Solvolysis

Many SNSN reactions are carried out using the solvent as the nucleophilic agent. They are called solvolysis reactions and involve solvents such as water, ethanol, ethanoic acid, and methanoic acid. Two examples are

In these examples, solvolysis is necessarily a first-order reaction, because normally the solvent is in such great excess that its concentration does not change appreciably during reaction, and hence its contribution to the rate does not change. However, that the overall rate is first order does not mean the reaction necessarily proceeds by an S_N1 mechanism, particularly in solvents such as water, alcohols, or amines, which are reasonably good nucleophilic agents. The solvent can act as the displacing agent in an S_N2 reaction.

To distinguish between S_N1 and S_N2 mechanisms of solvolysis requires other criteria, notably stereochemistry, and the effect of added nucleophiles on the rate and nature of the reaction products. For example, it often is possible to distinguish between SN1SN1 and SN2SN2 solvolysis by adding to the reaction mixture a relatively small concentration of a substance that is expected to be a more powerful nucleophile than the solvent. If the reaction is strictly SN1, the rate at which RX disappears should remain essentially unchanged because it reacts only as fast as R^⊕ forms, and the rate of this step is not changed by addition of the nucleophile, even if the nucleophile reacts with R⊕. However, if the reaction is S_N2, the rate of disappearance of RX should increase because RX reacts with the nucleophile in an SN2 reaction and now the rate depends on both the nature and the concentration of the nucleophile.

Many organic chemists, and indeed the previous versions of this book, use the term "carbonium ion" for species of this kind. However, there is well-established usage of the -onium suffix, for ammonium, oxonium, chloronium, and so on, to denote positively charged atoms with filled valence shells. In the interest of greater uniformity of nomenclature we shall use "carbocation" for carbon positive ions that have unfilled valence shells (6 electrons).