Ethyl propyl ether, protonated

Selective ether cleavage comes about during the substitution step, which obeys an Sn2 mechanism. Therefore, selective cleavage requires selective attack by Y on one of the electrophilic carbons in the protonated ether. Determine if selective attack is likely by examining the shape of the lowest-unoccupied molecular orbital (LUMO) in protonated ethyl propyl ether. Is this orbital larger near one carbon than the other If so, what product combination will result What other atom(s) contribute to the LUMO What would happen if 1 attacked this atom(s) ... 

Repeat this analysis for the reaction of phenyl methyl ether with HI leading to phenol and methyl iodide or methanol and phenyl iodide and involving protonated phenyl methyl ether as an intermediate. (Note In this case, the appropriate empty molecular orbital is LUMO+2 the LUMO is concentrated primarily on the CO bond.) Which reaction, with ethyl propyl ether or phenyl methyl ether, appears to be more likely to give selective ether cleavage ... 

LUMO of protonated ethyl propyl ether reveals likely site of nucleophilic attack. 

These data permit us to complete the structure by adding an additional carbon and the requisite number of hydrogens in such a way that the signals of the protons attached to the carbons of the ether linkage are not split further. The correct structure is ethyl propyl ether. 

Figure 5. Interaction maps between a proton and a triazinone [4-amino-6-phenyl-3-ethyl-l, 2,4-triazine-5(4H)-one], nitrofen (2,4-dichlorophenyl p-nitrophenyl ether), and a dinitrophenol (2-iso-propyl-4,6-dinitrophenol). For the latter compound, only two carbon atoms of the isopropyl group...

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