MECHANISM FOR THE ACID CATALYSED FORMATION OF ACETALS | |
| 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 π bond and giving the electrons to the positive O. | |
| Step 3: An acid/base reaction. Deprotonation of the alcoholic oxonium ion neutralises 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 π bond move to neutralise the charge on the positive O. | |
| Step 7: An acid/base reaction. Deprotonation of the alcoholic oxonium neutralises the charge and produces the acetal product and regenerates the acid catalyst. | |
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Now, let's delve into the intricacies of the mechanism for the acid-catalyzed formation of acetals, breaking down each step and providing insights into the underlying concepts:
Step 1: An acid/base reaction. In this initial step, an acid/base reaction takes place. The weak nucleophile requires activation of the carbonyl compound by protonating the oxygen atom. Protonation facilitates the subsequent nucleophilic attack by enhancing the electrophilicity of the carbon in the carbonyl group.
Step 2: Nucleophilic attack on the carbonyl carbon. The nucleophilic oxygen in the alcohol attacks the electrophilic carbon in the carbonyl group. This results in the breaking of the π bond, and the electrons are transferred to the positively charged oxygen, leading to the formation of an oxyanion intermediate.
Step 3: Formation of the hemi-acetal. Another acid/base reaction occurs, leading to the deprotonation of the alcoholic oxonium ion, neutralizing the charge and yielding the hemi-acetal. Subsequently, there is a need to substitute the hydroxyl (-OH) group with an ethoxy group (-OEt).
Step 4: Preparation for leaving group substitution. To facilitate the departure of the hydroxyl group, an acid/base reaction is employed. Protonation is carried out to convert the hydroxyl group into a better leaving group, setting the stage for the subsequent substitution.
Step 5: Leaving group departure. Utilizing the electrons from the adjacent oxygen atom, the leaving group (protonated hydroxyl) departs, creating a positively charged species that resembles a protonated ketone.
Step 6: Nucleophilic attack on the protonated ketone. The nucleophilic oxygen of the alcohol attacks the electrophilic carbon in the protonated ketone. The movement of electrons from the π bond neutralizes the charge on the positively charged oxygen, resulting in a species ready for the final transformation.
Step 7: Formation of the acetal product. An acid/base reaction occurs once again, leading to the deprotonation of the alcoholic oxonium species. This neutralizes the charge and produces the desired acetal product, simultaneously regenerating the acid catalyst for future catalytic cycles.
Understanding these sequential steps provides a comprehensive insight into the acid-catalyzed formation of acetals, showcasing the intricate interplay of acid/base reactions and nucleophilic attacks in organic synthesis.
