Aldehydes & Ketones: Structure, Nomenclature, Properties, Preparation, and Reactions

Aldehydes & Ketones: Introduction

Carbonyl Group

Aldehydes and ketones are organic compounds containing the carbonyl group (C=O). This functional group is highly significant in organic chemistry due to its reactivity and prevalence in biological and industrial molecules.

• Aldehyde: Carbonyl carbon bonded to at least one hydrogen atom.

• Ketone: Carbonyl carbon bonded to two other carbon atoms.

General Structure:

• Aldehyde: R-CHO

• Ketone: R-CO-R'

Nomenclature of Aldehydes & Ketones

Rules for Aldehydes

• Replace the terminal -e of the corresponding alkane name with -al.

• The parent chain must contain the -CHO group; number the carbonyl carbon as C1.

• If the -CHO group is attached to a benzene ring, use the suffix -aldehyde (e.g., benzaldehyde).

Common Names of Aldehydes

• Some aldehydes are known by their common names, such as formaldehyde (methanal), acetaldehyde (ethanal), and benzaldehyde.

Rules for Ketones

• Replace the terminal -e of the alkane name with -one.

• The parent chain is the longest one containing the ketone group.

• Numbering begins at the end nearer to the carbonyl carbon.

Common Names of Ketones

• IUPAC retains some common names for a few ketones, such as acetone (propanone) and acetophenone.

Additional Nomenclature Rules

• Prefix oxo- is used when other functional groups are present, and the doubly bonded oxygen is labeled as a substituent.

• When several functional groups are present, the carbonyl group takes precedence over hydroxy, C=C, and C≡C bonds in numbering.

• In cyclic compounds, the carbonyl carbon of a cyclic ketone is C-1, and the number does not appear in the name (e.g., cyclohexanone).

Practice Problems: Nomenclature & Structure

• Draw structures for: 3-Methylbutanal, tert-Butylcyclohexanealdehyde

• Write IUPAC names for given structures (e.g., branched aldehydes and cyclic ketones).

• Draw structural formulas for: 4-methylpentanal, butan-2-ol, 2,3-dichloropropanal, 3-methylpentan-3-ol, 3-methyl-2-butanone

Physical Properties of Aldehydes & Ketones

Polarity

• The carbon-oxygen double bond (C=O) is polar, making these compounds more reactive and affecting their physical properties.

Boiling Points

• Higher than alkanes due to polarity.

• Lower than alcohols because carbonyl groups cannot hydrogen bond to each other.

Solubility

• More soluble in water than alkenes, but less soluble than alcohols.

Preparation of Aldehydes & Ketones

1. Oxidation of Alcohols

• Primary alcohols oxidize to aldehydes; secondary alcohols oxidize to ketones.

• Common oxidizing agents: Dess-Martin periodinane (DMP), Pyridinium chlorochromate (PCC).

2. Cleavage of Alkenes

• Alkenes with vinylic hydrogens undergo oxidative cleavage with ozone (O3) and dimethyl sulfide (DMS) to split the double bond, forming aldehydes and/or ketones.

3. Reduction of Esters

• Partial reduction of esters with diisobutylaluminum hydride (DIBAH) in toluene at -78°C produces aldehydes.

Additional Methods for Ketones

• Hydration of alkynes.

• Preparation from benzene derivatives.

Reactions of Aldehydes & Ketones

Oxidation

• Aldehydes are oxidized to carboxylic acids.

• Ketones are generally inert to oxidation.

• Tollens' Test: Uses Ag+ to oxidize aldehydes, producing a silver mirror.

• Benedict's Test: Uses Cu2+ to oxidize aldehydes, producing a brick-red precipitate.

Reduction

• Aldehydes reduce to primary alcohols; ketones reduce to secondary alcohols.

• Reduction can be catalytic (e.g., H2/Pd) or by metal hydrides (e.g., NaBH4, LiAlH4).

Addition Reactions

• Hydration: Addition of water forms geminal diols (hydrates). The yield depends inversely on the number of alkyl groups attached to the carbonyl carbon.

• Addition of Alcohols:

  • Aldehydes form hemiacetals and acetals.

  • Ketones form hemiketals and ketals.

• Intramolecular Cyclization: Hydroxy aldehydes can cyclize to form cyclic hemiacetals, important in monosaccharide chemistry (e.g., glucose cyclization).

• Addition with Amines:

  • Primary amines (RNH2) yield imines ($R_2C=NR$).

  • Secondary amines (R2NH) yield enamines ($R_2N-CR=CR_2$).

• Addition of HCN: Forms cyanohydrins via nucleophilic addition of CN- to the carbonyl group.

• Addition of Hydrazine: Aldehydes/ketones react with hydrazine (H2NNH2) and KOH to convert to alkanes (Wolff-Kishner reduction).

• Cannizzaro Reaction: Aldehydes without α-hydrogens react with OH- to yield carboxylic acid and alcohol.

Everyday Importance of Aldehydes & Ketones

Biological Significance

• Vitamin B6 forms pyridoxal phosphate (PLP), an important coenzyme.

Industrial Applications

• Formaldehyde: Used in building products and insulation materials.

• Acetone: Common solvent (e.g., nail polish remover).

• Butanedione: Used as butter flavoring in margarine.

Flavorings & Fragrances

• Many aldehydes (e.g., benzaldehyde, vanillin, cinnamaldehyde) are used as flavorings and fragrances.

Summary Table: Key Properties of Aldehydes & Ketones

Property	Aldehydes	Ketones
General Formula	R-CHO	R-CO-R'
Nomenclature Suffix	-al	-one
Oxidation	To carboxylic acids	Generally inert
Reduction	To primary alcohols	To secondary alcohols
Common Tests	Tollens', Benedict's	Negative for both
Physical Properties	Polar, moderate bp, moderate solubility	Polar, moderate bp, moderate solubility

Key Equations

• Oxidation of Aldehyde: $RCHO + [O] \rightarrow RCOOH$

• Reduction of Aldehyde: $RCHO + 2[H] \rightarrow RCH_2OH$

• Reduction of Ketone: $RCOR' + 2[H] \rightarrow RCH(OH)R'$

• Hydration (Addition of Water): $RCOR' + H_2O \rightarrow R_2C(OH)_2$

• Acetal Formation: $RCHO + 2ROH \xrightarrow{H^+} RCH(OR)_2 + H_2O$

• Ketal Formation: $RCOR' + 2ROH \xrightarrow{H^+} RCR'(OR)_2 + H_2O$

• Cyanohydrin Formation: $RCOR' + HCN \rightarrow R_2C(OH)CN$

• Imine Formation: $RCOR' + RNH_2 \rightarrow R_2C=NR + H_2O$

Additional info: Some example reactions, structures, and practice problem answers were not provided in the original notes and should be supplemented from a textbook for full mastery.