Reactivity
Oxygen is more electronegative than carbon, and thus draws electron density away from carbon to increase the bond's polarity. Therefore, the carbonyl carbon becomes electrophilic, and thus more reactive with nucleophiles. Also, the electronegative oxygen can react with an electrophile; for example a proton in an acidic solution or other Lewis Acid forming a oxocarbenium ion.
The alpha hydrogens of a carbonyl compound are much more acidic (~103 times more acidic) than a typical C-H bond. For example, the pKa values of acetaldehyde and acetone are 16.7 and 19, respectively. This is because a carbonyl is in tautomeric resonance with an enol. The deprotonation of the enol with a strong base produces an enolate, which is a powerful nucleophile and can alkylate electrophiles such as other carbonyls.
Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the nitrogen-carbon and carbon-oxygen bonds.
Carbonyl groups can be reduced by reaction with hydride reagents such as NaBH4 and LiAlH4, or catalytically by hydrogen and a catalyst such as copper chromite, Raney nickel, rhenium, ruthenium or even rhodium. Ketones give secondary alcohols; aldehydes, esters and carboxylic acids give primary alcohols.
Carbonyls would be alkylated by nucleophilic attack by organometallic reagents such as organolithium reagents and Grignard reagents. Carbonyls also may be alkylated by enolates as in aldol reactions. Carbonyls are also the prototypical groups with vinylogous reactivity, e.g. the Michael reaction where an unsaturated carbon in conjugation with the carbonyl is alkylated instead of the carbonyl itself.
Other important reactions include:
- Wittig Reaction a phosphonium ylid is used to create an alkene
- Wolff-Kishner reduction into a hydrazone and further into a saturated alkane
- Clemmensen reduction into a saturated alkane
- Mozingo reduction into a saturated alkane
- Conversion into thioacetals
- Hydration to hemiacetals and hemiketals, and then to acetals and ketals
- Reaction with ammonia and primary amines to form imines
- Reaction with hydroxylamines to form oximes
- Reaction with cyanide anion to form cyanohydrins
- Oxidation with oxaziridines to acyloins
- Reaction with Tebbe's reagent and phosphonium ylides to alkenes.
- Perkin reaction, an aldol reaction variant
- Aldol condensation, a reaction between an enolate and a carbonyl
- Cannizzaro reaction, a disproportionation of aldehydes into alcohols and acids
- Tishchenko reaction, another disproportionation of aldehydes that gives a dimeric ester
- Nucleophilic abstraction is used to produce carbon dioxide
Read more about this topic: Carbonyl