86 Unit 11: Ketones and aldehydes
Aldehydes ( ) and ketones ( ) are both carbonyl compounds. They are organic
compounds in which the carbonyl carbon is connected to C or H atoms on either side. An
aldehyde has one or both vacancies of the carbonyl carbon satisfied by a H atom, while a
ketone has both its vacancies satisfied by carbon.
86.1 Naming Aldehydes and Ketones
Ketones are named by replacing the-ein the alkane name with-one. Thecarbon
chainis numbered so that the ketone carbon, calledthe carbonyl group, gets the lowest
number. For example, would be named 2-butanone because the root structure is butane
and the ketone group is on the number two carbon.
Alternatively, functional class nomenclature of ketones is also recognized by IUPAC, which
is done by naming the substituents attached to the carbonyl group in alphabetical order,
ending with the word ketone. The above example of 2-butanone can also be named ethyl
methyl ketone using this method.
If two ketone groups are on the same structure, the ending-dionewould be added to the
alkane name, such as heptane-2,5-dione.
Aldehydesreplace the-eending of an alkane with-alfor an aldehyde. Since an aldehyde
is always at the carbon that is numbered one, a number designation is not needed. For
example, the aldehyde of pentane would simply be pentanal.
The-CH=O group of aldehydesis known as a formyl group. When a formyl group is
attached to a ring, the ring name is followed by the suffix”carbaldehyde”. For example,
a hexane ring with a formyl group is named cyclohexanecarbaldehyde.
86.2 Boiling Points and Bond Angles
Aldehyde and ketone polarity is characterized by the high dipole moments of their
carbonylgroup, whichmakesthemratherpolarmolecules. Theyaremorepolarthanalkenes
andethers, thoughbecausetheylackhydrogen, theycannotparticipateinhydrogenbonding
like alcohols, thus making their relative boiling points higher than alkenes and ethers, yet
lower than alcohols.