Aldehydes, Ketones, and Carbonyl Group Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Aldehydes and Ketones contain the carbonyl group (C=O) — aldehydes have H on one side (RCHO), ketones have two alkyl groups (RCOR)
• Nucleophilic Addition is the characteristic reaction: nucleophile attacks C=O → tetrahedral alkoxide intermediate → protonation
• Reagents for Reduction: NaBH₄ (mild, selective for aldehydes/ketones, not esters) | LiAlH₄ (strong, reduces almost everything)
• Grignard reagents (RMgX) add to C=O to give alcohols after aqueous workup; form C–C bonds
• Hemiacetal formation: Aldehyde/ketone + alcohol (1:1) → hemiacetal (reversible, unstable) | + excess alcohol → acetal (stable, protected)
• ⚡ Aldehydes are more reactive than ketones — steric (two alkyl groups in ketone) and electronic (two EDG vs one in aldehyde) reasons

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Carbonyl Compounds — Aldehydes and Ketones

Aldehydes and ketones contain the carbonyl functional group — one of the most important and versatile functional groups in organic chemistry. Understanding carbonyl chemistry is foundational for pharmacy students, as it underlies many drug structures, drug synthesis pathways, and metabolic transformations.

The Carbonyl Group — Structure and Reactivity

The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom.

Key Structural Features:

• C=O bond is short (about 1.22 Å), strong, and highly polar
• Carbon is electrophilic (δ+), oxygen is nucleophilic (δ−) — this polarity drives reactivity
• The carbonyl carbon is sp² hybridized, trigonal planar, ~120° bond angles
• Due to the C=O double bond, there is no free rotation (unlike single bonds)

Why Aldehydes are More Reactive than Ketones:

1. Steric: Ketones have two alkyl groups creating more steric hindrance around the carbonyl carbon; aldehydes have only one alkyl group + hydrogen
2. Electronic: Alkyl groups are electron-donating (+I), reducing the partial positive on the carbonyl carbon in ketones

Reactivity Order: Aldehydes > Ketones > Esters/Amides (esters and amides are much less reactive)

Nucleophilic Addition — The General Mechanism

All reactions of aldehydes and ketones proceed by the same fundamental mechanism:

Step 1: Nucleophile attacks the electrophilic carbonyl carbon → forms a tetrahedral alkoxide intermediate Step 2: The alkoxide (negatively charged oxygen) is protonated → gives the addition product

Important: This is addition to the C=O double bond (not substitution of a leaving group like in SN1/SN2). The carbonyl π-bond is consumed; no π-bond remains in the product.

Factors Affecting Nucleophilic Addition Rate:

Faster Addition when:

• Aldehyde rather than ketone (less steric hindrance)
• Less electron donation to carbonyl (more positive carbon)
• Stronger nucleophile

Slower Addition when:

• Ketone with bulky groups
• Electron-donating groups on the α-carbon (reduce electrophilicity of carbonyl)
• Conjugation with C=C (reduces electrophilicity; e.g., α,β-unsaturated carbonyls)

Important Reactions of Aldehydes and Ketones

1. Addition of Hydride (Reduction)

Reagents: NaBH₄ (Sodium Borohydride)

• Mild, selective reducing agent
• Reduces aldehydes and ketones to primary and secondary alcohols respectively
• Does NOT reduce esters, amides, carboxylic acids, or nitriles
• Mechanism: H⁻ from NaBH₄ attacks carbonyl; the O⁻ is protonated by workup (usually methanol or ethanol)

Reagents: LiAlH₄ (Lithium Aluminium Hydride)

• Very strong reducing agent
• Reduces ALL carbonyl compounds including esters, amides, carboxylic acids
• More dangerous (reacts violently with water)
• Etheral solvent required; aqueous workup

Chemoselectivity: NaBH₄ reduces aldehydes/ketones but not esters; LiAlH₄ reduces everything. Use NaBH₄ when you need selective reduction.

2. Addition of Grignard Reagents (RMgX)

Result: Formation of alcohols with creation of new C–C bonds

Mechanism:

• Grignard reagent (R⁻Mg²⁺X⁻) provides R:⁻ equivalent
• R:⁻ attacks carbonyl → alkoxide
• Aqueous workup protonates O⁻ → alcohol

Products:

• Aldehyde + RMgX → Secondary alcohol (after 1° alcohol with excess → tertiary alcohol)
• Ketone + RMgX → Tertiary alcohol

Important: Grignard reagents are destroyed by protic solvents (ROH, H₂O, NH₃), acids, and functional groups with acidic hydrogens (–OH, –SH, –COOH). Anhydrous conditions essential.

3. Formation of Imines (Schiff Bases)

Reagents: Primary amines (R–NH₂)

Aldehyde/Ketone + R–NH₂ → Imine (C=N–R) + H₂O

Mechanism: Nucleophilic attack by amine → carbinolamine intermediate → loss of water → imine

Application: This is the basis for pyridoxamine (vitamin B₆) chemistry in amino acid metabolism — enzymes use Schiff base intermediates

4. Cyanohydrin Formation

Reagents: HCN (or NaCN + acid)

Aldehyde/Ketone + HCN → Cyanohydrin (HO–C–CN)

Product: Contains both –OH and –CN groups; the new C–C bond is formed

Significance: Cyanohydrins can be hydrolyzed to α-hydroxy acids (lactic acid from acetaldehyde cyanohydrin); nitriles can be further converted to other functional groups

5. Acetal and Hemiacetal Formation

Aldehyde/Ketone + Alcohol (1 equivalent) → Hemiacetal (unstable, equilibrium) Aldehyde/Ketone + Alcohol (excess + acid catalyst) → Acetal (stable)

Mechanism: Nucleophilic attack by alcohol on carbonyl → carbinolamine-type intermediate → loss of water → oxocarbenium ion → second nucleophilic attack → acetal

Significance in Pharmacy: Many drugs contain acetal or hemiketal groups (e.g., certain general anesthetics like chloral hydrate); these can hydrolyze in aqueous biological environments to release active drugs

6. Oxidation Reactions

Aldehydes are easily oxidized to carboxylic acids:

• Tollens Reagent (Ag(NH₃)₂⁺): Gives silver mirror on aldehyde oxidation; ketones do NOT react
• Fehling’s Solution (Cu²⁺ tartrate): Gives brick-red Cu₂O precipitate with aldehydes; ketones do not react
• These tests distinguish aldehydes from ketones

Ketones are generally resistant to oxidation under mild conditions (no reaction with Tollens or Fehling’s)

7. Wittig Reaction

Reagents: Phosphonium ylide (Ph₃P=CH–R)

Aldehyde/Ketone + Wittig reagent → Alkene (C=O replaced by C=C)

Significance: Creates carbon-carbon double bonds with specific geometry (E/Z determined by ylide structure); important in drug synthesis

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Alpha-Carbon Chemistry and Enolization

The α-carbon (adjacent to carbonyl) bears acidic hydrogens (pKa ~ 20, much more acidic than typical C–H at ~50).

Why α-H is Acidic:

• The enolate anion formed upon deprotonation is resonance-stabilized — negative charge is delocalized between the α-carbon and the carbonyl oxygen

Enolization: In acidic or basic conditions, aldehydes and ketones can tautomerize to enols:

• Keto form ↔ Enol form
• Under neutral conditions, keto form dominates overwhelmingly (K >> [enol])
• Under acidic or basic catalysis, enol concentration increases

Significance of Enolization:

1. Halogenation at Alpha Position: Enols react with halogens (Cl₂, Br₂) at the α-carbon — this is why aldehydes and ketones undergo α-halogenation
2. Drug Metabolism: Metabolic oxidation of drug molecules often occurs at α-carbons adjacent to carbonyl groups (CYP450-mediated oxidation)
3. Aldol Condensation: Self-condensation of aldehydes/ketones via enol attack on another carbonyl

Alpha, Beta-Unsaturated Carbonyl Compounds

Conjugation between C=O and C=C creates special reactivity:

Michael Addition:

• Nucleophiles (enolate, malonate, amine) can add to the β-carbon of α,β-unsaturated carbonyls
• 1,4-addition (conjugate addition) vs 1,2-addition (direct carbonyl addition)
• In biological systems, Michael additions occur with nucleophilic amino acid side chains (cysteine, histidine) — important in drug binding

Examples in Drugs:

• Prednisone and other corticosteroids contain α,β-unsaturated ketone systems
• The Michael acceptor moiety is crucial to biological activity

Semicarbazide and 2,4-DNP Tests

Semicarbazide Test: Aldehyde/ketone + semicarbazide → crystalline semicarbazone (melting point used for identification)

2,4-Dinitrophenylhydrazine (2,4-DNP) Test: Aldehyde/ketone + 2,4-DNP → yellow/orange 2,4-DNP derivative (crystalline, melting point for identification)

Both are qualitative tests for carbonyl compounds — all aldehydes and ketones give positive results.

Pharmaceutical Applications

Paraldehyde (Trimer of Acetaldehyde): Former sedative/hypnotic; formed by acid-catalyzed trimerization of acetaldehyde

Chloral Hydrate: Trichloroacetaldehyde monohydrate; sedative/hypnotic drug; contains aldehyde group; metabolic conversion to trichloroethanol responsible for activity

Thiamine (Vitamin B1): Contains an aminopyrimidine ring with a positively charged nitrogen — acts as a coenzyme in decarboxylation reactions involving α-ketoacid intermediates

Drug Metabolism:

• Many drugs containing aldehyde/ketone groups undergo reduction (alcohol dehydrogenase, aldo-keto reductases) or oxidation (CYP450)
• Example: Progesterone metabolism involves reduction of the ketone to an alcohol; tamoxifen (anticancer) undergoes metabolic activation via hydroxylation

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.