What are functional groups? Why is it logical and useful to classify organic compounds according to their functional groups?

They are groups of atoms arranged in a specific way which react in a specific way.

Some example of functional groups are alcohols, aldehydes, and carboxylic acids. We should be able to classify organic compounds by functional groups like those for the sake of communication.

Through IUPAC nomenclature and association with the structure of the molecule, we can communicate to others what a molecule is known to look like.

If we did not do that well, we would have a hard time keeping things straight as to what compound we're talking about or what compound does what.

If all I said was:

• "This compound with two carbons in a row (single bonded with three hydrogens, and then two) and single bonded to an #"OH"# reacts with a different compound with a sulfur double bonded to an oxygen and single bonded to two chlorines to form the same first reactant, but with a chlorine instead of an #"OH"#."

...it would be way harder to understand than saying:

• "Ethanol reacts with thionyl chloride to form chloroethane".

You can rest easy knowing that you can look up the compound names in the latter to find the structures and realize that the reaction is:

Thus, functional group classifications are essential for clear communication amongst chemists.