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This question is about NMR spectroscopy - AQA - A-Level Chemistry - Question 7 - 2020 - Paper 2
Question 7
This question is about NMR spectroscopy. 1. A compound is usually mixed with Si(CH₃)₄ and either CCl₄ or CDCl₃ before recording the compound's ¹H NMR spectrum. Sta... show full transcript
Worked Solution & Example Answer:This question is about NMR spectroscopy - AQA - A-Level Chemistry - Question 7 - 2020 - Paper 2
Step 1
State why Si(CH₃)₄, CCl₄ and CDCl₃ are used in ¹H NMR spectroscopy:
Answer
Si(CH₃)₄ serves as a reference compound because it does not exhibit signals in the ¹H NMR spectrum, allowing for a clear baseline calibration. Both CCl₄ and CDCl₃ are non-polar solvents that do not interfere with the NMR signals of the sample, ensuring a clean analysis of the compound without additional peaks.
Step 2
Deduce the splitting pattern for the peaks given by the H atoms labelled x, y and z:
Answer
For the hydrogen atom labeled x (on the CH₃ group adjacent to a carbon with both Cl and H), it will appear as a singlet since it has no hydrogens on adjacent carbons that could couple with it.
The hydrogen atom labeled y (on the CH next to the carbon with Cl) will be a triplet because it is adjacent to two equivalent hydrogens from the CH₃ group.
Finally, the hydrogen atom labeled z (on the CH bonded to the carbonyl) will also be a singlet as it is not adjacent to any other hydrogen atoms.
Thus, the splitting pattern is:
- x: singlet
- y: triplet
- z: singlet.
Step 3
Suggest why it is difficult to use Table B in the Data Booklet to predict the chemical shift (δ value) for the peak given by the H atom labelled y.
Answer
The difficulty arises because the chemical shift for H atoms can vary greatly depending on the local electronic environment created by nearby electronegative atoms or functional groups. Hence, the specific context of this peak may not be accurately represented in the generalizations provided in Table B.
Step 4
Deduce the structures of these two isomers:
Answer
Isomer 1: CH₃C(Cl)C(O)CH₃ (2-chloro-2-methylpropan-1-ol)
Isomer 2: CH₃C(O)CHClCH₃ (2-chloro-3-methylpropan-1-ol)
Both structures align with the integration ratio of 2:9, corresponding to their respective hydrogen environments in the ¹H NMR spectra.