Problem Solving Session: Aldehydes and Ketones

1. Nomenclature and Structure

1.1 IUPAC nomenclature of aldehydes and ketones

• Aldehydes are compounds with the functional group -CHO.
• Ketones are compounds with the functional group >C=O.
• IUPAC nomenclature for aldehydes and ketones:
  • The longest carbon chain containing the functional group is identified as the parent chain.
  • The suffix “-al” is used for aldehydes, and “-one” is used for ketones.
  • The carbon atom of the functional group is assigned the number 1.
  • Substituents on the parent chain are named and numbered according to their position.

1.2 Structural isomers and functional isomers

• Structural isomers have the same molecular formula but different structural formulas.
• Functional isomers have the same molecular formula but different functional groups.

2. Physical Properties

2.1 Boiling points, melting points, solubility

• Aldehydes and ketones have lower boiling points and melting points than alcohols of comparable molecular weight.
• This is due to the weaker intermolecular forces between aldehydes and ketones (dipole-dipole interactions and hydrogen bonding).
• Aldehydes and ketones are generally soluble in organic solvents and insoluble in water.

2.2 Spectroscopy: IR, NMR, and Mass spectrometry

• IR spectroscopy: Aldehydes and ketones show a strong absorption band in the region of 1650-1750 cm^-1 due to the C=O stretching vibration.
• NMR spectroscopy: Aldehydes and ketones show a signal in the region of 9-10 ppm due to the proton attached to the carbonyl carbon.
• Mass spectrometry: Aldehydes and ketones show a molecular ion peak and characteristic fragmentation patterns.

3. Chemical Reactions

3.1 Nucleophilic addition reactions

• Aldehydes and ketones undergo nucleophilic addition reactions with a variety of nucleophiles.
• Some common nucleophiles include: HCN, NH3, NaHSO3, Grignard reagents, and water.
• The products of nucleophilic addition reactions are typically alcohols or derivatives of alcohols.

3.2 Electrophilic addition reactions

• Aldehydes and ketones also undergo electrophilic addition reactions with a variety of electrophiles.
• Some common electrophiles include: HBr, HCl, HI, and H2SO4.
• The products of electrophilic addition reactions are typically halides or derivatives of halides.

3.3 Oxidation reactions

• Aldehydes are easily oxidized to carboxylic acids.
• Ketones are more difficult to oxidize, but they can be oxidized to a mixture of carboxylic acids.
• Some common oxidizing agents include: KMnO4, CrO3, and Na2Cr2O7.

3.4 Reduction reactions

• Aldehydes and ketones can be reduced to alcohols.
• Some common reducing agents include: H2, NaBH4, and LiAlH4.
• Dissolving metal reduction (Birch reduction) can also be used to reduce aldehydes and ketones to alcohols.

4. Condensation reactions

4.1 Aldol condensation

• The aldol condensation is a reaction between two aldehydes or ketones in the presence of a base.
• The product of the aldol condensation is a β-hydroxyaldehyde or β-hydroxyketone.

4.2 Claisen condensation

• The Claisen condensation is a reaction between two esters in the presence of a base.
• The product of the Claisen condensation is a β-ketoester.

4.3 Dieckmann condensation

• The Dieckmann condensation is an intramolecular Claisen condensation.
• The product of the Dieckmann condensation is a cyclic β-ketoester.

5. Acidity of alpha hydrogens

5.1 Keto-enol tautomerism

• Aldehydes and ketones exist in equilibrium with their enol tautomers.
• The enol tautomer is an isomer of the aldehyde or ketone that has a hydroxyl group and a double bond.
• Keto-enol tautomerism is catalyzed by acids and bases.

5.2 Enolate ions and their reactions

• Enolate ions are the conjugate bases of enols.
• Enolate ions are strong nucleophiles and can react with a variety of electrophiles.
• Some common reactions of enolate ions include: alkylation, acylation, and aldol condensation.

5.3 Halogenation of aldehydes and ketones

• Aldehydes and ketones can be halogenated with a variety of halogenating agents.
• Some common halogenating agents include: Br2, Cl2, and I2.
• The product of halogenation of an aldehyde or ketone is a haloketone.

6. Synthesis of aldehydes and ketones

6.1 From alkenes

• Aldehydes and ketones can be synthesized by the oxidation of alkenes.
• Some common oxidizing agents include: KMnO4, CrO3, and Na2Cr2O7.

6.2 From alkynes

• Aldehydes and ketones can be synthesized by the hydration of alkynes.
• The hydration of alkynes is catalyzed by a mercury salt (HgSO4 or Hg(OAc)2).

6.3 From alcohols

• Aldehydes and ketones can be synthesized by the oxidation of alcohols.
• Some common oxidizing agents include: KMnO4, CrO3, and Na2Cr2O7.

References

• Morrison, R. T., & Boyd, R. N. (1992). “Organic chemistry” (6th ed.). Prentice Hall.
• Solomons, T. W. G., & Fryhle, C. B. (2004). “Organic chemistry” (8th ed.). Wiley.
• NCERT Chemistry Textbook for Class 11 and Class 12.