Edexcel A-Level Chemistry Paper 3, November 2020: Question 6

✅ Correct Answer

C₆H₅OH + 7O₂ → 6CO₂ + 3H₂O

❌ Common Errors

• Writing the incorrect molecular formula for phenol (e.g., confusing it with cyclohexanol or failing to count the 6 carbon atoms in the ring).
• Balancing oxygen incorrectly by forgetting that oxygen gas exists as a diatomic molecule ( O₂ ).

📐 Step-by-Step Calculation

1. Find total mass of carbon in each molecule: Both phenylmethanol ( C₇H₈O ) and benzoic acid ( C₇H₆O₂ ) contain 7 carbon atoms. Mass of C = 12 × 7 = 84 g mol⁻¹.
2. Determine molar masses: Phenylmethanol = 108 g mol⁻¹; Benzoic acid = 122 g mol⁻¹.
3. Calculate percentages:
   Phenylmethanol: ( 84 ÷ 108 ) × 100 = 77.8% (or 78%)
   Benzoic acid: ( 84 ÷ 122 ) × 100 = 68.9% (or 69%)

❌ Calculation Traps

Students often mix up the molar masses of the two organic compounds or use the wrong number of carbon atoms. Remember that both compounds possess a benzene ring plus a functional carbon side chain, totaling 7 carbons each.

💡 Key Knowledge

(ii) Observation: Black smoke / soot / smoky yellow flame. (Incomplete combustion of carbon-rich aromatic rings produces unburned carbon particulates).

(iii) Organic Type: Alkenes (or cycloalkenes/alkynes) will produce the exact same smoky observation due to their high carbon-to-hydrogen ratio.

🧠 Exam Technique

Explanation: If the window is kept above the safety line, the exhaust ventilation system cannot maintain sufficient inward airflow/suction. Consequently, toxic fumes and soot particles will escape into the laboratory atmosphere.

✅ Correct Answer (Similarities & Differences)

• Similarity (M1): All three contain arene rings, showing C-H stretch absorptions at 3030 cm⁻¹ and out-of-plane bends at 750/700 cm⁻¹ .
• Difference (M2): Only phenol and phenylmethanol feature an alcoholic/phenolic O-H stretch at 3750 - 3200 cm⁻¹ .
• Difference (M3): Only benzoic acid has a carboxylic acid O-H broad stretch at 3300 - 2500 cm⁻¹ .
• Difference (M4): Only benzoic acid features a C=O carbonyl stretch at 1700 - 1680 cm⁻¹ .
• Difference (M5): Only phenylmethanol has alkyl C-H stretches at 2962 - 2853 cm⁻¹ or bending absorptions at 1485 - 1365 cm⁻¹ .

💡 Key Knowledge

Phenylmethanol ( C₆H₅CH₂OH ) has 5 distinct carbon environments due to symmetry in the monosubstituted benzene ring:

• Total peaks: 5 peaks present in the spectrum.
• Aromatic region: 4 peaks within 165 - 105 ppm (accounting for C1, C2/6, C3/5, and C4 of the benzene ring).
• Aliphatic region: 1 peak within 75 - 55 ppm corresponding to the carbon bonded to the hydroxyl group ( -CH₂OH ).

✅ Correct Answer

Formula: C₆H₅COO⁻ or C₆H₅CO⁺ (benzoyl cation / acylium ion)

Matching m/z value: 105 (or 121 for C₆H₅CH₂O⁺ or COOH⁺ at 45 )

❌ Common Errors

Do not quote fragments that are common to all three molecules (such as the phenyl ring fragment C₆H₅⁺ at m/z = 77 ). The fragment must be uniquely present in benzoic acid and absent in phenol/phenylmethanol.