Lesson 5: Functional groups

Acids and the C=O family

Section 3 of 5

The next three families all have a carbon double-bonded to an oxygen, C=O. Where that C=O sits, and what is next to it, decides the family.

Carboxylic acids: -⁠COOH

A carboxylic acid has a C=O and an -⁠OH on the same carbon at the end of a chain, written -⁠COOH. The C=O and the -⁠OH act together as one group, so an acid is not an alcohol and not a ketone. Carboxylic acids taste sour. Acetic acid, called ethanoic acid by chemists, is what makes vinegar sour, and the sting of some ants comes from methanoic acid. Their names end in -⁠oic acid.

Acetic acid (ethanoic acid), CH₃COOHTwo carbon atoms; the second carbon has a double bond to one oxygen atom and a single bond to an -OH group.HOHCCOHH
Acetic acid (ethanoic acid), CH₃COOH: the -⁠COOH group sits on the end carbon.

Ketones: C=O inside the chain

In a ketone, the C=O carbon is inside the chain, joined to a carbon on each side, with no -⁠OH. Ketone names end in -⁠one. The best known is propanone, usually called acetone, used in nail polish remover. It is highly flammable.

Acetone (propanone), CH₃COCH₃Three carbon atoms in a chain; the middle carbon has a double bond to an oxygen atom.HOHHCCCHHH
Acetone (propanone), CH₃COCH₃: the C=O is on the middle carbon.

Aldehydes: C=O at the end

In an aldehyde, the C=O is at the end of the chain, and that carbon also carries a hydrogen. Aldehyde names end in -⁠al, as in ethanal. Careful: -⁠al (an aldehyde) and -⁠ol (an alcohol) differ by just one letter.

Acetaldehyde (ethanal), CH₃CHOTwo carbon atoms; the second carbon is double-bonded to an oxygen atom and single-bonded to a hydrogen atom.HOHCCHH
Acetaldehyde (ethanal), CH₃CHO: the C=O carbon at the end also holds a hydrogen.

Quick check

Acetone has a C=O group on the middle carbon of its three-carbon chain, and no -⁠OH group. Which family is it in?