[Daxx]

Hi there,

Can Et2O undergo 'Ether Cleavage' in the presence of excess HBr.

I would say it cannot because there isn't a 'more susceptible' bond C atom, therefore there would be no reaction right? Considering that most examples of cleavege are where there is a phenyl group and then an alkyl group bonded to the central O (Ph--O--Alkyl).

Thanks

[azmanam]

It will react, but only if it's heated along with excess HBr.  At room temperature, it will not react.

[Daxx]

thanks, but why?

[azmanam]

At room temperature, it is an Sn1 process, and diethyl ether would give a primary carbocation if cleaved under an Sn1 process.  Typically only tertiary ethers are cleaved with excess HBr at room temperature.  Those are the only ones to undergo SN1 reaction.

[orgopete]

At room temperature, it is an SN1 process, and diethyl ether would give a primary carbocation if cleaved under an Sn1 process.  Typically only tertiary ethers are cleaved with excess HBr at room temperature.  Those are the only ones to undergo SN1 reaction.

I do not agree. I agree that higher temperatures increase the rates of SN2 reactions and there are plenty of SN2 reactions that occur at low temperatures, so I would not be inclined to judge by that basis. Azmanam may have practical experience with a dealkylation reaction and that they do proceed faster at higher temperatures. However, I believe the energetics of this reaction is such that a greater portion of the molecules possess sufficient energy to reach the activation energy barrier. Lower temperatures will result in a slower reaction. If the rule of thumb that for every 10°C increase in temperature doubles the reaction rate, the extent or time for completion of a reaction will depend on how fast it would be normally and the temperature at which it occurs. Practically, there may be little reaction at room temperature, but I don't know that. If we increase the HBr concentration, we can also increase the rate.

I don't think an SN1 mechanism will be preferred over an SN2 reaction. I think a similar argument for SN1 v SN2 should be made. The reaction with the lower activation energy will be the path that will proceed. I think the activation energy for SN1 reactions would be quite high and thus it would be unusual to see SN1 like reactions. If the activation energy of two reactions were similar, I think it is in that instance that you will see both. (I use that advisedly as reaction profiles can be biased by changing nucleophiles for example to increase the rate of SN2 reaction even if an SN1 reaction could take place or an SN1 might take place in the absence of a good nucleophile.)