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