Diethyl ether, autoxidation

AlkoxyaLkyl hydroperoxides are more commonly called ether hydroperoxides. They form readily by the autoxidation of most ethers containing a-hydrogens, eg, dioxane, tetrahydrofuran, diethyl ether, diisopropyl ether, di- -butyl ether, and diisoamyl ether (10,44). From certain ethers, eg, diethyl ether (in the following, R = H R = 35 — CH2CH2), the initially formed ether hydroperoxide can yield alcohol on standing, or with acid treatment... 

Peroxides. These are formed by aerial oxidation or by autoxidation of a wide range of organic compounds, including diethyl ether, allyl ethyl ether, allyl phenyl ether, dibenzyl ether, benzyl butyl ether, n-butyl ether, iso-butyl ether, r-butyl ether, dioxane, tetrahydrofuran, olefins, and aromatic and saturated aliphatic hydrocarbons. They accumulate during distillation and can detonate violently on evaporation or distillation when their concentration becomes high. If peroxides are likely to be present materials should be tested for peroxides before distillation (for tests see entry under "Ethers", in Chapter 2). Also, distillation should be discontinued when at least one quarter of the residue is left in the distilling flask. 

Common impurities found in aldehydes are the corresponding alcohols, aldols and water from selfcondensation, and the corresponding acids formed by autoxidation. Acids can be removed by shaking with aqueous 10% sodium bicarbonate solution. The organic liquid is then washed with water. It is dried with anhydrous sodium sulfate or magnesium sulfate and then fractionally distilled. Water soluble aldehydes must be dissolved in a suitable solvent such as diethyl ether before being washed in this way. Further purification can be effected via the bisulfite derivative (see pp. 57 and 59) or the Schiff base formed with aniline or benzidine. Solid aldehydes can be dissolved in diethyl ether and purified as above. Alternatively, they can be steam distilled, then sublimed and crystallised from toluene or petroleum ether. 

This group covers polymeric peroxides of indeterminate structure rather than polyfunctional macromolecules of known structure. These usually arise from autoxidation of susceptible monomers and are of very limited stability or explosive. Polymeric peroxide species described as hazardous include those derived from butadiene (highly explosive) isoprene, dimethylbutadiene (both strongly explosive) 1,5-p-menthadiene, 1,3-cyclohexadiene (both explode at 110°C) methyl methacrylate, vinyl acetate, styrene (all explode above 40°C) diethyl ether (extremely explosive even below 100°C ) and 1,1-diphenylethylene, cyclo-pentadiene (both explode on heating). 

Two ethers that are frequently used as solvents are relatively easy to autoxidize — unfortunately, because this reaction is not carried out intentionally. Diethyl ether and THF form hydroperoxides through a substitution reaction in the a position to the oxygen atom (Figure 1.36). These hydroperoxides are fairly stable in dilute solution. However, they are highly explosive in more concentrated solutions or in pure form. Concentration of a solution of an ether hydroperoxide on a rotatory evaporator can lead to a really big explosion... be careful to use pure ethers. 

Fig. 1.36. Autoxidation of diethyl ether and THF net equations (top) and mechanism (bottom). In terms of Figure 1.2 it is a "substitution including an addition." The mechanism is given in the middle, the electronic effect in the rate-and selectivity-determining step at the bottom.

B Reactions of compounds with oxygen with the development of flames are called combustions. In addition, flameless reactions of organic compounds with oxygen are known. They are referred to as autoxidations. Of the autoxidations, only those that take place via sufficiently stable radical intermediates can deliver pure compounds and at the same time appealing yields. Preparatively valuable autoxidations are therefore limited to substitution reactions of hydrogen atoms that are bound to tertiary, allylic, or ben-zylic carbon atoms. An example can be found in Figure 1.27. Unintentional autoxidations can unfortunately occur at the O—Cprtm—H of ethers such as diethyl ether or tetrahydrofuran (THF) (Figure 1.28). 

Fig. 1.28. Autoxidation of diethyl ether and THF net equations (top) and mechanism (bottom).

Methyl tert-butyl ether, MTBE, and ethyl tert-butyl ether, ETBE, are added to gasoline in order to increase the efficiency of gasoline combustion, which reduces the quantity of volatile organic compounds (VOCs) that escape into the atmosphere and cause smog. The chemical industry is very interested in using MTBE as a substitute for THF and diethyl ether, because autoxidation of THF and diethyl ether is a major safety problem for chemical companies. The industry is less interested in ETBE as a solvent substitute. 

Dihydroquinoxalines are formed in the photochemical reactions of quinoxalines with diethyl ether, tetrahydrofuran, and dioxan in the presence of benzophenone. Thus reaction of 2,3-dimethylquinoxaline and tetrahydrofuran yields the product 1 of 1,2-addition. With 2-t-butyl-quinoxaline a mixture of the diastereoisomers resulting from addition to the 3,4-bond is formed but with quinoxaline itself, 2-substituted quinoxalines are produced. These are thought to arise from autoxidation of the intermediate 1,2-adducts. ... 

Autoxidation of ethers is very important since the peroxides formed are often the cause of violent explosions for details see Rieche s review.328 Peroxide-containing ethers, particularly diethyl ether, tetrahydrofuran, diisopropyl ether, and dioxan are sources of great danger since violent explosions can result on distillation. Moreover, petrol, light petroleum, decalin, xylene, cumene, and tetralin may all also contain peroxides. 

The products of autoxidation and photo-oxidation of ethers are the same [187,287,288]. Aldehydes, alcohols, acids, and esters are the main products of hydroperoxide decomposition [186,187,202,283—285,289,290]. For example, ethanol, acetaldehyde, acetic acid, ethyl acetate, and ethyl formate were found in the products of diethyl ether oxidation [186,188, 202,203]. Their formation may be explained by the scheme... 

Lindgren,G., Autoxidation of diethyl ether and its inhihition hy diphenylamine, Acta Chir. Scand., 94, 110, 1946. 

Similarly, the a position in ethers is autoxidized quite readily to give hydroperoxides. Autoxidation of ethers to the a-hydroperoxy derivatives is not an important preparative reaction, but is the basis of a widely recognized laboratory hazard. The peroxides formed from commonly used ethers such as diethyl ether, tetrahydrofuran, diglyme, and diisopropyl ether are explosive. Appreciable amounts... 

A review of autoxidation and autoxidation kinetics has been published. A DFT study of autoxidation of diethyl ether (DEE) supported the basic mechanism involving steps such as chain initiation, propagation, and termination reactions as in alkane oxidations but inferred that the reaction could be different in the presence or absence of... 

Figure 6.13. Normal phase HPLC analyses of hydroperoxide with a 5 m silica column, 34 500 diethyl ether hexane (v/v) as eluting solvent. A, photosensitized oxidized methyl linoleate detected by postcolumn fluorescent reagent diphenyl-1-pyrenylphosphine (DPPP), exciting (352 nm) and emission at (380 nm) absorptions and B, by UV detection at 234 nm, C, Autoxidized methyl linoleate detected DPPP fluorescence, and D, UV detection at 234 nm. See Figure 6.10 caption for abbreviations. From Ohshima et al (1996). Courtesy of the American Oil Chemists Society.

Analytical conditions Hydroperoxide isomeric mixtures from autoxidized methyl linoleate are reduced to hydroxydiene derivatives by sodium borohydride, and separated by preparative TLC on plates coated with silica gel treated with UV marker and developed with diethyl ether-hexane (6 4, v/v) and UV active hydroperoxides are eluted with diethyl ether. The dienol isomers are separated by HPLC on a preparative 6 m porous silica column with ethanol-hexane (0.5 99.5, v/v) as mobile phase with a UV detector set at 234 nm. The weight percent composition is based on the weight of each fraction collected. 

Four autoxidation products of methyllinoleate (i.e., methyl-13-hydroperoxy-X-octadecadienoate, where X = m-9-r ms-l 1, franj-9-tra s-ll, cis- Q-trans- 2, and trans-l0-trans-l2) were separated, but not baseline resolved, on a silica column (A = 234nm) using an 88/12 heptane/diethyl ether mobile phase. Elution was complete in <10 min [689]. 

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