Chapter 18: Carbonyl Compounds

The carbonyl group (C double bond O) is key; in aldehydes, it is bonded to a hydrogen atom and is always at the end of the carbon chain, while in ketones, it is bonded to two other carbon atoms. The preparation of these compounds involves oxidation: primary alcohols are oxidized to form aldehydes, a process that requires immediate distillation to prevent further oxidation into carboxylic acids, while secondary alcohols are oxidized to form stable ketones. Both aldehydes and ketones can be reduced back to their respective alcohols (primary and secondary, respectively) using reducing agents like sodium tetrahydridoborate (NaBH 4​ ) or lithium tetrahydridoaluminate (LiAlH 4​ ). Due to the polarization of the carbonyl bond, these compounds readily undergo nucleophilic addition reactions. A significant example is the reaction with hydrogen cyanide (HCN), which generates a 2-hydroxynitrile product and is utilized by chemists to increase the length of the carbon chain. The chapter also describes key tests for identification: the general presence of a carbonyl group is confirmed by the formation of a deep-orange precipitate using 2,4-dinitrophenylhydrazine (2,4-DNPH). Aldehydes and ketones can be distinguished because aldehydes are easily oxidized; a positive result with Tollens’ reagent produces a silver mirror, and with Fehling’s solution the blue solution turns into a red/orange precipitate, while ketones show no observable change in either test. Additionally, the tri-iodomethane test (using alkaline aqueous iodine solution) is used to detect the CH 3​ CO– group (found in methyl ketones and ethanal) or the CH 3​ CH(OH)– group in certain secondary alcohols, yielding a yellow precipitate. Finally, the utility of Infrared Spectroscopy in organic chemistry is covered, explaining how different functional groups, such as the C double bond O bond, absorb characteristic wavelengths of infrared radiation, allowing scientists to identify them based on specific, predictable wavenumber ranges.