Description and Properties
High-Yield Summary
- Aldehyde: carbonyl carbon has ≥1 H attached, always terminal. Ketone: carbonyl carbon has 2 alkyl/aryl groups, always internal.
- Aldehyde IUPAC: -e → -al (methanal, ethanal, propanal); terminal position means no locant needed in simple cases. Cyclic aldehydes use -carbaldehyde (e.g. cyclopentanecarbaldehyde).
- Ketone IUPAC: -e → -one, with a locant required in chains of 4+ carbons (2-propanone, 2-butanone). If outranked by a higher-priority group (e.g. carboxylic acid), named as the prefix oxo- (3-oxobutanoic acid).
- Boiling point order: alkanes < aldehydes/ketones (dipole-dipole from the polar carbonyl) < alcohols (hydrogen bonding, strongest).
- The carbonyl carbon's partial positive charge makes it electrophilic — the key driver of nucleophilic addition reactivity.
Forming Aldehydes/Ketones from Alcohols
| Alcohol + reagent | Product |
|---|---|
| Primary + PCC (mild) | → Aldehyde (stops here) |
| Secondary + Na₂Cr₂O₇/H₂SO₄ (strong) | → Ketone (no H on carbonyl C, can't go further) |
Key Terms
- Oxo- prefix
- Used to name an aldehyde/ketone carbonyl when a higher-priority group (e.g. carboxylic acid) is present and takes the parent suffix instead.
- Carbonyl polarity
- O is more electronegative than C, giving the carbonyl carbon a partial positive charge and oxygen a partial negative charge — drives dipole-dipole interactions and carbon's electrophilicity.
Common MCAT Trap
- Aldehydes rarely need a locant (terminal by definition), but ketones in chains of 4+ carbons always do — don't drop the position number for a ketone just because you're used to skipping it for aldehydes.
- Aldehydes and ketones can't hydrogen bond with themselves (no O-H bond) — their intermediate boiling point comes purely from dipole-dipole interactions, not H-bonding.
Quick Recall
Structurally, what distinguishes an aldehyde from a ketone?
Why do aldehydes/ketones have higher boiling points than alkanes but lower than alcohols?
What suffix is used for a cyclic aldehyde, and give an example.