15.3: Properties of Aldehydes and Ketones
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- Explain why the boiling points of aldehydes and ketones are higher than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols.
- Compare the solubilities in water of aldehydes and ketones of four or fewer carbon atoms with the solubilities of comparable alkanes and alcohols.
- Name the typical reactions take place with aldehydes and ketones.
- Describe some of the uses of common aldehydes and ketones.
The carbon-to-oxygen double bond is quite polar, more polar than a carbon-to-oxygen single bond. The electronegative oxygen atom has a much greater attraction for the bonding electron pairs than does the carbon atom. The carbon atom has a partial positive charge, and the oxygen atom has a partial negative charge:
In aldehydes and ketones, this charge separation leads to dipole-dipole interactions that are great enough to significantly affect the boiling points. Table \(\PageIndex{1}\) shows that the polar single bonds in ethers have little such effect, whereas hydrogen bonding between alcohol molecules is even stronger.
Compound | Family | Molar Mass | Type of Intermolecular Forces | Boiling Point (°C) |
---|---|---|---|---|
CH3CH2CH2CH3 | alkane | 58 | dispersion only | –1 |
CH3OCH2CH3 | ether | 60 | weak dipole | 6 |
CH3CH2CHO | aldehyde | 58 | strong dipole | 49 |
CH3CH2CH2OH | alcohol | 60 | hydrogen bonding | 97 |
Formaldehyde is a gas at room temperature. Acetaldehyde boils at 20°C; in an open vessel, it boils away in a warm room. Most other common aldehydes are liquids at room temperature.
Although the lower members of the homologous series have pungent odors, many higher aldehydes have pleasant odors and are used in perfumes and artificial flavorings. As for the ketones, acetone has a pleasant odor, but most of the higher homologs have rather bland odors.
The oxygen atom of the carbonyl group engages in hydrogen bonding with a water molecule.
The solubility of aldehydes is therefore about the same as that of alcohols and ethers. Formaldehyde, acetaldehyde, and acetone are soluble in water. As the carbon chain increases in length, solubility in water decreases. The borderline of solubility occurs at about four carbon atoms per oxygen atom. All aldehydes and ketones are soluble in organic solvents and, in general, are less dense than water.
Oxidation of Aldehydes and Ketones
Aldehydes and ketones are much alike in many of their reactions, owing to the presence of the carbonyl functional group in both. They differ greatly, however, in one most important type of reaction: oxidation. Aldehydes are readily oxidized to carboxylic acids, whereas ketones resist oxidation.
The aldehydes are, in fact, among the most easily oxidized of organic compounds. They are oxidized by oxygen (O2) in air to carboxylic acids.
\[2RCHO + O_2 \rightarrow 2RCOOH \label{14.10.1} \]
The ease of oxidation helps chemists identify aldehydes. A sufficiently mild oxidizing agent can distinguish aldehydes not only from ketones but also from alcohols. Tollens’ reagent, for example, is an alkaline solution of silver (Ag+) ion complexed with ammonia (NH3), which keeps the Ag+ ion in solution.
\[H_3N—Ag^+—NH_3 \label{14.10.2} \]
When Tollens’ reagent oxidizes an aldehyde, the Ag+ ion is reduced to free silver (Ag).
Deposited on a clean glass surface, the silver produces a mirror (Figure \(\PageIndex{1}\)). Ordinary ketones do not react with Tollens’ reagent.
Although ketones resist oxidation by ordinary laboratory oxidizing agents, they undergo combustion, as do aldehydes.
Summary
The polar carbon-to-oxygen double bond causes aldehydes and ketones to have higher boiling points than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols that engage in intermolecular hydrogen bonding. Aldehydes are readily oxidized to carboxylic acids, whereas ketones resist oxidation.