<P> However, an excess of one stereoisomer can be observed, as the leaving group can remain in proximity to the carbocation intermediate for a short time and block nucleophilic attack . This stands in contrast to the S 2 mechanism, which is a stereospecific mechanism where stereochemistry is always inverted as the nucleophile comes in from the rear side of the leaving group . </P> <P> Two common side reactions are elimination reactions and carbocation rearrangement . If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene . At lower temperatures, S 1 and E1 reactions are competitive reactions and it becomes difficult to favor one over the other . Even if the reaction is performed cold, some alkene may be formed . If an attempt is made to perform an S 1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination . This will be especially true if the reaction is heated . Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product . </P> <P> Since the S 1 reaction involves formation of an unstable carbocation intermediate in the rate - determining step, anything that can facilitate this will speed up the reaction . The normal solvents of choice are both polar (to stabilize ionic intermediates in general) and protic (to solvate the leaving group in particular). Typical polar protic solvents include water and alcohols, which will also act as nucleophiles and the process is known as solvolysis . </P> <P> The Y scale correlates solvolysis reaction rates of any solvent (k) with that of a standard solvent (80% v / v ethanol / water) (k) through </P>

Reactions proceeding by this mechanism involve carbocation intermediates