In organic chemistry, a carbonyl group is a functional group which has a carbon double bonded to an oxygen atom: C=O (Figure 2).
When a carbonyl functional group is placed within a molecule, it is known as a ketone. Ketones are organic compounds with the structure RC(=O)R', where R and R' can be a variety of carbon-containing substituents. IUPAC nomenclature rules dictate that ketone molecules are named by changing the suffix of the parent carbon molecule to -one. If the position of the ketone must be specified, then a number is placed between the parent chain name and the -one prefix (e.g., propan-2-one), or at the beginning of the IUPAC name. The prefixes "oxo" and "keto" are used to describe the ketone functional group (Figure 4).
The ketone carbon is sp2 hybridized, and it adopts a trigonal planar geometry around the ketonic carbon. The C–C–O and C–C–C bond angles are approximately 120 degrees. Because of the carbonyl group, ketones are polar and are able to interact with other compounds through hydrogen bonding; this hydrogen bond capability makes ketones more soluble in water than related methyelene compounds. Ketones are not usually hydrogen bond donors, and they tend not to "self associate" with other molecules of itself. As a result, ketones are often more volatile than alcohols and carboxylic acids of comparable molecular weights. Ketones have alpha-hydrogens which participate in keto-enol tautomerism. In the presence of strong base, enolate formation and subsequent deprotonation of the enolate will occur.
An aldehyde is an organic compound containing a carbonyl group, with the central carbon being bonded to a hydrogen and R group (R-CHO). Aldehydes differ from ketones in that the carbonyl is placed at the end of the carbon skeleton rather than between two carbon atoms of the backbone. Like ketones, aldehydes are sp2 hybridized and can exist in the keto or enol tautomer. Aldehydes are named by dropping the suffix of the parent molecule, and adding the suffix -al. If there are higher order functional groups on the compound, the prefix oxo- can be used to indicate which carbon atom is part of the aldehyde group. If the location of the aldehyde must be specified, a number can be used in between the parent chain and suffix, or at the beginning of the compound name (Figure 3).
Similarities of Aldehydes and Ketones
Both aldehydes and ketones exist in an equilibrium with their enol forms; the enol form is defined as an alkene with a hydroxyl group affixed to one of the carbon atoms composing the double bond. The keto form predominates at equilibrium for most ketones. However, the enol form is important for some reactions because the deprotonated enolate form is a strong nucleophile. The equilibrium is strongly thermodynamicaly driven, and at room temperature the keto form is favored. The interconversion can be performed in the presence of acid or base (Figure 1).
Both ketones and aldehydes can be identified by spectroscopic methods. They display strong vCO absorption bands near 1700 cm-1. In NMR spectroscopy, the carbonyl hydrogen shows a strong absorption peak, and any coupling to protons on the alpha carbon will also show strong signals.
Ketones and aldehydes can both be readily be reduced to alcohols, usually in the presence of a strong reducing agent such as sodium borohydride. In the presence of strong oxidants, they can be oxidized to carboxylic acids. As electrophiles, they are subject to attack by nucleophiles, meaning they participate in many nucleophilic addition reactions.