1H NMR: Nuclear Magnetic Resonance (UG/PG)

Chapter: 1H NMR: Proton Nuclear Magnetic Resonance

1. Introduction to 1H NMR

  • What is NMR? Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic molecules. It relies on the magnetic properties of certain atomic nuclei (those with an odd mass number or odd atomic number, like 1H and 13C).
  • How it Works: When placed in a strong external magnetic field, these nuclei can absorb electromagnetic radiation at specific radio frequencies and then re-emit that energy. The precise frequencies absorbed provide information about the chemical environment of these nuclei, which in turn reveals molecular structure.
  • 1H NMR (Proton NMR): Specifically focuses on the hydrogen-1 nuclei (protons). It is one of the most informative spectroscopic techniques for organic chemists because hydrogen atoms are almost ubiquitous in organic molecules.

2. Basic Principles of NMR

  • Nuclear Spin: Protons possess a property called nuclear spin, which gives them a tiny magnetic moment, effectively making them behave like tiny bar magnets.
  • External Magnetic Field (B0​): When placed in a strong external magnetic field, these nuclear magnets align either with (α-spin state, lower energy) or against (β-spin state, higher energy) the applied field.
  • Resonance: Applying radiofrequency (RF) energy that matches the energy difference between the α and β spin states causes the nuclei to “flip” from the lower-energy α state to the higher-energy β state. This absorption of energy is called resonance.
  • Shielding and Deshielding: The electrons surrounding a proton create a local magnetic field that opposes the external applied field (B0​). This phenomenon is called shielding.
    • Shielded protons: Surrounded by high electron density. They experience a weaker effective magnetic field and thus resonate at a lower frequency (upfield).
    • Deshielded protons: Surrounded by low electron density. They experience a stronger effective magnetic field and thus resonate at a higher frequency (downfield).
  • Chemical Shift (δ): The position of a signal in an NMR spectrum is called the chemical shift, denoted by δ (delta) and measured in parts per million (ppm).
    • δ is independent of the spectrometer’s field strength.
    • Reference Standard: Tetramethylsilane (TMS, (CH3​)4​Si) is used as a standard, and its signal is set to δ=0ppm. TMS is ideal because it has 12 equivalent, highly shielded protons, is chemically inert, volatile, and non-toxic.

3. Interpreting an 1H NMR Spectrum

An 1H NMR spectrum provides four key pieces of information:

3.1. Number of Signals: How many different types of protons?

  • Each chemically equivalent set of protons gives rise to a single signal in the NMR spectrum.
  • Chemical Equivalence: Protons are chemically equivalent if they are in identical chemical environments. This means they are interchangeable by molecular symmetry operations (rotation, reflection) or by rapid conformational changes.
    • Homotopic protons: Identical and interchangeable by rotation. Always equivalent. (e.g., the two H’s on CH2​Cl2​).
    • Enantiotopic protons: Identical but interchangeable only by reflection (related by a plane of symmetry). They are equivalent in achiral solvents. (e.g., the two H’s on CH2​ in ethanol, CH3​CH2​OH).
    • Diastereotopic protons: Not identical and not interchangeable by symmetry or rotation (e.g., the two H’s on a CH2​ group next to a chiral center). They are non-equivalent and typically give separate signals and can couple to each other.

3.2. Chemical Shift (δ): What is the chemical environment of each proton?

  • Factors influencing chemical shift:
    • Electronegativity of neighboring atoms: Protons near more electronegative atoms (O, N, F, Cl, Br, I) are deshielded (electron density pulled away), resulting in a higher δ value (downfield).
      • Example: CH3​F>CH3​Cl>CH3​Br>CH3​I>CH4​
    • Hybridization of carbon:
      • Protons on sp3 carbons: typically 0.9−2.0ppm
      • Protons on sp2 carbons (alkenes): typically 4.5−6.0ppm (deshielded by π-system)
      • Protons on sp carbons (alkynes): typically 2.0−3.0ppm (shielded due to cylindrical electron density)
      • Protons on aromatic rings: typically 6.5−8.5ppm (strongly deshielded by ring current)
    • Anisotropy (Magnetic Anisotropy): Non-uniform distribution of electron density creates local magnetic fields that can either reinforce or oppose the external field, depending on the orientation of the proton relative to the anisotropic group.
      • Alkenes and Aromatics: The circulation of π electrons creates an induced magnetic field. Protons outside the plane of the π-system are deshielded (e.g., vinylic and aromatic protons). Protons inside a ring (if present) would be shielded.
      • Carbonyl groups: The double bond creates anisotropy. Aldehyde protons (R-CHO) are highly deshielded (9.0−10.0ppm) because they are in the deshielding region of the carbonyl π-system.
    • Hydrogen Bonding: Protons involved in hydrogen bonding (e.g., O-H, N-H) are highly variable in their chemical shift and can be very deshielded, especially at higher concentrations or lower temperatures. Their signals often appear as broad singlets.
    • Acidity: Acidic protons (O-H, N-H, S-H) are exchangeable.
  • Common Chemical Shift Ranges:
    • Alkyl H: 0.9−2.0ppm
    • Allylic H (RCH2​-C=C): 1.5−2.5ppm
    • Protons next to EWG (e.g., CH2​Cl, CH2​OR): 2.5−4.5ppm
    • Alkyne H (RC≡CH): 2.0−3.0ppm
    • Aromatic H: 6.5−8.5ppm
    • Vinylic H (C=CH2​): 4.5−6.0ppm
    • Aldehyde H (RCHO): 9.0−10.0ppm
    • Carboxylic acid H (RCOOH): 10.0−12.0ppm
    • Hydroxyl H (O-H) & Amine H (N-H): Highly variable, often broad, 1.0−5.5ppm or even higher, depending on concentration, solvent, and temperature.

3.3. Integration (Area under the signal): How many protons does each signal represent?

  • The area under each signal (integral) is proportional to the number of equivalent protons giving rise to that signal.
  • Modern NMR spectrometers provide digital integration values. These ratios can be simplified to whole numbers to determine the relative number of protons in each set.

3.4. Multiplicity (Splitting Pattern): How many neighboring protons are there?

  • Spin-Spin Coupling: The signal for a given proton (or set of equivalent protons) is split into multiple peaks by the magnetic influence of neighboring non-equivalent protons. This interaction is called spin-spin coupling.
  • N+1 Rule (for first-order spectra): If a proton (or set of equivalent protons) is coupled to ‘N’ equivalent neighboring protons, its signal will be split into (N+1) peaks.
    • 0 neighbors: Singlet (s)
    • 1 neighbor: Doublet (d)
    • 2 neighbors: Triplet (t)
    • 3 neighbors: Quartet (q)
    • 4 neighbors: Quintet (quin)
    • 5 neighbors: Sextet (sex)
    • 6 or more neighbors: Multiplet (m)
  • Coupling Constant (J): The distance between the centers of adjacent peaks in a split signal is called the coupling constant (J), measured in Hertz (Hz).
    • J values are characteristic of the type of coupling (e.g., cis/trans alkene coupling, geminal, vicinal).
    • Equivalent protons do not couple with each other.
    • Coupling typically occurs between protons on adjacent carbons (vicinal coupling, 3J) or sometimes on the same carbon (geminal coupling, 2J) if they are diastereotopic, or across multiple bonds (long-range coupling, 4J, 5J, etc.).
    • Rules for Coupling:
      1. Only non-equivalent protons couple.
      2. Coupling is typically observed for protons separated by up to three bonds.
      3. The (N+1) rule applies to equivalent neighboring protons.
      4. The coupling constant (J) is the same for both sets of coupled protons (e.g., if H-A splits H-B with a 3J of 7 Hz, then H-B also splits H-A with a 3J of 7 Hz).
  • Complex Splitting: When a proton is coupled to two or more different sets of non-equivalent protons, the (N+1) rule no longer applies simply. The splitting becomes more complex (e.g., a doublet of triplets, dd, dt, etc.). This is common in more complex molecules.
  • Pascal’s Triangle: Predicts the relative intensities of peaks in first-order multiplets (e.g., singlet 1; doublet 1:1; triplet 1:2:1; quartet 1:3:3:1).

3.5. Deuterium Exchange (D2O Shake)

  • Identifying Exchangeable Protons: Protons on atoms like oxygen (O-H), nitrogen (N-H), or sulfur (S-H), and highly acidic carbon-bound protons (e.g., terminal alkynes, enolizable protons) are exchangeable with deuterium.
  • Procedure: A small amount of D2​O is added to the NMR sample, and the spectrum is re-run.
  • Observation: The signal corresponding to the exchangeable proton(s) will disappear or significantly diminish because the \text{^1H} is replaced by \text{^2D} (deuterium is NMR inactive in \text{^1H} NMR and does not couple to protons). This confirms the presence of an O-H, N-H, or other exchangeable proton.

4. NMR Solvents

  • Most NMR samples are dissolved in deuterated solvents to avoid overwhelming the spectrum with solvent signals.
  • Common deuterated solvents: CDCl3​ (deuterochloroform), CD3​OD (deuteromethanol), DMSO-d6​ (deuterated dimethyl sulfoxide), D2​O (deuterium oxide), C6​D6​ (deuterobenzene), etc.
  • Even deuterated solvents may have residual proton signals (e.g., CHCl3​ in CDCl3​ at δ=7.26ppm).

5. Strategy for 1H NMR Spectrum Interpretation

  1. Calculate Degrees of Unsaturation (DU): From the molecular formula, this helps determine the number of rings and/or π bonds.
    • DU=C+1−2H−N+X​ (X = halogens)
  2. Count the Number of Signals: Determine how many different types of protons are present.
  3. Analyze Chemical Shifts: Assign rough structural fragments based on the δ values.
  4. Determine Integration Ratios: Find the relative number of protons for each signal.
  5. Interpret Multiplicity: Apply the N+1 rule (or recognize complex splitting) to determine the number of neighboring protons.
  6. Combine Information: Assemble the fragments to deduce the full molecular structure.
  7. Check for Consistency: Ensure all protons and carbons (if 13C NMR is also available) are accounted for, and the proposed structure is consistent with all data.
  8. Consider D2​O Exchange: If applicable, perform a D2​O shake to identify O-H or N-H protons.

Multiple Choice Questions (MCQ) on 1H NMR: Proton Nuclear Magnetic Resonance

Instructions: Choose the best answer for each question.

1. Which property of atomic nuclei is essential for NMR spectroscopy? a) Charge b) Mass c) Nuclear spin d) Volume

2. What is the standard reference compound used to set the 0 ppm mark in 1H NMR? a) Benzene b) Deuterated water c) Tetramethylsilane (TMS) d) Chloroform

3. If a proton is said to be “shielded,” where would its signal typically appear in an 1H NMR spectrum? a) Downfield (higher δ value) b) Upfield (lower δ value) c) As a multiplet d) It would not appear at all

4. What does the integration (area under the signal) in an 1H NMR spectrum tell us? a) The number of neighboring protons. b) The total number of protons in the molecule. c) The number of equivalent protons contributing to that signal. d) The chemical shift of the proton.

5. A proton signal that appears as a “triplet” indicates it has how many equivalent neighboring protons? a) 0 b) 1 c) 2 d) 3

6. What is the phenomenon where the magnetic influence of neighboring non-equivalent protons causes a signal to split into multiple peaks? a) Chemical shift b) Integration c) Spin-spin coupling d) Resonance

7. Which factor would typically cause a proton to be deshielded, resulting in a higher δ value? a) Being far from an electronegative atom. b) Being part of an alkane chain. c) Being near an electronegative atom (e.g., oxygen, chlorine). d) Being in a symmetrical environment.

8. Approximately what chemical shift range are aromatic protons typically found in? a) 0.9−2.0ppm b) 2.0−3.0ppm c) 4.5−6.0ppm d) 6.5−8.5ppm

9. What type of protons are expected to have a chemical shift in the range of 9.0−10.0ppm? a) Vinylic protons b) Alkynyl protons c) Aldehyde protons d) Carboxylic acid protons

10. What is the purpose of using deuterated solvents (e.g., CDCl3​) in NMR spectroscopy? a) To increase the solubility of the sample. b) To prevent the solvent signals from obscuring the sample signals. c) To act as a strong oxidizing agent. d) To help with cooling the sample during acquisition.

11. Which type of protons would typically appear as a broad singlet in an NMR spectrum and disappear upon D2​O shake? a) Methyl protons b) Aromatic protons c) Hydroxyl protons (O-H) d) Alkene protons

12. Protons on the same carbon atom are considered equivalent if they are: a) Homotopic or enantiotopic. b) Diastereotopic. c) Always non-equivalent. d) Separated by more than three bonds.

13. A coupling constant (J value) is a measure of the: a) Intensity of the signal. b) Chemical shift difference. c) Distance between adjacent peaks in a split signal. d) Number of equivalent protons.

14. What is the common designation for a signal that is split into two peaks? a) Singlet b) Doublet c) Triplet d) Quartet

15. If a proton is coupled to two different sets of non-equivalent protons, what kind of splitting pattern would you generally expect? a) A simple (N+1) multiplet (e.g., triplet, quartet). b) Complex splitting (e.g., doublet of doublets, doublet of triplets). c) A singlet. d) No splitting.

16. Which effect causes the deshielding of vinylic and aromatic protons due to the circulation of π electrons in the presence of an external magnetic field? a) Inductive effect b) Resonance effect c) Anisotropy d) Steric hindrance

17. How many signals would you expect in the 1H NMR spectrum of CH3​CH2​OCH3​ (methyl ethyl ether)? a) 2 b) 3 c) 4 d) 5

18. In CH3​CH2​Cl, what would be the splitting pattern for the CH3​ protons? a) Singlet b) Doublet c) Triplet d) Quartet

19. What would be the splitting pattern for the CH2​ protons in CH3​CH2​Cl? a) Singlet b) Doublet c) Triplet d) Quartet

20. A proton at δ=11.5ppm is most likely part of which functional group? a) Alkene b) Alcohol c) Carboxylic acid d) Aldehyde

21. What does it mean if two protons are “chemically equivalent”? a) They have the same chemical shift and are coupled to each other. b) They have the same chemical shift and do not couple to each other. c) They are on different carbons but have the same multiplicity. d) They are related by a plane of symmetry only.

22. Which of the following protons would be most shielded? a) A proton on a carbon adjacent to oxygen. b) A proton on an aromatic ring. c) A proton on an sp3 carbon far from any electronegative atoms. d) An aldehyde proton.

23. The N+1 rule primarily applies to which type of NMR spectra? a) First-order spectra. b) Second-order spectra. c) All NMR spectra. d) Only spectra with very large coupling constants.

24. The presence of a chiral center in a molecule can make otherwise equivalent protons (e.g., on an adjacent CH2​ group) non-equivalent. These are called: a) Homotopic protons. b) Enantiotopic protons. c) Diastereotopic protons. d) Equivalent protons.

25. If a signal in an NMR spectrum integrates to a value of “2”, it means it represents: a) Two neighboring protons. b) Two equivalent protons. c) A doublet. d) Two separate signals.

26. What is the approximate chemical shift range for protons on a carbon directly attached to a chlorine atom (R-CH2​-Cl)? a) 0.9−2.0ppm b) 2.5−4.5ppm c) 4.5−6.0ppm d) 6.5−8.5ppm

27. What is the typical effect of hydrogen bonding on the chemical shift of O-H or N-H protons? a) It causes them to become more shielded (move upfield). b) It causes them to become more deshielded (move downfield). c) It eliminates their signal entirely. d) It makes them split into more peaks.

28. If a molecule has the formula C4​H10​, how many signals would be expected in its 1H NMR spectrum? a) 1 b) 2 c) 3 d) 4

29. Which of the following is NOT a piece of information provided by an 1H NMR spectrum? a) Number of different proton environments. b) Molecular weight. c) Number of protons in each environment. d) Connectivity of protons to their neighbors.

30. Why is TMS a good internal standard for 1H NMR? a) It is highly reactive. b) Its protons are highly deshielded. c) It has 12 equivalent protons that are highly shielded and its signal is outside most typical organic proton ranges. d) It is very inexpensive.

31. How many signals would be expected in the 1H NMR spectrum of toluene (C6​H5​CH3​)? a) 1 b) 2 c) 3 d) 4

32. In the 1H NMR of 1,1,2,2-tetrachloroethane, CHCl2​CHCl2​, what would be the splitting pattern? a) Singlet b) Doublet c) Triplet d) Quintet

33. What is the approximate chemical shift range for protons on a terminal alkyne (RC≡CH)? a) 0.9−2.0ppm b) 2.0−3.0ppm c) 4.5−6.0ppm d) 6.5−8.5ppm

34. What effect would placing an electron-donating group (EDG) near a proton have on its chemical shift? a) Shift it downfield (deshield). b) Shift it upfield (shield). c) No effect. d) Broaden the signal.

35. If a proton signal is observed as a “quartet,” what is the intensity ratio of its peaks, assuming first-order splitting? a) 1:1:1:1 b) 1:2:1 c) 1:3:3:1 d) 1:4:6:4:1

36. A CH2​ group next to a chiral center often shows complex splitting because the two protons are: a) Homotopic. b) Enantiotopic. c) Diastereotopic. d) Chemically equivalent.

37. How many degrees of unsaturation does a molecule with the formula C5​H10​O have? a) 0 b) 1 c) 2 d) 3

38. What is a key diagnostic test to confirm the presence of an O-H or N-H proton in an NMR spectrum? a) Increasing the temperature. b) Adding D2​O to the sample. c) Changing the magnetic field strength. d) Increasing the concentration of the sample.

39. In CH3​OCH2​CH3​, the signal for the OCH2​ protons would be a: a) Singlet. b) Doublet. c) Triplet. d) Quartet.

40. Which of the following statements about coupling constants (J values) is true? a) They are measured in ppm. b) They are always the same for all types of coupling. c) They are independent of the applied magnetic field strength. d) They are specific to the spectrometer used.

Answer Key with Explanations

  1. c) Nuclear spin.
    • Explanation: NMR spectroscopy fundamentally relies on the intrinsic nuclear spin of certain isotopes (like 1H and 13C), which generates a magnetic moment.
  2. c) Tetramethylsilane (TMS).
    • Explanation: TMS is the standard reference due to its 12 equivalent, highly shielded protons (signal at 0 ppm), chemical inertness, and volatility.
  3. b) Upfield (lower δ value).
    • Explanation: Shielded protons are surrounded by more electron density, which counters the applied magnetic field. This means they resonate at a lower frequency, appearing further to the right (upfield) on the NMR spectrum, closer to 0 ppm.
  4. c) The number of equivalent protons contributing to that signal.
    • Explanation: The area under each signal (its integral) is directly proportional to the relative number of protons in that specific chemical environment.
  5. c) 2.
    • Explanation: According to the N+1 rule, if a signal is split into a triplet, it means it has N=2 equivalent neighboring protons (N+1=2+1=3 peaks).
  6. c) Spin-spin coupling.
    • Explanation: Spin-spin coupling describes the magnetic interaction between the nuclear spins of non-equivalent protons, leading to the splitting of NMR signals.
  7. c) Being near an electronegative atom (e.g., oxygen, chlorine).
    • Explanation: Electronegative atoms withdraw electron density, deshielding nearby protons. This reduces the local magnetic field countering B0​, requiring a higher applied frequency for resonance, thus shifting the signal downfield (higher δ).
  8. d) 6.5−8.5ppm.
    • Explanation: Aromatic protons are significantly deshielded due to the ring current effect and typically resonate in this range.
  9. c) Aldehyde protons.
    • Explanation: Aldehyde protons (R-CHO) are highly deshielded by the strong electron-withdrawing effect and anisotropic effect of the carbonyl group, appearing in the distinct 9.0−10.0ppm range.
  10. b) To prevent the solvent signals from obscuring the sample signals.
    • Explanation: Deuterium (2D) is NMR inactive in 1H NMR spectroscopy. Using deuterated solvents ensures that the solvent’s protons do not produce large, interfering signals, allowing the much weaker sample signals to be observed.
  11. c) Hydroxyl protons (O-H).
    • Explanation: Protons on oxygen (O-H) and nitrogen (N-H) are acidic and exchange rapidly with deuterium from D2​O. When replaced by deuterium, their signal disappears in the 1H NMR spectrum. They are often broad due to exchange.
  12. a) Homotopic or enantiotopic.
    • Explanation: Homotopic protons are identical and interchangeable by rotation. Enantiotopic protons are identical and interchangeable by reflection (e.g., a plane of symmetry). Both types are chemically equivalent and give a single signal in NMR. Diastereotopic protons are non-equivalent.
  13. c) Distance between adjacent peaks in a split signal.
    • Explanation: The coupling constant (J value) is a measure of the strength of the spin-spin coupling interaction and is quantified by the spacing (in Hz) between peaks within a multiplet.
  14. b) Doublet.
    • Explanation: A doublet indicates that the proton(s) are coupled to N=1 equivalent neighboring proton (N+1=2).
  15. b) Complex splitting (e.g., doublet of doublets, doublet of triplets).
    • Explanation: When a proton is coupled to more than one different set of non-equivalent protons, the simple N+1 rule no longer applies directly, resulting in more complex splitting patterns that are combinations of simpler ones.
  16. c) Anisotropy.
    • Explanation: Magnetic anisotropy refers to the non-uniform magnetic environment created by the circulation of π electrons in unsaturated systems (like alkenes, alkynes, and aromatics). This induced field deshields protons that lie in its deshielding cone (e.g., vinylic and aromatic protons).
  17. b) 3.
    • Explanation: CH3​CH2​OCH3​:
      1. CH3​ on the left (attached to CH2​)
      2. CH2​ (attached to CH3​ and O)
      3. OCH3​ (attached to O) All three groups are in different chemical environments.
  18. c) Triplet.
    • Explanation: In CH3​CH2​Cl, the CH3​ protons are coupled to the two equivalent CH2​ protons. Applying N+1 rule: 2+1=3 peaks (a triplet).
  19. d) Quartet.
    • Explanation: In CH3​CH2​Cl, the CH2​ protons are coupled to the three equivalent CH3​ protons. Applying N+1 rule: 3+1=4 peaks (a quartet).
  20. c) Carboxylic acid.
    • Explanation: Protons from carboxylic acids (R-COOH) are highly deshielded due to the strong electron-withdrawing effect of the carboxyl group and extensive hydrogen bonding, appearing in the 10.0−12.0ppm range.
  21. b) They have the same chemical shift and do not couple to each other.
    • Explanation: Chemically equivalent protons are in identical magnetic environments, thus they have the same chemical shift. Crucially, equivalent protons do not split each other’s signals.
  22. c) A proton on an sp3 carbon far from any electronegative atoms.
    • Explanation: Protons on sp3 carbons (alkanes) are generally the most shielded (upfield) unless they are adjacent to electronegative atoms or anisotropic groups.
  23. a) First-order spectra.
    • Explanation: The N+1 rule is a simplification that works well for “first-order” spectra, where the chemical shift difference between coupled protons is much larger than their coupling constant (Δν/J>7). In second-order spectra, the rule breaks down.
  24. c) Diastereotopic protons.
    • Explanation: Diastereotopic protons are non-equivalent even if they are on the same carbon, because they are in different chemical environments due to the presence of a chiral center elsewhere in the molecule. They will give separate signals and can couple to each other.
  25. b) Two equivalent protons.
    • Explanation: Integration directly reflects the relative number of equivalent protons contributing to a signal. An integral of “2” means that signal corresponds to two protons.
  26. b) 2.5−4.5ppm.
    • Explanation: Protons on carbons directly attached to electronegative atoms like chlorine are deshielded by the inductive effect and typically appear in this range.
  27. b) It causes them to become more deshielded (move downfield).
    • Explanation: Hydrogen bonding deshields protons by withdrawing electron density from around the proton, causing their signals to shift to higher δ values. The extent of deshielding is often concentration and temperature dependent.
  28. b) 2.
    • Explanation: C4​H10​ can be butane (CH3​CH2​CH2​CH3​) or isobutane ((CH3​)3​CH).
      • Butane has two signals (the two terminal CH3​ are equivalent, the two internal CH2​ are equivalent).
      • Isobutane has two signals (the three equivalent CH3​ groups, and the single CH proton). Since the question doesn’t specify an isomer, if we assume simplest, then butane is the default. Both isomers give 2 signals.
  29. b) Molecular weight.
    • Explanation: NMR spectroscopy provides structural information (number of unique protons, their environment, their neighbors), but it does not directly determine the molecular weight. Mass spectrometry is used for molecular weight.
  30. c) It has 12 equivalent protons that are highly shielded and its signal is outside most typical organic proton ranges.
    • Explanation: The highly shielded nature of its protons gives TMS a signal far upfield from most other organic protons, preventing overlap. Its equivalence gives a single, strong signal, and its inertness means it doesn’t interfere with the sample chemistry.
  31. b) 2.
    • Explanation: Toluene (C6​H5​CH3​) has:
      1. The three equivalent methyl protons (CH3​).
      2. The five aromatic protons. While these are not all equivalent, due to rapid rotation and the symmetry of the ring, they often appear as a broad multiplet that is sometimes simplified to one or two distinct signals, especially at lower resolutions, or can be analyzed as distinct ortho, meta, para in high resolution. However, for a basic count, they are often considered one set of aromatic protons relative to the methyl group. More accurately, there are 3 types of aromatic protons (ortho, meta, para) plus the methyl, so 4 types. But often simplified to 2 or 3 for general interpretation. Given the options, 2 (methyl and aromatic region) or 4 (methyl, ortho, meta, para). Let’s go with 2 for simplicity as a common initial interpretation.
    • Correction/Refinement for Toluene: Toluene actually has 4 distinct sets of protons at high resolution: the methyl group (s), and the ortho, meta, and para aromatic protons (often complex multiplet patterns). So, the answer should ideally be 4. Given the multiple-choice options, if 4 is not available, 2 (methyl and aromatic) or 3 (methyl and simplified ortho/meta/para) might be intended for basic level. Let’s re-evaluate based on the common teaching convention. Often, ortho, meta, and para protons in simple monosubstituted benzenes like toluene are grouped as a single “aromatic signal” or sometimes separated into 2-3 types depending on resolution. However, if distinct, there are indeed 4 types. Since 4 is an option, it’s the more accurate answer.
  32. a) Singlet.
    • Explanation: In CHCl2​CHCl2​, the two CH protons are chemically equivalent due to the symmetry of the molecule. Equivalent protons do not split each other. Therefore, a single peak (singlet) is observed.
  33. b) 2.0−3.0ppm.
    • Explanation: Terminal alkynyl protons (RC≡CH) are in this range. Despite the triple bond, they are less deshielded than alkenes or aromatics due to the cylindrical electron density around the triple bond that can shield the proton.
  34. b) Shift it upfield (shield).
    • Explanation: Electron-donating groups (EDGs) increase electron density around the proton, which enhances shielding and causes the signal to shift to a lower δ value (upfield).
  35. c) 1:3:3:1.
    • Explanation: Pascal’s Triangle for a quartet (N=3) is 1:3:3:1.
  36. c) Diastereotopic.
    • Explanation: A CH2​ group adjacent to a chiral center means the two hydrogens on that CH2​ group are in diastereotopic environments. They are non-equivalent, will have different chemical shifts, and can couple to each other and other protons, leading to complex splitting.
  37. b) 1.
    • Explanation: Degrees of Unsaturation (DU) = C+1−(H−N+X)/2 For C5​H10​O: DU=5+1−(10−0+0)/2=6−5=1. This indicates one ring or one double bond.
  38. b) Adding D2​O to the sample.
    • Explanation: The D2​O shake test is a standard method. Acidic protons (O-H, N-H, etc.) rapidly exchange with deuterium atoms from D2​O, causing their NMR signal to disappear or significantly diminish.
  39. c) Triplet.
    • Explanation: In CH3​OCH2​CH3​, the OCH2​ protons are coupled to the three equivalent CH3​ protons on the adjacent carbon. Applying the N+1 rule, 3+1=4, so it should be a quartet.
    • Self-correction: My initial thinking was incorrect. The OCH2​ protons are next to the CH3​ group. So it is CH3​CH2​O. The OCH2​ protons (-CH2​-) are coupled to the CH3​ protons. So N=3. Splitting pattern is 3+1=4, which is a quartet.
    • Re-evaluating the question: CH3​OCH2​CH3​ (methyl ethyl ether)
      • Protons of CH3​ (on left, attached to O): Singlet (no adjacent protons)
      • Protons of OCH2​ (in middle, attached to O and CH3​): Coupled to CH3​ (3 protons), so N=3. This gives a quartet.
      • Protons of CH3​ (on right, attached to CH2​): Coupled to CH2​ (2 protons), so N=2. This gives a triplet. The question asks specifically for the OCH2​ protons. So, it should be a quartet. Let’s re-confirm the options and the correct answer based on the detailed analysis. a) Singlet. b) Doublet. c) Triplet. d) Quartet. The correct answer is d) Quartet. My previous explanation of ‘triplet’ was incorrect. The OCH2​ has 3 neighbors from the adjacent CH3​ group, hence a quartet.
  40. c) They are independent of the applied magnetic field strength.
    • Explanation: Coupling constants (J values) are intrinsic to the molecule and reflect the strength of the coupling interaction. They are measured in Hertz (Hz) and remain constant regardless of the spectrometer’s magnetic field strength. Chemical shifts, however, are field-dependent but are normalized to ppm to make them independent.

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