Spectroscopic and Theoretical Investigation on the Charge Transfer Complex of Aldehydes with Molecular Oxygen Concerning Epoxidation of Alkenes
Md.Kudrat-E-Zahan1* and Hiroshi Sakiyama2
1Associate Professor, Dept. of Chemistry, Faculty of Science, Rajshahi University, Rajshahi-6205, Bangladesh
2Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Japan
*Corresponding Author E-mail: Kudrat.email@example.com
The aerobic epoxidation of alkenes with an aldehyde as a co-reagent is an efficient and useful method for the production of fine chemicals. In this study, the interactions of molecular oxygen with its each component are measured. Dichloromethane is choosen as solvent. Pivalaldehyde, 2-ethylbutyraldehyde and heptanal are chosen as aldehydes. Aldehydes having tertiary and secondary carbon next to carbonyl carbon are found to produce ultraviolet contact charge transfer (CCT) absorption band with molecular oxygen (O2).
The aerobic epoxidation of alkenes with an aldehyde as a co-reagent is one of the most useful synthetic intermediates for the preparation of oxygen containing fine chemicals [1-2]. Several mechanisms for this reaction have been proposed [3-5]. The aldehyde plays an important role in these reactions when used with molecular oxygen at room temperature. When aldehydes having secondary and tertiary carbon next to the carbonyl carbon were employed yield of the epoxide increased . The reaction proceeds in presence of oxygen, study of its interaction with each reactants, may be a valuable support to understand the epoxidation mechanism. Recently, we studied electronic spectral properties of several complexes [7-10]. In this study, we reported the interaction of molecular oxygen with three different types of aldehydes to understand the role of aldehyde on the aerobic epoxidation of alkenes.
All the chemicals were commercial products and were used as supplied.
Electronic spectra were measured in dichloromethane on Jasco V-560 (200-800 nm) at room temperature. The ionization potentials (IP) of the aldehyde molecules were obtained using MOPAC 2007 RM1 restricted Hartree-Fock method [11-12].
2.3 Procedure for spectroscopic measurements
Dichloromethane was saturated with nitrogen and oxygen gas for 30 minute at room temperature. The absorption spectra of the dichloromethane were measured in the following order: (a) nitrogen saturated (b) oxygen saturated and (c) nitrogen saturated. For aldehydes, 0.02 molar solutions were prepared in dichloromethane saturated with appropriate gas. The aldehyde solution to be studied was filled into a quartz cell with rubber guard, nitrogen was bubble through for 1 minute and absorption spectrum was measured. Then oxygen was bubbled into the cell for 1 minute, the cell was sealed and the spectrum was measured. All the experiments were done at room temperature.
3. RESULTS AND DISCUSSION:
3.1 The absorption spectra of dichloromethane.
The absorption spectra of dichloromethane under nitrogen and oxygen are shown in Figure-1. The intensity of CCT absorption of dichloromethane was found about 0.03 at saturated condition at 230 nm. The absorption was completely eliminated when oxygen is removed by bubbling nitrogen gas through the solvent. This CCT pair formation phenomenon of dichloromethane is consisted with the other organic solvents [13-16].
Figure-1. Observed electronic spectra of dichloromethane under N2 (–) and under O2 (---).
3.2 The absorption spectra of pivalaldehyde
The absorption spectra of pivalaldehyde under nitrogen and oxygen are shown in Figure-2. In both spectrum the band at around 290 nm is due to the excitation of unshared electron pairs of oxygen to n → π* orbital. The band below 200 nm wavelengths is generally characteristic of π → π* transition of the carbonyl group. The maximum of this band was not observed due to upper absorption limit of the solvent.
Figure-2. Absorption spectra of pivalaldehyde in dichloromethane under N2 (–) and under O2 (---).
Generally, aldehyde forms corresponding acid or decomposed into CO2 after oxidation of carbonyl functional group. The absorption spectra of pivalaldehyde under nitrogen and oxygen conditions does not indicate any effect of molecular oxygen on the n → π* transition. Thus, no oxidation product was formed from pivalaldehyde after oxygen insertion to pivalaldehyde solution. Bastienne  studied interaction of isobutyraldehyde with molecular oxygen quantitatively. They reported that no reaction was took place between isobutyraldehyde with molecular oxygen; no oxygen was taken up and neither carboxylic acid nor CO2 formed which is partially consistent with present study.
The absorption intensity of pivalaldehyde under oxygen increases below 260 nm without reaching a maximum. The increase in absorption is about 0.2 at 230 nm for pivalaldehyde under oxygen. The absorption band disappeared when the dissolved oxygen is removed by purging with nitrogen. The resulting differences of the spectra under nitrogen and oxygen are due to the molecular oxygen. This is also evidence that the extra absorption bands found are not due to oxidation products. The CCT absorption for dichloromethane was much lower than this absorption. Thus, this can be concluded that the extra absorption is due to the formation of CCT complex between pivalaldehyde and molecular oxygen. The heat of formation of the contact donor-acceptor complex is negligible at room temperature and so isolation of CCT complex is impossible .
3.3 The absorption spectra of 2-ethylbutyralaldehyde
The absorption spectra of 2-ethylbutyralaldehyde were measured in dichloromethane under nitrogen and oxygen and shown in Figure-3. The n → π* transition band was observed at 300 nm. This transition band shows a red shift compared with that of pivalaldehyde. Generally, conjugated carbonyl group shows red shifts than the less conjugated or unconjugated one . The red shift of n → π* transition band of 2-ethylbutyralaldehyde indicates that the unpaired electrons of carbonyl oxygen are much more delocalized than that of pivalaldehyde. 2-ethylbutyralaldehyde also form CCT band with molecular oxygen. The CCT absorption for 2-ethylbutyralaldehyde is found about 0.2 at 230 nm. The obtained intensity of CCT absorption normal compared with other organic molecules [15, 18].
Figure-3. Absorption spectra of 2-ethylbutyraldehyde in dichloromethane, under N2 (–) and under O2 (---).
3.4 The absorption spectra of heptanal
The absorption spectra of heptanal were measured in dichloromethane under nitrogen and oxygen and shown in Figure-4. In this spectra n → π* transition band was observed at 290 nm. This shows a hypochromic shift compared with that of pivalaldehyde which indicates the localization of unpaired electron on carbonyl oxygen. The increase in intensity was found about 0.03 at 230 nm due to presence of oxygen. This extra absorption is equal to the CCT absorption by dichloromethane under oxygen. Thus, the increase in the intensity of absorption band under oxygen is due to the CCT absorption of dichloromethane.
Table-1: Absorbance due to CCT formation.
Name of molecule
Absorbance due to CCT formation (wave length)
0.03 (230 nm)
0.2 (230 nm)
0.2 (230 nm)
0.03 (230 nm), which is for dichloromethane
Figure-4. Absorption spectra of heptanal in dichloromethane, under N2 (–) and under O2 (---).
The CCT absorption bands are broad and are related to the ionization potentials (IP) of the molecules . The lower the IP value of the molecule, the lower energy of the CCT band and so the longer the wavelengths at which absorption occurs. The IP (eV) values for pivalaldehyde, 2-ethylbutyralaldehyde, and heptanal using MOPAC 2007 RM1 restricted Hartree-Fock method were found 10.21±0.02, 10.26±0.02, and 10.40±0.02 eV, respectively. According to reported relationship between the wavelength of CCT absorption and IP, CCT absorption with IP value 10.4 should found at about 220-230 nm. The present absorption spectra study is comparable with choi et.al. .
The aldehyde under present study formed CCT pair with molecular oxygen having secondary and tertiary carbon next to the carbonyl carbon atom Table-1. On the other hand, the charge transfer interaction was not observed for primary aldehyde, heptanal. So, for aldehydes, the carbon next to the carbonyl carbon atom has an effect on CCT pair formation. Molecular structure calculation shows that the probability of finding number of electrons on carbon next to the carbonyl carbon atom increases in the order heptanal > 2-ethylbutyralaldehyde > pivalaldehyde. This seems to be explainable that after formation of CCT pair between aldehyde and molecular oxygen, the electron density on carbonyl carbon increases. For the aldehyde having secondary and tertiary carbon next to the carbonyl carbon atom, like pivalaldehyde can able to compensate by sharing the extra negative charge which seems impossible for heptanal.
This study demonstrated the formation of CCT pair between pivalaldehyde (tertiary aldehyde) and 2-ethylbutyralaldehyde (secondary aldehyde) with molecular oxygen. Whereas, for straight chain aldehyde, heptanal no CCT complex was found to form with molecular oxygen.
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Received on 07.12.2013 Modified on 14.01.2014
Accepted on 19.01.2014 © AJRC All right reserved