19.1 Naming Aldehydes and Ketones

Nomenclature of Aldehydes and Ketones

Aldehydes and ketones both contain a carbonyl group ($C=O$), but they differ in where that group sits on the carbon chain. Aldehydes have the carbonyl at the terminal carbon, while ketones have it somewhere in the interior. Naming them correctly under IUPAC rules is the foundation for everything else in this unit, since you need to read a name and draw the structure (or vice versa) quickly and accurately.

Systematic Naming and IUPAC Nomenclature

IUPAC (International Union of Pure and Applied Chemistry) nomenclature gives every organic compound one unambiguous name. For carbonyls, the key suffixes are "-al" for aldehydes and "-one" for ketones. You'll still encounter common names (acetone, formaldehyde) in everyday use, but systematic names are what you're expected to use on exams and in scientific writing.

IUPAC Nomenclature for Aldehydes

An aldehyde has a formyl group ($-CHO$) at the end of a carbon chain. Because it's always terminal, you don't need a position number for the carbonyl itself.

Naming steps:

1. Find the longest carbon chain that includes the $-CHO$ carbon.
2. Replace the "-e" ending of the parent alkane with "-al" (e.g., propane → propanal, hexane → hexanal).
3. The aldehyde carbon is automatically C1. Number the rest of the chain starting from that end.
4. Name and number any substituents using the chain numbering from step 3.

Examples:

• 2-Methylbutanal: 4-carbon chain, methyl group on C2, aldehyde at C1.
• 3-Ethylhexanal: 6-carbon chain, ethyl group on C3, aldehyde at C1.

IUPAC Nomenclature for Ketones

A ketone has a carbonyl group ($C=O$) bonded to two carbon atoms within the chain, so you do need to specify its position.

Naming steps:

1. Find the longest carbon chain that includes the carbonyl carbon.
2. Replace the "-e" ending of the parent alkane with "-one" (e.g., pentane → pentanone).
3. Number the chain from the end that gives the carbonyl the lowest possible locant.
4. Place that number before the parent name (e.g., 2-pentanone, not 4-pentanone).
5. Name and number substituents as usual, using the same chain numbering.

Examples:

• 2-Pentanone: 5-carbon chain, carbonyl on C2.
• 3-Methyl-2-butanone: 4-carbon chain, carbonyl on C2, methyl on C3.
• 4-Ethyl-3-hexanone: 6-carbon chain, carbonyl on C3, ethyl on C4.

Note that propanone doesn't need a position number because there's only one possible location for the carbonyl in a 3-carbon ketone.

Common Names of Simple Carbonyls

Several simple aldehydes and ketones are still widely known by their common names. You should recognize both the common and IUPAC names for these:

Drawing Structures from Carbonyl Names

Being able to go from a name to a correct structure is a skill you'll use constantly. Here's the process for each type.

For aldehydes (e.g., 4-methylhexanal):

1. Read the root name to get the chain length (hexanal → 6 carbons).
2. Draw a straight chain of that length.
3. Place the $-CHO$ group at C1 (it's always terminal).
4. Attach substituents at the numbered positions (methyl on C4).
5. Fill in hydrogens so every carbon has four bonds.

For ketones (e.g., 4-heptanone):

1. Read the root name for chain length (heptanone → 7 carbons).
2. Draw a straight chain of that length.
3. Place the $C=O$ at the position indicated (C4 in this case).
4. Attach any substituents at their numbered positions.
5. Fill in hydrogens to complete the structure.

The carbonyl group ($C=O$) is the defining functional group for both aldehydes and ketones. Everything in this unit, from naming to reactivity (nucleophilic addition), traces back to the chemistry of that double bond.