Oxidation Reactions of 1- and 2-Naphthols: An Experimental and Theoretical Study

Abstract

The transients formed during the reactions of oxidizing radicals with 1-naphthol (1) and 2-naphthol (2) in aqueous medium have been investigated by pulse radiolysis with detection by absorption spectroscopy and density functional theory (DFT) calculations. The transient spectra formed on hydroxyl radical ((•)OH) reactions of 1 and 2 exhibited λ(max) at 340 and 350 nm at neutral pH. The rate constants of the (•)OH reactions of 1 (2) were determined from build-up kinetics at λ(max) of the transients as (9.63 ± 0.04) × 10(9) M(-1) s(-1) ((7.31 ± 0.11) × 10(9) M(-1) s(-1)). DFT calculations using the B3LYP/6-31+G(d,p) method have been performed to locate favorable reaction sites in both 1 and 2 and identification of the pertinent transients responsible for experimental results. Calculations demonstrated that (•)OH additions can occur mostly at C1 and C4 positions of 1, and at C1 and C8 positions of 2. Among several isomeric (•)OH adducts possible, the C1 adduct was found to be energetically most stable both in 1 and 2. Time-dependent density functional theory (TDDFT) calculations in the solution phase has shown that the experimental spectrum of 1 was mainly attributed by 1a4 (kinetically driven (•)OH-adduct) formed via the addition of (•)OH at the C4 position which was 0.73 kcal/mol endergonic compared to 1a1 (thermodynamic (•)OH adduct), whereas 2a1 (thermodynamic/kinetic (•)OH-adduct) was mainly responsible for the experimental spectrum of 2. Naphthoxyl radicals of 1 and 2 have been predicted as the transient formed in the reaction of (•)OH at basic pH. In addition, the same transient species resulted from the reactions of oxide radical ion (O(•-)) at pH ≈ 13 and azide radical (N3(•)) at pH 7 with 1 and 2. Further, UV photolysis of aqueous solutions of 1 and 2 containing H2O2 (UV/H2O2) were used for the (•)OH induced oxidation product formations up on 60% degradations of 1 and 2; profiling of the oxidation products were performed by using an ultraperformance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) method. According to the UPLC-Q-TOF-MS analyses, the preliminary oxidation products are limited to dihydroxy naphthalenes and naphthoquinones with N2-saturation, while some additional products (mainly isomeric monohydroxy-naphthoquinones) have been observed in the degradations of 1 and 2 in the presence of O2. We postulate that dihydroxy naphthalenes are derived explicitly from the most favorable (•)OH-adducts speculated (preference is in terms of the kinetic/thermodynamic dominancy of transients) by using theoretical calculations which in turn substantiate the proposed reaction mechanisms. The observations of (•)OH-adducts for an aromatic phenol (herein for both 1 and 2 at pH 7) rather than phenoxyl type radical in the pulse radiolysis experiments is a distinct and unique illustration. The present study provides a meaningful basis for the early stages associated with the (•)OH initiated advanced oxidation processes of 1- and 2-naphthols.

Transformation Reactions of Radicals from the Oxidation of Diphenhydramine: Pulse Radiolysis and Mass Spectrometric Studies

“…In the absence of molecular oxygen (applicable to our pulse radiolysis experiments), it is reported that the hydroxycyclohexadienyl‐type radicals derived from the reaction of • OH with mono and di‐hydroxy naphthalene undergoes disproportionation reactions ultimately led to the formation of stable hydroxylated compounds . In the present case also, we propose a similar mechanism which results hydroxylated derivatives of DPH as a result of disproportionation of hydroxycyclohexadienyl‐type radicals ‘ a’ (Scheme ).…”

“…In the case of DPH, due to the presence of two identical aromatic rings in the structure, it is highly logical to expect the addition of • OH on the aromatic ring. Such reactions generally led to a resonance stabilized hydroxycyclohexadienyl – type radical . It is thus proposed that the main reaction of • OH is on one of the identical aromatic ring of DPH leading to the formation of unresolved hydroxycyclohexadienyl – type radicals (collectively represented as a ) of this compound.…”

“…Exploring the mechanism of oxidative degradation of organic pollutants instigated by free radicals in various advanced oxidation processes (AOPs) is exceedingly important due to the possible formation of harmful transformation products . The oxidation reactions initiated by various radical entities are known to generate radical cations and hydroxycyclohexadienyl‐type radicals as immediate intermediates in the case of aromatic compounds –. Elucidation of the further transformation pathways of these secondary radicals, that are utmost important to probe the underlying science behind the environmentally important processes like AOPs, are highly challenging due to the very short lifetime of these processes .…”

Identification of position isomers by energy‐resolved mass spectrometry

“…The crucial role of hydroxyl radicals ( • OH) in the oxidative destruction of environmental pollutants in Advanced Oxidation Processes (AOPs) and in many biological processes including DNA damage, mutation and ageing is well established. [1][2][3][4][5][6][7][8] The non positionselective reaction of • OH with aromatic compounds generates isomeric hydroxylated compounds in various chemical processes. [6,9,10] The exact structural assignments of the transient absorption spectra obtained by pulse radiolysis experiments (a widely used technique to probe the spectroscopic details of the transient intermediate species) are consequently very difficult, and hence many authors interpret the experimental spectra in to a cumulative effect of undistinguishable OH adducts.…”

Facile Synthesis and Photocatalytic Performance of WO3/rGO Nanocomposite for Degradation of 1-Naphthol

“…If the photocatalysis process was allowed to be completed, the organic molecules would transform to CO 2 and H 2 O eventually. 38 In addition, broadening the light absorption range of semiconductor can occur when rGO is combined with WO 3 . A number of W-C and W-O-C chemical bonds are formed which is analogous to doping a semiconductor with carbon.…”