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Organic Chemistry Lecture - 02: Aldehydes, Ketones & Carboxylic Acids

Topics to be Covered

  • Chemical Properties of Aldehydes & Ketones

  • Nucleophilic Addition Reactions (NAR)

  • Practice Problems

Recap of Previous Lecture

  1. Methods of Preparation of Aldehydes

  2. Methods of Preparation of Ketones

Key Reaction Mechanisms

  • Ozonolysis of Hydrocarbons

    • Reaction Name: Ozonolysis

    • Reagents/Conditions: O₃, then Zn/H₂O

    • Key Features/Product: Breaks C=C; identifies alkene structure.

  • Wurtz Reaction

    • Reagents/Conditions: Na / Dry Ether

    • Key Features/Product: Alkyne halide → Symmetrical Alkane.

  • Lucas Test for Alcohols

    • Reagents/Conditions: Conc. HCl + Anhyd. ZnCl₂

    • Key Features/Product: Identifies 1°, 2°, 3° alcohols (Turbidity).

  • Cleavage of C-O bond Reaction with HX for Ethers

    • Reaction Type: SN₁ or SN₂ depending on stability

  • Reimer-Tiemann Reaction for Phenols

    • Reagents/Conditions: CHCl₃ + aq. NaOH

    • Key Features/Product: Formation of Salicylaldehyde.

  • Kolbe's Electrolysis for Phenols

    • Reagents/Conditions: CO₂ + NaOH, then H+

    • Key Features/Product: Formation of Salicylic Acid.

Nucleophilic Addition Reactions (NAR)

  • Definition: These reactions involve the addition of a nucleophile to the electrophilic carbon of a carbonyl compound (aldehydes and ketones).

Mechanism of Nucleophilic Addition Reactions

  1. Initial Attack: A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group roughly perpendicular to the plane of sp² hybridized orbitals.

  2. Hybridization Change: The hybridization changes from sp² to sp³, creating a tetrahedral alkoxide intermediate.

  3. Protonation Step: This intermediate captures a proton from the medium to yield a neutral product.

  4. Final Product Formation: Thus, there is an addition of the nucleophile and H+ to the carbon-oxygen double bond.

Reactivity of Aldehydes vs. Ketones

  • Observation: Aldehydes are generally more reactive than ketones due to:

    • Steric Factors: Ketones have two larger substituents hindering nucleophilic approach.

    • Electronic Factors: Two alkyl groups in ketones reduce the electrophilicity of the carbonyl carbon more than one alkyl group in aldehydes.

Specific Reactions

Hydration of Carbonyl Compounds

  • Addition of HCN: Aldehydes and ketones react with hydrogen cyanide to yield cyanohydrins. The reaction is slow with pure HCN and is catalyzed by a base due to the stronger nucleophilicity of the cyanide ion (CN-).

Sodium Hydrogen Sulphite Addition

  • Test: Sodium bisulfite reacts with aldehydes and methyl ketones to form water-soluble adducts. Most aldehydes and methyl ketones give this test. The reaction can be reversed to recover the original carbonyl compounds by treatment with dilute acid.

Formation of Hemiacetals and Acetals

  • Mechanism: Aldehydes react with alcohol in the presence of dry HCl to form hemiacetals, which on further reaction with alcohol yield acetals. Ketones react similarly with ethylene glycol to form cyclic ketals.

  • Hydrolysis: Acetals and ketals can be hydrolyzed with aqueous mineral acids back to the corresponding aldehydes and ketones.

Ethical and Practical Considerations

  • Understanding these reactions is crucial to predict chemical behavior in organic synthesis and in pharmaceutical applications where aldehydes and ketones play significant roles.

Practice Questions

  • Example questions from NCERT Exemplar focusing on concepts such as alkyl substitution in acylation reactions, the results of Lucas test, and the comparative reactivity of carbonyl compounds towards nucleophilic addition.

    • Assertion: Formaldehyde is planar due to sp² hybridization.

    • Identifying correct statements regarding the carbonyl group and various addition reactions.

Conclusion

  • These detailed insights into the properties and reactivity of aldehydes and ketones are foundational for mastering organic chemistry, particularly in the domain of functional group transformations and reaction mechanisms.