Nucleophilic substitution in ethers and Alcohols

In Alcohols(secondary,tertiary,benzylic,allyic) on reaction with HX, X=(Cl,Br,I) SN1 takes place as H2O leaves and forms a carbonation.But in ethers on reaction with HX, X=(Cl,Br,I) (except tertiary,benzylic) SN2 takes place. Is ROH not as good leaving group as H2O.what is the reason for SN1 in Alcohols and SN2 in ethers??




From what I understand, the generalization you have constructed is not based on any gross rule that - "ROH not as good leaving group as H2O". I think what you mean is that the protonated alkoxy group is a poorer leaving group than the OH2+ group, in which case I would still not agree to a generalization unless I know the identity of the R group. There are many ethers that show SN1 substitution reactions as well, especially under acidic conditions. A good example is the highly strained oxirane ring, which opens via an SN1 reaction in low pH.


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