Regiochemistry relates to where (i.e. which region) in a molecule a reaction happens or where a functional group is placed. Knowing which products are formed as major and minor under various conditions helps us decide how they were formed mechanistically. The main factors involved will be steric and/or electronic in terms of concepts like crowding in transition states (steric) or stabilization of transition states or products by factors such as hyperconjugation or induction (electronic). Consider the following two alkene additions that are used in synthesis to produce regioisomeric alcohols.
In both reactions the first reagent is adding to a flat alkene so it can’t be that geometry that is causing the different outcomes. In the first process the OH group ends up on the tertiary, more substituted and more crowded carbon, while in the second process the hydroxyl ends up on the less substituted less crowded end of the alkene. What causes the difference; transition state energies, intermediate stabilities, or both? If the OH adds to the tertiary carbon in the first reaction then it can’t be a steric issue but could be in the second reaction where the large borane reagent (e.g. the parachute, 9-BBN) appears to avoid the large alkyl groups on the alkene. We know that the alkyl groups will stabilize reactive intermediates such as carbocations in stepwise mechanisms (by hyperconjugation) but those same alkyl groups will deflect large incoming groups in a concerted approach. These ideas help direct us to the first reaction being stepwise via carbocations while the first step in the second process is most likely governed by steric considerations.