NEET Chemistry Notes Aldehydes, Ketones and Carboxylic Acids – Properties

NEET Chemistry Notes Aldehydes, Ketones and Carboxylic Acids – Properties

Properties

Physical Properties

•  Aldehydes and ketones are liquid or solid at room temperature. Methanal is a gas at room temperature. Ethanal is a volatile liquid.
• The boiling point of aldehydes and ketones are higher than ethers of comparable molecular masses.
•  The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions because they form hydrogen bond with water. However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.

Chemical Properties

Aldehydes and ketones due to the presence of polar carbonyl group exhibit the following characteristics:

Nucleophilic Addition Reactions

A nucleophile attacks at the electrophilic carbon atom of the polar carbonyl group.

As the number of carbon atoms increases, reactivity decreases due to steric hindrance. Hence, the order of reactivity is

This reaction is used for the separation and purification of aldehydes and ketones.
The reason for this is that the hydrogen sulphite addition compound formed, is water soluble and can be converted back to the original carbonyl compound by treating with dilute mineral acid or alkali.

Addition of Ammonia and Its Derivative

This reaction is reversible and catalysed by acid (pH<<4). The equilibrium favours the product formation due to rapid dehydration of the intermediate

Oxidation

Aldehydes are easily oxidised to carboxylic acids on treatment with strong oxidising agents (HN0₃, K₂Cr₂0₇,KMn0₄, etc). Oxidation of primary alcohols and aldehydes cannot be carried out by alkaline K₂Cr₂0₇ because under these conditions, potassium chromate is formed which does not act as an oxidising agent.
Mild oxidising agents, mainly Tollen’s reagent and Fehling’s reagent also oxidise aldehydes

This reaction is not given by aromatic aldehydes. Ketones are oxidised only in the presence of strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to give a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone, e.g.

Haloform Reaction

Methyl ketones (—COCH₃) are also oxidised by haloform reaction in which they are treated with halogen in the presence of alkali or hypohalite salt.

—CH₃ group of COCH₃ is converted into haloform as it contains acidic hydrogen atoms. Acid salt is obtained corresponding to total number of carbon atoms apart from —CH₃ of RCOCH₃ .
Mechanism of reaction is as follows:

Halogenation

Alkali Hydrolysis of Trihaloketone

Reactions Due to Acidic

Aldehydes or ketones having atleast one a-hydrogen atom, undergo a reaction in the presence of dilute alkali , as catalyst to form aldehydes (aldol) or  ketones (ketol) respectively, e.g. aldol condensation

When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation, e.g

Cannizzaro Reaction

Aldehydes which do not have any a-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali.

Electrophilic Substitution Reaction

Aromatic aldehydes and ketones undergo electrophilic substitution. Carbonyl group’ shows +R-effect, therefore acts as a deactivating and meta directing group.

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