The best protecting group for the hydroxyl group in a molecule containing both alcohol and carboxylic acid functionalities is the tert-butyldimethylsilyl TBS group. The TBS group can be selectively introduced to the alcohol using tert-butyldimethylsilyl chloride TBSCl and a suitable base, such as imidazole, without affecting the carboxylic acid functionality.The TBS group is advantageous for several reasons:1. Selectivity: The TBS group selectively reacts with the alcohol functionality without affecting the carboxylic acid group. This allows for selective chemical modification of the carboxylic acid group without interference from the protected alcohol.2. Stability: The TBS-protected alcohol is stable under a wide range of reaction conditions, including acidic and basic conditions, as well as various reagents used for carboxylic acid modifications.3. Orthogonality: The TBS group can be selectively removed without affecting other protecting groups or functionalities in the molecule. Common deprotection methods include treatment with tetrabutylammonium fluoride TBAF or mild acidic conditions, such as aqueous acetic acid.4. Ease of introduction and removal: The TBS group can be easily introduced and removed using readily available reagents and mild reaction conditions, making it a practical choice for protecting the hydroxyl group in a synthetic sequence.In summary, the TBS group is an excellent choice for protecting the hydroxyl group in a molecule containing both alcohol and carboxylic acid functionalities due to its selectivity, stability, orthogonality, and ease of introduction and removal. This allows for selective chemical modification of only one of these functionalities without interference from the protected group.