Catalysts make alcohols more reactive by taking away hydrogen to create carbonyl compounds and then returning the hydrogen to the final products. Alcohols are relatively common starting materials for chemical reactions, even though they are quite unreactive. For example, reactions that would substitute another functional group (a nucleophile) for OH often fail because the hydroxide group (HO − ) is difficult to displace—it is a poor leaving group. Alcohols are usually activated by turning the hydroxide into a better leaving group, either by protonating the alcohol or by converting it into a sulfonate or halide. However, both of these activation methods have some disadvantages ( 1 ). The acidic environment required for protonating the alcohol also protonates and deactivates the incoming nucleophile, especially amines. Conversion of the alcohol into a sulfonate or halide can lead to toxicity problems; many alkyl halides and alkyl sulfonates are mutagenic.