Mechanism for Hemiacetal and Acetal Formation 1) Protonation of the carbonyl 2) Nucleophilic attack by the alcohol 3) Deprotonation to form a hemiacetal 4) Protonation of the alcohol 5) Removal of water 6) Nucleophilic attack by the alcohol 7) Deprotonation by water An example is shown below. 10.3A) cyclohexanone cyclohexanone ethylene acetal (85% yield) ... As we have just shown, the mechanism for acetal formation is really a combination of other familiar mechanisms. 3) Deprotonation to form a hemiacetal. equivalent of a 1,2- or 1,3-diol can react to form a cyclic acetal, in which the acetal group is part of a ﬁve- or six-membered ring, respectively. It involves an … Step 1: An acid/base reaction. Acetal derivatives of aldehydes and ketones are prepared by an acid-catalyzed dehydration reaction with alcohols or diols. 4) Protonation of the alcohol. 5) Removal of water. 1) Protonation of the carbonyl. Writing a mechanism for this reaction provides a good test of ones' understanding of acid-catalyzed processes. Formation is also kinetically favoured because the intramolecular ring-closing reaction is fast. 2) Nucleophilic attack by the alcohol. The reaction produces two products, the acetal plus water, so the usually unfavourable entropy of acetal formation is not a factor. 6) Nucleophilic attack by the alcohol. Cyclic acetals are readily formed by the reaction of two molecules, a ketone and a diol. 7) Deprotonation by water Step 2: The nucleophilic O in the alcohol attacks the electrophilic C in the C=O, breaking the π bond and giving the electrons to the positive O. Step 3: An acid/base reaction. (19.46) (H 2O O O O HO HO + " + % %CH 2 CH 2 0 0 p-toluenesulfonic acid (Sec. The mechanism shown here applies to both acetal and hemiacetal formation. Mechanism for Hemiacetal and Acetal Formation. Since there is only a weak nucleophile we need to activate the carbonyl by protonating on O. Acetal Hydrolysis Mechanism We have learned that the reactions of aldehydes and ketones with alcohols and amines are all reversible and one application of this feature was the use of acetals as protecting groups for aldehydes and ketones: The Mechanism of Acetal Formation.