MCAT Organic Chemistry: Aldehydes and Ketones
Presenter: Iman
Chapter Overview
- Topic: Aldehydes and Ketones
- Focus: Nucleophilic addition reactions
- Key Objective: Understanding generalized reaction mechanism of nucleophilic addition to a carbonyl
Key Concepts
Nucleophilic Addition Reaction Mechanism
- Carbonyl Bond Polarization: Partial positive charge on the carbon, partial negative charge on the oxygen
- Nucleophilic Attack: Nucleophile forms a covalent bond with the carbon, breaking the ฯ bond in the carbonyl
- Tetrahedral Intermediate: Formed after nucleophilic attack; question if carbonyl can reform based on leaving group
Outcomes Based on Leaving Group Presence
- No Good Leaving Group (Aldehydes & Ketones): Carbonyl will not reform, oxygen accepts a proton forming an alcohol
- Good Leaving Group (Carboxylic Acids and Derivatives): Carbonyl double bond can reform, pushing off the leaving group
Hydration of Aldehydes and Ketones
- Geminal Diols Formation: Reaction with water forms 1,1-diols (geminal diols)
- Catalysis: Reaction rate can be increased with small amount of acid or base
Acetals and Hemi-Acetals; Ketals and Hemi-Ketals
- 1 Equivalent Alcohol + Aldehyde: Forms a hemiacetal
- 1 Equivalent Alcohol + Ketone: Forms a hemiketal
- 2 Equivalents of Alcohol: Forms acetal (from aldehyde) or ketal (from ketone)
- Identification: Hemiacetals/hemiketals retain hydroxy group; Acetals/ketals replace it
Reaction Mechanisms
- Formation Steps: Electrons from ฯ bonds are pushed to oxygen, oxygen accepts protons -> product formation
Nitrogen-Based Functional Groups
Imines and Enamines
- Primary Amine + Aldehyde/Ketone: Forms imine (double bond between carbon and nitrogen)
- Secondary Amine + Aldehyde/Ketone: Forms enamine (double bond adjacent to nitrogen)
- Mechanism: Addition followed by elimination (condensation reaction)
- Examples: Provided of primary vs. secondary amine reactions
Cyanohydrins Formation
Mechanism
- Hydrogen Cyanide: Classic nucleophile, dissociates to form cyanide anion
- Attack on Carbonyl Carbon: Forms covalent bond, oxygen accepts electrons and gets protonated -> cyanohydrin formation
Oxidation-Reduction Reactions
Oxidation of Aldehydes
- Result: Further oxidation forms carboxylic acids
- Agents: Potassium permanganate, chromium trioxide, hydrogen peroxide, but not PCC
Reduction by Hydride Reagents
- Result: Aldehydes/Ketones reduced to alcohols
- Agents: Lithium aluminum hydride (LiAlH4), sodium borohydride (NaBH4)
Summary
- Objectives Covered: General mechanisms, specific reactions with aldehydes and ketones
- Next Steps: Practice problems in following video
Note: Additional in-depth content available in Organic Chemistry playlist (Chap 19 for advanced topics)