20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters

Carbonyl compounds are essential in organic chemistry, featuring a carbon-oxygen double bond. Aldehydes, ketones, carboxylic acids, and esters all contain this group, giving them unique properties and reactivity. Their structures influence polarity, boiling points, and solubility.

These compounds play crucial roles in various industries and biological processes. Aldehydes and ketones undergo oxidation and reduction reactions, while carboxylic acids and esters participate in esterification and hydrolysis. Understanding their properties and reactions is key to grasping organic chemistry concepts.

Aldehydes and Ketones

Structure of carbonyl compounds

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• Aldehydes contain a carbonyl group ($\ce{C=O}$) at the end of a carbon chain with the general formula $\ce{R-CHO}$, making them polar molecules due to the carbonyl group and resulting in boiling points higher than alkanes and ethers of similar molar mass (ethanal, propanal)
• Aldehydes are soluble in water and organic solvents because of their polarity and ability to form hydrogen bonds with water molecules (methanal, benzaldehyde)
• Ketones contain a carbonyl group ($\ce{C=O}$) within a carbon chain with the general formula $\ce{R-CO-R'}$, making them polar molecules due to the carbonyl group and resulting in boiling points higher than aldehydes of similar molar mass (propanone, butanone)
• Ketones are soluble in organic solvents but less soluble in water than aldehydes because they lack the ability to form hydrogen bonds with water molecules as effectively as aldehydes (acetone, cyclohexanone)

Oxidation states in organic molecules

• In alcohols, the carbon bonded to the hydroxyl group ($\ce{-OH}$) has an oxidation state of -1 because the oxygen atom is more electronegative and draws electron density away from the carbon (methanol, ethanol)
• In aldehydes, the carbonyl carbon has an oxidation state of +1 because it is bonded to one carbon atom and one oxygen atom, and the oxygen atom draws more electron density (ethanal, propanal)
• In ketones, the carbonyl carbon has an oxidation state of +2 because it is bonded to two carbon atoms and one oxygen atom, and the oxygen atom draws more electron density (propanone, butanone)

Reactions of carbonyl compounds

• Aldehydes undergo oxidation reactions to form carboxylic acids, reduction reactions to form primary alcohols, and nucleophilic addition reactions with alcohols to form hemiacetals and acetals (oxidation of ethanal to ethanoic acid, reduction of propanal to propan-1-ol)
• Aldehydes are used in the production of plastics, dyes, and fragrances (formaldehyde in resins, benzaldehyde in perfumes)
• Ketones undergo reduction reactions to form secondary alcohols and nucleophilic addition reactions with alcohols to form ketals (reduction of propanone to propan-2-ol, formation of dimethyl ketal from propanone and methanol)
• Ketones are used as solvents and in the production of pharmaceuticals and polymers (acetone as a solvent, cyclohexanone in nylon production)

Carboxylic Acids and Esters

Structure of carbonyl compounds

• Carboxylic acids contain a carboxyl group ($\ce{-COOH}$) at the end of a carbon chain with the general formula $\ce{R-COOH}$, making them polar molecules due to the carboxyl group and resulting in high boiling points due to the formation of hydrogen bonds (ethanoic acid, propanoic acid)
• Carboxylic acids are soluble in water and organic solvents because of their polarity and ability to form hydrogen bonds with water molecules (methanoic acid, benzoic acid)
• Carboxylic acids have acidic properties due to their ability to donate protons from the carboxyl group (dissociation of ethanoic acid in water)
• Esters are formed by the condensation reaction between a carboxylic acid and an alcohol with the general formula $\ce{R-COO-R'}$, making them less polar than carboxylic acids due to the replacement of the hydroxyl group with an alkoxy group and resulting in lower boiling points than carboxylic acids of similar molar mass (ethyl ethanoate, methyl benzoate)
• Esters are soluble in organic solvents but less soluble in water than carboxylic acids because they lack the ability to form hydrogen bonds with water molecules as effectively as carboxylic acids (ethyl acetate, isoamyl acetate)

Oxidation states in organic molecules

• In carboxylic acids, the carbonyl carbon has an oxidation state of +3 because it is bonded to one carbon atom, one oxygen atom, and one hydroxyl group, and the oxygen atoms draw more electron density (ethanoic acid, propanoic acid)

Reactions of carbonyl compounds

• Carboxylic acids undergo esterification reactions with alcohols to form esters, reduction reactions to form aldehydes or primary alcohols, and deprotonation reactions to form carboxylate salts (esterification of ethanoic acid with ethanol to form ethyl ethanoate, reduction of propanoic acid to propanal)
• Carboxylic acids are used in the production of polymers, pharmaceuticals, and food additives (acrylic acid in plastics, acetylsalicylic acid (aspirin), citric acid as a preservative)
• Esters undergo hydrolysis reactions to form carboxylic acids and alcohols (reverse of esterification) and reduction reactions to form primary alcohols (hydrolysis of ethyl ethanoate to ethanoic acid and ethanol, reduction of methyl propanoate to propan-1-ol)
• Esters are used as solvents, fragrances, and flavorings (ethyl acetate as a solvent, isoamyl acetate in banana flavoring)

Properties and Structural Characteristics

• Functional groups: The carbonyl group ($\ce{C=O}$) is the key functional group in aldehydes, ketones, carboxylic acids, and esters, determining their chemical properties and reactivity
• Polarity: The presence of electronegative oxygen atoms in these functional groups creates dipole moments, influencing their solubility and boiling points
• Hydrogen bonding: Carboxylic acids and aldehydes can form hydrogen bonds with water and other molecules, affecting their physical properties and solubility
• Resonance structures: The carbonyl group in these compounds can form resonance structures, contributing to their stability and reactivity
• Acidity and basicity: Carboxylic acids are weak acids due to the stabilization of the conjugate base by resonance, while aldehydes and ketones can exhibit weak basicity through their carbonyl oxygen