Aldehydes, Ketones, Introduction, Nomenclature, Methods of Preparation of Aldehydes and Ketones, Reactions of Aldehydes and Ketones (For CBSE, ICSE, IAS, NET, NRA 2022)

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Aldehydes, Ketones

Introduction

The organic compound with C = O functional group are called as carbonyl compounds. It includes aldehydes, ketones, carboxylic acids and their derivatives.

Carbonyl Compounds
  • It is important part of organic compound because these compounds are useful in synthesis of various chemical compounds such as reagents, solvents which are used in industries and other commercial values such as flavoring and aroma agent in food industries.
  • Aldehydes are carbonyl group with one hydrogen and other group may be alkyl or aryl group.
  • Ketones have 2 alkyl or aryl group bonded to the carbon.
The Carbon

Nomenclature

  • Aldehydes are known as alkanals after substituting ‘e’ in alkanes to ‘al’
  • The carbaldehyde is named when CHO group is attached with the ring.
  • Ketones are also called as alkanones by replacing final letter ‘e’ to ‘one’
  • The carbon chain is numbered by lowest number of the carbonyl groups. E. g. :
The Carbonyl Groups

Methods of Preparation of Aldehydes and Ketones

  • Both primary alcohols and secondary alcohols are oxidized to give aldehydes and ketones respectively.
  • The aldehydes and ketones are obtained based on the initial structure of the alkenes.
  • Markovnikov՚s hydration of alkynes gives ketones and aldehydes are obtained from anti-Markovnikov hydration of alkynes.
Anti-Markovnikov Hydration
  • The alkanoylation (formation of alkyl benzene from alkyl halides) method yields aromatic ketones.
Alkanoylation
  • The carbonyl carbon and oxygen atoms in aldehydes and ketones are sp2 hybridized so all atoms are present in a plane.
Carbonyl Carbon
  • -bond is formed by overlapping of p-orbitals of carbon and oxygen atoms.
  • The p-orbitals are present in a perpendicular plane to the molecule plane.
  • The oxygen attracts electrons from bond to form polarization because oxygen is electronegative, and carbon is electropositive in nature.
  • This polarization makes oxygen atom a nucleophilic and basic and carbon atom as electrophilic.
  • The polarization of aldehydes and ketones affects the physical and chemical properties of aldehydes and ketones.
  • The dipole-dipole attraction between the aldehydes and ketones results in their higher boiling points as compared to the hydrocarbons of similar molecular weight.
  • The water solubility natures of aldehydes are due to the hydrogen bonding between the oxygen atoms which are present in the aldehyde with hydrogen atoms of water molecules.
Hydrogen Atom
  • The 3 centers of reactivity in the structure of carbonyl compounds are:
    • Oxygen atom is nucleophilic so prone to attack by electrophiles
    • Carbonyl carbon is electrophilic so it is attacked by nucleophilic
    • Hydrogen atom which are present in α-carbon atom is acidic in nature
Hydrogen Atom

Aldehydes are more reactive than ketones because of:

  • The carbonyl carbons in ketones are bonded by 2 alkyl groups which are less electrophilic than carbonyl carbon in aldehydes. So, it is less susceptible to attack by nucleophiles.
  • The aldehydic carbonyl carbons are more accessible for attack by nucleophiles than the carbonyl carbon of the ketones as they are more crowded than carbonyl carbon in aldehyde.

Reactions of Aldehydes and Ketones

Nucleophilic Addition Reactions

It is represented by following reaction

Nucleophilic Addition Reactions

Mechanism of Nucleophilic Addition

The nucleophiles attack at carbonyl group yields a tetrahedral intermediate

Step 1: Protonation from a solvent (i.e.. Water or alcohol) takes place in the acidic addition.

Acidic

Step 2: Nucleophile attacks carbonyl carbon and thus addition process is completed.

Nulceophile

Some of the reactions are:

Formation of Cyanohydrins

The carbonyl compounds react with hydrogen cyanide to yield cyanohydrins. It is useful in the synthesis of carboxylic acids.

Carbonyl Compounds
Formation of Hemiacetals
  • Aldehydes and ketones react with alcohols to give Hemiacetals.
  • They have an – OH and an – OR group attached to the same carbon atom in their molecules.
  • When excess of alcohol is used, a second molecule of the alcohol reacts to give an acetal.
  • Acetals are stable in basic solutions, so, they are used as protecting groups for aldehydes and ketones.
  • Acetals are converted back to carbonyl compounds by treating them with dilute acids because of the reversible nature of the following reaction.
Dilute Acids
Formation of Alcohols

The Grignard reagents (RMgX) react with aldehydes and ketones to give alcohols

Grignard Reagents

Addition-Elimination or Condensation Reactions

Reaction with Ammonia and Its Derivatives
  • Aldehydes and ketones react with ammonia and primary amines to give imines (compounds having carbon-nitrogen double bond) .
  • During this reaction the water molecules is lost from primary amine and carbonyl compounds.
Amine and Carbonyl Compounds
  • The compounds which are formed from ammonia derivatives are insoluble solids which have characteristic melting points.
Ammonia Derivatives
Ammonia Derivatives

De-Oxygenation Reactions

It involves removal of oxygen from aldehydes and ketones and it can be reduced to the corresponding alkanes by following 2 reactions such as:

  • Wolff-Kishner Reduction: It involves heating of aldehydes or ketones in hydrazine in high boiling alcohol, thus the carbonyl group get converted to a methylene group.
Ethylbenzene
  • Clemmensen Reduction: It is carried out in acidic medium using amalgamated zinc and HCl.
Heptanal

Oxidation of Aldehydes

  • Aldeydes can be easily oxidized to carboxylic acids using oxidizing agents such as chromic acid, chromium trioxide, permanganate or silver oxide.
  • In Tollen՚s test it involves the addition of silver nitrate and aqueous ammonia to the carbonyl compound.
  • The aldehyde gets oxidized to carboxylic acid whereas silver ions are reduced to silver metal which are deposited on the walls of the test tubes and gives shining appearance.
Aldehydes
  • Aldehydes are also oxidized by containing Fehling solution complexed with tartarate ions as the oxidants.
  • These Cu2 + ions are reduced by aldehydes in alkaline medium to give brick red precipitate of cuprous oxide.
Aldehydes

Reaction at Α-Carbon

  • The -hydrogen in aldehydes and ketones is acidic and easily abstracted by strong base.
  • The end product enolate is anion and it is stabilized by resonance. When keto form and enol form are in equilibrium and is called as keto-enol tautomerism.
Reaction at Α-Carbon

Other reactions are:

Halogenation
  • Ketones have -hydrogen atom which reacts with halogens in presence of both acids and bases gives -haloketones.
Propanone
  • In presence of base the reaction gives trihalo products such as trihaloketones.
  • This trihaloketone reacts with hydroxyl radicals to give carboxylate ions and halo forms and this reaction is called as halo form reaction.
  • The iodoform () is a bright yellow soild have characteristic melting point, this is the base of iodoform test.
Trihaloketone
Aldol Condensation
  • Aldehydes with -hydrogen atoms with diluted NAOH gives aldols. Its illustrated by using ethanol:
    • The resultant product contains both aldehyde and alcohol functional groups called as aldol.
    • This aldol on heating undergoes dehydration to give -unsaturated aldehydes which are condensation product. This is called as aldol condensation.
Aldol Condensation
Aldol Condensation
Cannizzaro Reaction
  • Aromatic or aliphatic aldehydes don՚t have α-hydrogen when it reacts with KOH or NAOH gives alcohol and sodium salts of carboxylic acids. This reaction is Cannizzaro reaction.
  • One molecule of aldehyde is oxidized to carboxylic acid salt and another molecule is reduced to alcohol molecule.
  • Reaction with aliphatic compounds
Methanol

Reaction with aromatic compounds:

Benzaldehyde
  • It is placed by nucleophilic addition of – OH to aldehyde to give tetrahedral intermediate.
  • Here, disproportionation, oxidation and reduction take place to give one molecule of carboxylic acid and alcohol molecules of the starting aldehyde.
  • It is limited to that aldehyde which has no hydrogen on carbon next to CHO group.

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