Oxidation of α, Β -unsaturated alcohols by quinaldinium fluorochromate

Available online at www.ilcpa.pl International Letters of Chemistry, Physics and Astronomy 5 (2012) 8-19 ISSN 2299-3843 Oxidation of α,β-unsaturated alcohols by Quinaldinium Fluorochromate Krishnamoorthy Guna Sekar, Seplapatty Kalimuthu Periyasamy Department of Chemistry, National College, Tiruchirappalli - 620 001, Tamilnadu, India E-mail address: , ABSTRACT The kinetics of oxidation of α,β-unsaturated alcohols (allyl alcohol, Crotyl alcohol, Cinnamyl alcohol) by quinaldinium fluorochromate has been studied in aqueous acid medium at 313 K. α,βunsaturated alcohols were converted to the corresponding acrolein, crotonaldehyde and cinnamaldehyde. The reaction is first order each in oxidant, substrate and H+. The decrease in dielectric constant of the medium increases the rate of the reaction. Increase in ionic strength by the addition of sodium perchlorate has no effect on the rate constant. There is no polymerization with acrylonitrile. The reaction has been conducted at four different temperatures and activation parameters were calculated. From the observed kinetic results a suitable mechanism consistent with rate law has been proposed. The relative reactivity order was found to be Cinnamyl alcohol > Crotyl alcohol > Allyl alcohol. Keywords: Oxidation, Kinetics, α,β-unsaturated alcohols, Quinaldinium fluorochromate 1. INTRODUCTION Chromium compounds have been used in aqueous and non aqueous medium for the oxidation of a variety of organic compounds1. Chromium especially Cr (VI) reagents have been proved to be versatile reagents and capable of oxidizing almost all the oxidizable organic functional groups2,3. Numerous reagents and experimental procedures have been developed to carry out this transformation4 in particular reagents containing chromium(VI) reactive species are widely used for the oxidation of alcohols to carbonyl compounds5,6.Quinaldinium fluorochromate (QnFC) has been used in oxidation of many organic substrates. Quinaldinium fluorochromate is a mild, efficient and stable oxidizing agent. It is a bright yellow crystalline solid. The probable structure of quinaldinium fluorochromate (QnFC) is the following Scheme 1. Quinaldinium fluorochromate oxidises anthracene into anthraquinone in the presence of dichloromethane. α,β-unsaturated alcohols are used in many ways. Allyl alcohol finds a number of industrial applications in the preparation of resins, plasticisers, pharmaceuticals and many organic compounds. Crotyl alcohol was used in organic compound products, as pharmaceutical raw materials and dope. Cinnamyl alcohol was used in perfumery and as a deodorant. Kinetic studies on the oxidation of allyl alcohol with different oxidants such as Chloramine–T,7,8, potassium hexacyanoferrate(II)9, pyridinium fluorochromate10, potassium bromate11, acid bromate12, quinolinium dichromate13, and imidazolium dichromate14 have been reported. International Letters of Chemistry, Physics and Astronomy 5 (2012) 8-19 The present study on the oxidation of α,β-unsaturated alcohols by quinaldinium fluorochromate is to ascertain the nature and the order of reactivity of these compounds under the given kinetic conditions. FCrO3CH3 N H Quinaldinium Fluorochromate 2. EXPERIMENTAL METHODS 2. 1. Reagents All the α,β-unsaturated alcohols such as allyl alcohol, crotyl alcohol and cinnamyl alcohol (Aldrich) were purified by distillation. This purity was checked from physical constants. 2. 2. Preparation of Quinaldinium fluorochromate [QnFC]15 Chromium trioxide (7 g) was dissolved in 8ml of water in a polythene beaker and 11 ml of 40% hydrofluoric acid were added with stirring at room temperature. A clear orange red solution was formed, 9 ml of quinaldine were added dropwise with stirring. The mixture was heated on a water bath for about 15 min, then cooled to room temperature, and allowed to stand for 1h. The bright yellow crystalline quinaldinium fluorochromate was isolated by filtration. It was recrystallized from water and dried vacuo for about 2 h. m.pt.146 – 148 ºC. 2. 3. Acetic Acid Glacial acetic acid (AR) (2 litre) was partially frozen and about 1 litre of the liquid was removed. The residue was melted and refluxed with chromium trioxide (30 g) for 4 h and fractionally distilled .The distilled portion was collected between 116 – 118 ºC, partially frozen and about half of the acid was discarded as liquid. The remaining residue was melted and fractioned again after treating with chromium trioxide (30 g). The boiled fraction was collected 116 – 118 ºC and kept in brown bottles. All other chemicals were used as AR grade. Triply distilled water was used for the preparation of solutions. 2. 4. Kinetic measurements The reactions were performed in aqueous acetic acid medium under pseudo-first order conditions by maintaining a large excess of substrate over quinaldinium fluorochromate. The kinetic measurements were carried out spectrophotometrically in a thermostated cell compartment of a spectrophotometer (Perkin Elmer-Lambda 35) at 470 nm. This wavelength of the maximum absorption due to quinaldinium fluorochromate has been observed and absorption due to other reaction species was negligible16. Oxidation reaction was studied only in the concentration range of quinaldinium fluorochromate where the Beer’s law is obeyed. The reactions were followed upto 70% completion. The rate constants were evaluated from the linear plot of log (absorbance) versus time by the least square method and were reproducible within ±3%. 9 International Letters of Chemistry, Physics and Astronomy 5 (2012) 8-19 2. 5. Stoichiometry and product analysis Reaction mixtures containing an excess of the oxidant over α,β-unsaturated alcohols were kept at room temperature in presence of perchloric acid for two hours. Estimation of the unreacted oxidant proved that one mole of oxidant consumes one mole of substrate. The same experimental conditions were used for kinetic determinations; a solution of the reaction mixture was kept under nitrogen for 24 h. The solution was extracted with ether, the organic layer washed with water was dried over anhydrous sodium sulphate and then concentrated. The products were acrolein (from allyl alcohol), Crotonaldehyde (from Crotyl alcohol), and Cinnamaldehyde (from Cinnamyl alcohol) identified by spot tests, the products were confirmed by its 2:4 DNP derivative (80% yield) and silver mirror test. The products were further characterised by IR spectra and GC-MS. The following stoichiometric reactions obtained from experimental results are given below. O 6+ O -QnH+ Cr O F O-QnH+ OH + R CH CH CH2OH R CH CH CHO + OH- Cr + + H + F Here R = H, CH3 & C6H5 3. RESULTS AND DISCUSSION Oxidation of α,β-unsaturated alcohols by quinaldinium fluorochromate has been conducted in 40% acetic acid and 60% water medium at 313 K under pseudo - first order conditions and the observed results were discussed below. Effect of varying Oxidant concentration The kinetic data revea (...truncated)