Aldehydes, Ketones and Carboxylic Acids

Aldehydes, ketones, and carboxylic acids are carbonyl compounds—they contain the C=O (carbonyl) functional group—one of the most important in organic chemistry. This versatile group is a chemical "hotspot": electrons can be pulled toward the electronegative oxygen, creating a reactive carbon. Aldehydes (R-CHO) are found in vanilla extract, caraway seeds, and industrial synthesis. Ketones (R-CO-R') range from acetone (solvent) to steroid hormones. Carboxylic acids (R-COOH) are the "core" of fat molecules, amino acids, and countless pharmaceuticals. Understanding their structures, preparations, and reactions unlocks organic chemistry.

Carbonyl Compounds: The C=O Bond

The carbonyl group (C=O) is the foundation. The double bond consists of a σ-bond and a π-bond. Oxygen, being more electronegative than carbon, pulls electron density toward itself, creating a polar C=O. The carbon becomes electron-deficient (electrophilic), making it susceptible to nucleophilic attack—the dominant reactivity of carbonyls.

Aldehydes: The Reactive End

Aldehydes contain a carbonyl at the terminal position, bonded to a hydrogen: R-CHO. The hydrogen makes them far more reactive than ketones—it's easily oxidized and acts as a handle for chemical modifications.

Structure and Nomenclature

IUPAC names end in -al: formaldehyde (H-CHO), acetaldehyde (CH₃-CHO), propanal (CH₃CH₂-CHO).

Common names are older: formaldehyde, acetaldehyde, benzaldehyde.

The carbonyl carbon counts as position 1 in the parent chain.

Physical Properties of Aldehydes

Boiling points are higher than alkanes of similar molecular weight (C=O is polar) but lower than alcohols (no hydrogen bonding between aldehydes).

Solubility: Aldehydes with fewer than 6 carbons are water-soluble (hydrogen bonding with water), becoming less soluble with longer chains.

Odor: Many aldehydes have distinctive, often pleasant smells. Benzaldehyde smells like bitter almonds; vanillin (vanilla aldehyde) is aromatic; malonaldehyde smells like fresh-cut grass.

Preparation of Aldehydes

Oxidation of Primary Alcohols (mild conditions preserve aldehyde; strong oxidizers convert to carboxylic acid): R-CH₂OH + [O] → R-CHO

Reagents: PCC (pyridinium chlorochromate) oxidizes selectively to aldehyde; KMnO₄ goes further to carboxylic acid.

Reduction of Carboxylic Acids or Esters: R-COOH + LiAlH₄ → R-CH₂OH (gives alcohol, not aldehyde) R-COOR' + LiAlH₄ → R-CH₂OH + R'OH (usually goes to primary alcohol)

To stop at aldehyde, use Diisobutylaluminum hydride (DIBAL-H) at low temperature.

From Alkynes: R-C≡C-H + H₂O → R-CH₂-CHO (via hydration, followed by tautomerization)

Reactions of Aldehydes

Oxidation: The signature reaction of aldehydes. The C-H is easily oxidized to C-OH (carboxylic acid). R-CHO + [O] → R-COOH

This is so characteristic that it's used diagnostically: aldehydes turn Tollens reagent (silver mirror test) or Fehling reagent blue/brick-red.

Nucleophilic Addition to C=O: The polar C=O is attacked by nucleophiles. The π-electrons move to oxygen (which can stabilize a negative charge), and the nucleophile bonds to carbon:

R-CHO + Nu⁻ → R-CH(OH)-Nu (after protonation)

Cyanohydrin Formation (HCN adds): R-CHO + HCN → R-CH(OH)-CN

Used to extend carbon chains or introduce the nitrile (future carboxylic acid) group.

Grignard Addition (R-MgX adds): R-CHO + R'-MgX → R-CH(OH)-R'

Forms secondary alcohols. Useful for building complex molecules.

Aldol Condensation (two aldehydes couple): 2 R-CHO → R-CH(OH)-CH(R)-CHO (aldol) → R-CH=C(R)-CHO (α,β-unsaturated aldehyde)

Creates a new C-C bond; a classic carbon-skeleton-building reaction.

Ketones: Stable Carbonyls

Ketones have the carbonyl between two carbons: R-CO-R'. They're more stable and less reactive than aldehydes because:

• No easily oxidizable hydrogen on the carbonyl carbon
• The carbonyl carbon is shielded by two alkyl groups, reducing nucleophile access

Nomenclature and Properties

IUPAC names end in -one: propanone (acetone), butanone (methyl ethyl ketone), cyclohexanone.

Physical properties similar to aldehydes but slightly different: ketones are generally less reactive toward oxidation, and many have distinctive smells (acetone used in nail polish remover; cyclohexanone has a peppermint-like odor).

Preparation of Ketones

Oxidation of Secondary Alcohols: R-CH(OH)-R' + [O] → R-CO-R'

Mild oxidizers (like dichromate) stop here, unlike primary alcohols which further oxidize to carboxylic acids.

From Alkynes: R-C≡C-R' + H₂O → R-CO-CH₂-R' (via hydration and tautomerization)

Friedel-Crafts Acylation (aromatic ketones): Ar-H + R-COCl → Ar-CO-R + HCl (with AlCl₃ catalyst)

Reactions of Ketones

Ketones resist oxidation but undergo nucleophilic addition:

Aldol Condensation: 2 R-CO-R' → products (similar to aldehydes but slower because the carbonyl carbon is more hindered)

Grignard Addition: R-CO-R' + R''-MgX → R-C(OH)(R')-R''

Forms tertiary alcohols.

Haloform Reaction (ketones with methyl group): CH₃-CO-R + X₂ (Br₂, Cl₂, or I₂) → R-COOH + CHX₃

The methyl group is oxidized and cleaved; useful for preparing carboxylic acids and the haloform.

Carboxylic Acids: Acidic Carbonyls

Carboxylic acids contain the carboxyl group (-COOH): R-COOH. The hydroxyl makes them far more reactive and acidic than aldehydes or ketones. They're ubiquitous in nature: fatty acids (fats), amino acids (proteins), citric acid (fruits).

Structure and Nomenclature

The carboxyl group is electron-withdrawing: the C=O pulls electrons, and the C-OH is activated for various reactions.

IUPAC names end in -oic acid: methanoic acid (formic), ethanoic acid (acetic), benzoic acid.

Common names: formic acid (from ants), acetic acid (from vinegar), butyric acid (from butter).

Physical Properties

Boiling points: Very high due to strong hydrogen bonding (carboxylic acids form dimers through O-H...O hydrogen bonds between the OH of one molecule and the C=O of another). Acetic acid boils at 118°C while acetaldehyde boils at 21°C!

Solubility: Carboxylic acids are water-soluble, even long-chain ones, because the carboxyl group is very hydrophilic.

Acidity: The defining feature. Unlike alcohols (pKa ~ 15-16), carboxylic acids have pKa ~ 4-5. The carboxylate ion (RCOO⁻) is stabilized by resonance: the negative charge delocalizes across both oxygen atoms of the C=O and the C-OH.

Stronger acids than alcohols/phenols because the carboxylate ion is more stable (resonance from the C=O helps delocalize the negative charge).

Preparation of Carboxylic Acids

Oxidation of Primary Alcohols or Aldehydes: R-CHO + [O] → R-COOH

Strong oxidizers like KMnO₄ or Na₂Cr₂O₇.

From Carboxylic Acid Derivatives (esters, acid chlorides): R-COOR' + NaOH → R-COONa + R'OH (saponification; ester hydrolysis) R-COCl + H₂O → R-COOH + HCl (acid chloride hydrolysis)

From Grignard Reaction: R-MgX + CO₂ → R-COOMgX → R-COOH (after acidification)

Elegant way to add a carboxylic acid group to an alkyl chain.

Oxidation of Alkylbenzenes: Ar-CH₃ + KMnO₄ → Ar-COOH (any alkyl side chain oxidizes to carboxylic acid)

Reactions of Carboxylic Acids

Esterification (reaction with alcohols): R-COOH + R'-OH → R-COO-R' + H₂O (with H₂SO₄ catalyst)

Reverse of saponification; crucial in making fats, oils, and polyesters.

Decarboxylation (loss of CO₂): R-COOH → R-H + CO₂ (requires heating and catalyst)

Reduction to Alcohols: R-COOH + LiAlH₄ → R-CH₂OH

Difficult reduction; only strong reagents work.

Halogenation (Alpha) (H adjacent to C=O replaced by halogen): R-CHX-COOH (X = Cl, Br, etc.)

Used in drug synthesis.

Formation of Acid Chlorides and Amides: R-COOH + SOCl₂ → R-COCl + SO₂ + HCl R-COOH + NH₃ → R-CO-NH₂ + H₂O (with coupling reagents in practice)

Comparing the Three Carbonyl Classes

Socratic Questions

1. Why is formaldehyde (H-CHO) more reactive toward nucleophilic addition than acetone ((CH₃)₂CO)?

1. An aldehyde is oxidized to a carboxylic acid. Why is the carboxylic acid more acidic than the aldehyde, despite both having the same carbon skeleton?

1. In an aldol condensation, two aldehyde molecules combine. Why does one molecule act as a nucleophile and the other as an electrophile, even though they're identical?

1. Why does the haloform reaction (like iodoform from acetone) only work on methyl ketones (CH₃-CO-R), not on other ketones?

1. Carboxylic acids form dimers in nonpolar solvents through hydrogen bonding. What structural feature enables this, and how would this affect the actual molecular weight determined by freezing-point depression?

Related Topics

Alcohols, Phenols and Ethers - Oxidation to aldehydes and ketones Haloalkanes and Haloarenes - Halogenation of carbonyl compounds Amines - Formation of imines and enamines with carbonyls