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) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}, \mathrm{COOH}\)

Step-by-Step Solution

Answer

Option (c) 3{\mathrm{CH}_3 \mathrm{COCOOH}\) is the thermally most unstable.

Step 1: Understanding thermal instability

To determine which acid is thermally most unstable, we need to consider the possibility of decarboxylation. This process involves the loss of \( ext{CO}_2\) from a carboxylic acid group. Thermally unstable acids often undergo decarboxylation easily.

Step 2: Evaluating decarboxylation readiness

Decarboxylation is facilitated when a \(- ext{COOH}\) group is positioned \(\beta\) to a carbonyl group (\(\text{C}=\text{O}\)). Compounds with such an arrangement tend to lose \( ext{CO}_2\) more easily when heated due to the stabilization of the transition state.

Step 3: Analysis of given options

- Option (a) \( ext{CH}_3 ext{COCH}_2 ext{COOH}\) has the \(- ext{COOH}\) group \(\beta\) to the ketone.- Option (b) \( ext{CH}_3 ext{COCH}_2 ext{CH}_2 ext{CH}_2 ext{COOH}\) has the \(- ext{COOH}\) four carbons away from the ketone.- Option (c) \( ext{CH}_3 ext{COCOOH}\) has the \(- ext{COOH}\) group adjacent to the ketone, positioning it for easy decarboxylation.- Option (d) \( ext{CH}_3 ext{COCH}_2 ext{CH}_ ext{,} ext{COOH}\) exhibits a similar pattern to option (b).

Step 4: Conclusion based on analysis

Option (c) \( ext{CH}_3 ext{COCOOH}\) is the most thermally unstable because the carboxylic group is directly adjacent to the carbonyl group, allowing ready decarboxylation compared to the other options.

Key Concepts

Decarboxylation

Decarboxylation is a fascinating chemical reaction involving the removal of a carboxyl group, typically in the form of carbon dioxide (\(\text{CO}_2\)), from an organic molecule.
This process is crucial in organic chemistry, especially when analyzing the thermal stability of different compounds. Carboxylic acids are prone to decarboxylation, especially when specific structural conditions are met.

• When a carboxylic acid has a \(-\text{COOH}\) group positioned \(\beta\) to a carbonyl (\(\text{C} = \text{O}\)) group, the acid is more likely to undergo decarboxylation when heated.
• During heating, the \(\beta\)-positioning allows for the stabilization of the transition state, which facilitates the loss of \(\text{CO}_2\).

This outcome results because the movement of electrons is more favorable, reducing the energy needed for the transition state. Thus, anticipating decarboxylation potential is vital when predicting the thermal stability of various acids.

Carboxylic Acids

Carboxylic acids are a category of organic acids characterized by the presence of a carboxyl group (\(-\text{COOH}\)).
They play a significant role in both natural processes and industrial applications. Understanding their properties helps in predicting their chemical behavior, including tendencies for decarboxylation.

• The carboxyl group is what makes these acids reactive and prone to decomposition through thermal means.
• Factors influencing their reactivity include the presence and position of other functional groups near the carboxyl group.

In the context of decarboxylation, when a carbonyl group is nearby, it alters electron distribution and stability, making the carboxyclic acid behave differently when exposed to heat.

Beta-Keto Acids, Carbonyl Compounds

Beta-keto acids belong to a special category of carboxylic acids that contain a ketone group (carbonyl) positioned at the \(\beta\)-position relative to the carboxyl group.
This structural arrangement is critical as it significantly influences the stability and reactivity of the compound.

• These acids are prone to rapid decarboxylation because the \(\beta\)-carbonyl group provides a stabilizing effect during the transition state when \(\text{CO}_2\) is lost.
• The proximity of the carbonyl group aids electron shifts that lower the activation energy required for the reaction.

For instance, in the exercise, among the given options, it's the beta-keto arrangement in option (c) \(\text{CH}_3 \text{COCOOH}\) that makes it the most unstable.
Hence, understanding the traits of beta-keto acids helps predict their readiness for decarboxylation when heated.