Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following acids is thermally most unstable? (a) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COCOOH}\) (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\)

Step-by-Step Solution

Answer

(c) CH₃COCOOH is the most thermally unstable acid.

Step 1: Understand Acidic Stability

Thermal stability of organic acids often depends on the ability of the molecule to release CO₂ upon heating. Molecules that form stable intermediates upon loss of CO₂ are more thermally stable.

Step 2: Compare Acids for Decarboxylation Ability

Thermal instability often comes from the ease with which the acid can lose CO₂. Examine each option to see which can easily form more stable structures or intermediates upon loss of CO₂.

Step 3: Analyze Benzoyl Formic Acid (c)

The compound (c) CH₃COCOOH can decarboxylate to form acetone. This decarboxylation process readily occurs, making it thermally unstable as it can form a relatively stable ketone.

Step 4: Evaluate Other Options

The other acids, (a), (b), and (d), are less likely to decarboxylate easily since their resulting structures would be less stable compared to acetone. Thus, they remain more stable over heat exposure.

Step 5: Conclusion

Identify (c) CH₃COCOOH as being the most thermally unstable acid since it readily undergoes decarboxylation to form acetone, which is a stable compound.

Key Concepts

Decarboxylation

Decarboxylation is a chemical reaction that involves the removal of a carboxyl group from a molecule, usually in the form of carbon dioxide (CO₂). Although it sounds complex, this process is quite common in organic chemistry. When certain organic acids are heated, they release CO₂ and form a new compound, often a ketone or alcohol. This change can significantly impact the stability of the original molecule.

A molecule prone to decarboxylation usually has a carboxyl group (-COOH) attached to a structure that becomes more stable once the CO₂ is removed. This concept underscores the thermal stability of acids. When an acid easily loses CO₂ and forms a stable product, it is considered thermally unstable.

• Decarboxylation becomes favorable when the resulting compound is stable.
• Acids with groups leading to stable intermediates post-decarboxylation are less stable themselves.

Understanding the decarboxylation process can help predict the thermal behavior of different organic acids.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their properties. It explores the chemical reactions and structures of carbon-based molecules. One crucial aspect of organic chemistry is understanding how different functional groups within a molecule influence its reactivity and stability.

• Functional groups define the chemical reactions a molecule can undergo.
• The presence of specific groups, like carboxyl, affects an acid's ability to decarboxylate.

In the context of this exercise, organic chemistry helps explain why certain acids undergo decarboxylation more readily than others. The structure of a molecule, such as proximity of the carboxyl group to a keto group, influences its thermal stability.
By applying principles of organic chemistry, we can predict reaction pathways and potential products when acids are heated. This understanding aids in identifying thermally unstable compounds and their possible transformations.

Molecular Stability

Molecular stability refers to how resistant a molecule is to breaking down or changing under various conditions, such as temperature. In the context of thermal stability, it specifically highlights the ability of a molecule to maintain its structure when heated.
A molecule's structure dictates its stability. Compounds that can form stable by-products upon losing a part of their structure, like decarboxylation, tend to be less stable initially.

• Stable intermediates lead to thermal instability in parent molecules.
• Keto groups adjacent to carboxyl groups often facilitate decarboxylation.

In our exercise, the acid (c) \( \text{CH}_3\text{COCOOH} \) exemplifies this principle. Its ability to easily decarboxylate into acetone, a stable ketone, renders it thermally unstable. Analyzing molecular stability helps in understanding why some acids resist heat, while others transform into more stable entities.