* Hydrocarbons are molecules composed solely of hydrogen and carbon atoms.

* Ethanol (CH3CH2OH) has an oxygen atom in its structure, which means it's not purely a hydrocarbon.

Instead of being a substituted hydrocarbon, ethanol is classified as an alcohol. This classification arises from the presence of the hydroxyl group (-OH) attached to the carbon chain.

Here's how to think about it:

* Substituted hydrocarbons involve replacing one or more hydrogen atoms in a hydrocarbon chain with other atoms or functional groups.

* Ethanol is not formed by substituting a hydrogen atom in a hydrocarbon. It has a distinct functional group (hydroxyl) that defines its properties.

Therefore, ethanol is not a substituted hydrocarbon but rather an alcohol due to the presence of the hydroxyl group.