OCR A-Level Chemistry AS Depth in chemistry (02), June 2019: Question 3

✅ Correct Answer

• Curly arrow starting from the lone pair (or negative charge) of the hydroxide ion ( :OH⁻ ) to the carbon atom attached to iodine.
• Partial charges correctly labelled ( δ+ on C, δ- on I) on the C-I bond.
• Curly arrow starting from the C-I bond and moving entirely onto the iodine atom.
• Final organic product drawn as CH₃CH₂CH₂CH₂CH₂OH (pentan-1-ol) along with the leaving iodide ion ( I⁻ ).

🧠 Exam Technique

• Arrow origins: Ensure the first curly arrow strictly originates from the lone pair or negative charge of the OH⁻ ion, pointing precisely at the carbon atom.
• Bond breaking: The second arrow must originate from the middle of the C-I bond and point directly to the iodine atom.

❌ Common Errors

• Starting the nucleophile arrow from somewhere in empty space rather than the lone pair or charge.
• Using double-headed or half-headed (radical) arrows instead of standard single-barbed curly arrows.
• Omitting the partial dipoles ( δ+ / δ- ).

✅ Correct Answers

(ii) Measurement & Observation: Measure the time taken for a precipitate to appear.

(iii) Explanation: The C-I bond is weaker than the C-Br bond (has a lower bond enthalpy), requiring less energy to break, meaning the carbon-halogen bond breaks faster.

💡 Key Knowledge

• Silver nitrate in ethanol reacts with the halide ions produced during hydrolysis to form insoluble silver halide precipitates ( AgI is yellow, AgBr is cream).
• Bond enthalpy decreases down Group 7: C-F > C-Cl > C-Br > C-I. Therefore, iodoalkanes react the fastest.

❌ Common Errors

• Failing to state a comparison in part (iii) (e.g., just saying "C-I has a weak bond" without mentioning C-Br).

Forgetting to include time as the measurement parameter in part (ii).

✅ Correct Answers

(i) Peak X (m/z = 198): Represents the molecular mass (or relative molecular mass / molar mass) of the molecule.

(ii) Fragment ions:

• Y (m/z = 71): CH₃CH₂CH₂CH₂CH₂⁺ (or C₅H₁₁⁺ )
• Z (m/z = 43): CH₃CH₂CH₂⁺ (or C₃H₇⁺ )

🧠 Exam Technique & Formatting

• Always include the positive charge sign ( ⁺ ) when writing formulae for ions in mass spectrometry. Losing the charge can cost marks.
• Structural, skeletal, or displayed formulae are accepted as long as they are unambiguous.

❌ Common Errors

• Stating that peak X represents the "molecular ion alone" without linking it clearly to mass/Mr.
• Miscalculating fragment masses or omitting structural connectivity.

✅ Correct Answers

(i) Structure of 2-iodo-2-methylbutane: A branched five-carbon chain where carbon-2 holds both a methyl group ( -CH₃ ) and an iodine atom ( -I ).

(ii) Spectra Comparison:

• Similarity: Both have a peak at the same molecular ion / Mr value ( m/z = 198 ) or share other common fragment peaks like m/z = 71 or 29 .
• Difference: 2-iodo-2-methylbutane has no peak at m/z = 43 (due to different fragmentation patterns caused by branching).

💡 Key Knowledge

Isomers share the same molecular formula (hence the same molecular ion mass peak, m/z = 198 ), but their different carbon skeletons lead to distinct fragmentation pathways and unique fingerprint regions in mass spectrometry.

❌ Common Errors

• Drawing an incorrect carbon skeleton for 2-iodo-2-methylbutane (e.g., miscounting the chain length).
• Giving vague differences like "different ion peaks" without specifying which m/z value is missing or present.