Physical Properties of Aldehydes and Ketones
- The intermolecular forces of attraction in aldehydes and ketones are dipole-dipole interactions. These are stronger than van der Waals forces in alkanes but weaker than H- bonding in alcohols. Therefore, the boiling point order is:
Alkanes < Aldehyde / Ketones < Alcohols
- Due to the polarity of the carhonyl group, lower aldehydes and ketones are capable of forming hydrogen bond with water and are soluble while solubility decreases down the homologous series as size of R group increases.
- All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc.
Chemical Reaction of Aldehydes and ketones:
Nucleophilic addition reactions
Mechanism of nucleophilic addition reactions:
A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon.
The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate capture a proton from the reaction medium to give the electrically neutral product. The net result is the addition of Nu– and H+ across the carbon-oxygen double bond.
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent.
Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:
- Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN–) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin. Cyanohydrins are useful synthetic intermediates.
- Addition of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products.
Addition of alcohols: Aldehydes on addition of monohydric alcohol in presente of dry HC1 forms hemiacetal which further on reacting with one more molecule of alcohol turns to and acetal.
Ketones do not react with monohydric alcohols. Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.
- Addition of ammonia and its derivatives: Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones.
Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
Reduction to alcohols:
Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4).
Reduction to alkanes:
- Chemmensen reduction:
- Wolff-Kishner reduction:
Oxidation of Aldehydes and Ketones:
Due to the presence of hydrogen at carbonyl carbon, aldehydes are easily oxidisable while ketones are difficult to oxidise.
Aldehydes are oxidized to acids in presence of common oxidising agents HNO3, K2Cr2O7, KMnO4.
Ketones are oxidized under drastic conditions i.e. with powerful oxidising agents like HNO3, K2Cr2O7/H2SO4 , KMnO4/H2SO4 at higher temperature to give a mixture of acids, each containing lesser number of carbon atoms than the parent ketones.
Test to Distinguish between Aldehydes and Ketones:
Tollen’s reagent test:
Tollen’s reagent is ammoniacal silver nitrate and is a mild oxidising agent. Aldehydes give silver mirror with Tollen’s reagent.
Fehling’s solution test:
Fehling’s solution is an alkaline solution of CuSO4 (Fehling A) and sodium potassium tartrate, Rochelle salt (Fehling B). Aldehyde gives reddish brown ppt. of brown precipitates of cuprous oxide (Cu2O). Aromatic aldehyde gives poor yield.
Reaction due to α-hydrogen
Acidity of α-hydrogen of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen.
Aldehydes and ketones containing at least one α -hydrogen undergo self-condensation reactions in the presence of dilute alkali to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively which on heating in the presence of H+ gives α, β-unsaturated aldehydes or ketones.
Cross aldol condensation:
When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products
Aldehydes which do not have an α -hydrogen atom undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali to form alcohol and salt of acid.
Electrophilic substitution reaction:
Aromatic aldehyde and ketones undergo electrophilic substitution at the meta position. Carbonyl group shows + R effect, therefore acts as a deactivating and meta directing group.
Example of electrophilic susbstitution reaction:
Uses of aldehydes and ketones:
- Aldehyde is mostly used in the formation of resins when it is combined with melamine, urea, phenol etc.
- Formaldehyde in the form of formalin (40%) solution, is used to preserve biological specimens.
- Benzaldehyde is used in perfumery and in dye industries.
- Acetophenone is used as ingredients in flavors and deodorants.
- Ketones are used as solvents and intermediates in the chemical industryBottom of Form