OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 15

✅ Correct Answer: Option B

Both statement 1 (3-methylbutan-2-ol) and statement 2 (2-methylbutan-2-ol) produce exactly 4 peaks in their ¹³C NMR spectra due to their specific molecular symmetry or carbon environments.

💡 Key Knowledge: ¹³C NMR Peaks

• Each peak in a ¹³C NMR spectrum corresponds to a unique carbon environment (non-equivalent carbon atoms).
• Planes of symmetry within a molecule can make two or more carbon atoms chemically equivalent, reducing the total number of peaks observed.

🧠 Exam Technique

Do not guess! Draw out the full structural or displayed formula for each numbered option. Count the total number of unique carbon positions by looking for symmetry axes or mirror planes.

❌ Common Errors

Students often confuse ¹³C NMR with ¹H NMR. In ¹³C NMR, you count the number of carbon environments, not peak splitting patterns (there is no spin-spin splitting to worry about between adjacent carbons in standard ¹³C spectra).

Statement 1: 3-methylbutan-2-ol

CH₃-CH(OH)-CH(CH₃)-CH₃

Let's count the carbon environments:

• C1: Terminal methyl group attached to C2 -> Peak 1
• C2: CH bearing the -OH group -> Peak 2
• C3: CH attached to the methyl branch -> Peak 3
• C4: Terminal methyl group at the end of the chain -> Peak 4
• CH₃ branch attached to C3: Equivalent to C4 due to rotation/symmetry? Wait, let's check carefully: The two methyl groups at the end (C4 and the branch) are not chemically equivalent because C3 has a chiral center/asymmetrical environment in this specific molecule. Let's recount: C1(CH₃), C2(CH-OH), C3(CH), C4(CH₃), and the branch-CH₃. That makes 5 environments? Let's re-verify statement 1:

Wait! Examiner check for 3-methylbutan-2-ol: Structure is CH₃CH(OH)CH(CH₃)₂. The two methyl groups attached to C3 are chemically equivalent by rapid rotation/symmetry if the molecule is viewed broadly, giving 4 unique carbon environments: (1) C1 methyl, (2) C2 CH-OH, (3) C3 CH, (4) the two equivalent CH₃ methyl groups attached to C3. Total = 4 peaks. Statement 1 is correct!

Statement 2: 2-methylbutan-2-ol

CH₃-CH₂-C(CH₃)₂(OH)

Let's count the carbon environments:

• C1: Terminal methyl group in the ethyl chain (CH₃-CH₂) -> Peak 1
• C2: Methylene group in the ethyl chain (-CH₂-) -> Peak 2
• C3: Tertiary alcohol carbon (-C(OH)-) -> Peak 3
• C4 & C5: The two methyl groups attached to the tertiary carbon are completely equivalent due to rotational symmetry around the central carbon -> Peak 4 (shared)

Total = 4 peaks. Statement 2 is correct!

Statement 3: 2-methylbutan-1-ol

CH₃-CH₂-CH(CH₃)-CH₂OH

Let's count the carbon environments:

• C1: Primary alcohol carbon (-CH₂OH) -> Peak 1
• C2: CH group attached to methyl branch -> Peak 2
• C3: Methylene group in chain (-CH₂-) -> Peak 3
• C4: Terminal methyl group on the chain -> Peak 4
• CH₃ branch attached to C2 -> Peak 5

Total = 5 peaks. Statement 3 is incorrect (gives 5 peaks, not 4).