Isotope effects in acid‐catalyzed aromatic hydrogen exchange

Cox, Robin A.

doi:10.1002/poc.610040406

Cited by 21 publications

(32 citation statements)

References 42 publications

(1 reference statement)

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“…Product formation was quantitative. Results were also obtained at four other temperatures in 69.26% H2S0,, enabling calculation of activation parameters as before (9,11,17), see Table 3. The rate data were analyzed using the excess acidity method (8), and to this end plots of log k+ -log CH+ against X are given for 1 and 2 in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…The statistical information regarding these lines is given in Table 3 with the activation parameters. Points at one X-value were rejected as being off the line for each compound (17), probably due to an acid concentration error.…”

Section: Figmentioning

confidence: 99%

“…Phone: (416) 978-458 1. Fax: (416) 978-8775. thioester (lo), amide (ll), and acylhydrazine (12) hydrolyses, enolizations (13,14), aldol condensations (14), hydrations of alkenes and alkynes (15,16), and most recently aromatic hydrogen exchange (17) and phenylazo ether cleavage (18).…”

Section: Introductionmentioning

confidence: 99%

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The cyclization of 2-substituted imidazolines in sulfuric acid

Cox¹,

Moore²,

McDonald³

1994

Can. J. Chem.

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A study of the rates of the cyclizations 1 + 3 and 2 + 4 as a function of medium acidity and temperature in aqueous sulfuric acid has been performed. The latter reaction is five times faster at all acidities. An excess acidity kinetic analysis reveals the probable involvement of a water molecule in the reaction in both cases. Mechanistic possibilities suggested by the observations are discussed; it is proposed that the water molecule acts as a base catalyst during the rate-determining ring closure. On a effectut une ttude des vitesses des cyclisations 1 + 3 et 2 + 4 en fonction de I'aciditC du milieu et de la temptrature, dans de I'acide sulfurique aqueux. A tous les niveaux d'aciditk, la deuxibme rtaction est cinq fois plus rapide. Une analyse de I'aciditC cinttique en excbs rCvble qu'une molCcule d'eau est probablement impliquCe dans la reaction, dans chacun des cas. On discute des diverses possibilitts de mtcanismes suggkrtes par ces observations; on suggbre que la moltcule d'eau agit comme catalyseur basique au cours de l'ttape dtterminant la vitesse de la fermeture du cycle.[Traduit par la rCdaction]

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Section: Resultsmentioning

confidence: 99%

Section: Figmentioning

confidence: 99%

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The cyclization of 2-substituted imidazolines in sulfuric acid

Cox¹,

Moore²,

McDonald³

1994

Can. J. Chem.

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“…The results are analyzed using the excess acidity method (l), which has been widely applied to organic reactions in sulfuric acid in recent years, most recently to aromatic hydrogen exchange (12), phenylazo ether cleavage (13), imidazole cyclizations (14), and the hydrations of fluorinated alkyl vinyl ethers (15).…”

Section: F 'mentioning

confidence: 99%

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

Cox¹

1996

Can. J. Chem.

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Abstract:In aqueous sulfuric acid, aliphatic N-nitro amines decompose to N,O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonated aci-nitro tautomer, the nucleophile being a water molecule at acidities below 82-85% H,S04, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the u* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly the m'm* slopes also correlate with u*, although the scatter is bad.Key words: N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.

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“…7, as transient intermediates in the disproportionation. The technique used to analyse the observed rate constants is the excess acidity method, 18 which has seen extensive use in recent years for the determination of reaction mechanisms in aqueous strong acid media; for instance, aromatic hydrogen exchange processes, 19 imidazoline cyclizations, 20 alkene hydrations, 21 ether hydrolyses, 13,22 thioacetal hydrolyses 23 and many other reactions have been studied. Most recently it has been used to determine the acid hydrolysis mechanisms of alkylnitramines 24 and nitramide, 25 several N-nitro amides 26 and acylimidazoles.…”

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confidence: 99%

Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism 1

Cheon¹,

Cox²,

Keum³

et al. 1998

J. Chem. Soc., Perkin Trans. 2

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A kinetic and product analysis study of the reactions of the three isomeric phenylazopyridines (PAPys) in aqueous sulfuric acid media (30-97 wt% H 2 SO 4 ) is reported. The final products obtained from the reaction of 4-(phenylazo)pyridine (4-PAPy) are the hydroxylated product 4-(4-hydroxyphenylazo)pyridine, the reduction products 4-aminophenol and 4-aminopyridine, and a small amount of a dimerized product. 3-(Phenylazo)pyridine is unreactive, but 2-(phenylazo)pyridine gives the equivalent 2-(4-hydroxyphenylazo)pyridine, 4-aminophenol and 2-aminopyridine products. This product pattern, an oxidized azo-compound and two reduced amines, is similar to that found in the disproportionation of di-psubstituted hydrazinobenzenes observed in benzidine rearrangement studies. Consequently it has been proposed that the corresponding [NЈ-(4-hydroxyphenylhydrazino)]pyridines were formed as reaction intermediates in the present system; this is confirmed by showing that [NЈ-4-(4-hydroxyphenylhydrazino)pyridine synthesized independently gave the same products as 4-PAPy under the same conditions. The kinetic study shows that the 4-isomer reacted faster than the 2-isomer at all the acid concentrations investigated (the 3-isomer being inert). Rate maxima are observed, at ~72 wt% H 2 SO 4 for 4-PAPy and ~86 wt% H 2 SO 4 for 2-PAPy. To facilitate the kinetic analysis, values of pK BH 2 2؉ for the protonation of the substrates and the possible hydroxy products at the azo-group were determined, using the excess acidity method; the first protonation occurs on the pyridine nitrogen in the pH region. An excess acidity analysis of the observed pseudo-first-order rate constants as a function of acidity indicate an A2 mechanism, with the diprotonated substrate and either one HSO 4 ؊ ion or one H 2 O molecule in the activated complex. The proposed mechanism thus involves nucleophilic attack of HSO 4 ؊ or H 2 O at an aryl carbon of the diprotonated substrate in the slow step, resulting in an intermediate hydrazo species which gives the observed products in a subsequent fast step (cf. benzidine rearrangement).

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Isotope effects in acid‐catalyzed aromatic hydrogen exchange

Cited by 21 publications

References 42 publications

The cyclization of 2-substituted imidazolines in sulfuric acid

The cyclization of 2-substituted imidazolines in sulfuric acid

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism 1

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