Question

Which of the following reaction is not possible? (A) \(2 \mathrm{R}-\mathrm{COOH}+2 \mathrm{Na} \longrightarrow 2 \mathrm{R}-\mathrm{COONa}+\mathrm{H}_{2}\) (B) \(\mathrm{R}-\mathrm{COONa}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{R}-\mathrm{COOH}+\mathrm{NaOH}\) (C) \(\mathrm{R}-\mathrm{COOH}+\mathrm{NaHCO}_{3} \longrightarrow \mathrm{R}-\mathrm{COO}^{-} \mathrm{Na}^{\oplus}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}\) (D) \(\mathrm{R}-\mathrm{COOH}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{\oplus}+\mathrm{R}\)

Short answer

Based on the analysis of feasibility, it is concluded that Reaction D (\(R-COOH + H_{2}O \to H_{3}O^+ + R\)) is not possible due to the formation of an unstable alkyl radical (\(R\)). Hence, the correct answer is (D).

Step-by-step solution

Reaction A

For the reaction: \(2 R-COOH + 2 Na \to 2 R-COONa + H_{2}\) Here, a carboxylic acid (\(R-COOH\)) reacts with sodium metal (\(Na\)) to produce a sodium carboxylate salt (\(R-COONa\)) and hydrogen gas (\(H_{2}\)). This is a classic acid-base reaction between a carboxylic acid and an alkali metal, leading to the formation of a salt and hydrogen gas. Therefore, this reaction is possible.

Reaction B

For the reaction: \(R-COONa + H_{2}O \to R-COOH + NaOH\) This reaction represents the hydrolysis of a sodium carboxylate salt (\(R-COONa\)) in the presence of water (\(H_2O\)). The reaction results in the formation of the corresponding carboxylic acid (\(R-COOH\)) and sodium hydroxide (\(NaOH\)). This is a well-known reaction and thus, is possible.

Reaction C

For the reaction: \(R-COOH + NaHCO_{3} \to R-COONa + H_{2}O + CO_{2}\) This reaction happens between a carboxylic acid (\(R-COOH\)) and sodium bicarbonate (\(NaHCO_3\)). The products formed are a sodium carboxylate salt (\(R-COONa\)), water (\(H_2O\)), and carbon dioxide gas (\(CO_2\)). This is another common acid-base reaction involving a carboxylic acid and a weak base. Therefore, this reaction is possible.

Reaction D

For the reaction: \(R-COOH + H_{2}O \to H_{3}O^+ + R\) In this reaction, a carboxylic acid (\(R-COOH\)) is proposed to react with water (\(H_2O\)) to form a hydronium ion (\(H_3O^+\)) and what appears to be an alkyl radical (\(R\)). Since alkyl radicals are extremely unstable species and are not formed under normal reaction conditions, this reaction is not feasible under normal conditions. In summary, reactions A, B, and C are all possible based on their feasibility as common acid-base reactions involving carboxylic acids and various bases. Reaction D, however, is not possible due to the formation of an unstable alkyl radical.

Key concepts

Carboxylic Acid

Carboxylic acids are a class of organic compounds characterized by the presence of a carboxyl group \((R-COOH)\). This group consists of a carbonyl group \((C=O)\) and a hydroxyl group \((O-H)\) linked to the same carbon. Carboxylic acids are known for their acidic properties, primarily due to the ability of the carboxyl hydrogen to dissociate easily.

• They have a pungent odor similar to vinegar, which contains acetic acid, a simple carboxylic acid.
• Carboxylic acids are commonly found in nature and can be either aliphatic or aromatic.
• They participate in various chemical reactions, notably in forming esters and amides, and in acid-base reactions.

In acid-base reactions, carboxylic acids react with bases like sodium bicarbonate \((NaHCO_3)\) to release carbon dioxide (\(CO_2)\), water, and a salt. This reaction is exothermic and happens due to the formation of a more stable carboxylate salt.

Sodium Metal

Sodium metal \((Na)\) is a highly reactive alkali metal. It is stored under oil or in an inert atmosphere to prevent its reaction with moisture and oxygen in the air. When sodium metal comes into contact with water, it reacts vigorously, producing sodium hydroxide \((NaOH)\) and hydrogen gas \((H_2)\).

• When sodium reacts with carboxylic acids, it forms sodium carboxylates \((R-COONa)\) and releases hydrogen gas. This is also an example of an acid-base reaction where the carboxylic acid acts as an acid and sodium as a base.
• This reaction is often used to test the presence of carboxylic acids. Evolving hydrogen gas during the reaction indicates the acidity of the compound.

This reaction's feasibility is confirmed since both hydrogen gas evolution and carboxylate salt formation are expected outcomes.

Sodium Bicarbonate

Sodium bicarbonate \((NaHCO_3)\), commonly known as baking soda, is a weak base frequently used in cooking and cleaning. It effectively neutralizes acids and in the process produces carbon dioxide gas, a feature that makes it useful in baking as a leavening agent.

• In chemical reactions involving carboxylic acids, sodium bicarbonate acts as a base, reacting to form carbon dioxide, water, and a sodium salt of the acid.
• This reaction is used in laboratories to test for carboxylic acids; effervescence indicates the release of carbon dioxide gas.

The reaction is highly favored in equilibrium and is an essential part of gas evolution reactions, showcasing the release of carbon dioxide when reacting with acids.

Hydronium Ion

A hydronium ion \((H_3O^+)\) is a cation that forms when an acid dissolves in water, increasing the concentration of \([H^+]\) ions in the solution. The presence of hydronium ions is what gives an aqueous solution its acidic character.

• Hydronium ions are crucial in acid-base chemistry because they represent acids in solution and partake in transferring a proton during reactions.
• The reaction proposing a carboxylic acid forming a hydronium ion fails because it improbably generates an unstable radical (\(R)\) in normal conditions.

Acid-base reactions with realistic pathways typically involve the exchange of protons to form a more stable configuration, such as water and its associated ions, rather than unstable free radicals.