Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 139

— Cont'd
Because the reaction mixture is still acidic, equilibrium protonation of the basic
oxygens in the hemiacetal can occur. Protonation of the hydroxyl group gives a good
leaving group which is easy to replace with a second alcohol molecule. This is similar
to the acid-catalyzed substitution in Program 19E and F.
Try to draw a mechanism for this nucleophilic substitution step.
D Following Program 19, you should have drawn:
Because they are acid-catalyzed, the reaction steps are drawn as equilibria. The
reactions can be reversed by changing the concentration of the reaction components.
E Reactions with primary amino nucleophiles RNH 2 are related to the addition
reactions seen in D. Initial nucleophilic addition occurs in exactly the same way
as it does with alcohols. It can be written as:
The difference between this and the earlier hemiacetal intermediate is that the
trivalent nitrogen still has a further hydrogen attached. Therefore, the molecule
has the substituents which are needed for an elimination step to occur.
Try to draw a representation of this step.