Addition of Alcohols to Form Hemiacetals and Acetals

In this section we shall see that alcohols add to carbonyl groups in much the same manner as water does. Both acids and bases catalyze the process. In addition, acids catalyze the further transformation of the initial addition product to furnish acetals by replacement of the hydroxy group with an alkoxy substituent. 

Not surprisingly, alcohols also undergo addition to aldehydes and ketones, by a mechanism virtually identical to that outlined for water. The adducts formed are called hemiacetals (hemi, Greek, half) because they are intermediates on the way to acetals. 

Like hydration, these addition reactions are governed by equilibria that usually favor the starting carbonyl compound. Hemiacetals, like hydrates, are therefore usually not isolable. Exceptions are those formed from reactive carbonyl compounds such as formaldehyde or 2,2,2-trichloroacetaldehyde. Hemiacetals are also isolable from hydroxy aldehydes and ketones when cyclization leads to the formation of relatively strain-free five- and six-membered rings. 

Cyclic Hemiacetals Are More Stable than Acyclic Hemiacetals 

Intramolecular hemiacetal formation is common in sugar chemistry (Chapter 24). For example, glucose, the most common simple sugar in nature, exists as an equilibrium mixture of an acyclic pentahydroxyaldehyde and two stereoisomeric cyclic hemiacetals. The cyclic forms constitute more than 99% of the mixture in aqueous solution. 

---

Addition of Alcohols to Form Hemiacetals and Acetals CHAPTER 17... 

---