Aqueous hydrochloric acid (aq. HCl) is often used in the workup procedure of ketone reduction for the following reasons:
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pH Adjustment and Protonation: During the reduction of ketones (for example, using lithium aluminum hydride - LiAlHâ‚„), the reaction can produce alkoxide intermediates. Aqueous HCl is used to quench this reaction by hydrolyzing the alkoxide and providing protons to regenerate the ketone product or convert the intermediate into a more stable alcohol. This protonation step neutralizes the basic species from the reduction and helps to stabilize the products.
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Separation of Organic and Aqueous Layers: After the reduction and protonation steps, HCl helps to create a clear separation between the organic phase (containing the reduced alcohol or other products) and the aqueous phase. The acidic conditions can also help in the extraction of any remaining organic compounds into the organic layer, facilitating easier purification through techniques such as liquid-liquid extraction.
Overall, aq. HCl plays a crucial role in neutralizing the reaction conditions and aiding in the workup process.