http://www.ck12.org Chapter 25. Organic Chemistry
For aldehydes, the R group may be a hydrogen atom or a carbon chain of any length. To name an aldehyde, first
name the parent compound by finding the longest continuous chain that contains the carbonyl group. Then, change
the –eat the end of the name of the alkane to –al. Because an aldehyde only occurs at the end of the chain, no
numbers are needed to indicate its position. If other substituents are present, begin numbering the parent chain on
the end where the aldehyde is found. Two examples of aldehydes are shown below.
For ketones, R and R’ are both carbon chains, which can be the same or different. The steps for naming ketones,
followed by two examples, are shown below.
- Name the parent compound by finding the longest continuous chain that contains the carbonyl group. Change
the –eat the end of the name of the alkane to –one. - Number the carbon atoms in the chain so that the carbonyl group has the lowest possible number.
- Add the numerical prefix into the name before the name of the ketone.
- Use a hyphen between the number and the name of the ketone.
Properties of Aldehydes and Ketones
Aldehydes and ketones can participate in weak hydrogen bonds with water through the carbonyl oxygen atom. The
smallest aldehydes and ketones (3 carbons or fewer) are soluble in water in all proportions. As the length of the
carbon chain increases, water solubility decreases. Like ethers, aldehydes and ketones cannot hydrogen bond with
themselves because they do not contain any hydrogen atoms in sufficiently polar bonds. As a result, their boiling
points are generally lower than those of alcohols. However, aldehydes and ketones are polar molecules due to the
polarized carbonyl group. The dipole-dipole interactions are stronger than the dispersion forces present in alkanes of
a similar size. The boiling points of aldehydes and ketones are intermediate between those of alkanes and alcohols.
For example, the boiling points of propane, ethanal, and ethanol (all of which have molar masses in the range of
44-46 g/mol) are−42°C, 20°C, and 78°C, respectively.