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

Question

Three moles of ethanol react with one mole of phosphorus tribromide to form 3 moles of bromoethane and one mole of \(X\). Which of the following is \(X ?\) (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 byproduct \(X\) is \(\text{H}_3\text{PO}_3\).

Step 1: Understanding the Reaction

When ethanol reacts with phosphorus tribromide (PBr₃), the bromine atoms from phosphorus tribromide replace the hydroxyl groups in ethanol, resulting in the formation of bromoethane and a phosphorus oxyacid as byproduct (X). This is a substitution reaction.

Step 2: Balancing the Chemical Equation

The balanced chemical equation for the reaction is: \[ 3 ext{ C}_2 ext{H}_5 ext{OH} + ext{PBr}_3 ightarrow 3 ext{ C}_2 ext{H}_5 ext{Br} + ext{H}_3 ext{PO}_3 \] This equation shows that the phosphorus compound produced is phosphorous acid, \( ext{H}_3 ext{PO}_3\).

Step 3: Identifying the Byproduct (\(X\))

From the balanced chemical equation, we identify the byproduct \(X\) as \( ext{H}_3 ext{PO}_3\) because that is the phosphorus compound formed when three moles of ethanol react with phosphorus tribromide.

Key Concepts

Substitution Reactions

In organic chemistry, substitution reactions are a fundamental type of reaction where one atom or a group of atoms in a molecule is replaced by another atom or group of atoms. A classic example is the reaction between ethanol and phosphorus tribromide (PBr₃). Here, the hydroxyl groups (-OH) in ethanol are replaced by bromine atoms from PBr₃ to form bromoethane. This is how new compounds are formed in substitution reactions.
Substitution reactions often occur in two main types:

• **Nucleophilic substitution:** Where a nucleophile replaces a leaving group. Nucleophiles are electron-rich species like -OH, Cl⁻, or Br⁻.
• **Electrophilic substitution:** Typically occurs in aromatic compounds where an electrophile replaces a hydrogen atom on a benzene ring.

Understanding these types helps in predicting the products when various reactants are combined in chemical reactions.

Balancing Chemical Equations

Balancing chemical equations is a crucial step in understanding and performing chemical reactions. It involves ensuring the number of each type of atom on the reactant side equals the number on the product side. This upholds the Law of Conservation of Mass, which states matter cannot be created or destroyed.
For the reaction of ethanol with PBr₃, the balanced chemical equation is:
\[ 3 \text{ C}_2\text{H}_5\text{OH} + \text{PBr}_3 \rightarrow 3 \text{ C}_2\text{H}_5\text{Br} + \text{H}_3\text{PO}_3 \]
This shows three moles of ethanol react with one mole of phosphorus tribromide to yield three moles of bromoethane and one mole of phosphorous acid. Balancing is achieved by adjusting coefficients to maintain the same number of each atom on both sides. This ensures that the equation follows the conservation of mass and provides a quantitative relationship between reactants and products.

Byproducts in Reactions

In many chemical reactions, especially organic reactions, byproducts are substances produced in addition to the desired product. Byproducts can provide valuable information about the reaction mechanism and conditions. For our example of ethanol reacting with PBr₃, the byproduct is phosphorous acid (\( \text{H}_3\text{PO}_3 \)).
Byproducts in reactions can be:

• **Desirable:** Sometimes they are useful compounds and can be isolated for other applications.
• **Undesirable:** Other times, they may be waste products that require proper disposal.

Understanding the formation of byproducts is essential. It helps in designing reactions to minimize waste or in altering reaction conditions to maximize yield of desired products.