Nucleophilic addition of an alcohol to the carbonyl group initially yields a hydroxy ether called a hemiacetal, analogous to the gem diol formed by addition of water (Section 19.6). Hemiacetals are formed reversibly, with the equilibrium normally favoring the carbonyl compound. In the presence of acid, however, a further reaction can occur. Protonation of the -OH group followed by an El-like loss of water leads to an oxonium ion, R2C=OR+, which undergoes a second nucleophilic addition of alcohol to yield the acetal. For example, reaction of cyclohexanone with methanol yield the dimethyl acetal. The mechanism is shown in Figure 19.12 (p.778).