Azo-hydrazone Equilibrium Research Articles

Semi-empirical design and synthesis of novel 4-hydroxy-6-sulphonaphthyl azohydroxynaphthalenes with predominant hydrazone form

Semi-empirical design and synthesis of novel 4-hydroxy-6-sulphonaphthyl azohydroxynaphthalenes with predominant hydrazone form

Azo-hydrazone tautomerism in a simple coumarin azo dye and its contribution to the naked-eye detection of Cu2+ and other potential applications

A new tailor-made azo dye of coumarin connected to phenolic derivative is presented herein. Azo-hydrazone tautomerism in aqueous solution of the dye was observed and studied using spectroscopic assays such as 1H NMR, absorption and emission assays, and theoretical studies. Tautomerism was attributed to the presence of a labile phenolic hydrogen in the ortho position to the azo functionality and the hydrazone was found to be the more dominant tautomer. Influence of metal ions on the azo-hydrazone chemical equilibrium and how the accompanying colour and spectroscopic changes can be exploited for various functions, especially the detection and quantification of Cu2+ in aqueous environments was explored. The presence of Cu2+ affects the azo-hydrazone equilibrium resulting in visual appearance and spectroscopic changes and the likely binding sites for Cu2+ were evaluated. Cu2+ pushes the azo-hydrazone equilibrium towards the more conjugated form and the presence of other metal ions does not have any perceivable impact on this mechanism. The dye showed potential applications as a sensor in colorimetric and spectroscopic detection and quantification of Cu2+ in domestic and environmental water samples, photo-imprinting and as a logic gate. The limits of detection (LOD) and quantification (LOQ) for Cu2+ were found to be 0.0779mg/L and 0.236mg/L, respectively, much lower than the World Health Organization (WHO) guideline limit for Cu2+ levels in drinking water.

Preferential Solvation of 4-Carboxyl-2,6-Dinitrophenylazohydroxynaphthalenes in Aqueous Dimethylformamide and Dimethylsulfoxide Binary Mixtures by UV-VIS Spectroscopy

The objective of this study was to evaluate the influence of partly aqueous solvent mixtures on the solubility and azohydrazone equilibrium processes for a group of phenylazohydroxynaphthalenes, AZ-01, AZ-02, AZ-03, and AZ-04, whose applications as potential color additives and chemosensors have been demonstrated in previous studies. The UV-visible spectrum was acquired between 190–900 nm at concentrations of the dyes that precluded molecular aggregation for each dye in aqueous dimethylformamide and dimethylsulfoxide binary mixtures of varying compositions. The plots of E12 against mole fractions of the co-solvent showed deviation from ideality in the behaviors of the four dyes in the aqueous solvent mixtures. The solvation data were largely influenced by the structural chemistry of the dyes. In particular, AZ-01, which contains a free parahydroxyl group that can donate hydrogen to hydrogen bond acceptor solvents showed substantial bathochromic shifts in the aqueous DMF and DMSO mixtures as well as a local accumulation of the organic solvent in its solvation sphere. Conversely, the positional isomer AZ-02 with its ortho hydroxyl group being involved in intramolecular hydrazone rearrangement exhibited dielectric enrichment in both aqueous solvent mixtures. In addition, synergism through formation of the water–DMSO and water–DMF complexes was observed with all the dyes in the solvent mixtures with AZ-01 being solvated by the more polar component of the complex while AZ-02 and AZ-04 were solvated by the less polar solvent mixture component. Thus, the preferential solvation of the phenylazohydroxynaphthalene series from AZ-01 to AZ-04 in the partly aqueous DMSO and DMF solvent mixtures has been successfully studied using UV-visible spectroscopy.

Azo-Hyd] Tautomerism and Structure of Selected Metal Complex Dyes AM1 and ZINDO/1 Methods

Quantum-chemical calculation methods have been used to examine an influence of tautomeric equilibrium [Azo Hydrazo] on a structure of 1:2 chromium metal complex dyes called Gryfalan Navy Blue RL (CI 15 707, Acid Blue 193) and Gryfalan Black RL (Acid Black 194). Chromatographic analysis indicates that synthesis yields a mixture of several dyes with different shades. Studies conducted to date have suggested that such dye complexes can constitute Drew-Pfitzner or Pfeiffer-Schetty structural isomers [1]. It is a know fact, that o-hydroxy-azo dyes exist in equilibrium of tautomeric azo and hydrazone forms. We decided to examine, whether color properties of examined metal-complex dyes can be influenced also by an azo-hydrazone equilibrium and what kind of influence on it has sulphonic groups present in molecules. Calculation and optimization of the geometrical structure were performed using the AM1 methods for monoazoo-hydroxy-azo dyesand ZINDO/1 for 1:2 chromium metal complex dyes. It was stated that monoazo dyes can create complexes in both forms: azo and hydrazone, because energy differences between each form of the metal complex dye are so small, that the monoazo dye can, in practice, create complexes in both the azo and the hydrazone form, with energy differences not exceeding about 4 - 11 kcal/mol. It is calculated that spatial structure changes with an angle of about 90° between two molecules of the dye, and angles of 10° - 20° between the naphthalene moieties. Similar electron density on the hydroxyl and ketone groups suggests that the mixture contains a dominant share of the azo form. The presence of an ionised sulphonic group was also found to affect on the tautomer equilibrium.

The research of the azo–hydrazone equilibrium by means of AM1 method based on the example of an azo dye – the Schäffer salt derivative

An AM1 and ZINDO/S method, azo–hydrazone equilibrium of an azo dye (Schäffer acid derivative) and a model dye (2-naphthol derivative) were examined by means of quantum-chemical methods of calculations. Calculations performed for an isolated molecule (in vacuum) evidenced a higher stability of an azo form [A]. Hydration of dye molecule shows that a hydrazone form [H] is more stable in water solution. These researches confirm practical conclusions about the superiority or the exclusive occurrence of dyes in hydrazone form in real structures of o-hydroxyazo dyes.

15N, 13C and 1H NMR spectra and azo-hydrazone tautomerism of some dyes prepared by coupling of 1-naphthalenediazonium salt

Abstract 2-(1-Naphthyl)azo-5-tert.butylphenol ( 1 ), 3-methyl-1-phenylpyrazole-4,5-dione 4-(1-naphthyl)hydrazone ( 2 ), 1-(1-naphthyl)azo-2-naphthol ( 3 ), and 4-(1-naphthyl)azo-1-naphthol ( 4 ) were prepared by coupling naphthalene-diazonium chloride with the appropriate passive components and their 1 H, 13 C and 15 N NMR spectra were measured and analysed. Two-dimensional NMR experiments were used to assign 1 H and 13 C chemical shifts unambiguously. The values of 1 J( 15 N, 1 H) and 15 N chemical shifts show that compounds 1 and 4 exist almost completely in azo tautomeric forms, whilst compound 2 exists completely in the hydrazone form and compound 3 predominantly in the hydrazone form. The increase of hydrazone content in compound 3 compared with the hydrazone content in 1-phenylazo-2-naphthol is in the range 1–2.7%, calculated from δ ( 15 N a ) and δ ( 15 N b ) chemical shifts and 1.2–1.6% using 1 J( 15 N, 1 H), i.e. the annelation of the benzene ring in the active component has, contrary to the annelation of the benzene ring in the passive component, practically negligible influence on the azo-hydrazone equilibrium in compounds studied.

Tautomerisation in 1-(4-Methylphenylazo)naphthalen-2-ol and 2-(4-Methylphenylazo)-4-methylphenol: A Crystallographic and 13C{1H}NMR Study

The acetic acid esters of 1-(4-methylphenylazo)naphthalen-2-ol 1 and 2-(4-methylphenylazo)-4-methylphenol 3 are prepared and characterised by single crystal X-ray diffraction studies and 13C{1H}NMR spectroscopy; the position of the C(2)13C resonance for the ester is used to predict the position of resonant frequency of the equivalent carbon in the parent alcohols and hence, calculate the position of the azo-hydrazone equilibrium in these compounds.

Tautomerisation in 1-(4-Methylphenylazo)naphthalen-2-ol and 2-(4-Methylphenylazo)-4-methylphenol: A Crystallographic and 13C{1H}NMR Study

The acetic acid esters of 1-(4-methylphenylazo)naphthalen-2-ol 1 and 2-(4-methylphenylazo)-4-methylphenol 3 are prepared and characterised by single crystal X-ray diffraction studies and 13C{1H}NMR spectroscopy; the position of the C(2) 13C resonance for the ester is used to predict the position of resonant frequency of the equivalent carbon in the parent alcohols and hence, calculate the position of the azo-hydrazone equilibrium in these compounds.

Nuclear magnetic resonance investigations of azo-hydrazone, acid-base equilibria of FD&C Yellow No. 6

Azo-hydrazone, acid-base equilibria were examined for the water-soluble dye FD&C Yellow No. 6 (Y6), with the observation of dynamic NMR effects near, and at, pH 12, where the dye converts from a predominantly hydrazone tautomer (acidic to moderately basic pH) to a prevailing azo-anion resonance hybrid (high pH). Semilogarithmatic pH-13C chemical shift plots and a 2-bond, 15Nβ-13C8a coupling constants vs. pH relationship both indicate that the midpoint of the azo-hydrazone equilibrium occurs at ca. pH 12. Potentiometric titrations further indicate that thepKa of Y6 is ca. 12. These data suggest that the observed NMR line broadening is due to slow proton transfer between the hydrazone-NH and water. Download full-size image

Deuterium isotope effects on 13C and 15N nuclear shielding in o‐hydroxyazo dyes

AbstractDeuterium isotope effects on the 13C and 15N nuclear shieldings of o‐hydroxyazo compounds are described. Both the direct and the equilibrium contributions were determined. Large direct deuterium isotope effects on 15N nuclear shieldings for 1Δ, 2Δ and 4Δ and negative Δ1J(15NH) [D] values were observed. Isotope effects on 15N nuclear shieldings, because of their magnitudes, are shown to be very useful in determining changes in the azo–hydrazone equilibrium. Isotope effects on 13C nuclear shieldings are smaller, but the effects observed at the carbon resonances of the N‐phenyl ring are likewise very useful in determining the shift in the equilibrium. Deuterium substitution leads in all cases to a shift in the equilibrium so that the content of the predominant form of the protio compound is increased.

Synthesis and properties of acid dye derivatives of arylsulphonanilides

Abstract Thirty-five acid dyes suitable for dyeing wool and polyamide fibres were synthesized, using derivatives of arylsulphonanilides prepared from o-nitrotoluene or chloronitrotoluene. As coupling components, derivatives of 1-phenyl-3-methylpyrazolone-5 and 2-naphthol were used. Fastness and colour properties of the prepared dyes were determined. It was found that some of the prepared dyes on polyamide fibres show batho- and hypso-chromic effects as well as hypo- and hyper-chromic effects during testing of their fastness to washing and to acid and alkaline perspiration. Spectroscopic examinations and determination of pK a of the sulphonamide and hydroxyl groups of the dyes showed that the observed changes are associated with aggregation or disaggregation of the dyes on fibres or with a change in the azo-hydrazone equilibrium. Some of the prepared dyes are characterized by very good dyeing and fastness properties.

Substituent effects on the tautomerism and photochromism exhibited by a series of hydroxyazo cationic dyes for polyester fibres

Abstract A critical study of the substituent effects on different properties exhibited by a series of thirteen cationic dyes of the hydroxyazo type is presented. This series is well representative of the polarity scale (o, m, p: OCH 3 , CH 3 , Cl, NO 2 ). The first part deals with the constant K = H/A , the thermodynamic characteristics Δ H , Δ S of the azo—hydrazone equilibrium and the p K , e, of these tautomers. Some deviations encountered are explained. In a second part, the substituent effect on the kinetic relaxation of these photochromic dyes is easily accounted for by the empirical σ T constants. However, more advanced models are envisaged and comparison between the different results enables some considerations on the validity of the linear free energy relationships to be presented.

Photooxidation von Arylazonaphtholen mit Singulett‐Sauerstoff

Photooxidation of Arylazo‐Naphtholes with Singlet OxygenThe reaction of p‐substituted 4‐arylazo‐1‐naphtholes 1 and 1‐arylazo‐2‐naptholes 5 with singlet oxygen (produced by methylene blue‐sensitized photolysis) has been studied spectroscopically in methanol as a solvent. The rate constants of dye bleaching are the same, irrespective whether the extinctions of the azo or the hydrazone forms of the tautomeric systems are measured. Both reaction series 1 and 5 afford linear Hammett plots, which is discussed in the light of the azohydrazone equilibrium of the dyes and an „ene”︁ mechanism of the reaction with singlet oxygen. It could be shown by short‐time spectroscopy that the azo‐hydrazone isomerization proceeds much faster than the attack of singlet oxygen on the hydrazone form, which explains all observed facts.

Protonation equilibria of 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes

The spectroscopic behaviour of substituted 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes has been investigated in a solvent system consisting of 20% v/v ethanol and 80% v/v sulphuric acid–water in order to measure the protonation equilibrium constants. Some naphthols (I; X = H, o-OCH3, p-OCH3, o-CH3, p-Cl) and all the methoxynaphthalenes (II) show two absorption bands due to the free base and the conjugate acid, with a characteristic isosbestic point. For these compounds, pkBH+ values have been calculated using Jaffé's acidity function. Other naphthols (I; X =m-OCH3, m-CH3, p-CH3, o-Cl, m-Cl) show a single absorption band which shifts on changing the acidity of the medium. The lack of an isosbestic point has been ascribed to the overlap of the azo–hydrazone equilibrium with the protonation equilibrium of the azo-group. For methoxynaphthalenes, separate correlations of pKBH+ values with σm and σp of the substituents are found, owing to the presence in the molecules of two distinct basic centres.

Tautomerism in arylazonaphthols by dipole moment analysis

Abstract The azo-hydrazone equilibrium in some arylazonaphthols has been studied by dipole moment measurements and HMO calculations. Results show that in 1,4-derivatives comparable amounts of both forms are present. The 1,2-derivatives appear to be essentially azo-compounds, but an overestimate of azo proportion cannot be excluded.