Aldehydes and ketones

Chemical structure and physical properties

The core of oxo compounds is the carbonyl group. Due to the shift of the $\pi$-bond electron pair toward oxygen, a partial positive charge ($\delta+$) arises on the carbon atom, and a negative charge ($\delta-$) on the oxygen. This positive charge on the carbon enables it to accept a reagent electron pair, which determines the tendency toward nucleophilic addition reactions.

The physical properties of oxo compounds depend directly on the polarity of the $C=O$ group, yet their molecules lack hydrogen bonds. Consequently, their volatility is significantly higher than that of alcohols with a comparable carbon skeleton.

• Methanal (formaldehyde) is a gas under normal conditions.
• Ethanal (acetaldehyde) is a volatile liquid or easily liquefiable gas (boiling point only $20^\circ C$).
• Lower aldehyde homologs are highly soluble in water.

The first member of the aldehyde homologous series is methanal ($n=1$), and of the ketone series, propanone (acetone, $n=3$). On an industrial scale (over 3 million tons per year), acetone is used as a powerful solvent for varnishes, paints, and fats.

Main preparation methods

Different approaches are used for the synthesis of oxo compounds in laboratory and industrial settings.

1. Oxidation of alcohols Strong oxidizing agents, such as potassium dichromate ($K_2Cr_2O_7$) in an acidic medium ($H_2SO_4$), are frequently used in laboratories.

• Oxidation of primary alcohols (e.g., 1-propanol) yields aldehydes.
• Oxidation of secondary alcohols (e.g., 2-butanol) synthesizes ketones.

2. Hydrolysis of halogen derivatives Dihalogenoalkanes are used to obtain oxo compounds. Mandatory condition: both halogen atoms must be located on the same carbon atom.

3. Industrial methods

• Formaldehyde is produced in massive volumes by the catalytic oxidation of methanol (using a $Cu/CuO$ catalyst upon heating) or by the direct oxidation of methane.
• Acetaldehyde is produced by the hydration of acetylene or the catalytic oxidation of ethylene (in the presence of $PdCl_2$).

Chemical properties: addition and condensation

Due to the charge on the carbonyl carbon, aldehydes and ketones add a variety of substances (water, alcohols, ammonia, acetylene).

Hydration and interaction with alcohols Formaldehyde alone is almost completely hydrated in aqueous solution, existing as a dihydroxy derivative that decomposes upon isolation attempts. Ketones do not react with water. When aldehydes react with alcohols (in the presence of $H^+$ ions), hemiacetals are formed first. With an excess of alcohol, the hydroxyl group is replaced by an alkoxy group, yielding a stable acetal, a compound in which two oxygen atoms are attached to a single carbon atom. These are more stable than hydrates due to the presence of hydrocarbon radicals instead of labile hydrogen atoms.

Aldol condensation In an alkaline medium, aldehyde molecules can interact with each other. Oxygen withdraws electrons, causing the hydrogen at the adjacent ($\alpha$-) carbon atom to acquire acidic properties and mobility. One aldehyde molecule acts as an electrophile and adds another. As a result, an aldol is formed — a product containing both alcohol and aldehyde groups simultaneously.

Identification tests for aldehydes

Unlike ketones, aldehydes are active reducing agents and are easily oxidized to carboxylic acids even by weak reagents. This property forms the basis of identification tests.

1. Silver mirror test.

The aldehyde reacts with an ammoniacal silver oxide solution (diamminesilver(I) complex). As a result, a fine layer of metallic silver precipitates on the cleanly washed walls of the vessel. The aldehyde itself is converted into a salt (for example, ethanal yields ammonium acetate).

1. Oxidation with copper(II) hydroxide.

Freshly precipitated $Cu(OH)_2$ in an alkaline medium (e.g., with $NaOH$) is used as a reagent. The visual sign of the reaction is the formation of an orange-red precipitate of copper(I) oxide ($Cu_2O$).

In addition, aldehydes are oxidized by strong reagents, such as potassium permanganate ($KMnO_4$) in an acidic medium, converting into the corresponding carboxylic acids.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of nucleophilic addition reactions ($A_N$) to the carbonyl group.
• Features of formation and stability of hemiacetals and acetals.
• Synthesis of imines via the reaction of oxo compounds with ammonia.
• Acetylene addition reaction with carbon chain elongation.
• Polymerization processes of formaldehyde: paraform, trioxane, and carbohydrate synthesis.

Other topics in "Chapter 16. Oxygen-containing organic substances"

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