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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, CCl₄, and CDCl₃ are used in 1H NMR spectroscopy due to their non-polar nature and lack of proton signals.
- Si(CH₃)₄ acts as a reference compound with known chemical shifts and provides a standard for chemical shift measurements.
- CCl₄ and CDCl₃ are good solvents for organic compounds, ensuring that the samples remain in solution and do not interfere with the NMR signals.
- Additionally, they do not produce signals in the 1H NMR spectrum, allowing for clearer observation of the sample's protons.
Step 2
Deduce the splitting pattern for each of the peaks given by the H atoms labelled x, y and z in the 1H NMR spectrum:
Answer
For the H atoms in the compound:
- x: doublet (due to coupling with adjacent protons)
- y: quartet (due to coupling with three adjacent protons)
- z: singlet (no adjacent protons causing splitting)
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
It is difficult to use Table B to predict the chemical shift for peak y because the local environment around the hydrogen atom affects its chemical shift significantly.
Factors such as electronegative atoms nearby, hybridization, and steric hindrance can alter the expected δ value beyond typical reference values provided in the data table.
Step 4