carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom : C=O.

The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex (a metal carbonyl, e.g. nickel carbonyl); in this situation, carbon is triple-bonded to oxygen : C≡O.

A carbonyl group characterizes the following types of compounds:

 aldehyde
 ketone
 carboxylic acid
 ester
 amide
 enone
 acyl halide
 acid anhydride

Note that the most specific labels are usually employed. For example, R(CO)O(CO)R’ structures are known as acid anhydride rather than the more generic ester, even though the ester motif is present.

Other organic carbonyls are urea and carbamates, the derivatives of acyl chlorides chloroformates and phosgene, carbonate esters, thioesters, lactones, lactams, hydroxamates, and isocyanates.

Examples of inorganic carbonyl compounds are carbon dioxide and carbonyl sulfide.

Reactivity

Oxygen is more electronegative than carbon, and thus pulls 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.

The alpha hydrogens of a carbonyl compound are much more acidic ( 1030 times more acidic) than a typical C-H bond. For example, the pKa values of acetaldehyde and acetone are 16.7 and 19, respectively.[1] 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 can be alkylated by nucleophilic attack by organometallic reagents such as organolithium reagents and Grignard reagents. Carbonyls also 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
 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