Kinetische Wasserstoffisotopeneffekte und allgemeine Basenkatalyse der Azokupplung. Zur Kenntnis der Kupplungsreaktion, 9. Mitteilung
TL;DR: In this paper, the 4-chloro-benzenediazonium ion with 2-naphthol-6,8-disulphonic acid has been investigated kinetically.
Abstract: 1. The reaction of the 4-chloro-benzenediazonium ion with 2-naphthol-6,8-disulphonic acid has been investigated kinetically. It shows a salt effect which is smaller than the theoretically expected effect (Bronsted's equation).
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TL;DR: In this paper, a linear free-energy relationship was used to correlate the data for aromatic substitution reactions with side-chain reactions, and the correlation was shown to be robust to modifications of the selectivity of the reagent.
Abstract: Publisher Summary This chapter focuses on the quantitative treatment of directive effects in aromatic substitution. The observations for the electrophilic substitution reactions of the monosubstituted benzenes are examined for adherence to a linear free-energy relationship. The selectivity relationship, or the extended selectivity relationship, correlates the data for aromatic substitution simply and accurately. Comparison of the correlations of substitution reactions with side-chain reactions illustrates the more precise nature of the selectivity treatment. This result must be a reflection of the greater similarity of transition states for all aromatic substitution reactions as contrasted to the large variations in the character of the equilibrium states and activated complexes encountered in ordinary and electrophilic side-chain reactions. The introduction of additional parameters in the correlation equation does not yield a significant improvement in the correlation of substitution data. The discrepancies found are not associated with the more apparent properties of these reactions—that is, with influences, such as the charge on the electrophile or variations in resonance interactions as a consequence of the modification of selectivity of the reagent.
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TL;DR: In this article, the SEAr substitutions of a number of 5-X-substituted indoles (1a, f), 5-x-substantituted 2-methylindoles, 1g-j and N-methyl indole (1k) by 4,6-dinitrobenzofuroxan (DNBF) have been kinetically studied in 70-30 (v/v) H2O-Me2SO, 50-50 (v /v) h2O-, methanol and acetonitri
Abstract: The SEAr substitutions of a number of 5-X-substituted indoles (1a–f), 5-X-substituted 2-methylindoles (1g–j) and N-methylindole (1k) by 4,6-dinitrobenzofuroxan (DNBF) have been kinetically studied in 70–30 (v/v) H2O–Me2SO, 50–50 (v/v) H2O–Me2SO, methanol and acetonitrile. The absence of a significant dependence of the rates of reactions on the hydrogen or deuterium labelling at C-3 of the indole ring indicates that electrophilic attack (k1DNBF)by DNBF at this position is the rate-limiting step of the substitutions. However, the k1DNBF rate constants are strongly sensitive to the solvent polarity, the observed reactivity sequence being 70–30 H2O–Me2SO > 50–50 H2O–Me2SO > methanol > acetonitrile. This trend is consistent with a highly dipolar transition state where the development of negative charge in the DNBF moiety is concomitant with that of a partial positive charge on the indole nitrogen. The finding of relatively large negative ρ values (–3.85) for the Hammett plots log k1DNBF=f(σpX)supports this idea. An interesting result is that the effect of the X substituent on the rates (k1DNBF) is the same in the four solvents studied and does not depend upon the substitution at C-2 of the indole ring. However, the Bronsted or Hammett lines for the 5-X-2-methylindoles are located about 1.5 log k unit below those for the 5-X-indoles in a given solvent, showing that the 2-methyl group causes significant steric hindrance to the approach of DNBF from the adjacent C-3 position. Rates of protiodetritiation of a large number of [3-3H]indoles have also been measured in aqueous solution. The data define a unique Bronsted line (βInH= 0.65), implying that the 2-substituents do not exert any steric effect on the rates of the exchange process and suggesting that the correlation can be used to estimate unknown pKa values of indoles with a free 3-position. Comparison of the rates of coupling of indoles with 4,6-dinitrobenzofuroxan with similar data for coupling of these heterocycles with p-nitrobenzenediazonium cation reveals that, despite its neutral character, DNBF is the most electrophilic species. DNBF also appears to be a much stronger electrophile than the proton.
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