A functional group is the atom or group of atoms that gives a family of compounds its typical reactions. In alcohols the functional group is -OH. This is why methanol, ethanol, propanol and butanol react in similar ways.
Alcohols can be dehydrated. This means water is removed from the molecule. One way is to pass ethanol vapour over a hot aluminium oxide catalyst. The alcohol loses its -OH group and a hydrogen atom from the next carbon, and these leave as a molecule of water.
The product is an alkene, which has a C=C double bond. Ethanol gives ethene and water, and propanol gives propene and water. For ethanol, the equation is C2H5OH → C2H4 + H2O. The alkene has the same number of carbon atoms as the alcohol it came from.
