mesylates and tosylates. A mesylate is a compound containing the functional group –SO 3 CH 3 ,
derived from methanesulfonic acid; its anionic form is shown in Figure 5.9.
Figure 5.9. Structure of the Mesylate AnionMesylates are prepared using methylsulfonyl chloride and an alcohol in the presence of a base.
Tosylates contain the functional group –SO 3 C 6 H 4 CH 3 , derived from toluenesulfonic acid. These
compounds are produced by reaction of alcohols with p-toluenesulfonyl chloride, forming esters of
toluenesulfonic acid. A tosylate is shown in Figure 5.10.
Figure 5.10. Structure of a TosylateIn addition to making hydroxyl groups of alcohols into better leaving groups for nucleophilic
substitution reactions, mesyl and tosyl groups can also serve as protecting groups when we do not
want alcohols to react. These groups are protective in that they will not react with many of the other
reagents that would attack alcohols, especially oxidizing agents. Thus, reacting an alcohol to form a
mesylate or tosylate is sometimes performed before multistep reactions in which the desired
products do not derive from the alcohol.
PROTECTING GROUPS
Alcohols can also be used as protecting groups for other functional groups themselves. For example,
aldehydes and ketones can be reacted with two equivalents of an alcohol or a diol (dialcohol),
forming acetals (primary carbons with two –OR groups and a hydrogen atom) or ketals (secondary
carbons with two –OR groups). Carbonyls are very reactive with strong reducing agents like lithium
aluminum hydride (LiAlH 4 ). Acetals and ketals, on the other hand, do not react with LiAlH 4 . The
acetal or ketal functionality thereby protects the aldehyde or ketone from reaction. After reducing