Are carbocations that bad?
Are all mechanisms involving carbocations concerted?


Many carbocations are stable, observable entities.  Many large, delocalized carbocations like trityl perchlorate form very stable salts (reference see p. 196), other carbocations are easily observed by nmr.



If free carbocations didn't exist as discrete intermediates how could you explain the stereochemical results from an SN1 reaction where a chiral starting material generates a planar, long-lived carbocation that can be attacked from both sides to produce an achiral product?  Any type of concerted process would be expected to proceed with inversion of configuration (SN2).


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