Kaplan ch 6

Aldehydes and Ketones I: Electrophilicity and Oxidation–Reduction

Introduction

Aldehydes and ketones are fundamental functional groups in organic chemistry, characterized by the presence of a carbonyl group (C=O). Their reactivity, physical properties, and roles in oxidation-reduction reactions are central to understanding organic synthesis and biological processes. This study guide summarizes key concepts and mechanisms relevant to college-level organic chemistry and MCAT preparation.

Description and Properties

Nomenclature

• Aldehydes are named by replacing the -e ending of the parent alkane with -al (e.g., ethanal, propanal).

• Ketones are named by replacing the -e ending with -one (e.g., propanone, butanone), or by using the prefix oxo- or keto- when as a substituent.

• Aldehydes are terminal functional groups (at the end of a carbon chain), while ketones are internal (within the chain).

Physical Properties

• The carbonyl group is highly polar due to the electronegativity difference between carbon and oxygen.

• This polarity increases intermolecular forces (dipole-dipole interactions), resulting in higher boiling points than alkanes.

• However, aldehydes and ketones do not form hydrogen bonds as alcohols do, so their boiling points are lower than those of alcohols.

• Example: For compounds of similar molecular weight: $\text{alkane} < \text{aldehyde/ketone} < \text{alcohol}$ in boiling point.

Electrophilicity of the Carbonyl Carbon

• The carbonyl carbon is electrophilic due to electron withdrawal by the oxygen atom.

• Resonance structures place a partial positive charge on the carbonyl carbon, making it susceptible to nucleophilic attack.

• Aldehydes are generally more reactive than ketones due to less steric hindrance and fewer electron-donating alkyl groups.

Nucleophilic Addition Reactions

General Mechanism

• Nucleophiles attack the carbonyl carbon, breaking the $\pi$ bond and pushing electrons onto the oxygen atom.

• If no good leaving group is present (as in aldehydes and ketones), the oxygen is protonated to form an alcohol.

• If a good leaving group is present (as in carboxylic acids and derivatives), the carbonyl reforms and the leaving group is expelled.

Hydration

• Water adds to the carbonyl group, forming a geminal diol (two hydroxyl groups on the same carbon).

• Equation: $\ce{R_2C=O + H_2O \rightarrow R_2C(OH)_2}$

Acetals and Hemiacetals

• Alcohols react with aldehydes/ketones to form hemiacetals (one -OH, one -OR group) and acetals (two -OR groups).

• Hemiacetals and hemiketals are usually unstable and react further to form acetals/ketals, especially under acidic conditions.

• Equation (hemiacetal formation): $\ce{R_2C=O + R'OH \rightarrow R_2C(OH)(OR')}$

• Equation (acetal formation): $\ce{R_2C=O + 2R'OH \rightarrow R_2C(OR')_2 + H_2O}$

Reactions with Nitrogen Derivatives

• Aldehydes and ketones react with ammonia and its derivatives to form imines, oximes, hydrazones, and semicarbazones.

• Equation (imine formation): $\ce{R_2C=O + R'NH_2 \rightarrow R_2C=NR' + H_2O}$

• Imines can tautomerize to enamines.

Reaction with Hydrogen Cyanide

• Hydrogen cyanide (HCN) adds to the carbonyl carbon to form a cyanohydrin.

• Equation: $\ce{R_2C=O + HCN \rightarrow R_2C(OH)CN}$

Oxidation-Reduction Reactions

Oxidation of Aldehydes

• Aldehydes can be oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate ($\ce{KMnO_4}$), chromium trioxide ($\ce{CrO_3}$), silver oxide ($\ce{Ag_2O}$), or hydrogen peroxide ($\ce{H_2O_2}$).

• Pyridinium chlorochromate (PCC) is a mild, anhydrous oxidant that can oxidize primary alcohols to aldehydes but cannot further oxidize aldehydes to carboxylic acids.

• Equation: $\ce{RCHO + [O] \rightarrow RCOOH}$

Oxidation of Ketones

• Ketones are generally resistant to further oxidation under normal conditions because they are the most oxidized form of secondary carbons.

Reduction of Aldehydes and Ketones

• Aldehydes and ketones can be reduced to alcohols using hydride reagents such as lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).

• Equation (aldehyde reduction): $\ce{RCHO + 2[H] \rightarrow RCH_2OH}$

• Equation (ketone reduction): $\ce{R_2C=O + 2[H] \rightarrow R_2CHOH}$

Key Concepts Table

Functional Group	Oxidation Product	Reduction Product	Common Reagents
Aldehyde	Carboxylic acid	Primary alcohol	Oxidation: KMnO4, CrO3, Ag2O, H2O2 Reduction: LiAlH4, NaBH4
Ketone	None (resistant)	Secondary alcohol	Reduction: LiAlH4, NaBH4

Sample MCAT-Style Questions (with Concepts)

• Electrophilicity: The carbonyl carbon is electrophilic due to electron withdrawal by oxygen.

• Boiling Point Order: Alkane < Ketone < Alcohol (due to dipole and hydrogen bonding effects).

• Nucleophilic Addition: Alcohols add to carbonyls to form hemiacetals/acetals; water adds to form geminal diols.

• Reactions with Nitrogen Derivatives: Ammonia and derivatives form imines, oximes, hydrazones, and semicarbazones.

• Reduction: LiAlH4 and NaBH4 reduce aldehydes/ketones to alcohols.

• Oxidation: Strong oxidants convert aldehydes to carboxylic acids; PCC is selective for aldehyde formation from primary alcohols.

Summary Table: Hemiacetals and Hemiketals

Compound	Central Carbon Bonds	Stability
Hemiacetal	-OH, -OR, -H, -R	Unstable; rapidly converts to acetal
Hemiketal	-OH, -OR, -R, -R'	Unstable; rapidly converts to ketal

Additional info:

• Common aldehydes and ketones are found in nature and flavorings (e.g., cinnamaldehyde in cinnamon, vanillin in vanilla).

• MCAT questions often focus on mechanisms, reagent selection, and product prediction for these reactions.