Everything about Carbonyl Group totally explained
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, for example
nickel carbonyl); in this situation, carbon is
triple-bonded to oxygen : C≡O.
The remainder of this article concerns itself with the
organic chemistry definition of
carbonyl.
Carbonyl compounds
A carbonyl group characterizes the following types of compounds (where -CO denotes a C=O carbonyl group):
Other organic carbonyls are
urea and
carbamates. Examples of inorganic carbonyl compounds are
carbon dioxide,
carbonyl sulfide and
phosgene
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 hydrogen in a carbonyl compound is much more acidic (~10
30 times more acidic) than a typical CH bond. For example the pK
a values of
acetaldehyde and
acetone are 16.7 and 19, respectively.
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 LiAlH
4, and by
organometallic reagents such as
organolithium reagents and
Grignard reagents.
Other important reactions include:
α,β-Unsaturated carbonyl compounds
α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure C
β=C
α−C=O. In these compounds the carbonyl group is
conjugated with an
alkene (hence the
adjective unsaturated), from which they derive special properties. Examples of unsaturated carbonyls are
acrolein,
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, be it an
aldehyde or
acid or a
ketone, draws electrons away from the alkene and the alkene group in unsaturated carbonyls are therefore deactived towards an
electrophile such as
bromine or
hydrochloric acid. As a general rule with unsymmetric electrophiles hydrogen attaches itself at the α position in an
electrophilic addition.
On the other hand, these compounds are activated towards
nucleophiles in
nucleophilic addition.
Spectroscopy
Infrared spectroscopy: the C=O double bond absorbs infrared light at wavenumbers between approximately 1600–1900 cm−1. The exact location of the absorption is well understood with respect to the geometry of the molecule. This absorption is known as the "carbonyl stretch" when displayed on an infrared absorption spectrum.
Nuclear magnetic resonance: the C=O double-bond exhibits different resonances depending on surrounding atoms, generally a downfield shift. The 13C NMR of a carbonyl carbon is in the range of 160-220 ppm.Further Information
Get more info on 'Carbonyl Group'.
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