Aldehydes and Ketones, Name reactions

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

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