One of the more complex hydration reactions utilises sulfuric acid as a catalyst. This reaction occurs in a similar way to the addition reaction but has an extra step in which the OSOH group is replaced by an OH group, forming an alcohol:
:CH + HO → CHOH
As can be seen, the HSO does take part in the overall reaction, however it remains unchanged so is classified as a catalyst.
This is the reaction in more detail:
#The H–OSOH molecule has a δ+ charge on the initial H atom. This is attracted to and reacts with the double bond in the same way as before.
#The remaining (negatively charged) OSOH ion then attaches to the carbocation, forming ethyl hydrogensulphate (upper way on the above scheme).
#When water (HO) is added and the mixture heated, ethanol (CHOH) is produced. The "spare" hydrogen atom from the water goes into "replacing" the "lost" hydrogen and, thus, reproduces sulfuric acid. Another pathway in which water molecule combines directly to the intermediate carbocation (lower way) is also possible. This pathway become predominant when aqueous sulfuric acid is used.
Overall, this process adds a molecule of water to a molecule of ethene.
This is an important reaction in industry, as it produces ethanol, whose purposes include fuels and starting material for other chemicals.