The pH of a reaction mixture is one of the most important indicators or what can and cannot happen in a reaction mechanism. Knowing if the environment is basic, acidic, or neutral will limit the kind of species present in solution. For example, in acids there are no strong bases present, and in base there are no strong acids. This is important in deciding the course of events when bonds are formed and broken on organic substrates. Consider the following three situations.
In the first equation the conditions are very basic since a Grignard reagent is employed. This means no acids are present since they would react immediately and quench the Grignard species. No water, no alcohol, no acids of any kind can survive at this pH. A polar aprotic solvent such as ether must be used and all species involved other than metals will be negatively charged. Basic pH also means that the reagent, in this case the Grignard, will attack the organic substrate and not the other way around.
In the second equation the environment is acidic so no strong bases will be present. If the substrate contains a lone pair, for example on O or N, it will attack the acid and activate the organic material as a positively charged species. This makes the material more reactive in general with positively charged O or N being able to accept electrons in subsequent steps. There will be no highly basic species present here, only weak and typically unreactive conjugate bases.
In the third equation the reaction mixture is neutral so there are no strong acids or strong bases present. This typically means the conditions are mild, the organic substrate is neither attacked nor attacks and may react differently than in acidic or basic conditions. For example, tertiary alkyl halides may dissociate in neutral polar protic solvents to give both positive and negative ions.