[Jennifer010576]

Okay...hopefully this is the last question I'll have for awhile.

My lab partner and I are working on this problem...

"Ethers can undergo cleavage by the addition of HI or HBr.  However, diphenyl ether is an exception in the fact that it can't undergo acidic cleavage.  Why?"

I think...
    Acidic cleavage is a nucleophili substitution reaction and in order for cleavage to take place, the C-O bond must break.  However, because the geometry of the diphenyl ether, the
C-O bonds are "protected" by the bulkiness of the molecule. Therefore, the C-O bonds are not susceptible to nucleophilic attack, so cleavage by the addition of HI or HBr is not possible.

She thinks...
    That ethers can only undergo acidic cleavage IF a alkyl group is present (to be attacked by the nucleophile).  Since, technically, benzenes aren't alkenes, they in themselves can't be concidered an ALKYL group, so acidic cleavage would not apply to diphenyl ether.

Which one of us is correct...or are we both way off...and if so, where can I find the answer, because neither our notes or text explain this specific example.

Thanks
Jenn

[GCT]

You may wish to consider the basicity of the oxygen first...would you consider it to be delocalized?  If so to what extent.

Also, consider if diphenyl ether was protonated, how unstable do you think it would be relative to an alkyl ether?

[movies]

I would explain it the same way that you and your partner did, but it's really a combination of the two.  Remember that S_N2 reactions can't occur on sp² carbons because the backside attack doesn't work out the way that it does with an sp³ carbon.  Furthermore, since the carbon in question is in a ring there is no chance to access the backside of the C-O bond.  So it's a consequence of sterics and the geometry of an sp² carbon.

GCT raised a good point about the basicity of the oxygen as well.  I'm not certain that this difference would be sufficient to account for a complete lack of reactivity, but certainly would account for a much slower rate of reaction.