Aldehydes and Ketones: Structure, Properties, and Reactions

Compounds with Carbonyl Group

Features of Aldehydes and Ketones

Aldehydes and ketones are organic compounds containing the carbonyl functional group (C=O). The position of the carbonyl group distinguishes aldehydes from ketones.

• Aldehydes: The carbonyl group is bonded to at least one hydrogen atom. General formula: R-CHO.

• Ketones: The carbonyl group is bonded to two carbon atoms. General formula: R-CO-R'.

• Physical properties: Both are typically polar, with higher boiling points than alkanes of similar molecular weight due to dipole-dipole interactions.

Structure, Properties, and Reactions of Aldehydes

Physical and Chemical Properties of Aldehydes

Aldehydes exhibit distinct physical and chemical properties due to the presence of the carbonyl group.

• Solubility: Lower aldehydes are soluble in water due to hydrogen bonding; solubility decreases with increasing chain length.

• Melting and Boiling Points: Aldehydes have higher boiling points than hydrocarbons but lower than alcohols of similar mass.

Effect of Structure on Physical Properties

The structure of aldehydes influences their physical properties such as boiling point and solubility.

• Branching in the carbon chain generally lowers boiling point.

• Presence of electron-withdrawing or donating groups affects reactivity and polarity.

Conversion Between Names and Structures of Aldehydes

Understanding IUPAC and common naming conventions is essential for identifying aldehydes.

• IUPAC Naming: Replace the terminal '-e' of the parent alkane with '-al'.

• Example: Methanal (formaldehyde), Ethanal (acetaldehyde).

Reactions of Aldehydes

Aldehydes undergo various chemical reactions, including oxidation, reduction, and addition reactions.

• Oxidation: Aldehydes are easily oxidized to carboxylic acids.

• Reduction: Aldehydes can be reduced to primary alcohols.

• Tests: Tollens' test and Benedict's test are used to distinguish aldehydes from ketones.

Test	Positive for	Observation
Tollens' Test	Aldehydes	Silver mirror forms
Benedict's Test	Aldehydes	Red precipitate forms
Additional info:	Ketones	No reaction

Synthesis of Aldehydes

Aldehydes can be synthesized by controlled oxidation of primary alcohols.

• Equation:

$\text{R-CH}_2\text{OH} + [O] \rightarrow \text{R-CHO} + \text{H}_2\text{O}$

Structure, Properties, and Reactions of Ketones

Physical and Chemical Properties of Ketones

Ketones share many properties with aldehydes but differ in reactivity due to the absence of a hydrogen atom attached to the carbonyl carbon.

• Solubility: Lower ketones are soluble in water; solubility decreases with chain length.

• Boiling Points: Similar to aldehydes, but generally slightly higher due to increased molecular weight.

Effect of Structure on Physical Properties

The structure of ketones affects their physical properties and reactivity.

• Branching lowers boiling point.

• Substituents can affect polarity and reactivity.

Conversion Between Names and Structures of Ketones

Ketones are named by replacing the '-e' of the parent alkane with '-one'.

• Example: Propanone (acetone), Butanone.

Reactions of Ketones

Ketones are less reactive than aldehydes in oxidation reactions but can undergo reduction and addition reactions.

• Reduction: Ketones can be reduced to secondary alcohols.

• Equation:

$\text{R}_2\text{CO} + [H] \rightarrow \text{R}_2\text{CHOH}$

• Reaction with Alcohols: Ketones react with alcohols to form acetals and ketals.

Synthesis of Ketones

Ketones are commonly synthesized by oxidation of secondary alcohols.

• Equation:

$\text{R}_2\text{CHOH} + [O] \rightarrow \text{R}_2\text{CO} + \text{H}_2\text{O}$

Structure of Derivatives of Aldehydes and Ketones

Reactions with Alcohols: Formation of Acetals, Ketals, Hemiacetals, and Hemiketals

Aldehydes and ketones react with alcohols to form derivatives such as acetals, ketals, hemiacetals, and hemiketals. These reactions are important in organic synthesis and carbohydrate chemistry.

• Acetals: Formed from aldehydes and alcohols.

• Ketals: Formed from ketones and alcohols.

• Hemiacetals: Intermediate formed when one alcohol adds to an aldehyde.

• Hemiketals: Intermediate formed when one alcohol adds to a ketone.

Derivative	Formed from	General Reaction
Acetal	Aldehyde + 2 Alcohols	$\text{R-CHO} + 2\text{R'OH} \rightarrow \text{R-CH(OR')}_2$
Ketal	Ketone + 2 Alcohols	$\text{R}_2\text{CO} + 2\text{R'OH} \rightarrow \text{R}_2\text{C(OR')}_2$
Hemiacetal	Aldehyde + 1 Alcohol	$\text{R-CHO} + \text{R'OH} \rightarrow \text{R-CH(OH)(OR')}$
Hemiketal	Ketone + 1 Alcohol	$\text{R}_2\text{CO} + \text{R'OH} \rightarrow \text{R}_2\text{C(OH)(OR')}$

• Recognition: Acetals and ketals are stable in neutral and basic conditions but hydrolyze in acid.

• Synthesis: Used for protecting carbonyl groups during multi-step organic synthesis.

Example: Glucose forms a cyclic hemiacetal in aqueous solution.

Additional info: Acetals and ketals are important in carbohydrate chemistry and as protecting groups in organic synthesis.