This reaction is the formal addition of H2O across a double bond to make an alcohol.
However, the reaction cannot take place if water is the only reagent. That is because
water is not acidic enough to produce a strong enough electrophile to begin the reaction.
Remember the pKa of H2O is 15.7. That means that when H2O goes to H+and OH-,
there is only one proton for every 1016 water molecules present. To make the reaction work, we need to
add a very small amount of protons usually in the form of sulfuric acid, pKa=-5.2. We need only a small
amount of acid, because a proton is given off in the last step of the reaction and this proton can go off
and start another series of reaction steps. Since a proton is taken in the first step and returned in the
last step, we say that protons are catalysts in this reaction.
