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Carbonyl compounds [edit ]

A carbonyl group characterizes the following types of compounds:

Compound Aldehyde

Ketone

Carboxylic acid

Ester

Amide

Structure

General formula RCHO RCOR' RCOOH RCOOR' RCONR'R''

Compound Enone Acyl halide Acid anhydride Imide

Structure

General formula RC(O)C(R')CR''R''' RCOX (RCO) 2O RC(O)N(R')C(O)R'''

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.

Carbon dioxide

Other organic carbonyls are urea and the 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 .

A special group of carbonyl compounds are 1,3-dicarbonyl compounds that have acidic protons in the

central methylene unit. Examples are Meldrum's acid , diethyl malonate and acetylacetone .

Reactivity [edit ]

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Carbonyl group

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 an oxocarbenium ion.

The alpha hydrogens of a carbonyl compound are much more acidic (~10 3 times more acidic) than a typical

C-H bond. For example, the pK a 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 NaBH 4 and LiAlH4, orcatalytically 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:

Carbonyl Alpha-Substitution Reactions

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

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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

α,β -Unsaturated carbonyl compounds [edit ]

Acrolein , an α,β -unsaturated carbonyl compound.

α,β -Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general

structure −(O=C)−C α=C β−. In these compounds the carbonyl group is conjugated with an alkene (hence the

adjective unsaturated ), from which they derive special properties. Unlike the case for simple carbonyls, α,β -

unsaturated carbonyl compounds are often attacked by nucleophiles at the β carbon. This pattern of

reactivity is called vinylogous . Examples of unsaturated carbonyls ar e acrolein (propenal), mesityl

oxide , acrylic acid , and maleic acid . Unsaturated carbonyls can be prepared in the laboratory in an aldol

reaction and in the Perkin reaction .

The carbonyl group draws electrons away from the alkene , and the alkene group is, therefore, deactivated

towards an electrophile , such as bromine or hydrochloric acid . As a general rule with asymmetric

electrophiles, hydrogen attaches itself at the α -position in an electrophilic addition . On the other hand,

these compounds are activated towards nucleophiles in nucleophilic conjugate addition .

Since α,β -unsaturated compounds are electrophiles, many α,β -unsaturated carbonyl compounds are toxic,

mutagenic and carcinogenic. DNA can attack the β carbon and thus be alkylated . However, the

endogenous scavenger compoun dglutathione naturally protects from toxic electrophiles in the body.

Spectroscopy [edit ]

Infrared spectroscopy : the C=O double bond absorbs infrared light at wavenumbers betweenapproximately 1600 – 1900 cm −1 . The exact location of the absorption is well understood with respect to

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the geometry of the molecule. This absorption is known as the "carbonyl stretch" when displayed on an

infrared absorption spectrum .[2]

Nuclear magnetic resonance : the C=O double-bond exhibits different resonances depending on

surrounding atoms, generally a downfield shift. The 13 C NMR of a carbonyl carbon is in the range of

160-220 ppm.

See also [edit ]

Organic chemistry

Functional group

Bridging carbonyl