Chemical Reactions of Aldehydes and Ketones

Nucleophilic Addition Reaction

Addition of Hydrogen Cyanide (HCN)

• Aldehydes and ketones on reacting with HCN give cyanohydrins

• Very slow reaction with pure HCN

• So, it is catalyzed by a base and the generated CN- being a stronger nucleophile readily attacks on carbonyl carbon to yield corresponding cyanohydrin

• Cyanohydrins are a useful synthetic intermediate

Nucleophilic Addition Reaction

Addition of Sodium Hydrogen Sulphite (NaHSO3)

• Most aldehydes and aliphatic methyl ketones which are not sterically hindered on reaction with NaHSO3 give bisulphite addition product

• Position of equilibrium lies largely to the right for most aldehydes and left for ketones due to steric reasons

• Bisulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali

• Useful for separation and purification of aldehydes

Nucleophilic Addition Reaction

Addition of Grignard’s Reagent

• Aldehydes and ketones on reaction with Grignard’s reagent give addition products which upon hydrolysis with water or dilute mineral acids give alcohols

• The type of alcohol depends upon the aldehyde/ketone used

  • Formaldehyde → 1* alcohols

  • Other aldehydes - 2* alcohols

  • Ketone → 3* alcohols

Nucleophilic Addition Reaction

Addition of Alcohols

• Aldehydes react with one equivalent of monohydric alcohol in the presence of dry HCl gas to yield alkoxy alcohol intermediates known as hemiacetals

• Hemiacetals on further reaction with one more molecule of alcohol give acetals

• Ketones do not react with monohydric alcohols but they react with dihydric alcohol such as ethylene glycol to give cyclic ketals

• Acetals and ketals hydrolysed by dilute acids generate original aldehydes and ketones

Nucleophilic Addition Elimination Reaction

Addition of Ammonia and its Derivatives

• Aldehydes and ketones react with ammonia and many derivatives (H2N-Z) in weakly acidic mediums to form compounds containing >C=N group

• Reactions are reversible and catalysed by acids

• Equilibrium favors product formation due to rapid dehydration of the intermediated to form >C=N-Z type compounds

Quick Note

Brady’s Reagent

• 2,4-dinitrophenol hydrazine

• Aldehydes and ketones react to form yellow, orange or red precipitate of 2,4-dinitrophenyl hydrazone

• Useful for characterization of aldehydes and ketones

Oxidation

Difference in Oxidation of Aldehyde and Ketone

• Aldehydes can be oxidized to carboxylic acids easily in the presence of both weak and strong oxidizing agents due to the presence of H-atom on the carbonyl group. This helps to easily convert into -OH group without any cleavage. The carboxylic acid contains the same number of carbons as the parent aldehyde.

• Ketones do not contain the H-atom on carbonyl group, therefore can only be oxidized via strong oxidizing agents like KMnO4, HNO3 etc. It involves cleavage of carbon-carbon bond therefore reducing the number of carbons in the carboxylic acid compared to parent ketone.

Oxidation

Tollen’s Test

• Known as silver mirror test ; aliphatic and aromatic aldehydes reduce Tollen’s reagent

• Aldehyde heated with freshly prepared Tollen’s reagent (ammoniacal solution of silver nitrate) a silver mirror is formed due to formation of silver metal

• Aldehyde oxidizes to corresponding carboxylate anion

• Reaction occurs in alkaline medium

Oxidation

Fehling’s Test

• Fehling’s Solution A - aqueous copper sulphate solution

• Fehling’s Solution B - alkaline sodium potassium tartarate (Rochelle’s salt)

• These solutions are mixed in equal proportion before test

• When aldehyde is heated with Fehling’s reagent, reddish brown precipitate is formed. Aldehyde oxidizes to corresponding carboxylate anion

• Aromatic aldehydes do not reduce Fehling’s solution

Oxidation

Benedict’s Test

• Benedict’s reagent : Cu2+ ions that are complexed with citrate ions (unlike Fehling’s)

• Reacts same way as Fehling’s

Oxidation

Oxidation of Methyl Ketones by Haloform Reaction

• Aldehydes and ketones having one methyl group attached to the carbonyl carbon (methyl ketones) are oxidized by sodium hypohalite to corresponding sodium salts of carboxylic acids

• Methyl group gets converted to haloform

• Carbon-carbon double bond remains unaffected by this reaction, if present

• Iodoform reaction is used to detect CH3CO- group/CH3CH(OH)- group that oxidizes to CH3CO-

Reduction

Reduction to Hydrocarbons (Clemmensen Reduction)

• Reducing the carbonyl group to a -CH2 group

• Carried out in the presence of Zn amalgam and conc. HCl

• Generally used for aldehydes and ketones that are sensitive to alkalis

Reduction

Reduction to Hydrocarbons (Wolff-Kishner Reduction)

• Aldehydes and ketones are heated with hydrazine and NaOH/KOH in high boiling solvent like ethylene glycol

• Useful for carbonyl compounds that are sensitive to acids

Reduction

Reduction with HI and Red P

• Aldehydes and ketones on reacting with hydroiodic acid (HI) and red phosphorus at 423K get reduced to their corresponding alkanes

Reduction

Reduction to Alcohols

• Aldehydes and ketones can be reduced to 1* and 2* alcohols respectively either by catalytic dehydrogenation or chemically via LiAlH4/NaBH4

Reduction due to α-Hydrogen

Aldol Condensation

• Aldehydes/ketones with α-H atom undergo reduction in the presence of dilute alkali as catalyst to form aldols/ketols

• Name comes from the two functional groups : aldehyde/ketone + alcohol

• Aldols readily lose water to α, β unsaturated carbonyl compounds (aldol condensation products)

• Formaldehyde, benzaldehyde, benzophenone do not undergo aldol condensation due to absence of α-H

Reduction due to α-Hydrogen

Cross-Aldol Condensation

• When aldol condensation comprises of two different aldehydes/ketones

• Both reactants having α-H atom will result in a mixture of 4 products due to cross condensation and self condensation

• Great synthetic use even if one of the compounds is not having α-H atoms

• Ketones can also be one component

Cannizzaro Reaction

• Aldehydes that do not have α-H atoms undergo self oxidation and reduction disproportionately on heating with conc. alkali

• One molecule reduces to alcohol, other oxidizes as salt of carboxylic acid

Electrophilic Substitution Reaction

• Aromatic aldehydes and ketones undergo electrophilic substitution reactions like halogenation, sulphonation, nitration

• Due to electron withdrawing nature, deactivating and meta-directing