Dialkyloxonium ions

A mechanism for the cleavage of diethyl ether by hydrogen bromide is outlined in Figure 16.4. The key step is an SN2-like attack on a dialkyloxonium ion by bromide (step 2). 

Step 1 Proton transfer to the oxygen of the ether to give a dialkyloxonium ion. 

Step 2 Nucleophilic attack of the halide anion on car bon of the dialkyloxonium ion. This step gives one molecule of an alkyl halide and one molecule of an alcohol. 

Secondary Oxonium Ions [RR OH ]. 1H NMR and IR investigation75 of protonated ether salts (hexachloroantimonates) in dichloromethane solutions showed the formation of both (a) dialkyloxonium ions in which the proton is bound to only one oxygen (21) and (b) bidentate complexes in which the proton is shared between two ether molecules (22). Structural analysis of such a bidentate complex of diethyl ether with a complex anion shows a broad H+ resonance at 81 H 16.3 and unequal O—H bond distances (1.39 and 1.11 A).76... 

The oxygen of the ether can be protonated, and the resulting highly polar dialkyloxonium ion is soluble in sulfuric acid ... 

There are no data available on the rate of formation of dialkyloxonium ions like protonated 1,3-dioxolane in Eq. (16). It is remarkable that the rate constants of formation of secondary onium ions from linear ethers, acetals, sulfides etc. are also utdcnovra. These should however be lower than the rate constants of proton transfer in water (an upper limit) being close to 10 mole 1 s but certainly higher than the rate constantsof protonation of olefins... 

Ethers also protonate in superacid media45 to give dialkyloxonium ions. 

---