Properties of Aldehydes and ketones
Carbonyl group
The carbonyl group is a C=O group. The carbonyl group is planar with bond angles of 120°, and consists of two sp2 hybridized atoms (C and O) linked by a strong σ bond and a weaker π bond. The carbonyl group is polarized such that oxygen is slightly negative and carbon is slightly posi-tive. In aldehydes and ketones, the substituents must be one or more of the following – an alkyl group, an aromatic ring, or a hydrogen.
Properties
Aldehydes and ketones have higher boiling points than alkanes of compa-rable molecular weight due to the polarity of the carbonyl group. However, they have lower boiling points than comparable alcohols or carboxylic acids due to the absence of hydrogen bonding. Aldehydes and ketones of small molecular weight are soluble in aqueous solution since they can participate in intermolecular hydrogen bonding with water. Higher molecular weight aldehydes and ketones are not soluble in water since the hydrophobic char-acter of the alkyl chains or aromatic rings outweighs the polar character of the carbonyl group.
Nucleophilic and electrophilic centers
The oxygen of the carbonyl group is a nucleophilic center. The carbonyl car-bon is an electrophilic center.
Keto–enol tautomerism
Ketones are in rapid equilibrium with an isomeric structure called an enol. The keto and enol forms are called tautomers and the process by which they interconvert is called keto–enol tautomerism. The mechanism can be acid or base catalyzed.
Spectroscopic analysis of aldehydes and ketones
Aldehydes and ketones show strong carbonyl stretching absorptions in their IR spectra as well as a quaternary carbonyl carbon signal in their 13C nmr spectra. Aldehydes also show characteristic C–H stretching absorp-tions in their IR spectra and a signal for the aldehyde proton in the 1H nmr which occurs at high chemical shift. The mass spectra of aldehydes and ketones usually show fragmentation ions resulting from cleavage next to the carbonyl group. The position of the uv absorption band is useful in the structure determination of conjugated aldehydes and ketones.
Carbonyl group
Both aldehydes and ketones contain a carbonyl group (C=O). The substituents attached to the carbonyl group determine whether it is an aldehyde or a ketone, and whether it is aliphatic or aromatic.
The geometry of the carbonyl group is planar with bond angles of 120°. The carbon and oxygen atoms of the carbonyl group are sp2 hybridized and the double bond between the atoms is made up of a strong σ bond and a weaker π bond. The carbonyl bond is shorter than a C−O single bond (1.22 Å vs. 1.43 Å) and is also stronger since two bonds are present as opposed to one (732 kJ mol−1 vs. 385 kJ mol−1). The carbonyl group is more reactive than a C−O single bond due to the relatively weak π bond.
The carbonyl group is polarized such that the oxygen is slightly negative and the carbon is slightly positive. Both the polarity of the carbonyl group and the presence of the weak π bond explains much of the chemistry and the physical properties of aldehydes and ketones. The polarity of the bond also means that the carbonyl group has a dipole moment.
Properties
Due to the polar nature of the carbonyl group, aldehydes and ketones have higher boiling points than alkanes of similar molecular weight. However, hydrogen bonding is not possible between carbonyl groups and so aldehydes and ketones have lower boiling points than alcohols or carboxylic acids.
Low molecular weight aldehydes and ketones (e.g. formaldehyde and acetone) are soluble in water. This is because the oxygen of the carbonyl group can partic- ipate in intermolecular hydrogen bonding with water molecules. As molecular weight increases, the hydrophobic character of the attached alkyl chains starts to outweigh the water solubility of the carbonyl group with the result that large molecular weight aldehydes and ketones are insoluble in water. Aro-matic ketones and aldehydes are insoluble in water due to the hydrophobic aro- matic ring.
Nucleophilic and electrophilic centers
Due to the polarity of the carbonyl group, aldehydes and ketones have a nucleophilic oxygen center and an electrophilic carbon center as shown for propanal. Therefore, nucleophiles react with aldehydes and ketones at the carbon center, and electrophiles react at the oxygen center.
Keto–enol tautomerism
Ketones which have hydrogen atoms on their α-carbon (the carbon next to the carbonyl group) are in rapid equilibrium with an isomeric structure called an enol where the α-hydrogen ends up on the oxygen instead of the carbon. The two isomeric forms are called tautomers and the process of equilibration is called tautomerism (Fig. 4). In general, the equilibrium greatly favors the keto tautomer and the enol tautomer may only be present in very small quantities.
The tautomerism mechanism is catalyzed by acid or base. When catalyzed byacid (Fig. 5), the carbonyl group acts as a nucleophile with the oxygen using a lone pair of electrons to form a bond to a proton. This results in the carbonyl oxygen gaining a positive charge which activates the carbonyl group to attack by weak nucleophiles (Step 1). The weak nucleophile in question is a water molecule which removes the α-proton from the ketone, resulting in the formation of a new C = C double bond and cleavage of the carbonyl π bond. The enol tautomer is formed thus neutralizing the unfavorable positive charge on the oxygen (Step 2).
Under basic conditions (Fig. 6), an enolate ion is formed, which then reacts with water to form the enol.
Spectroscopic analysis of aldehydes and ketones
The IR spectra of aldehydes and ketones are characterized by strong absorptions due to C=O stretching. These occur in the region 1740–1720 cm−1 for aliphatic aldehydes and 1725–1705 cm−1 for aliphatic ketones. However conjugation to aromatic rings or alkenes weakens the carbonyl bond resulting in absorptions at lower wavenumbers.
For example, the carbonyl absorptions for aromatic aldehydes and ketones are in the regions 1715–1695 cm−1 and 1700–1680 cm−1 respectively. For cyclic ketones, the absorption shifts to higher wavenumber with increasing ring strain. For example, the absorptions for cyclohexanone and cyclobutanone are 1715 and 1785 cm−1 respectively.
In the case of an aldehyde, two weak absorptions due to C–H stretching of the aldehyde proton may be spotted, one in the region 2900–2700 cm−1 and the other close to 2720 cm−1. The aldehyde proton gives a characteristic signal in the 1H nmr in the region 9.4–10.5 ppm. If the aldehyde group is linked to a carbon bearing a hydrogen, coupling will take place, typically with a small coupling constant of about 3 Hz. Indications of an aldehyde or ketone can be obtained indirectly from the 1H nmr by the chemical shifts of neighboring groups. For example, the methyl signal of a methyl ketone appears at 2.2 ppm as a singlet.
The carbonyl carbon can be spotted as a quaternary signal in the 13C nmr spec-trum in the region 200–205 ppm for aliphatic aldehydes and 205–218 ppm for aliphatic ketones. The corresponding regions for aromatic aldehydes and ketones are 190–194 ppm and 196–199 respectively.
The mass spectra of aldehydes and ketones often show fragmentation ions resulting from bond cleavage on either side of the carbonyl group (α-cleavage). Aromatic aldehydes and ketones generally fragment to give a strong peak at m/e 105 due to the benzoyl fragmentation ion [PhC=O]+.
The carbonyl groups of saturated aldehydes and ketones give a weak absorp-tion band in their uv spectra between 270 and 300 nm. This band is shifted to longer wavelengths (300–350 nm) when the carbonyl group is conjugated with a double bond. The exact position of the uv absorption band can be useful in the structure determination of conjugated aldehydes and ketones.