OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 4

✅ Correct Answer: D

Reaction D is an elimination (dehydration) reaction where a single organic reactant forms an organic product and a single by-product ( H₂O ). Crucially, let's look at reaction types:

• Reaction D (Elimination): 1 reactant molecule yields 1 organic product + 1 by-product. Atom economy is less than 100%, but higher than multi-product systems. Wait, let's check Addition:
• Reaction A (Addition): CO₂ + 3H₂ → CH₃OH + H₂O has two products ( CH₃OH and H₂O ).
• Reaction D: CH₃CH₂CH₂CH₂OH → CH₃CH₂CH=CH₂ + H₂O also has two products, but let's inspect the stoichiometric coefficients and total masses. Alternatively, look for addition reactions which have 100% atom economy. However, none of options A-D are pure addition reactions where all atoms form a single product. Let's evaluate total molecular mass of products vs desired product.

💡 Key Knowledge

• Definition: Atom Economy measures how efficiently a reaction uses its reactant atoms.
• Formula: Atom Economy = (M_r of desired product / Sum of M_r of all products) × 100%
• Addition Reactions: Generally have 100% atom economy because all reactants are incorporated into the single product (e.g., alkene + hydrogen).
• Substitution & Elimination: Always produce at least two products, meaning their atom economy is strictly less than 100%.

🧠 Exam Technique

In multiple-choice questions involving atom economy, you do not always need to calculate the exact numerical value for every option:

• Count the number of products on the right-hand side. Reactions with a single product automatically have a higher atom economy.
• Compare the molar mass of the lost by-product ( H₂O , NaBr , NaHSO₄ ) relative to the desired organic product. Lighter by-products like H₂O ( M_r = 18.0 ) retain a much higher atom economy than heavy inorganic salts like NaBr ( M_r = 102.9 ) or NaHSO₄ ( M_r = 120.1 ).

❌ Common Errors

• Confusing Yield with Atom Economy: Students often think about percentage yield or reaction completeness instead of mass efficiency.
• Ignoring Stoichiometry: Forgetting to multiply molar masses by big numbers (coefficients) when calculating total product mass in Reaction A ( 3H₂ ).
• Guessing based on reagents: Assuming inorganic reagents like NaOH automatically make a process "unsustainable" without checking the balanced equation.