Question

Aldol additions also occur in the presence of acidic catalysts. For example, 2-propanone with dry hydrogen chloride slowly yields \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCOCH}_{3}\) (mesityl oxide) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCOCH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (phorone). Write mechanisms for the formation of these products, giving particular attention to the way in which the new carbon-carbon bonds are formed.

Short answer

The mechanism involves protonation, enol formation, nucleophilic attack, and dehydration leading to mesityl oxide and phorone.

Step-by-step solution

Protonation of the Carbonyl Group

The mechanism begins with the protonation of the carbonyl oxygen of 2-propanone by the acidic hydrogen chloride. This makes the carbonyl carbon more electrophilic, facilitating nucleophilic attack.

Formation of the Enol

The alpha hydrogen of the protonated 2-propanone is removed by a chloride ion, leading to the formation of an enol. This structure allows for isomerization between keto and enol forms.

Nucleophilic Attack on Protonated Carbonyl

Another molecule of 2-propanone undergoes protonation and the enol form acts as a nucleophile, attacking the electrophilic carbon of the protonated carbonyl group. This forms a new C-C bond.

Formation of beta-hydroxy ketone intermediate

The intermediate formed is a beta-hydroxy ketone. In acidic conditions, dehydration occurs, leading to the formation of a double bond and resulting in mesityl oxide.

Formation of Phorone

For phorone formation, mesityl oxide undergoes further aldol condensation with another molecule of 2-propanone. Similar steps are followed—protonation, enol formation, nucleophilic attack, and additional dehydration—producing phorone.

Key concepts

Acidic Catalysis

In the Aldol Addition Reaction, acidic catalysis plays an essential role. When an acid, like hydrogen chloride, is present, it protonates the carbonyl oxygen in 2-propanone. This step is vital as it increases the electrophilicity of the carbonyl carbon. By doing so, it makes the molecule more reactive and susceptible to further chemical reactions. Protonation essentially adds a positive charge, which makes the carbonyl group a better target for nucleophiles. This enhanced reactivity sets the stage for new bond formations and is key to the sequential steps that follow in the aldol reaction process.

Carbon-Carbon Bond Formation

One of the most interesting aspects of aldol reactions is the formation of new carbon-carbon bonds. This is a hallmark of the aldol addition. After the enol form of 2-propanone is generated, which we will discuss later, it acts as a nucleophile. This nucleophile then attacks the carbonyl carbon of another protonated 2-propanone molecule. The result is the creation of a new C-C bond. Such new bonds are the backbone for synthesizing complex molecules from simpler ones. The capability of forming these bonds is precisely why aldol reactions are a highly prized mechanism in organic chemistry, allowing scientists to build larger, more complex molecular structures.

Protonation Mechanism

Protonation is a critical mechanism in the aldol addition process, particularly under acidic conditions. The procedure begins with the addition of a proton (H⁺) to the carbonyl oxygen of 2-propanone. This protonation is facilitated by the acidic environment provided by hydrogen chloride. By making the carbon more electrophilic, it becomes a more attractive site for nucleophilic attack. This alteration in reactivity helps to guide the reaction forward, ensuring that subsequent steps like enol formation and nucleophile attack can smoothly proceed. The protonation steps in acidic catalysis underscore the fundamental nature of how acids enhance reaction pathways by altering molecular electronics.

Enol Formation

Enol formation in the aldol addition begins after hydrogen chloride facilitates the protonation of the carbonyl group. Enols are a special tautomeric form of ketones, often resulting from the removal of an alpha hydrogen, helped along by chloride ions. In the mechanism, this formation entails shifting to a configuration that has the double-bond character moved to a carbon-carbon link adjoining the carbonyl group. Enolization plays a vital role as it places the molecule in an arranged state for nucleophilic attack. It acts as a nucleophile, participating actively in the subsequent step of bond formation by attacking the electrophilic carbonyl carbon. This duality, where the molecule can be present in keto or enol form, adds flexibility and dynamic reactivity to the aldol addition process.