Aldehydes and Ketones

Overview of Aldehydes and Ketones

Aldehydes and ketones are carbonyls. In IUPAC nomenclature, aldehydes are indicated by changing the suffix from -ane to -anal. Ketones are indicated by changing the suffix from -ane to -anone. In general, aldehydes are more reactive than ketones.

An aldehyde is an organic compound that contains a carbonyl group (C=O{\rm{C{=}O}}) bound to one alkyl (R{-}{\rm {R}} ) fragment and one hydrogen atom, with RC(=O)H{ \rm{RC({=}O)H}} or RCHO{\rm{R{-}CHO}} stoichiometry. A carbonyl is a functional group in which a carbon atom is double bonded to an oxygen atom. Aldehydes have a carbon that is attached to an R group and a hydrogen and double bonded to an oxygen. The R group is any generic alkyl or aryl group.

A ketone is an organic compound that contains a carbonyl group (C=O{\rm{C{=}O}}) bound to two alkyl (R{-}{\rm {R}} ) or aryl (aromatic ring) fragments, with RC(=O)R {\rm{RC({=}O)R'}} or RCOR′ stoichiometry. The alkyl fragments can be the same (R and R) or different (R and R′). Ketones have a carbon attached to two R groups and double bonded to an oxygen.

The International Union of Pure and Applied Chemistry (IUPAC) is the organization that sets rules for nomenclature. IUPAC rules state that the longest chain attached to the CHO{-}{\rm{CHO}} group is the parent chain. The corresponding alkane suffix -ane is replaced by -anal. Propanal is the IUPAC name for the organic compound with the formula CH3CH2CHO.

The common names of aldehydes use the corresponding carboxylic acid's name by replacing the -ic acid with -aldehyde. For example, propionaldehyde (CH3CH2CHO) can be oxidized to propionic acid.
Propionaldehyde (CH3CH2CHO) is the common name for 1-propanal, or propanal.
In numbering the chain for substituents, numbering begins with the carbonyl carbon always being C-1.
Hexanal (C6H12O) is the IUPAC name for a six-carbon aldehyde. The locant numbers in the chain are assigned starting at the end that gives the carbonyl carbon the lowest possible locant, which for hexanal is 1.
In IUPAC nomenclature for ketones, the longest chain with the carbonyl group is the parent structure. The corresponding alkane suffix -ane is replaced with -anone. The positions of the carbonyl groups are indicated with a locant number. The carbonyl carbon has the lowest number possible. Common nomenclature for ketones names the two groups attached to the carbonyl carbon, followed by the word ketone. If the carbonyl group is attached to a benzene ring, it is named as a –phenone. For example, propiophenone (C6H5C(O)CH2CH3) is the common name for 1-phenylpropan-1-one, also called benzoylethane or BzEt.
Isobutyl methyl ketone ((CH3)2CHCH2C(O)CH3) is the common name for 4-methyl-2-pentanone.
Benzyl methyl ketone (C6H5CH2COCH3) is the common name for 1-phenyl-2-propanone.
Electronics includes the charge or electrical factors that influence an atom or molecule. Both aldehydes and ketones are polar groups. The oxygen of the carbonyl is electronegative, which means it attracts electrons and thus pulls electrons away from the carbonyl carbon. The carbonyl carbon is more partially positive in aldehydes than in ketones because of the electron-donating nature of the alkyl groups attached to ketones and aldehydes. Ketones have two electron donor groups, while aldehydes only have one.

Sterics includes the size, arrangement, and spatial factors that influence an atom or molecule. Aldehydes are generally more reactive than ketones. They are less sterically hindered because hydrogen is smaller than any other R group, and they are less stabilized by electron-donating alkyl groups (two alkyl groups for ketones versus one alkyl group and one H for aldehydes).

Ketones and aldehydes can be distinguished by 1H nuclear magnetic resonance (NMR) spectroscopy. Electron distribution in a molecule causes a chemical shift in hydrogens, or protons. Moving electrons create a small magnetic field that affects the hydrogen. Electrons around the atom create a shield about the hydrogen from the applied magnetic field. This shield allows for less energy to excite the proton from one spin state to another. If there is a lower electron density around an atom, the protons become deshielded, or have a larger chemical shift. Hydrogens attached to the carbonyl carbon of aldehydes appear around 10 ppm in a 1H NMR spectrum.
1H NMR of butanal shows a peak near 10 ppm.
The hydrogen atoms on the carbons adjacent to the carbonyl appear near 2–3 ppm for ketones.
1H NMR of butanone shows peaks at 2.45 and 2.12 ppm for the protons on the carbons attached to the carbonyl.
The π\pi and delocalized electrons create an induced magnetic field that may deshield or shield an atom. 13C NMRs are influenced by the same factors as 1H NMRs. 13C NMRs detect 13C isotope rather than 12C because 12C does not have a nuclear spin. A carbonyl carbon is deshielded because of its lowered electron density. The carbonyl carbons of aldehydes and ketones appear between 190 and 220 ppm in the 13C NMR spectrum.
13C NMR of butanal shows a peak at 202.2 ppm for the carbonyl carbon.
13C NMR of butanone shows peaks at 208.8, 36.8, 29.5, and 7.6 ppm. The peak at 208.8 ppm is the carbonyl carbon of butanone, and the peaks at 36.8, 29.5, and 7.6 ppm are the alkane carbons of butanone.