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Aldehydes and ketones are two important classes of organic compounds that contain the carbonyl group (C=O). The carbonyl group is highly reactive and plays a central role in the chemical behavior of these compounds. Aldehydes have at least one hydrogen atom attached to the carbonyl carbon, while ketones have two alkyl or aryl groups attached to the carbonyl carbon. These compounds are widely used in industrial and pharmaceutical applications, and they are crucial in organic synthesis.
Aldehydes: The general structure of an aldehyde is R-CHO, where R can be a hydrogen atom or an alkyl/aryl group. The simplest aldehyde is formaldehyde (HCHO), and the next in line is acetaldehyde (CH₃CHO). Aldehydes are named by replacing the "-e" ending of the corresponding alkane name with "-al."
Example: CH₃CHO is named acetaldehyde.
Ketones: The general structure of a ketone is R-CO-R', where R and R' are alkyl or aryl groups. Ketones are named by replacing the "-e" ending of the corresponding alkane name with "-one."
Example: CH₃COCH₃ is named acetone (propan-2-one).
Aldehydes can be prepared by several methods, including:
Oxidation of Alcohols: Primary alcohols can be oxidized to aldehydes using mild oxidizing agents such as PCC (Pyridinium chlorochromate) or DMP (Dess-Martin periodinane).
Example: Ethanol (CH₃CH₂OH) is oxidized to acetaldehyde (CH₃CHO) using PCC.
Reduction of Acids: Acid chlorides can be reduced to aldehydes using reagents like diborane (B₂H₆) or lithium tri-tert-butoxyaluminum hydride (L-Selectride).
Example: Reduction of acetic acid (CH₃COOH) gives acetaldehyde.
Ozonolysis of Alkenes: Alkenes can undergo ozonolysis to produce aldehydes if the double bond is cleaved and the products are appropriately stabilized.
Example: Ozonolysis of propene (CH₃-CH=CH₂) forms acetaldehyde.
Ketones can be synthesized by several methods, including:
Oxidation of Secondary Alcohols: Secondary alcohols can be oxidized to ketones using oxidizing agents like potassium dichromate (K₂Cr₂O₇) or chromium trioxide (CrO₃).
Example: Isopropanol (CH₃CH(OH)CH₃) is oxidized to acetone (CH₃COCH₃).
Friedel-Crafts Acylation: Ketones can be synthesized through the Friedel-Crafts acylation reaction, where an aryl or alkyl group reacts with an acyl chloride in the presence of an aluminum chloride (AlCl₃) catalyst.
Example: The reaction of benzene (C₆H₆) with acetyl chloride (CH₃COCl) in the presence of AlCl₃ forms acetophenone (C₆H₅COCH₃).
Reduction of Acids and Acid Derivatives: Acid chlorides or esters can be reduced to ketones using reducing agents like diborane or lithium aluminum hydride (LiAlH₄).
Example: The reduction of acetyl chloride (CH₃COCl) gives acetone.
Boiling Points: Both aldehydes and ketones have relatively higher boiling points than hydrocarbons of similar molecular weight due to the presence of the polar carbonyl group. However, they have lower boiling points than alcohols because they lack hydrogen bonding.
Solubility: Smaller aldehydes and ketones (e.g., formaldehyde, acetone) are soluble in water due to hydrogen bonding between the carbonyl group and water molecules. As the size of the molecule increases, their solubility in water decreases.
Odor: Many aldehydes and ketones have distinctive odors. For example, formaldehyde has a pungent smell, while acetone is odorless or mildly sweet.
Nucleophilic Addition Reactions: The carbonyl group is electrophilic, and it is susceptible to attack by nucleophiles. This leads to nucleophilic addition reactions, which are the hallmark of aldehydes and ketones.
Example: The reaction of aldehydes or ketones with water leads to the formation of hydrates (geminal diols).
Reduction: Both aldehydes and ketones can be reduced to alcohols by reducing agents such as sodium borohydride (NaBH₄) or lithium aluminum hydride (LiAlH₄).
Example: Acetone (CH₃COCH₃) is reduced to isopropanol (CH₃CH(OH)CH₃) using NaBH₄.
Oxidation of Aldehydes: Aldehydes can be easily oxidized to carboxylic acids using mild oxidizing agents such as Tollen's reagent (ammoniacal silver nitrate) or Fehling's solution (copper sulfate in alkaline solution).
Example: Acetaldehyde (CH₃CHO) is oxidized to acetic acid (CH₃COOH).
Aldol Condensation: Aldehydes and ketones can undergo aldol condensation in the presence of a base to form β-hydroxy ketones or aldehydes. This reaction is important in the synthesis of larger molecules.
Example: Acetaldehyde (CH₃CHO) can undergo aldol condensation to form crotonaldehyde (CH₃CH=CHCHO).
Clemmensen Reduction: Aldehydes and ketones can be reduced to hydrocarbons using zinc amalgam and hydrochloric acid in a reaction known as Clemmensen reduction.
Example: Acetone (CH₃COCH₃) is reduced to propane (CH₃CH₂CH₃).
Haloform Reaction: Aldehydes with a methyl group attached to the carbonyl carbon undergo halogenation in the presence of a halogen and a base, producing a trihalogenated compound and a haloform.
Example: Acetaldehyde (CH₃CHO) reacts with iodine to form chloroform (CHCl₃).
Aldehydes in Industry: Formaldehyde (HCHO) is widely used in the production of plastics (e.g., bakelite), disinfectants, and as a preservative in biological specimens. Acetaldehyde (CH₃CHO) is used in the synthesis of acetic acid and other chemicals.
Ketones in Industry: Acetone (CH₃COCH₃) is a common solvent used in paints, varnishes, and nail polish removers. It is also used in the production of plastics and synthetic fibers.
Aldehydes in Medicine: Formaldehyde and other aldehydes are used as disinfectants and in the preservation of biological samples. Acetaldehyde is an intermediate in the metabolism of ethanol.
Ketones in Medicine: Ketones, such as acetone, are used in various pharmaceutical applications. Acetone is used as a solvent for many drugs and in the formulation of certain ointments and creams.
Predicting the products of nucleophilic addition reactions involving aldehydes and ketones.
Solving problems based on the oxidation and reduction of aldehydes and ketones.
Writing the mechanisms for aldol condensation and other related reactions.
Identifying the reagents required for the preparation and functional group transformation of aldehydes and ketones.
Mastering the concepts related to aldehydes and ketones will help in tackling a wide range of questions in JEE Chemistry, including reaction mechanisms, synthesis, and applications. Understanding their reactivity and how to manipulate their properties is crucial for solving problems in organic chemistry.
