NEET Chemistry Notes Aldehydes, Ketones and Carboxylic Acids – 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.
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
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.
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:
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
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.