Question

Show the complete mechanism for the acid-catalyzed reaction of acetaldehyde with ethanol to form an acetal.

Answer 1

Step 1: An acid/base reaction. Since there is only a weak nucleophile we need to activate the carbonyl by protonating on O.
Step 2: The nucleophilic O in the alcohol attacks the electrophilic C in the C=O, breaking the p bond and giving the electrons to the positive O.
Step 3: An acid/base reaction. Deprotonation of the alcoholic oxonium ion neutralizes the charge giving the hemi-acetal. Now we need to substitute the -OH by -OEt.
Step 4: An acid/base reaction. In order for the -OH to leave we need to make it into a better leaving group by protonation.
Step 5: Using the electrons from the other O, the leaving group departure is facilitated.
Step 6: We now have what resembles a protonated ketone (compare with step 2). The nucleophilic O of the alcohol attacks the electrophilic C and the electrons of the p bond move to neutralize the charge on the positive O.
Step 7: An acid/base reaction. Deprotonation of the alcoholic oxonium neutralizes the charge and produces the acetal product and regenerates the acid catalyst.

P.S.- the mechanism uses a ketone. please use a H molecule instead of one of the CH₃ for acetaldehyde reaction.