Naming Ketones

These are some common aldehydes:

• H2CO—this is methanal or formaldehyde, the smallest aldehyde

• CH3CHO—this is ethanal, commonly referred to as acetaldehyde

• CH3CH2CH2CHO—this is butanal as it has four carbon atoms, commonly called butyraldehyde

• CH3CH2CH2CH2CHO—this is pentanal as it has five carbon atoms, commonly called valeraldehyde

• Cyclohexane plus a CHO group is referred to as cyclohexane carbaldehyde

• Benzene plus a CHO group is referred to as benzene carbaldehyde or simply benzaldehyde

These basic aldehydes have common names and should be memorized.

NAMING KETONES

The suffix “-one” is used to define ketones. A ketone is similar to an aldehyde but it has two R groups on either side of the carbonyl group—neither of which is hydrogen. Because it isn’t at the end of the chain, it needs to be numbered. Numbering starts at the carbon atom that gives the carbonyl group the lowest possible number. Propanone and phenylethanone don’t require a locator number because the carbonyl chain can only happen at one spot in the molecule.

Another way to name a ketone is just to say the names of the two side groups attached to the carbonyl group and then add “ketone” to the end of the name. The R chains are listed alphabetically. There are common names, such as “acetone”, which is the name for propanone. Another example is something like 3-hexanone, which can also be called ethyl propyl ketone (named for the R and R-prime side chains). Figure 41 shows some ketones that should be memorized:

Figure 41.

What should you do when there are aldehydes and ketones within the same molecular structure? In such cases, the aldehyde takes precedence over the ketone group, so it is called an aldehyde. The ketone group is called an “oxo” side group. The aldehyde, of course, does not need to be numbered; however, the “oxo” group must be numbered and named accordingly. The carbonyl group of the aldehyde is given the number “one” so that 2-methyl-3-oxo-butanal has 4 carbon atoms in the main chain, a methyl group at carbon number two and another carbonyl group at carbon number 3.

It is entirely possible to have diketones and dialdehydes. The aldehyde doubled would have a carbonyl group at either end and would end in “-dial” to reflect two aldehyde groups. Butanedial has 4 carbon atoms, including two that participate in the aldehyde ends of the molecule. In a “dione” molecule, there are two ketone carbonyl groups— both of which need numbering. The molecule 2, 4, pentanedione is a five-carbon chain that has a carbonyl group on the second and fourth carbon atoms.

It is also possible to have a cyclic ketone molecule. The number one carbon atom in the ring is the carbonyl carbon, going around the ring to have the lowest possible numbers. Of course, cyclo is added before the parent chain to indicate a cyclic molecule. The suffix “-one” is added to indicate that the molecule is a ketone. This would lead to molecules like cyclopentanone—a five carbon molecule with a carbonyl group as part of a ring structure. Dione is the suffix given when two carbonyl groups are in the cyclic molecule. This would lead to a molecule like 1,2-cyclopentanedione.

In the case of the aldehyde or ketone plus an alcohol side chain, the carbonyl group takes precedence over the alcohol group. It is named as a ketone or aldehyde with a