WHY DOES ETHANOL ENCOURAGE E2 (ELIMINATION) REACTIONS WHILE WATER ENCOURAGES SN1/SN2 (NUCLEOPHILIC SUBSTITUTION) REACTIONS??

The terms S_N1 and E1 mean "substitution, nucleophilic, unimolecular" and "elimination, unimolecular," respectively. These two reaction types are being considered together for two reasons:

1. They often occur simultaneously and competitively with one another, under the same reaction conditions.

    

2. They each involve the formation of a carbocation as the crucial intermediate in the rate-determining step; these reactions exhibit unimolecular (or "first-order") kinetics, because only one molecule -- the immediate precursor of the carbocation -- is involved in the rate-determining step.

The generalized mechanism for each of these reaction types has been depicted below, using tert-butyl chloride as the starting material:

Notice that the product of an S_N1 substitution reaction has simply replaced the chlorine atom with the new substituent, "Nu" in this case. The alkyl group (t -butyl) present in the starting material is still intact, and the hybridization of the substituent-bearing carbon has not changed (it's still sp³). Conversely, the product of the elimination reaction is an alkene: the starting material has lost the elements of HCl, and the hybridization of the carbon originally bearing the chlorine atom has changed from sp³ to sp².

Note also that the nucleophile in an S_N1 reaction does not have to bear a negative charge. In fact, these reactions are typically performed under "solvolysis" conditions, i.e., simply heating the starting material in a protic solvent (an alcohol or carboxylic acid) that can also act as a nucleophile. In these cases, of course, the product of carbocation capture by the solvent will bear a "+" charge, and it will have to lose H⁺ in order to form a neutral product. Similarly, the base in an E1 reaction does not have to be strong. In fact, the base must not be strong, otherwise the E2 mechanism will be followed. It is common for the solvent to act as the base in an E1 reaction, just as it acted as the nucleophile in an S_N1 process.