Question

Acetic acid with HI in presence of red phosphorus gives (a) methane (b) ethane (c) ethanal (d) methanal

Short answer

Answer: The product formed in this reaction is ethane (CH3CH3).

Step-by-step solution

Identifying the reactants

The reactants are acetic acid (CH3COOH) and hydroiodic acid (HI). The reaction also involves red phosphorus (P).

Identifying the reaction mechanism

The reaction of acetic acid with hydroiodic acid in the presence of red phosphorus is a reduction reaction. Red phosphorus acts as a reducing agent, and HI as a nucleophile.

Breaking down the acetic acid molecule

Acetic acid (CH3COOH) has a carboxylic acid functional group (-COOH). During the reduction reaction with red phosphorus and HI, the carbonyl group (C=O) will be reduced to an alcohol group (-OH). The hydroiodic acid will replace the hydroxyl group (-OH) in the acetic acid with iodine (I).

Writing the reaction equation

The reaction process can be written as: CH3COOH + HI -> CH3COI + H2O Then, the reaction with red phosphorus and another molecule of hydroiodic acid reduces the carbonyl group to a single bond and replaces iodine with a hydrogen atom. CH3COI + 2HI + P -> CH3CH2I + H2O + PI3 The last step of the reaction is intramolecular hydrogen extraction, which leads to the formation of ethane. CH3CH2I + HI + P -> CH3CH3 + H2 + PI3

Identifying the final product

As a result of this reaction, the final product is ethane (CH3CH3). Therefore, the correct answer is option (b).

Key concepts

Reduction Reaction

A reduction reaction is a chemical process where a molecule gains hydrogen atoms or loses oxygen atoms. In the specific case of the reaction between acetic acid and hydroiodic acid, the process takes place in the presence of red phosphorus. This reaction is an example of how acetic acid is converted into a simpler molecule through the removal of an oxygen atom and addition of hydrogen atoms.

The concept is crucial as it explains the transformation of molecules. In this reaction, the acetic acid is reduced, specifically by removing the carboxylic's oxygen and hydrogen atoms and replacing it with hydrogen. This makes the final molecule more hydrogen-rich, in line with the typical characteristics of a reduction reaction.

Red Phosphorus

Red phosphorus plays an essential role in the reduction reaction involving acetic acid and hydroiodic acid. It acts as a reducing agent, which facilitates the chemical change. A reducing agent is a substance that induces reduction by losing electrons and thus promotes the adding of hydrogen or removing oxygen in other substances.

In this context, red phosphorus helps in breaking down acetic acid and converting it to ethane. Its ability to transfer electrons to other chemicals in the reaction is a critical element in transitioning from acetic acid's initial form to ethane, the final product.

Hydroiodic Acid

Hydroiodic acid (HI) is a strong acid that plays a dual role in the reaction with acetic acid. It acts as a nucleophile, a chemical species that donates an electron pair to form a chemical bond in relation to the reaction. The HI focuses on the carboxyl group within acetic acid, facilitating changes needed for reduction.

As the reaction progresses, hydroiodic acid helps in both replacing functional groups within acetic acid and stabilizing the intermediate compounds formed throughout the process. Ultimately, it aids in proceeding to the formation of ethane as the output.

Reaction Mechanism

The reaction mechanism is the step-by-step process through which reactants are transformed into products. In the case of acetic acid reacting with hydroiodic acid in the presence of red phosphorus, the mechanism involves several stages.

It starts with the action of hydroiodic acid and red phosphorus on acetic acid, where the carbonyl group is reduced to an alcohol group. Then, the hydroiodic acid replaces the hydroxyl group, leading to the formation of an intermediate compound (CH3COI). Red phosphorus and HI further bridge this towards the formation of ethane through repeated reductions and molecular alterations. This mechanism effectively demonstrates how complex transformations can be broken down into simpler steps, leading to the final product.

Ethane Formation

The culmination of this chemical process is ethane formation. Ethane is a relatively simple hydrocarbon with the formula CH3CH3, and it arises as the final product when acetic acid is reacted with hydroiodic acid in the presence of red phosphorus.

The process involves the careful removal and addition of atoms facilitated by the reducing agent and acid acting together. Ethane forms through multiple steps, which include converting the carboxyl group of acetic acid to an alkyl chain., The final product is a stable molecule that marks the success of the attempted reductions and transformations throughout the reaction process.