Slide 1: Introduction to Aldehydes and Ketones

• Aldehydes and ketones are functional groups containing a carbonyl group.
• The general formula for aldehydes is RCHO, and for ketones is RCOR'.
• They have distinct physical and chemical properties.
• Examples of aldehydes include formaldehyde, acetaldehyde, and benzaldehyde.
• Examples of ketones include acetone, butanone, and cyclohexanone.

Slide 2: Nomenclature of Aldehydes and Ketones

• Aldehydes are named by replacing the -e ending of the corresponding alkane with -al.
  • Methane becomes methanal, ethane becomes ethanal, and so on.
• Ketones are named by replacing the -e ending of the corresponding alkane with -one.
  • Propane becomes propanone, butane becomes butanone, and so on.
• In cyclic ketones, the carbonyl carbon is assigned position 1, and the ring carbon atoms are numbered accordingly.

Slide 3: Structure and Bonding in Aldehydes and Ketones

• The carbonyl group consists of a carbon atom bonded to an oxygen atom by a double bond.
• The carbon atom in the carbonyl group is sp2 hybridized, with trigonal planar geometry.
• The oxygen atom has two lone pairs of electrons, contributing to its bent shape.
• The carbonyl group is highly polar due to the electronegativity difference between carbon and oxygen.
• The polarity of the carbonyl group makes aldehydes and ketones more reactive compared to alkanes.

Slide 4: Physical Properties of Aldehydes and Ketones

• Aldehydes and ketones have higher boiling points than corresponding alkanes of similar molecular weights.
• The dipole-dipole interactions in aldehydes and ketones contribute to their higher boiling points.
• Aldehydes and ketones with lower molecular weights are soluble in water due to hydrogen bonding with water molecules.
• As the carbon chain length increases, the solubility decreases since the hydrophobic alkyl group dominates.

Slide 5: Preparation of Aldehydes

• Oxidation of primary alcohols can produce aldehydes.
  • The process is typically carried out using mild oxidizing agents.
  • For example, primary alcohols can be oxidized using pyridinium chlorochromate (PCC) or chromic acid.
• Reduction of carboxylic acids or esters can also yield aldehydes.
  • LiAlH4 is commonly used as a reducing agent in these reactions.

Slide 6: Preparation of Ketones

• Oxidation of secondary alcohols leads to the formation of ketones.
  • Stronger oxidizing agents such as chromic acid or potassium permanganate can be used for this purpose.
• Friedel-Crafts acylation is another method to prepare ketones.
  • It involves the reaction of an aromatic compound with an acyl halide in the presence of a Lewis acid catalyst.

Slide 7: Reactions of Aldehydes and Ketones - Addition Reactions

• Aldehydes and ketones can undergo addition reactions with nucleophiles.
• One important example is the addition of water molecule to form hydrates.
  • This reaction is catalyzed by acid or base.
• Another example is the addition of alcohols to form hemiacetals or acetals.
  • Acidic conditions are required for this reaction.

Slide 8: Reactions of Aldehydes and Ketones - Oxidation Reactions

• Aldehydes can be further oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate.
• Ketones, on the other hand, are resistant to oxidation due to the absence of an alpha-hydrogen.
• Tollens test and Fehling’s test are commonly used to distinguish between aldehydes and ketones.

Slide 9: Reactions of Aldehydes and Ketones - Reduction Reactions

• Aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
• Reduction of aldehydes yields primary alcohols, while reduction of ketones yields secondary alcohols.
• The reduction reactions are typically carried out under mild conditions.

Slide 10: Reactions of Aldehydes and Ketones - Aldol Condensation

• Aldol condensation involves the reaction between an aldehyde or ketone and another aldehyde or ketone molecule.
• It leads to the formation of a beta-hydroxyaldehyde or beta-hydroxyketone.
• The reaction occurs under basic conditions, where the carbonyl group acts as a nucleophile attacking the electrophilic carbon.
• The aldol condensation can proceed further to form a larger molecule, resulting in the formation of a conjugated system.

Intramolecular aldol

• Intramolecular aldol is a type of aldol condensation reaction that occurs within a single molecule.
• It involves the reaction between a carbonyl group and an enolizable alpha carbon within the same molecule.
• The reaction proceeds through nucleophilic addition of the enolate ion formed by deprotonation of the alpha carbon to the carbonyl carbon.
• This leads to the formation of a new carbon-carbon bond and the elimination of a water molecule.
• Intramolecular aldol reactions are often used in natural product synthesis and can lead to the formation of complex cyclic compounds.

Mechanism of Intramolecular aldol

• Step 1: Deprotonation of the alpha carbon by a base to form the enolate ion.
• Step 2: Nucleophilic attack by the enolate ion on the carbonyl carbon, resulting in the formation of a new carbon-carbon bond.
• Step 3: Proton transfer from the oxygen of the carbonyl group to the alpha carbon.
• Step 4: Elimination of a water molecule to regenerate the carbonyl group.
• The overall reaction is a condensation reaction as a water molecule is eliminated.

Example of Intramolecular aldol

• 2-cyclohexenone can undergo an intramolecular aldol reaction to form a cyclic compound.
• The reaction takes place between the carbonyl group and the enolizable alpha carbon within the same molecule.
• After the nucleophilic attack and elimination of water, a cyclic compound called 2-cyclohexen-1-ol is formed.

Problem 1

• Predict the product formed in the intramolecular aldol reaction of 3-pentanone.
• Solution:
  • Deprotonation of the alpha carbon forms the enolate ion.
  • The enolate ion attacks the carbonyl carbon within the same molecule.
  • After water elimination, a cyclic compound called 2-cyclopenten-1-one is formed.

Problem 2

• Predict the product formed in the intramolecular aldol reaction of 2-butanone.
• Solution:
  • Deprotonation of the alpha carbon forms the enolate ion.
  • The enolate ion attacks the carbonyl carbon within the same molecule.
  • After water elimination, a cyclic compound called 3-methyl-2-cyclopenten-1-one is formed.

Retro-aldol Reaction

• Retro-aldol reaction is the reverse of the aldol reaction.
• It involves the cleavage of a carbon-carbon bond adjacent to the carbonyl group in a molecule, resulting in the formation of two smaller molecules.
• The reaction requires a strong base, such as hydroxide ion or lithium diisopropylamide (LiDA), to abstract the alpha hydrogen.
• Retro-aldol reaction can be useful in synthesis to access precursors for further transformations.

Mechanism of Retro-aldol Reaction

• Step 1: Proton abstraction by a base from the alpha carbon adjacent to the carbonyl group.
• Step 2: Nucleophilic attack by the enolate ion on the carbonyl carbon, breaking the carbon-carbon bond.
• Step 3: Deprotonation of the alpha carbon to form an enolate ion.
• The overall reaction is a cleavage reaction as a carbon-carbon bond is broken.

Example of Retro-aldol Reaction

• Benzaldehyde can undergo a retro-aldol reaction to form benzyl alcohol and formaldehyde.
• The reaction involves the cleavage of the carbon-carbon bond adjacent to the carbonyl group.
• After proton abstraction and nucleophilic attack, the carbon-carbon bond is cleaved, resulting in the formation of two smaller molecules.

Problem 3

• Predict the products formed in the retro-aldol reaction of 3-hydroxy-2-butanone.
• Solution:
  • Proton abstraction by a base breaks the carbon-carbon bond.
  • The resulting fragments are acetone and formaldehyde.

Problem 4

• Predict the products formed in the retro-aldol reaction of 3-hydroxybutanal.
• Solution:
  • Proton abstraction by a base breaks the carbon-carbon bond.
  • The resulting fragments are propanal and formaldehyde.

Slide 21: Problem Solving Session - Intramolecular aldol

• Predict the product formed in the intramolecular aldol reaction of 2-pentanone.
  • Solution:
    • Deprotonation of the alpha carbon forms the enolate ion.
    • The enolate ion attacks the carbonyl carbon within the same molecule.
    • After water elimination, a cyclic compound called 3-methyl-2-cyclopenten-1-one is formed.

Slide 22: Problem Solving Session - Retro-aldol Reaction

• Predict the products formed in the retro-aldol reaction of 4-hydroxy-2-butanone.
  • Solution:
    • Proton abstraction by a base breaks the carbon-carbon bond.
    • The resulting fragments are propanone and formaldehyde.

Slide 23: Spectroscopic Analysis of Aldehydes and Ketones

• Aldehydes and ketones can be identified and characterized using various spectroscopic techniques.
• Nuclear Magnetic Resonance (NMR) spectroscopy can provide information about the chemical environment of the carbonyl carbon and neighboring atoms.
• Infrared (IR) spectroscopy can be used to detect the stretching vibrations of the carbonyl group.
• Mass spectrometry (MS) can determine the molecular weight and fragmentation patterns of aldehydes and ketones.

Slide 24: Chemical Properties of Aldehydes and Ketones - Oxidation

• Aldehydes can be oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate (KMnO4) or chromic acid (H2CrO4).
• Ketones are resistant to oxidation due to the absence of an alpha-hydrogen.
• Tollens test and Fehling’s test can be used to distinguish between aldehydes and ketones based on their oxidation reactions.

Slide 25: Chemical Properties of Aldehydes and Ketones - Reduction

• Aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
• Reduction of aldehydes yields primary alcohols, while reduction of ketones yields secondary alcohols.
• The reduction reactions are typically carried out under mild conditions.

Slide 26: Chemical Properties of Aldehydes and Ketones - Nucleophilic Addition Reactions

• Aldehydes and ketones can undergo nucleophilic addition reactions with a wide range of nucleophiles.
• Examples include the reaction with ammonia or primary amines to form imines or enamines.
• Reaction with hydrazine or its derivatives can yield hydrazones.
• Reaction with sodium bisulfite or sodium cyanide can lead to the formation of addition products.
• The choice of nucleophile and reaction conditions determines the specific product formed.

Slide 27: Chemical Properties of Aldehydes and Ketones - Aldol Condensation

• Aldol condensation involves the reaction between an aldehyde or ketone and another aldehyde or ketone molecule.
• It leads to the formation of a beta-hydroxyaldehyde or beta-hydroxyketone.
• The reaction occurs under basic conditions, where the carbonyl group acts as a nucleophile attacking the electrophilic carbon.
• The aldol condensation can proceed further to form a larger molecule, resulting in the formation of a conjugated system.

Slide 28: Problem Solving Session - Aldol Condensation

• Predict the product formed in the aldol condensation of propanal and propanal.
  • Solution:
    • The carbonyl group of one propanal molecule acts as a nucleophile attacking the other propanal molecule.
    • After elimination of water, the product is 2-methyl-2-pentenal.

Slide 29: Problem Solving Session - Nucleophilic Addition Reactions

• Predict the product formed in the reaction between acetone and ammonia.
  • Solution:
    • The carbonyl group of acetone reacts with ammonia to form an imine.
    • The product is N-methyl-2-propanimine.

Slide 30: Summary

• Aldehydes and ketones are important functional groups containing a carbonyl group.
• They have distinct physical and chemical properties.
• Aldehydes can be prepared by oxidizing primary alcohols or reducing carboxylic acids/esters.
• Ketones can be prepared by oxidizing secondary alcohols or through Friedel-Crafts acylation.
• Aldehydes and ketones can undergo various reactions, including addition, oxidation, reduction, and condensation.
• Spectroscopic techniques can be used to identify and characterize aldehydes and ketones.