Aldehydes and Ketones
High-Yield Summary
- Both aldehydes and ketones contain a carbonyl group (C=O); carbonyl position distinguishes them.
- Aldehydes: carbonyl at the end of the chain, bonded to at least one H. Suffix -al, replacing -e. Always carbon 1, so no locant needed.
- Ketones: carbonyl within the chain, bonded to two other carbons. Suffix -one.
- When a higher-priority functional group is present, both use the prefix oxo- instead of their suffix; ketones can alternately use keto-.
- Worked example: 4-oxopentanoic acid (levulinic acid) — carboxylic acid outranks the ketone, so the ketone is demoted to the oxo- prefix.
Key Terms
- Carbonyl group
- A carbon atom double-bonded to an oxygen atom (C=O); shared by aldehydes and ketones.
- Aldehyde
- Terminal carbonyl bonded to at least one hydrogen; suffix -al (methanal/formaldehyde, ethanal/acetaldehyde, propanal/propionaldehyde).
- Ketone
- Internal carbonyl bonded to two carbons; suffix -one (2-propanone/acetone, but-3-en-2-one).
- Oxo- / keto- prefix
- Used for aldehyde/ketone carbonyls when a higher-priority group (e.g. carboxylic acid) takes the suffix instead; keto- is ketone-specific alternate.
Aldehyde vs. Ketone
| Feature | Aldehyde / Ketone |
|---|---|
| Carbonyl position | End of chain / Within chain |
| Carbonyl carbon bonded to | ≥1 hydrogen / 2 other carbons |
| Suffix | -al / -one |
| Locant needed? | No (always C1) / Yes (position within chain) |
| Prefix if demoted | oxo- / oxo- or keto- |
Common MCAT Trap
- An aldehyde is always carbon 1 by definition — never write a locant number for it.
- But-3-en-2-one: the ketone (higher priority) gets the lower locant (2), the double bond gets the higher one (3) — priority controls numbering, not just 'lowest set.'
- 4-oxopentanoic acid: the carboxylic acid, not the ketone, controls the suffix and numbering start — the ketone is forced into the oxo- prefix despite being a valid carbonyl group on its own.
Quick Recall
What structural feature determines whether a carbonyl-containing compound is an aldehyde or a ketone?
Why doesn't an aldehyde ever need a locant number in its name?
What prefix is used for a ketone or aldehyde carbonyl when a higher-priority group takes the suffix?
What is the complete name for a 5-carbon chain that's a carboxylic acid at C1 with a ketone at C4?