Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 120

7.7.1.4 ADDITION OF WATER AND ALCOHOLS
The nucleophiles in these reactions are weaker non-ionic species. The relative
stability of substrate and product determines the position of the equilibrium in
these reactions. To shift the equilibrium toward product, the rate of the forward
process must be increased. This is usually done by acid catalysis.
The first step is protonation of the negatively polarized carbonyl oxygen. This
increases the inductive polarization of the carbonyl carbon, and speeds up the
attack by the weak, uncharged nucleophile. Figure 7.30 shows the formation of
hydrate products if water is the nucleophile.
FIGURE 7.30
Hydration of carbonyl compounds.
As Figure 7.31 shows, if an alcohol is used as the nucleophile, a hemiacetal is
made. In this case, the reaction usually does not stop. Instead, nucleophilic
substitution of the hydroxyl by a second molecule of alcohol gives an acetal.
The reaction equilibrium can be moved to the right or left by addition or
removal of the H 2 O or ROH reagent.
FIGURE 7.31
Acetal formation from carbonyl compounds.
The hemiacetal and acetal functional groups are common in cyclic simple carbo-
hydrates and in complex carbohydrates. Chapter 8 discusses examples of these
compounds.
7.7.2 Condensation Reactions with 1° Amino Derivatives
A condensation reaction is made up of an addition and elimination reaction
sequence. As Figure 7.32 shows, ammonia and 1° amine derivatives can be used
in these reactions. Table 7.4 lists some condensation examples with aldehydes
and ketones to give products in which the nitrogen replaces the carbonyl oxygen.
These products are usually crystalline solids with sharp melting points. This fea-
ture means these derivative products can be used as standards to identify many
common aldehydes and ketones.