Structure elucidation by UV, IR, MS, 1H-NMR and 13C-NMR: carvone, citral, menthol MCQs With Answer

Introduction

This blog presents a focused set of MCQs tailored for M.Pharm students on structure elucidation of three important monoterpenes—carvone, citral and menthol—using UV, IR, MS, 1H‑NMR and 13C‑NMR. Questions emphasize interpretation of characteristic bands, chemical shifts, coupling constants, DEPT behavior, and mass spectral fragmentation patterns often encountered in phytochemical analysis. The aim is to deepen understanding of how complementary spectroscopic data confirm functional groups, configurations (E/Z), and connectivity in monoterpene scaffolds, and how to apply these principles to real spectral problems encountered during natural-product characterization and quality control.

Q1. Which spectroscopic technique is most sensitive to the extended conjugation present in citral and will show a bathochromic shift?

• Ultraviolet-visible (UV‑Vis) spectroscopy showing a bathochromic shift
• Infrared (IR) spectroscopy looking at C=O stretching only
• Mass spectrometry giving molecular ion only
• 13C‑NMR chemical shifts of aliphatic carbons

Correct Answer: Ultraviolet-visible (UV‑Vis) spectroscopy showing a bathochromic shift

Q2. In IR spectroscopy, the carbonyl (C=O) stretch of an α,β‑unsaturated ketone like carvone typically appears in which region?

• Around 1660–1680 cm‑1 (lowered by conjugation)
• Around 1720–1740 cm‑1 (typical ester)
• Around 1705–1715 cm‑1 (saturated ketone)
• Around 1600 cm‑1 (aromatic C=C)

Correct Answer: Around 1660–1680 cm‑1 (lowered by conjugation)

Q3. Which statement correctly describes the molecular ion observed for carvone in electron ionization mass spectrometry?

• Molecular ion observed at m/z 150 corresponding to C10H14O
• Molecular ion observed at m/z 152 corresponding to C10H16O
• Molecular ion observed at m/z 156 corresponding to C10H20O
• No molecular ion is observed for carvone under EI conditions

Correct Answer: Molecular ion observed at m/z 150 corresponding to C10H14O

Q4. In the 1H‑NMR of citral, where would the aldehydic proton be expected?

• Approximately 9.5–10.5 ppm (distinct aldehydic resonance)
• Approximately 5.0–6.5 ppm (vinylic region)
• Approximately 3.0–4.5 ppm (allylic/methylene)
• Approximately 0.8–1.5 ppm (methyl region)

Correct Answer: Approximately 9.5–10.5 ppm (distinct aldehydic resonance)

Q5. Which experimental 1H‑NMR observation confirms the presence of an exchangeable hydroxyl proton in menthol?

• The OH signal disappears after addition of D2O (D2O exchange)
• OH appears as a well‑resolved triplet coupling to adjacent methyls
• OH gives a large integration corresponding to three protons
• OH appears as a sharp singlet at 7–8 ppm

Correct Answer: The OH signal disappears after addition of D2O (D2O exchange)

Q6. In 13C‑NMR, the carbonyl carbon of an aldehyde (as in citral) typically resonates in which chemical shift range?

• Approximately 190–200 ppm
• Approximately 160–170 ppm
• Approximately 120–140 ppm
• Approximately 60–80 ppm

Correct Answer: Approximately 190–200 ppm

Q7. Which feature in the mass spectrum of menthol commonly indicates dehydration (loss of H2O)?

• A prominent M‑18 peak corresponding to [M − H2O]+
• An M‑28 peak corresponding to loss of CO
• A base peak at m/z 91 (tropylium ion)
• No fragmentation due to high stability of M+

Correct Answer: A prominent M‑18 peak corresponding to [M − H2O]+

Q8. The methyl groups of an isopropyl fragment (CH(CH3)2) in 1H‑NMR typically appear as which splitting pattern?

• Doublet (due to coupling with the methine proton)
• Triplet (coupling with two equivalent protons)
• Quartet (coupling with three protons)
• Singlet (no coupling)

Correct Answer: Doublet (due to coupling with the methine proton)

Q9. Which 13C‑NMR DEPT experiment differentiates CH, CH2 and CH3 with CH2 signals appearing negative?

• DEPT‑135 (CH and CH3 positive; CH2 negative)
• DEPT‑90 (shows only CH2)
• DEPT‑45 (shows only quaternary carbons)
• NOESY (gives proton–proton spatial correlations)

Correct Answer: DEPT‑135 (CH and CH3 positive; CH2 negative)

Q10. How does conjugation with a C=C affect the IR stretching frequency of a C=O group?

• It lowers the C=O stretching frequency (bathochromic shift in IR)
• It raises the C=O stretching frequency significantly
• It has no effect on the C=O stretching frequency
• It converts the C=O stretch into a C–O stretch only

Correct Answer: It lowers the C=O stretching frequency (bathochromic shift in IR)

Q11. Which technique most directly provides the accurate molecular weight and isotopic pattern to confirm a molecular formula of citral?

• Mass spectrometry (high‑resolution MS gives exact mass and isotopic pattern)
• IR spectroscopy (gives fingerprint region only)
• UV‑Vis spectroscopy (gives conjugation info but not molecular weight)
• 1H‑NMR (gives proton environment but not exact molecular weight)

Correct Answer: Mass spectrometry (high‑resolution MS gives exact mass and isotopic pattern)

Q12. In 1H‑NMR, which coupling constant (J value) between vinyl protons indicates a trans (E) double bond as present in geranial (citral isomer)?

• Approximately 15–17 Hz (large trans coupling)
• Approximately 6–8 Hz (typical vicinal aliphatic)
• Approximately 0–2 Hz (geminal coupling only)
• Approximately 2–3 Hz (allylic coupling)

Correct Answer: Approximately 15–17 Hz (large trans coupling)

Q13. Which IR absorption is diagnostic for a terminal alkene =C–H stretching vibration often seen in monoterpenes?

• Weak band near 3050 cm‑1 (=C–H stretch)
• Strong broad band at 3300 cm‑1 (O–H stretch)
• Sharp band at 2250 cm‑1 (C≡N stretch)
• Strong band near 1740 cm‑1 (ester C=O)

Correct Answer: Weak band near 3050 cm‑1 (=C–H stretch)

Q14. Which combined 13C‑NMR observations would best support that carvone contains a conjugated ketone?

• A downfield carbonyl resonance around 185–200 ppm together with sp2 carbons between 120–140 ppm
• Carbonyl at 160 ppm and aliphatic carbons at 10–30 ppm only
• All carbons clustered between 0–50 ppm indicating saturation
• A single peak at 77 ppm only (solvent artifact)

Correct Answer: A downfield carbonyl resonance around 185–200 ppm together with sp2 carbons between 120–140 ppm

Q15. Which spectroscopic feature allows differentiation of the geranial (E) vs neral (Z) isomers of citral?

• 1H‑NMR vinyl proton coupling constants (large J for E, smaller J for Z)
• Infrared fingerprint region only
• Mass spectrometry identical fragmentation, cannot distinguish
• UV‑Vis identical λmax for both isomers always

Correct Answer: 1H‑NMR vinyl proton coupling constants (large J for E, smaller J for Z)

Q16. To establish relative stereochemistry of substituents in menthol, which NMR experiment is most useful?

• NOESY or 1D‑NOE experiments to observe spatial (through‑space) correlations
• IR spectroscopy to measure OH stretching frequency
• UV‑Vis to detect chromophores for stereochemistry
• Simple 13C‑NMR without 2D experiments

Correct Answer: NOESY or 1D‑NOE experiments to observe spatial (through‑space) correlations

Q17. According to the nitrogen rule in mass spectrometry, an organic compound containing oxygen and no nitrogen will typically have what characteristic for the molecular ion?

• An even mass molecular ion (even m/z)
• An odd mass molecular ion (odd m/z)
• No detectable molecular ion
• Always a doubly charged molecular ion

Correct Answer: An even mass molecular ion (even m/z)

Q18. In a DEPT‑135 13C experiment which type of carbon signal appears with negative phase (inverted)?

• CH2 carbons appear negative (inverted)
• CH3 carbons appear negative
• Quaternary carbons appear negative
• All carbons appear positive

Correct Answer: CH2 carbons appear negative (inverted)

Q19. How does conjugation of a carbonyl (as in carvone) influence its UV absorption compared to a non‑conjugated ketone?

• Conjugation causes a bathochromic (red) shift to longer λmax and generally increased intensity
• Conjugation eliminates UV absorption completely
• Conjugation causes a hypsochromic (blue) shift to shorter λmax
• Conjugation changes only the IR signals, not UV absorption

Correct Answer: Conjugation causes a bathochromic (red) shift to longer λmax and generally increased intensity

Q20. Which combination of spectral observations most directly confirms the presence of an aldehyde functional group in citral?

• A strong IR C=O band near the aldehydic region plus a 1H‑NMR resonance around 9.5–10 ppm for the aldehydic proton
• An IR O–H broad band and a 1H‑NMR multiplet at 3.5 ppm
• A mass spectral base peak at m/z 91 only
• 13C‑NMR signals only between 10–40 ppm

Correct Answer: A strong IR C=O band near the aldehydic region plus a 1H‑NMR resonance around 9.5–10 ppm for the aldehydic proton

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