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Understanding Class 2 Carbonyl Chemistry

Mar 28, 2025

Organic Chemistry Lecture: Class 2 Carbonyls

Introduction

  • Class 1 vs Class 2 Carbonyls:
    • Class 1 carbonyls have good leaving groups (e.g., Cl, N, O).
    • Class 2 carbonyls lack good leaving groups.

Characteristics of Class 2 Carbonyls

  • Class 2 Examples: Aldehydes and Ketones.
  • Unstable Leaving Groups:
    • Carbanion and hydride are unstable leaving groups.
    • Ketones and aldehydes don't easily reform carbonyls after nucleophilic attack.

Reactions involving Class 2 Carbonyls

  • Nucleophile Attack:

    • Nucleophiles can attack the carbon of the carbonyl.
    • Forms a tetrahedral intermediate, breaking carbon-oxygen double bonds.
    • The intermediate can revert to starting material or progress to an alcohol-containing compound.

  • Reversibility:

    • Dependent on the strength of the nucleophile.
    • Strong nucleophile: favors formation of tetrahedral intermediate.
    • Weak nucleophile: tends to revert to starting materials.

Types of Nucleophiles

  • Water and Alcohols:
    • Water can attack carbonyls, leading to hydrate formation.
    • Alcohols can attack, forming hemiacetals or hemiketals.

Formation of Hemiacetals and Hemiketals

  • Hemiacetal Formation:

    • Reaction of aldehyde and alcohol.
    • Carbon is part of both ether and alcohol groups; called hemiacetal if derived from an aldehyde.

  • Hemiketal Formation:

    • Reaction of ketone and alcohol.
    • If carbon is part of two ethers, derived from a ketone, it is a hemiketal.

Acetals and Ketals

  • Acetals:

    • Formed from aldehydes.
    • Carbon part of two ether linkages; lacks alcohol functional group.

  • Ketals:

    • Derived from ketones.
    • Carbon also part of two ether linkages.

Biological Significance

  • Glucose:
    • Exists in a dynamic equilibrium between straight-chain and cyclic form.
    • Cyclic form prevents oxidation, protecting glucose in the body.

Equilibrium and Hydrolysis

  • Acetal/Ketal Hydrolysis:
    • In presence of H⁺ and excess water, breaks down into hemiacetal/hemiketal and corresponding alcohol.

Conclusion

  • Functional Group Identification:
    • Acetals and ketals are identified by the presence of two ether linkages.
    • Discussed the significance and reactivity of these functional groups in biological systems.

Tips for Further Study

  • Review Video: Recommended to re-watch the lecture video for better understanding.
  • Discussion and Queries: Encouraged to attend discussion sessions or email for doubts.

These notes summarize the key points discussed in the lecture regarding class 2 carbonyl chemistry, focusing on their reactions, intermediates, and significance, particularly in biological contexts such as carbohydrate chemistry.

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