Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

Three moles of ethanol react with one mol of phosphorus tribromide to form 3 moles \(\alpha\) bromoethane and one mole of \(\mathrm{X}\). Which of th following is \(\mathrm{X}\) ? (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (d) \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

The compound \( \text{X} \) is \( \text{H}_3 \text{PO}_3 \), option (a).

Step 1: Write the Chemical Equation

The balanced chemical equation for the reaction of ethanol with phosphorus tribromide is:\[ \begin{align*} 3 \text{C}_2 \text{H}_5 \text{OH} + \text{PBr}_3 \rightarrow 3 \text{C}_2 \text{H}_5 \text{Br} + \text{H}_3 \text{PO}_3 \end{align*} \]This shows that when ethanol reacts with phosphorus tribromide, the products are bromoethane and a phosphoric acid derivative.

Step 2: Identify the Product X

From the equation, it is clear that the other product besides \( \alpha \) bromoethane is \( \text{H}_3 \text{PO}_3 \). This is commonly known as phosphorous acid, which is formed as a by-product of the reaction.

Step 3: Match the Product X with Options

Compare the compound \( \text{H}_3 \text{PO}_3 \) with the given options:- (a) \( \text{H}_3 \text{PO}_3 \)- (b) \( \text{H}_3 \text{PO}_2 \)- (c) \( \text{H}_3 \text{PO}_4 \)- (d) \( \text{HPO}_3 \)The correct match is option (a), \( \text{H}_3 \text{PO}_3 \).

Key Concepts

Ethanol Reaction

Ethanol is a common alcohol, with the chemical formula \( \text{C}_2\text{H}_5\text{OH} \). It is often used in various reactions due to its unique properties. In chemical reactions, ethanol can act as both an acid and a base. In this exercise, ethanol reacts with phosphorus tribromide.

When ethanol interacts with phosphorus tribromide, it undergoes a substitution reaction. This type of chemical reaction involves the replacement of one atom or group of atoms by another. Here, the hydroxyl group (-OH) in ethanol is replaced by a bromine atom, producing bromoethane.

• Substitution reactions are a key mechanism in organic chemistry. They allow the transformation of reactants into useful products like bromoethane, which is important in various applications.
• This reaction shows the transformation of a simple molecule like ethanol, highlighting its versatile reactivity.

Understanding the ethanol reaction is crucial for grasping how complex organic compounds are synthesized.

Phosphorus Tribromide

Phosphorus tribromide, represented as \( \text{PBr}_3 \), is vital in converting alcohols to alkyl bromides. It is a colorless liquid that reacts vigorously with water. In organic chemistry, it primarily functions as a brominating agent.

This compound is highly reactive due to the strong electronegativity of bromine, which facilitates the replacement of the alcohol hydroxyl group with a bromine atom.

• Phosphorus tribromide not only reacts with alcohols but also influences how the entire chemical process unfolds.
• One significant aspect is the balanced chemical equation that shows how three moles of ethanol react with one mole of phosphorus tribromide.

This balanced equation is critical for understanding the stoichiometry of the reaction. It shows us how much phosphorus tribromide is needed to produce a certain amount of bromoethane.
Grasping how phosphorus tribromide works with ethanol illuminates its key role in chemical synthesis.

Bromoethane Production

Bromoethane, also known as ethyl bromide, is an organic compound that results from the reaction of ethanol with phosphorus tribromide.

This reaction’s main product is bromoethane - a versatile compound in organic synthesis, often used in producing pharmaceuticals and other organic compounds.

• The reaction equation \(3 \text{C}_2 \text{H}_5 \text{OH} + \text{PBr}_3 \rightarrow 3 \text{C}_2 \text{H}_5 \text{Br} + \text{H}_3 \text{PO}_3\) emphasizes how effectively bromoethane can be mass-produced from ethanol and phosphorus tribromide using simple chemical reaction processes.

The significance of this process is its applicability in industrial settings, where large-scale synthesis of organic compounds is necessary.
The creation of bromoethane from easily accessible substances like ethanol demonstrates the efficiency and utility of substitution reactions in industrial chemistry.

Phosphorus Compounds

In the reaction of ethanol with phosphorus tribromide, a by-product is a phosphorus compound, specifically phosphorous acid \( \text{H}_3 \text{PO}_3 \).

This acid is crucial for recognizing the complexity of phosphorus chemistry. Phosphorous acid differs from other similar acids because it contains a lower oxidation state of phosphorus, which gives it unique chemical properties.

• Understanding phosphorous acid is vital because it highlights the diversity in phosphorus compounds, showing variation in their structural and chemical characteristics.
• The ability to form different compounds like \( \text{H}_3 \text{PO}_3 \) demonstrates phosphorus's versatile chemistry.

Phosphorus compounds such as these are essential in various applications, including fertilizers and flame retardants.
Recognizing the formation of such acids in reactions helps one appreciate the intricacy and utility of chemical processes involving phosphorus.