20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters

Carbonyl compounds all share a carbon-oxygen double bond ($\ce{C=O}$), but the atoms attached to that carbon determine whether you're looking at an aldehyde, ketone, carboxylic acid, or ester. Each arrangement changes the molecule's polarity, boiling point, solubility, and reactivity. This section covers how to tell these four groups apart, predict their physical properties, and understand their most important reactions.

Aldehydes and Ketones

Structure of Carbonyl Compounds

Both aldehydes and ketones are built around the carbonyl group ($\ce{C=O}$), but they differ in where that group sits.

Aldehydes have the carbonyl at the end of a carbon chain, with the general formula $\ce{R-CHO}$. That terminal hydrogen is what distinguishes them. Common examples include methanal (formaldehyde) and propanal.

• The carbonyl makes aldehydes polar, so their boiling points are higher than alkanes or ethers of similar molar mass.
• Shorter-chain aldehydes are soluble in water because the carbonyl oxygen can accept hydrogen bonds from water molecules.

Ketones have the carbonyl within the carbon chain, bonded to two carbon groups, with the general formula $\ce{R-CO-R'}$. Propanone (acetone) and butanone are common examples.

• Ketones are also polar, and their boiling points are comparable to aldehydes of similar size.
• They dissolve well in organic solvents. Smaller ketones like acetone are water-soluble, but as the hydrocarbon portion grows, water solubility drops because the nonpolar carbon chains dominate.

Oxidation States in Organic Molecules

Tracking oxidation states helps you see the logical progression from alcohols to aldehydes to carboxylic acids. Each step is an oxidation (loss of electrons from carbon).

• Alcohols: The carbon bonded to $\ce{-OH}$ is at a relatively low oxidation state. Think of it as the most "reduced" form.
• Aldehydes: The carbonyl carbon has been oxidized one step up from a primary alcohol. It's now double-bonded to oxygen instead of single-bonded.
• Ketones: The carbonyl carbon is bonded to two carbon groups and one oxygen. Ketones sit at an oxidation level corresponding to secondary alcohols.

The key pattern: the more bonds to oxygen (or the fewer bonds to hydrogen) a carbon has, the more oxidized it is.

Reactions of Carbonyl Compounds

Aldehyde reactions:

1. Oxidation → carboxylic acids. For example, ethanal ($\ce{CH3CHO}$) oxidizes to ethanoic acid ($\ce{CH3COOH}$). This is a useful test: aldehydes can be oxidized easily, ketones cannot (under normal conditions).
2. Reduction → primary alcohols. Propanal reduces to propan-1-ol by adding hydrogen across the $\ce{C=O}$ bond.
3. Nucleophilic addition with alcohols → hemiacetals and acetals.

Aldehydes show up in industry as building blocks for plastics (formaldehyde in resins) and fragrances (benzaldehyde gives an almond scent).

Ketone reactions:

1. Reduction → secondary alcohols. Propanone reduces to propan-2-ol.
2. Nucleophilic addition with alcohols → ketals.
3. Ketones resist oxidation under mild conditions, which is one way to distinguish them from aldehydes in the lab.

Ketones are widely used as solvents (acetone) and as intermediates in pharmaceutical and polymer manufacturing (cyclohexanone in nylon production).

Carboxylic Acids and Esters

Structure of Carbonyl Compounds

Carboxylic acids have a carboxyl group ($\ce{-COOH}$) at the end of a carbon chain, with the general formula $\ce{R-COOH}$. This group combines a carbonyl and a hydroxyl on the same carbon. Ethanoic acid (vinegar) and propanoic acid are familiar examples.

• The $\ce{-COOH}$ group can both donate and accept hydrogen bonds. Carboxylic acids actually form hydrogen-bonded dimers (two molecules pairing up), which is why their boiling points are notably high.
• Short-chain carboxylic acids are very soluble in water. Methanoic acid and ethanoic acid mix freely with water.
• They are weak acids: the $\ce{-COOH}$ group donates a proton to water, forming a carboxylate ion ($\ce{RCOO^-}$). This conjugate base is stabilized by resonance, with the negative charge spread equally over both oxygen atoms. That resonance stabilization is why carboxylic acids are acidic enough to lower pH, even though they don't fully dissociate.

Esters form when a carboxylic acid reacts with an alcohol, replacing the $\ce{-OH}$ of the acid with an $\ce{-OR'}$ group. Their general formula is $\ce{R-COO-R'}$. Ethyl ethanoate and methyl benzoate are common examples.

• Esters are less polar than carboxylic acids because they've lost the $\ce{O-H}$ bond that enabled strong hydrogen bonding. This means lower boiling points and lower water solubility compared to carboxylic acids of similar size.
• They dissolve well in organic solvents. Many esters have pleasant, fruity smells: isoamyl acetate is responsible for banana flavoring.

Oxidation States in Organic Molecules

In a carboxylic acid, the carbonyl carbon is bonded to two oxygen atoms (one double-bonded, one in the $\ce{-OH}$). This makes it more oxidized than an aldehyde or ketone. Carboxylic acids represent the most oxidized single-carbon functional group you'll encounter in this course (short of $\ce{CO2}$ itself).

The oxidation progression for a primary alcohol looks like this:

Reactions of Carbonyl Compounds

Carboxylic acid reactions:

1. Esterification: A carboxylic acid reacts with an alcohol (usually with an acid catalyst) to produce an ester and water. For example, ethanoic acid + ethanol → ethyl ethanoate + water. This is a condensation reaction.
2. Deprotonation: Reacting with a base forms a carboxylate salt (e.g., sodium ethanoate).
3. Reduction: Strong reducing agents can convert carboxylic acids back to aldehydes or all the way to primary alcohols.

Carboxylic acids are everywhere in industry and biology: acrylic acid is used in plastics, acetylsalicylic acid is aspirin, and citric acid serves as a food preservative.

Ester reactions:

1. Hydrolysis: The reverse of esterification. Adding water (with acid or base catalyst) breaks an ester back into a carboxylic acid and an alcohol. Ethyl ethanoate hydrolyzes to ethanoic acid + ethanol.
2. Reduction: Esters can be reduced to primary alcohols.

Esters are used as solvents (ethyl acetate), fragrances, and flavorings throughout the food and cosmetics industries.

Properties and Structural Characteristics

A few cross-cutting ideas tie all four compound types together:

• The carbonyl group ($\ce{C=O}$) is the defining feature of all four classes. What's attached to the carbonyl carbon determines which class you're dealing with.
• Polarity and boiling points: Every compound here is polar because of the electronegative oxygen atoms. Boiling points follow this general trend: carboxylic acids > aldehydes/ketones > esters (for similar molar mass), driven mainly by differences in hydrogen bonding ability.
• Hydrogen bonding and solubility: Carboxylic acids are the strongest hydrogen bonders (they can both donate and accept). Aldehydes and ketones can only accept hydrogen bonds. Esters have the weakest interactions with water. This pattern directly predicts water solubility.
• Resonance: The carbonyl group participates in resonance in all four classes, but it matters most for carboxylic acids. Resonance stabilization of the carboxylate ion ($\ce{RCOO^-}$) is the reason carboxylic acids can act as acids.
• Acidity: Carboxylic acids are weak acids. Aldehydes and ketones are not meaningfully acidic under normal conditions, though the carbonyl oxygen can act as a very weak base by accepting a proton.