anionic nucleophiles in aromatic nucleophilic substitutions were studied The catalysis by the polyammonium host molecule 25 in most of these reactions could only be characterized by a Fig. of merit (FM = K ke ) but not by individual rate constants. 

Thus, a significant difference in the aromatic inductive constant (other hand, the aromatic electrophilic constant (<7p) is only slightly lowered by complexation. Comparison of the aromatic nucleophilic constants (electron-attracting group than the free phenyl group. Comparison of the Hammett constants and a demonstrates that the (CgH5)Cr(CO)3... 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

Further lowering the dielectric constants has been achieved by preparing highly fluorinated polyethers without any sulfone, ketone, or other polarizable groups.239 241 Typically, the /jara-lluorinc atoms on highly fluorinated aromatic compounds, such as hexafluorobenzene and decafluorobiphenyl, are activated and thus can go through aromatic nucleophilic substitution with HFBPA under typical reaction conditions (Scheme 6.31).217... 

For a number of reactions in functional micelles and comicelles second-order rate constants are similar in micelles and in water. Except for aromatic nucleophilic substitution they are slightly smaller in the micelles than in water, and the pattern of behavior is exactly that found for reactions of organic nucleophilic anions in non-functional micelles. Some examples of these comparisons are in Table 9. 

Rate constants of bimolecular, micelle-assisted, reactions typically go through maxima with increasing concentration of inert surfactant (Section 3). But a second rate maximum is observed in very dilute cationic surfactant for aromatic nucleophilic substitution on hydrophobic substrates. This maximum seems to be related to interactions between planar aromatic molecules and monomeric surfactant or submicellar aggregates. These second maxima are not observed with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

AN+- (Reitstoen and Parker, 1991). In other words, the triad of reactive fragments produced in (63) in the charge-transfer excitation of the EDA complex with A-nitropyridinium ion is susceptible to mutual (pairwise) annihilations leading to the Wheland intermediate W and the nucleophilic adduct N (Scheme 12), so that the observed second-order rate constant ku for the spectral decay of ArH+- in Table 3 actually represents a composite of k2 and k2. A similar competition between the homolytic and nucleophilic reactivity of aromatic cation radicals is observed in the reaction triad (55)... 

Fig. 17 Variation of the rate constants for the homolytic (k2) and nucleophilic (k2) annihilation of various aromatic cation radicals with N02 and pyridine, respectively, as a function of the oxidation potential E x (to gauge ArH+ stability).

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. 

The area of chemical reactivity for which there is most information regarding the electronic effects of NO is that involving processes to which a experimental evidence is not entirely straightforward. From studies of aromatic nucleophilic substitution (Section VII.B), Miller and Parker213 obtained a a ... 

TABLE 6. Second-order rate constants for nucleophilic aromatic substitution reactions of 2,4-dinitrochlorobenzene and picryl chloride. Reprinted with permission from Reference 77. Copyright (1992) American Chemical Society... 

ScoOiOPorph/RCoooporph-1 The principle of the method is illustrated in Fig. 18 with the example of reaction (142). The rate constants obtained with the investigated nucleophiles (or with single electron donors—that is the question ) are compared to those of the reaction of a series of anion radicals with the same alkyl halide in the same medium. As discussed on p. 59, aromatic anion radicals behave in this reaction as outer sphere electron donors and the alkyl halide undergoes a dissociative electron transfer. For... 

The reaction of bornyl and isobornyl bromides with the nucleophile (Scheme 18) is another case where the amount of inversion is small and the rate constant close to that observed with an aromatic anion radical of the same standard potential (Daasbjerg et al., 1989) it can therefore be rationalized along the same lines. Cyclizable radical-probe experiments carried out with the same nucleophile and 6-bromo-6-methyl-1-heptene, a radical clock presumably slower than the preceding one, showed no cyclized coupling product. It should be noted, on the other hand, that, unlike the case... 

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