Figure 6.6. Hydration Reaction
The carbonyl is hydrated by water, then protonated, resulting in a geminal diol.ACETALS AND HEMIACETALS
A similar reaction occurs when aldehydes and ketones are treated with alcohols. When one
equivalent of alcohol (the nucleophile in this reaction) is added to an aldehyde or ketone, the
product is a hemiacetal or hemiketal, respectively, as shown in Figure 6.7. Hemiacetals and
hemiketals can be recognized by the retention of the hydroxyl group. This “halfway” step (hence the
hemi– prefix) is the endpoint in basic conditions.
Figure 6.7. Hemiacetal Formation
The oxygen in the alcohol functions as the nucleophile, attacking the carbonyl carbon, and
generating a hemiacetal.When two equivalents of alcohol are added, the reaction proceeds to completion, resulting in the
formation of an acetal or ketal, as shown in Figure 6.8. This reaction proceeds by a nucleophilic
substitution reaction (SN1) and is catalyzed by anhydrous acid. The hydroxyl group of a hemiacetal
or hemiketal is protonated under acidic conditions and lost as a molecule of water. A carbocation is
thus formed, and another equivalent of alcohol attacks this carbocation, resulting in the formation
of an acetal or ketal. Acetals and ketals, which are comparatively inert, are frequently used as
protecting groups for carbonyl functionalities. Molecules with protecting groups can easily be
converted back to carbonyls with aqueous acid and heat.