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 \(\mathrm{X}\). 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 product \(X\) is \(\mathrm{H}_3\mathrm{PO}_3\).

Step 1: Write the Chemical Equation

Start by writing the chemical equation for the reaction between ethanol (\(C_2H_5OH\)) and phosphorus tribromide (\(PBr_3\)). The balanced equation is: \(3C_2H_5OH + PBr_3 \rightarrow 3C_2H_5Br + X\).

Step 2: Determine the Product Based on Atomic Balance

We know that phosphorus tribromide reacts with ethanol to substitute the \(OH\) group and form bromoethane (\(C_2H_5Br\)). The remaining atoms, mainly from the phosphorus tribromide that do not end up in bromoethane, must end up in the product \(X\). In this case, the phosphorus and oxygen atoms are left to form \(X\).

Step 3: Evaluate Phosphorus and Oxygen Compounds

From the balanced chemical equation, each reaction results in one phosphorus atom from \(PBr_3\) and one oxygen from \(C_2H_5OH\). Thus, \(X\) must be a phosphorus-oxygen compound. Checking the options, \(X\) should have one phosphorus atom and three oxygen atoms. Option (a) \(H_3PO_3\) matches this configuration.

Step 4: Verify the Oxidation States and Reaction Consistency

Phosphorus in \(PBr_3\) has an oxidation state of +3. Considering phosphorous acid, \(H_3PO_3\), phosphorus is also in +3 state, making this option consistent with the process since there is no change in the oxidation state for phosphorus. Thus, the reaction forms \(H_3PO_3\) as a byproduct. This concurs with forming no additional water, which matches typical reactions.

Key Concepts

Phosphorus Tribromide Reactions

Phosphorus tribromide (\( PBr_3 \)) is a widely used reagent in organic chemistry, primarily for converting alcohols to alkyl bromides. In this type of reaction, \( PBr_3 \) acts as a bromide donor, replacing the hydroxyl \( OH \) group in alcohols with a bromine atom. This transformation is significant for preparing compounds like bromoethane from ethanol.

• The reaction is highly efficient and generally yields a clean conversion to the alkyl bromide.
• A vital aspect is forming a phosphorus-oxygen byproduct due to the availability of oxygen from the alcohol.
• In reactions with \( PBr_3 \), there's typically no change in the oxidation state of phosphorus. It starts and remains at +3.

Understanding \( PBr_3 \) is crucial for aspiring chemists, as it appears often in synthetic pathways. It's important to ensure safety when handling this reagent due to its corrosive nature.

Bromoethane Formation

Bromoethane, also known as ethyl bromide, forms through the substitution reaction of ethanol with phosphorus tribromide. In the reaction, ethanol (\( C_2H_5OH \)) interacts with \( PBr_3 \) to produce bromoethane (\( C_2H_5Br \)) and a phosphorus-oxygen compound.

• This substitution process involves the replacement of the hydroxyl group by a bromine atom.
• Bromoethane has significant applications, especially as an alkylating agent in organic synthesis.
• The reaction is balanced, converting three moles of ethanol with one mole of \( PBr_3 \) to yield three moles of bromoethane.

Overall, bromoethane serves both as a synthetic intermediate and chemical reagent. Its preparation through alcohol substitution is a fundamental concept in organic synthesis.

Oxidation States in Reactions

Understanding oxidation states in chemical reactions allows you to track electron transfer and changes in compounds. An oxidation state is essentially the charge an atom would have if electrons were completely transferred.

• In the reaction between ethanol and \( PBr_3 \), no change occurs in the oxidation state of phosphorus. It starts at +3 in \( PBr_3 \) and remains +3 in the product, phosphorous acid (\( H_3PO_3 \)).
• This consistency confirms that no radical redox processes are taking place, allowing a straightforward prediction of products.
• Evaluating oxidation states helps determine the result of a reaction, especially when deciding what products form, like identifying \( H_3PO_3 \) as the compound "X."

Knowing how to assign and recognize oxidation states is a foundational skill in chemistry. It ensures clear understanding of not just \( PBr_3 \) reactions but many others.