9.27C - Alcohols and dehydration

The alcohol functional group

A functional group is an atom or group responsible for a family's characteristic reactions. Alcohols have a hydroxyl group, –OH, bonded to a carbon that is not part of a C=O group. For example, ethanol is CH3CH2OH.

The dash in –OH represents a bond to the rest of the molecule. It is not a minus charge: OH⁻ is the hydroxide ion found in alkaline solutions. Alcohol molecules are neutral. A molecular formula such as C2H6O gives the atom count but does not show which atoms connect.

Alcohols contain oxygen, so they are not hydrocarbons. An alkene such as ethene contains only carbon and hydrogen.

Removing water to form an alkene

Dehydration means removing water. Ethanol can lose the elements of water and form ethene:

ethanol → ethene + water
CH3CH2OH → CH2=CH2 + H2O

Think of the removed water as the OH group from one carbon together with H from the neighbouring carbon. A carbon–carbon double bond forms between those carbons. This describes the overall structural change, not a full reaction mechanism. Simply evaporating water from an ethanol solution does not cause this chemical change.