Aldehydes and Ketones, Name reactions

Contents

• Aldehyde and Ketones

• Acidity of Α-hydrogen Atom

• Mechanism Involved In Nucleophilic Addition Reaction

• Mechanism of Aldol and Crossed Aldol Condesation

• Cannizzaro

• Crossed cannizaro reaction

• Mechanism

Learning Objectives

At the end of this lecture, student will be able to

• Compare the reactivity of aldehyde and ketones

• Explain the acidity of α-hydrogen atom

• Explain the mechanism involved in nucleophilic addition reaction

• Outline the mechanism of aldol and crossed aldol condesation

• Explain the mechanism involved in cannizzaro and crossed cannizaro reaction

• Outline the mechanism involved in Benzoin condensation

Aldehydes and Ketones Structure

Example

Structure of the Carbonyl Group

• Carbonyl carbon atom is sp2 hybridized

• A trigonal planar structure

Resonance Structure for the Carbonyl Group

Physical Properties of the Carbonyl Group

• Carbonyl carbon is polar in nature

• Alpha hydrogen is acidic

Relative Reactivity of Aldehydes and Ketones

• Aldehydes are usually more reactive than ketones toward nucleophilic addition

• Characteristic reaction of carbonyl compounds

• An aldehyde has a greater partial positive charge on its carbonyl carbon than ketone

• Hydrogen is electron withdrawing compared with an alkyl group

• Aldehyde is less stable than a ketone

• More reactive toward nucleophilic attack

• Steric factors

• Hydrogen in aldehyde is smaller than alkyl group in ketone

Acidity of Alpha Hydrogen Atom

• Attached to carbon adjacent to carbonyl group

Nucleophilic Addition Reaction

Aldol Condensation

• Under the influence of dilute base or dilute acid

• 2 molecules of an aldehyde or a ketone with α hydrogen

• β-hydroxyaldehyde or β-hydroxyketone

• α carbon of the first molecule becomes attached to the carbonyl carbon of the second molecule

Examples 

• If aldehyde or ketone does not contain α hydrogen → simple aldol

• Condensation cannot take place

Mechanism of Aldol Condensation

• Base catalyzed

• Step 1 Hydroxide ion abstracts hydrogen ion from α carbon of aldehyde to form carbanion I

• Step 2 Carbanion I attacks carbonyl carbon to form ion II

• Step 3 Ion II abstracts hydrogen ion from water to form β hydroxy aldehyde /aldol

Crossed Aldol Condensation

• Aldol condensation between 2 different carbonyl compounds

• Mixture of 4 possible products

• Good yield of single product obtained under certain conditions

• One reactant contains no α hydrogens

• Reactant mixed with catalyst

• Other carbonyl compound with α hydrogen

• Examples

Mechanism of Crossed Aldol Condensation

• Step 1 Deprotonation and formation of nucleophile

• Step 2 Attack by nucleophile – formation of alkoxide ion

• Step 3 Hydrolysis and formation of aldol type product

Cannizzaro Reaction

• Aldehydes containing no α-hydrogens undergo self-oxidation and reduction

• In the presence of concentrated alkali

• Products - mixture of alcohol and salt of carboxylic acid

• Examples

• Step 1 Addition of hydroxide ion to give intermediate ion

• Step 2 Transfer of hydride ion to the carbonyl carbon of aldehyde

• Cannizzaro reaction between 2 different aldehydes

• If one of the aldehyde is formaldehyde, products - sodium formate and alcohol from other aldehyde

• Mechanism

Summary

• Examples for carbonyl compounds are aldehydes and ketones

• Carbonyl group is polar in nature

• Alpha hydrogen is acidic

• Aldehydes are more reactives than ketones

• Formaldehyde is the most reactive one

• Carbanion formed by the abstraction of alpha hydrogen is resonance stabilized

• Nucleophilic addition reaction is characteristic reaction of carbonyl compound

• Carbonyl compounds are trigonal planar compounds 

• Tetrahedral intermediate is formed which abstracts proton

• Product formed is trigonal planar

• In presence of concentrated alkali two molecules of aldehyde and ketones form aldol

• Mechanism involved is nucleophilic addition

• Carbon anion is formed as intermediate

• In crossed aldol condensation different types of aldehyde and ketones are involved

• Mechanism involved in cannizzaro and crossed cannizaro reaction is nucleophilic aromatic addition

• Transfer of hydride ion to one of the carbonyl compounds

• Formation of salt of carboxylic acid and an alcohol

• Formaldehyde and benzaldehyde is used as reactants for crossed aldol condensation