The oxidations of some simple secondary alcohols by hydrogen peroxide urea adduct (UHP) using microwaves as an energy source is described. The reaction results in appropriate carbonyl compound. The observed higher yield and shortening of the reaction time was observed for microwave-assisted process comparing to the conventional conditions.
microwaves, hydrogen peroxide urea adduct, oxidation, alcohols.
The application of microwaves in synthetic organic chemistry has already been shown in many publication [1]. The conclusions from the research, show that such kind of energy transport could have a big influence on both rate and selectivity the reaction [2]. The phenomenon seems to be a consequence of the interaction of microwaves with the matter by means of dielectric and conducting mechanism [3]. In the accelerating of the reaction the interphase polarization could also play an important role [4].
From the other hand the oxidation of alcohols results in obtaining of the very important group of compounds. Aldehydes or ketones as the reaction products finds many different application as both final products and intermediates [5]. The oxidation of alcohols could be conducted in many different manners but its very often suffer from not environment friendly oxidants or wastes [6]. On this field we have already applied hydrogen peroxide as the oxidant for many types of organic compounds[7]. The replacing of H2O2 by its urea adduct guides to simplifying of the reaction set-up because handling of a very large amount of water is not necessary. The absence of the water is also a very important factor for microwave-assisted reaction due to avoiding of the heating of the reaction mixtures by the conventional heat transfer from water which is the very good microwave absorber and heats rapidly.
As a continuation of our earlier research on microwave assisted oxidation reactions we now report the microwave oxidation of some alcohols using hydrogen peroxide urea adduct as the oxidant and Na2WO4 as the catalyst. The described reaction was conducted applying phase transfer catalysis (tetraethylammonium hydrogensulphate was used as the PTC catalyst) and methanol as a solvent.
All the reactions were carried out in multiphase system containing natrium tungstate, PTC catalyst (tetraethylammonium hydrogensulphate), UHP, and the alcohol dissolved in the acetonitrile (Scheme 1). The microwave reactions were done in multimode microwave reactor (Plazmatronika, Poland).
Scheme 1. Oxidation of alcohols using UHP
The comparable experiments were performed applying conventional conditions (thermostated water bath). All the experiments were carried out using temperature and power programs listed in Table 1.
Table 1. Reaction conditions for the oxidation of alcohols
multimode reactor
Plazmatronika Poland |
power: 0-80W
temperature: boiling of the mixture |
| conventional heating |
temperature: boiling of the mixture |
In the oxidation of 2-octanol in methanol, which is representative for all the reactions the alcohol (6,1mmol) was dissolved in the methanol (10ml).
Then solid Na
2WO
4 (0,6mmol) and tetrabutylammonium hydrogensulphate (0,62mmol) were added. The mixture was then irradiated by microwave or heated conventionally and after its reaches 60°C, UHP (18,7mmol) was added. The mixture was then stirred vigorously and irradiated (MW)/heated up (conventionally) to the boiling under the reflux. The process was monitored using GC. After the reaction is finished the mixture was extracted with ether. The organic layer was evaporated resulting the crude ketones. Products were separated by the distillation in vacuum and characterized by FT-IR, H-NMR and MS spectroscopy.