Displacement of leaving groups in organic reactions When I initially learned about substitution reactions, there would be times where a poor leaving group had to be changed into a better one in order to proceed with a substitution reaction. For instance, changing a poor leaving group (-OH) to a better one, such as water:
 

After the "good" leaving group is made, it is displaced via an SN2 reaction. The displacement of the water required a nucleophile. However, I recently came across this mechanism (dehydration):


In this case, the water leaves by itself. My question is:

If I ever come across an instance where water (a good leaving group), must leave, how should I assess whether it can leave BY ITSELF or not?
In that kind of reactions, the full explanation would require advanced quantic calculation to determine the energy of all the present entities.

In chemistry, we usually prefer giving a half-quantitative explanation to explain the formation of the observed products.

In your example A —> B + C + D, A is a secondary alcohol and it CAN be transformed to a carbocation leading to B... but you can see it is far from perfect since B represents only 50% of the yield. The result would be much more clear-cut with a primary alcohol (unstable carbocation) or a tertiary one (very stable carbocation). I think you are expected to explain all that.