molecules and the prevention of the formation of alternative products. Effectively, bulky groups
make it impossible for the nucleophile to reach the most reactive electrophile, making the
nucleophile more likely to attack another region.
Another way that sterics come into play is in the protection of leaving groups. One can temporarily
mask a reactive leaving group with a sterically bulky group during synthesis. For example, reduction
of a molecule containing both carboxylic acids and aldehydes or ketones can result in reduction of
all of the functional groups. To prevent this, the aldehyde or ketone is first converted to a
nonreactive acetal or ketal, which serves as the protecting group, and the reaction can proceed.
This reaction is shown in Figure 4.10. Another protective reaction is the reversible reduction of
alcohols to tert-butyl ethers.
Figure 4.10. Protection of a Ketone by Conversion to an AcetalBRIDGE
When an aldehyde is mixed with a diol (or two equivalents of alcohol), it forms an acetal.
When a ketone is mixed with a diol (or two equivalents of alcohol), it forms a ketal. Acetal
and ketal chemistry is discussed in Chapter 6 of MCAT Organic Chemistry Review.