# What stops intermediates of alcohol hydration and dehydration reacting in the same way?

I am aware they both occur under different conditions and that water attaches in a secondary position in elimination but primary position in hydration.

(Stage 2)

(Stage 3)

But both appear to be the same intermediate, why exactly do different reactions follow this stage?

• dehydration requires conc. acid, whereas hydration requires dilute acid, it is mostly the conc. of the reagents used , that determine the path of a reaction, in case of multiple paths possible. The conditions provided, like activation energy through heating or some other such means also play a major role, in such scenarios – Supernova Mar 22 '17 at 16:24
• This is one reversible reaction, you only change equilibrium with suitable conditions. – Mithoron Mar 22 '17 at 17:03

In concentrated $\ce{H2SO4}$, any water will be immediately ‘captured’ by the strongly hygroscopic sulphuric acid. Thus, water is actively drawn out of the equilibrium and the alkene is produced by elimination.