Review of Aldehydes and Ketones - SS3 Chemistry Lesson Note

Aldehydes and ketones are important classes of organic compounds that contain the carbonyl functional group (C=O). They play a crucial role in both natural and synthetic chemistry and have diverse applications in various fields. Let's review aldehydes and ketones in detail:

Aldehydes:

Aldehydes have a carbonyl group (C=O) at the terminal carbon of the carbon chain. The general formula for aldehydes is RCHO, where R represents an alkyl or aryl group. The carbonyl carbon of an aldehyde is bonded to a hydrogen atom and another substituent (R group).

Naming Aldehydes: Aldehydes are named by replacing the -e ending of the parent alkane with -al. For example, methanol becomes methanal (formaldehyde), and ethane becomes ethanal (acetaldehyde).

Physical Properties: Aldehydes with fewer carbon atoms are generally gases or liquids at room temperature, while those with larger carbon chains may be solids. Aldehydes have a distinctive and often pungent odour.

Ketones:

Ketones also contain a carbonyl group (C=O), but they are located within the carbon chain, not at the terminal position. The general formula for ketones is RCOR', where R and R' are alkyl or aryl groups.

Naming Ketones: Ketones are named by replacing the -e ending of the parent alkane with -one. For example, propanone is a ketone with the structure CH3COCH3 (commonly known as acetone).

Physical Properties: Ketones with smaller carbon chains are generally liquids, while those with larger carbon chains may be solids. Like aldehydes, ketones also have a distinct smell, especially in lower-molecular-weight examples.

Reactivity and Chemical Properties:

●     Both aldehydes and ketones undergo nucleophilic addition reactions at the carbonyl carbon. Nucleophiles attack the electrophilic carbon of the C=O group, leading to the formation of new carbon-carbon and carbon-oxygen bonds.

●     Aldehydes are more reactive than ketones due to the absence of an alkyl group on the carbonyl carbon, which makes the electron-deficient carbon more susceptible to attack.

●     Oxidation of aldehydes results in the formation of carboxylic acids, while ketones are not easily oxidised under normal conditions.

Applications:

●     Formaldehyde is a vital industrial chemical used in the production of resins, plastics, and various other chemicals.

●     Acetone is a common solvent and is used in the production of many chemicals, including methyl methacrylate (MMA) for acrylic plastics.

●     Ketones are essential in the synthesis of pharmaceuticals and agrochemicals.

Biological Significance:

●     Glucose, an essential sugar in metabolism, can be oxidised to form gluconic acid, which is an example of an aldehyde in its open-chain form.

●     Ketone bodies, such as acetoacetate and beta-hydroxybutyrate, are produced in the liver during periods of prolonged fasting or low carbohydrate intake and serve as an alternative energy source for the brain and muscles.

In summary, aldehydes and ketones are versatile classes of organic compounds with significant applications in chemistry, industry, and biology. Their reactivity and unique chemical properties make them valuable building blocks for the synthesis of various complex molecules, making them central to the field of organic chemistry.