Problem 14 - Chemistry in 30 days

Question

When acetone is saturated with dry HCl gas, it undergoes a condensation reaction to form a trimer. The trimer is (a) Phorone (b) mesityl oxide (c) mesitylene (d) diacetone alcohol

Step-by-Step Solution

Answer

The trimer formed is (c) mesitylene.

Step 1: Understand the Reaction

The problem states that acetone, when saturated with dry HCl gas, undergoes a condensation reaction. This reaction involves the combination of acetone molecules in the presence of an acid catalyst, such as HCl.

Step 2: Recall Acetone Reactions

Acetone ((CH_3)_2CO) is a well-known ketone that can undergo aldol condensation reactions under acidic conditions. This reaction can lead to the formation of a dimer or trimer, depending on the specific conditions and reactants involved.

Step 3: Identify Acid-Catalyzed Products

In the presence of an acid catalyst like HCl, acetone undergoes a self-condensation reaction. When condensation continues past the dimer stage, a trimer can form. One of the known trimers formed from acetone is mesitylene.

Step 4: Confirm the Product

Given the options, acetone's known trimer when reacted with an acid like HCl is mesitylene. Therefore, the correct answer is option (c) mesitylene.

Key Concepts

Acetone Reactions

Acetone is a simple yet fascinating compound with the chemical formula \((CH_3)_2CO\). As a member of the ketone family, acetone is highly polar, which makes it quite reactive in certain conditions.
Under specific circumstances, acetone can undergo various types of reactions, but one of the most interesting is the condensation reaction. This reaction involves two or more acetone molecules joining together to form a larger molecule.
When acetone is exposed to an acidic medium, particularly with the presence of HCl gas, it can participate in a self-condensation reaction. The reaction with dry HCl is significant because it acts as a catalyst, aiding the combination of acetone units. Initially, a dimer is formed, but continued reaction leads to larger and complex molecules like a trimer.

Acid-Catalyzed Products

An acid-catalyzed reaction is one where an acid plays an essential role in accelerating the chemical conversion. HCl (hydrochloric acid) is one of the acids that can catalyze condensation reactions in organic chemistry.
In the context of acetone, the presence of dry HCl gas facilitates the condensation process. The acid does not get consumed; instead, it accelerates the reaction by increasing the electrophilic nature of the carbonyl group in acetone.
The enhancement allows the carbonyl group to readily accept nucleophilic attacks, promoting stronger interactions among acetone molecules. Overcoming activation energy barriers becomes easier, leading to the formation of complex structures like trimers. Despite being a catalyst, the role of HCl is crucial as it helps determine the specific product outcomes of these reactions, such as forming mesitylene.

Aldol Condensation Reactions

Aldol condensation is a pivotal reaction mechanism in organic chemistry, valuably utilized in forming carbon-carbon bonds. It involves aldehydes and ketones with α-hydrogens reacting in the presence of an acid or base catalyst.
In an acidic medium, like with dry HCl, acetone undergoes aldol condensation. The primary step involves the formation of an enol or enolate ion, which then interacts with other acetone molecules.
For instance, under acidic catalysis, the enol form of acetone attacks the carbonyl carbon of another acetone molecule leading to the formation of a dimer. Following this, a further condensation might happen to form a trimer of acetone, known as mesitylene. The process continues based on reaction conditions, but the key here is how molecules link under catalytic conditions leading to complex structures.