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Protocol to deprotect a dimethylacetal with HBr. is it better than HCl ? my reaction with aq. HCl in large excess in THF typically never ends even at 50°c for 16h
Accepted Answer
HBr is generally a more effective reagent for deprotecting dimethylacetals than HCl. This is because bromide is a better nucleophile than chloride, making it more efficient at attacking the acetal carbon and initiating the deprotection process.
Your observation that the reaction with aqueous HCl in THF doesn't go to completion even at elevated temperatures suggests that the reaction is hindered. This could be due to several factors:
1. Insufficient Acid Concentration: The reaction may require a higher concentration of HCl than you are currently using. Consider increasing the concentration of HCl or using a stronger acid such as concentrated HCl.
2. Water Interference: The presence of water in the reaction mixture can be problematic, as it can compete with the deprotection reaction. Using anhydrous HCl in a dry solvent like THF or DCM can improve efficiency.
3. Reaction Kinetics: The reaction rate can be influenced by factors like temperature, solvent, and the nature of the substrate. Consider carefully optimizing these conditions to ensure the deprotection occurs efficiently.
4. Alternative Deprotection Method: If you are encountering persistent issues with HCl deprotection, you could consider using an alternative method like p-toluenesulfonic acid (TsOH) or a catalytic amount of acid in a different solvent (e.g., MeOH) for the deprotection.
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