Overview
This lecture covers the structure, nomenclature, synthesis, and reactivity of aldehydes and ketones, highlighting their importance in organic chemistry and biochemistry.
Structure and Nomenclature
- Aldehydes and ketones contain a carbonyl group (C=O).
- The carbonyl carbon is attached to one hydrogen and one carbon in aldehydes; to two carbons in ketones.
- Aldehydes are named by adding “-al” to the parent chain; common names include formaldehyde and acetaldehyde.
- Ketones are named by adding “-one” to the parent chain; examples include acetone and acetophenone.
- For molecules with multiple carbonyl groups, the aldehyde takes naming priority.
- The prefix "oxo-" is used for ketones in compounds with both functional groups.
Synthesis of Aldehydes and Ketones
- Primary alcohols oxidized with mild agents (like PCC) yield aldehydes; strong agents may further oxidize to carboxylic acids.
- Secondary alcohols oxidized with strong agents produce ketones.
- Reactions typically require heating; solvent loss is prevented by using reflux.
- Chromium (VI) agents are effective but toxic; alternatives include ozone (ozonolysis), which cleaves alkenes to form carbonyls.
- Hydroboration (anti-Markovnikov) of alkynes produces less substituted enols, which tautomerize to aldehydes or ketones.
- Oxymercuration (Markovnikov) yields more substituted enols, forming ketones on tautomerization.
- Acid chlorides with organocopper compounds or reduction of esters with bulky agents can create aldehydes and ketones.
Reactivity and Mechanisms
- The carbonyl carbon is electrophilic due to dipolar resonance, making it susceptible to nucleophilic addition.
- Nucleophiles like cyanide, acetylide anions, or hydride anions can be added to carbonyls.
- Hydride reductions (with NaBH4 or LiAlH4) convert aldehydes to primary and ketones to secondary alcohols.
- Attack can occur from either side, producing racemic mixtures at chiral centers.
- The Wittig reaction uses a phosphonium ylide (negatively charged carbon next to a positive phosphorus) to convert carbonyls to alkenes.
- Primary ylides give Z-alkenes; stabilized ylides favor E-alkenes.
Key Terms & Definitions
- Carbonyl group — a carbon atom double-bonded to oxygen (C=O).
- Aldehyde — compound with a carbonyl at the end of a chain (C=O attached to H).
- Ketone — compound with a carbonyl bonded to two carbons.
- Oxidation — addition of oxygen or removal of hydrogen, often converting alcohols to carbonyls.
- Reflux — heating with continuous condensation to prevent solvent loss.
- Ozonolysis — cleavage of double bonds in alkenes/alkynes with ozone to form carbonyls.
- Tautomerization — equilibrium between two isomers (e.g., enol and keto forms).
- Nucleophilic addition — nucleophile attacks an electrophilic carbonyl carbon.
- Wittig reagent — phosphonium ylide used to convert carbonyls to alkenes.
Action Items / Next Steps
- Review oxidation and reduction mechanisms for alcohols, aldehydes, and ketones.
- Read about organometallic chemistry for the next lecture.