If we can identify the conditions in a reaction as being acidic (sulfuric acid, HBr, generic hydronium, etc.) we already know what is going to happen first; the organic substrate is going to attack the acid and the substrate will become activated. This is of major importance in acid-catalyzed reactions where neutral conditions are not sufficient for reactivity but adding an acid gets things going by activating the organic substrate. This shows up in dozens of mechanisms in Organic 1 and 2 and gives an element of predictability to how pathways will unfold. Since we are dealing with a “positive” environment, any intermediates formed during these pathways will be positively charged (oxonium ions, carbocations, etc.).

 In the reaction here under acidic conditions, the substrate attacks the acid to yield a cationic oxonium ion intermediate, which may react further depending on what else is present. The environment in the next example has changed to strongly basic so the “attacker” and “attackee” change. Now the negative (electron-rich) base can attack the (electron-poor) carboxylic acid proton as a way to become stable through deprotonation. Notice how the mechanism arrows have changed in going from acidic to basic conditions. In general, organic substrates attack acids while bases attack organic substrates. This is a reliable indicator of what will happen in other mechanisms in the different pH environments.