Spectroscopy-2 NMR

Chemistry, HSSC-Punjab Second Year. This chapter is split into 27 topics and has 619 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Number of 13C NMR peaks: hydrocarbons 24 questions
2. NMR-active nuclei 21 questions
3. Predicting a 1H NMR spectrum 20 questions
4. Splitting patterns of proton signals 20 questions
5. Comparing 1H and 13C NMR spectra 24 questions
6. Carbon-13 NMR spectroscopy: principles 25 questions
7. High-resolution spectra and the n + 1 rule 24 questions
8. Identifying O-H and N-H protons by the D2O shake 21 questions
9. Chemical shift ranges of common protons (Table 30.1) 28 questions
10. Number of 1H NMR signals: alkanes, halogenoalkanes and others 25 questions
11. Low-resolution 1H NMR spectra 22 questions
12. Deducing a structure from 1H NMR data 23 questions
13. Principle of NMR spectroscopy and nuclear spin 21 questions
14. Peak areas: protons in each environment 26 questions
15. Number of 13C NMR peaks: oxygen, halogen and nitrogen compounds 27 questions
16. Chemical shifts of carbon environments (Table 30.3) 24 questions
17. Ratio of peak areas and total number of hydrogens 30 questions
18. Which carbon gives the highest and lowest 13C chemical shift 24 questions
19. Distinguishing isomers by 13C NMR 21 questions
20. Number of 1H NMR signals: alcohols, ethers and carbonyl compounds 23 questions
21. Number of sub-peaks and neighbouring protons (n + 1 rule) 22 questions
22. Procedure for deducing a structure from 1H NMR 20 questions
23. Deuterated solvents in NMR 22 questions
24. Which protons have the highest and lowest chemical shift 20 questions
25. Chemical equivalence of protons 20 questions
26. Tetramethylsilane as the reference standard 21 questions
27. Chemical shift and electron density around protons 21 questions

Sample questions: Spectroscopy-2 NMR

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. In 1-chloropropane, CH₃CH₂CH₂Cl, which protons give the signal at the highest chemical shift?

• A. none of them in particular, since every signal appears at the same δ
• B. the CH₂Cl protons(correct answer)
• C. the CH₃ protons
• D. the middle CH₂ protons

Answer: B.The CH₂Cl protons are alkyl protons next to an electronegative atom (O or Cl) (3.0–4.5 ppm), which is higher than every other kind of proton present, so their signal has the highest δ value.

Q2. Use Table 30.1 to decide which set of protons in CH₃COOCH₂CH₃ (ethyl ethanoate) appears furthest to the left (downfield).

From your book: Table 30.1 (page 205) Full page Open book

• A. the CH₃ of ethyl protons
• B. none of them in particular, since every signal appears at the same δ
• C. the OCH₂ protons(correct answer)
• D. the CH₃CO protons

Answer: C.The OCH₂ protons are alkyl protons next to an electronegative atom (O or Cl) (3.0–4.5 ppm), which is higher than every other kind of proton present, so their signal has the highest δ value.

Q3. Table 30.1 predicts that the highest chemical shift in CH₃CHClCH₂CH₃ is for the

From your book: Table 30.1 (page 205) Full page Open book

• A. the CHCl protons(correct answer)
• B. the CH₃ on C₁ protons
• C. none of them in particular, since every signal appears at the same δ
• D. the CH₃ on C₄ protons

Answer: A.The CHCl protons are alkyl protons next to an electronegative atom (O or Cl) (3.0–4.5 ppm), which is higher than every other kind of proton present, so their signal has the highest δ value.

Q4. Identify the highest-δ protons of methyl propanoate (CH₃CH₂COOCH₃).

• A. none of them in particular, since every signal appears at the same δ
• B. the OCH₃ protons(correct answer)
• C. the CH₃ of ethyl protons
• D. the CH₂ protons

Answer: B.The OCH₃ protons are alkyl protons next to an electronegative atom (O or Cl) (3.0–4.5 ppm), which is higher than every other kind of proton present, so their signal has the highest δ value.

Q5. Deduce the structure of the compound C₃H₈O whose proton NMR spectrum has the signals δ 0.9 (3H, triplet); δ 1.5 (2H, sextet); δ 2.5 (1H, singlet); δ 3.6 (2H, triplet).

• A. methyl ethanoate
• B. methoxyethane
• C. propan-2-ol
• D. propan-1-ol(correct answer)

Answer: D.Use the number of signals, the integration and the n + 1 splitting to build the structure CH₃CH₂CH₂OH: CH₃: 3H, n = 2; middle CH₂: 2H, n = 5; CH2OH: 2H, n = 2; OH: 1H, n = 0.

Q6. A compound of molecular formula C₄H₁₀O shows these ¹H NMR signals: δ 0.9 (3H, triplet); δ 1.2 (3H, doublet); δ 1.5 (2H, quintet); δ 2.5 (1H, singlet); δ 3.7 (1H, sextet). Which compound is it?

• A. 2-methylpropan-2-ol
• B. 2-methoxypropane
• C. butan-2-ol(correct answer)
• D. ethoxyethane

Answer: C.Use the number of signals, the integration and the n + 1 splitting to build the structure CH₃CH(OH)CH₂CH₃: CH₃ on C1: 3H, n = 1; CH(OH): 1H, n = 5; CH₂: 2H, n = 4; CH₃ on C4: 3H, n = 2; OH: 1H, n = 0.

Q7. Deduce the structure of the compound C₄H₈O whose proton NMR spectrum has the signals δ 0.9 (3H, triplet); δ 1.5 (2H, sextet); δ 2.4 (2H, quartet); δ 9.8 (1H, triplet).

• A. ethoxyethane
• B. butanal(correct answer)
• C. butane
• D. butanone

Answer: B.Use the number of signals, the integration and the n + 1 splitting to build the structure CH₃CH₂CH₂CHO: CH₃: 3H, n = 2; middle CH₂: 2H, n = 5; CH2CHO: 2H, n = 3; CHO: 1H, n = 2.

Q8. Which compound of formula C₄H₈O₂ fits the spectral data δ 0.9 (3H, triplet); δ 1.5 (2H, sextet); δ 2.3 (2H, triplet); δ 11.5 (1H, singlet)?

• A. propanoic acid
• B. methyl propanoate
• C. ethyl ethanoate
• D. butanoic acid(correct answer)

Answer: D.Use the number of signals, the integration and the n + 1 splitting to build the structure CH₃CH₂CH₂COOH: CH₃: 3H, n = 2; middle CH₂: 2H, n = 5; CH2CO: 2H, n = 2; COOH: 1H, n = 0.

Q9. Which compound of formula C₄H₁₀O fits the spectral data δ 0.9 (6H, doublet); δ 1.5 (1H, multiplet); δ 2.5 (1H, singlet); δ 3.4 (2H, doublet)?

• A. 2-methoxypropane
• B. 2-methylpropan-1-ol(correct answer)
• C. butan-2-ol
• D. ethoxyethane

Answer: B.Use the number of signals, the integration and the n + 1 splitting to build the structure (CH₃)₂CHCH₂OH: two CH₃ groups: 6H, n = 1; CH: 1H, multiplet; CH2OH: 2H, n = 1; OH: 1H, n = 0.

Q10. A compound C₃H₈O gives 3 peaks in ¹³C NMR and 3 signals in ¹H NMR. It is

• A. methoxyethane, CH₃OCH₂CH₃(correct answer)
• B. propan-1-ol, CH₃CH₂CH₂OH
• C. propan-2-ol, (CH₃)₂CHOH
• D. ethanol, CH₃CH₂OH

Answer: A.Methoxyethane has three carbon and three proton environments; propan-1-ol has three carbon but four proton environments; propan-2-ol has two carbon and three proton environments.