Steric Effects on substrate Look at the steric effect as we add substituents to the carbon undergoing reaction. If we use a non-basic nucleophile in a protic solvent a different order of reactivities is observed. Consider what happens when we reflux t-butyl bromide in methanol. Substrate The rate determining step is formation of the carbocation hence the more stable the carbocation intermediate, the faster the rxn. Since the nucleophile does not enter into the rate expression, we can have ionization even in the absence of nucleophile.
Solvent can help to stabilize carbocation.
If our postulate about S N 1 mech is correct, then the achiral carbocation intermediate would afford a racemic, optically inactive product. We will discuss each mechanism in detail 1. Strong bulky bases promote elimination 2. 1 R-X generally react by S N 2 or E2 because they are unhindered and they give poor carbocations 3.