Autoxidation of ethers

Boron tribromide (BBr3) cleaves ethers to give alkyl halides and alcohols. 

The reaction is thought to involve attack by a bromide ion on the Lewis acid-base adduct of the ether with BBr3 (a strong Lewis acid). Propose a mechanism for the reaction of butyl methyl ether with BBr3 to give (after hydrolysis) butanol and bromomethane. 

When ethers are stored in the presence of atmospheric oxygen, they slowly oxidize to produce hydroperoxides and dialkyl peroxides, both of which are explosive. Such a spontaneous oxidation by atmospheric oxygen is called an autoxidation. 

Organic chemists often buy large containers of ethers and use small quantities over several months. Once a container has been opened, it contains atmospheric oxygen, and the autoxidation process begins. After several months, a large amount of peroxide may be present. Distillation or evaporation concentrates the peroxides, and an explosion may occur. 

Such an explosion may be avoided by taking a few simple precautions. Ethers should be bought in small quantities, kept in tightly sealed containers, and used promptly. Any procedure requiring evaporation or distillation should use only peroxide-free ether. Any ether that might be contaminated with peroxides should be discarded or treated to destroy the peroxides. 

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The autoxidation of ethers occurs with self-acceleration as autoxidation of hydrocarbons. The kinetics of such reactions was discussed earlier (see Chapter 2). The autoacceleration of ether oxidation occurs by the initiating activity of the formed hydroperoxide. The rate constants of initiation formed by hydroperoxides were estimated from the parabolic kinetic... 

Any oxidation that proceeds spontaneously using oxygen in the air. Autoxidation of ethers gives hydroperoxides and dialkyl peroxides, (p. 641)... 

Autoxidation of ethers to ether peroxides also involves the formation of free radicals. THF forms the corresponding hydroperoxide 2.63 at the a-position. 

Organic peroxo compounds are also obtained by autoxidation of ethers, unsaturated hydrocarbons and other organic materials on exposure to air. The autoxidation is a free-radical chain reaction which is initiated almost certainly by radicals generated by interaction of oxygen and traces of metals such as Cu, Co, or Fe.27 The attack on specific reactive C—H bonds by a radical, X, gives first R and then hydroperoxides which can react further ... 

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. 

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... 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

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. 

Autoxidation of dimethylketene with oxygen in ether at —20°C gives the poly (peroxylactone) which as a dry solid is liable to undergo unpredictable and violent detonation. 

Propyl gallate Antioxidant (approved for use in oral concentrate), antimicrobial activity Prevents autoxidation of oils and peroxide formation in ether. Synergistic effects with other antioxidants such as butylated hydroxyanisole... 

The activity of the primary peroxides formed in hexane-air or ether-air mixtures is also demonstrated by the autoxidation of benzene and aniline, which occurs readily in the presence of hexane or ether, but cannot be induced at equally low temperatures in their absence. 

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). 

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.

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