INTOCHEMISTRY / FREE PRACTICE LAB
Spectra Practice Lab: IR, NMR & Mass Spectrometry
Interpret original simulated spectra, choose plausible structures, and check the evidence with explanations. Practice IR, proton NMR, carbon-13 NMR, combined spectroscopy and introductory EI mass spectrometry—all without an account or purchase.
Learning Center · Learn IR · Learn ¹H NMR · IR/GC Evidence Studio
Alcohol or carbonyl?
Provided molecular formula: C₃H₈O
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~3350 cm⁻¹ | strong, broad |
| ~2970 cm⁻¹ | medium |
| ~1110 cm⁻¹ | strong |
Show a hint
Examine the broad high-wavenumber absorption and check for a strong carbonyl.
Show worked explanation
Correct answer: A. Propan-2-ol
A broad O–H absorption near 3350 cm⁻¹ with a C–O band near 1110 cm⁻¹, but no strong C=O, supports propan-2-ol among these choices.
Identify the ketone
Provided molecular formula: C₃H₆O
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~1715 cm⁻¹ | very strong |
| ~2965 cm⁻¹ | medium |
| ~1360 cm⁻¹ | medium |
Show a hint
The strongest absorption lies near 1715 cm⁻¹.
Show worked explanation
Correct answer: B. Propanone
A strong saturated-ketone C=O band near 1715 cm⁻¹ and no broad O–H band favor propanone. The formula excludes the ester and acid options.
An ester signature
Provided molecular formula: C₄H₈O₂
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~1740 cm⁻¹ | very strong |
| ~1240 cm⁻¹ | strong |
| ~1040 cm⁻¹ | strong |
Show a hint
A strong C=O is accompanied by two strong C–O region peaks.
Show worked explanation
Correct answer: C. Ester
The strong ~1740 cm⁻¹ C=O plus strong C–O fingerprint bands near 1240 and 1040 cm⁻¹ favor an ester. There is no very broad carboxylic-acid O–H envelope.
Broad acid O–H
Provided molecular formula: C₂H₄O₂
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| 3300–2500 cm⁻¹ | very broad envelope |
| ~1710 cm⁻¹ | very strong |
| ~1280 cm⁻¹ | strong |
Show a hint
The very broad 3300–2500 cm⁻¹ absorption is a distinctive clue.
Show worked explanation
Correct answer: B. Carboxylic acid
The exceptionally broad O–H envelope spanning approximately 3300–2500 cm⁻¹ together with a strong ~1710 cm⁻¹ carbonyl supports a carboxylic acid.
Triple-bond region
Provided molecular formula: C₂H₃N
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~2250 cm⁻¹ | strong, sharp |
| ~2990 cm⁻¹ | medium |
Show a hint
Think about a nitrogen-containing multiple bond.
Show worked explanation
Correct answer: A. Nitrile
A fairly sharp absorption around 2250 cm⁻¹, together with the nitrogen-containing formula, is consistent with a C≡N nitrile stretching vibration.
Weak doublet by a carbonyl
Provided molecular formula: C₇H₆O
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~1700 cm⁻¹ | very strong |
| ~2820 and 2720 cm⁻¹ | weak pair |
| ~1600 cm⁻¹ | medium |
Show a hint
The weak pair is associated with the aldehyde C–H vibration.
Show worked explanation
Correct answer: C. Benzaldehyde
An aromatic carbonyl plus weak aldehydic C–H absorptions near 2820 and 2720 cm⁻¹ support benzaldehyde. Acetophenone would not show an aldehydic C–H pair.
Terminal alkyne evidence
Provided molecular formula: C₆H₁₀
Infrared spectrum
| Wavenumber | Shape / intensity |
|---|---|
| ~3300 cm⁻¹ | sharp, strong |
| ~2115 cm⁻¹ | weak–medium |
| ~2950 cm⁻¹ | medium |
Show a hint
The ~3300 cm⁻¹ peak is narrow, unlike a hydrogen-bonded O–H.
Show worked explanation
Correct answer: A. Terminal alkyne
A sharp ≡C–H stretch around 3300 cm⁻¹ and a weaker C≡C absorption near 2115 cm⁻¹ are consistent with a terminal alkyne.
One proton environment
Provided molecular formula: C₃H₆O
Proton NMR spectrum
| Chemical shift | Multiplicity / integration |
|---|---|
| δ 2.12 ppm | singlet, 6H |
Show a hint
How many different hydrogen environments does a symmetric ketone have?
Show worked explanation
Correct answer: A. Propanone
Acetone contains two equivalent methyl groups on either side of C=O. They produce one ¹H NMR singlet at approximately 2.1 ppm with a 6H integral.
The ethyl acetate pattern
Provided molecular formula: C₄H₈O₂
Proton NMR spectrum
| Chemical shift | Multiplicity / integration |
|---|---|
| δ 4.12 ppm | quartet, 2H |
| δ 2.05 ppm | singlet, 3H |
| δ 1.26 ppm | triplet, 3H |
Show a hint
A quartet and triplet can indicate an ethyl group, but a 4.12 ppm quartet is adjacent to oxygen.
Show worked explanation
Correct answer: C. Ethyl acetate
The 4.12 ppm quartet/1.26 ppm triplet pair identifies OCH₂CH₃; a 2.05 ppm methyl singlet identifies CH₃C(O)–. Together they form ethyl acetate.
Doublet and septet
Provided molecular formula: C₃H₈O
Proton NMR spectrum
| Chemical shift | Multiplicity / integration |
|---|---|
| δ 3.93 ppm | septet, 1H |
| δ 1.18 ppm | doublet, 6H |
| O–H | not shown; variable |
Show a hint
Six equivalent methyl H split into doublets by one methine H.
Show worked explanation
Correct answer: B. Propan-2-ol
The 6H doublet and 1H septet indicate two equivalent CH₃ groups coupled to one O-substituted CH. The formula fits propan-2-ol. The variable O–H resonance is omitted.
A monosubstituted benzene
Provided molecular formula: C₇H₈
Proton NMR spectrum
| Chemical shift | Multiplicity / integration |
|---|---|
| δ 7.1–7.3 ppm | multiplet, 5H |
| δ 2.32 ppm | singlet, 3H |
Show a hint
The 3H singlet occurs near 2.3 ppm, not at a methoxy shift.
Show worked explanation
Correct answer: C. Toluene
Five aromatic hydrogens and one methyl singlet near 2.3 ppm match toluene. Ethylbenzene gives a quartet/triplet pair rather than a methyl singlet.
A methyl ketone and an ethyl group
Provided molecular formula: C₄H₈O
Proton NMR spectrum
| Chemical shift | Multiplicity / integration |
|---|---|
| δ 2.43 ppm | quartet, 2H |
| δ 2.14 ppm | singlet, 3H |
| δ 1.02 ppm | triplet, 3H |
Show a hint
A 3H singlet at 2.14 ppm is consistent with a methyl next to a carbonyl.
Show worked explanation
Correct answer: A. 2-Butanone
2-Butanone has a CH₃CO singlet at ~2.14 ppm and CH₃CH₂– quartet/triplet at ~2.43/1.02 ppm. The formula and integration agree.
Only two carbon environments
Provided molecular formula: C₃H₆O
Carbon-13 NMR spectrum
| Chemical shift | Region |
|---|---|
| δ 205 ppm | carbonyl region |
| δ 30 ppm | alkyl region |
Show a hint
The two methyl carbons are symmetry-equivalent in the right structure.
Show worked explanation
Correct answer: A. Propanone
Propanone has a ketone carbonyl near 205 ppm and one type of symmetry-equivalent methyl carbon near 30 ppm. Propanal would show three carbon environments.
Carbonyl plus O–CH₂
Provided molecular formula: C₄H₈O₂
Carbon-13 NMR spectrum
| Chemical shift | Region |
|---|---|
| δ 171 ppm | carbonyl region |
| δ 60 ppm | C bonded to O |
| δ 21 ppm | alkyl region |
| δ 14 ppm | alkyl region |
Show a hint
The resonance around 60 ppm is a carbon directly bonded to oxygen.
Show worked explanation
Correct answer: C. Ethyl acetate
Ethyl acetate has ester carbonyl ~171, O–CH₂ ~60, acetyl CH₃ ~21, and terminal ethyl CH₃ ~14 ppm.
Aromatic aldehyde
Provided molecular formula: C₇H₆O
IR spectrum
¹H NMR spectrum
| Technique / position | Observation |
|---|---|
| IR ~1701 cm⁻¹ | very strong |
| IR ~2820/2720 cm⁻¹ | weak pair |
| ¹H δ 9.96 | singlet, 1H |
| ¹H δ 7.2–7.9 | multiplet, 5H |
Show a hint
The NMR resonance near 10 ppm is key.
Show worked explanation
Correct answer: B. Benzaldehyde
The aldehyde IR C–H pair near 2820/2720 cm⁻¹, carbonyl near 1700 cm⁻¹, and NMR singlet near 10 ppm (1H) with 5 aromatic H indicate benzaldehyde.
Conjugated ketone and aromatic ring
Provided molecular formula: C₈H₈O
IR spectrum
¹H NMR spectrum
| Technique / position | Observation |
|---|---|
| IR ~1685 cm⁻¹ | strong carbonyl |
| ¹H δ 7.4–8.0 | multiplet, 5H |
| ¹H δ 2.61 | singlet, 3H |
Show a hint
An aromatic 5H multiplet and 3H singlet at 2.6 ppm suggest C₆H₅COCH₃.
Show worked explanation
Correct answer: B. Acetophenone
Acetophenone has conjugated aryl ketone C=O near 1685 cm⁻¹, acetyl CH₃ singlet near 2.61 ppm, and 5 aromatic hydrogens.
Ester with an ethyl unit
Provided molecular formula: C₄H₈O₂
IR spectrum
¹H NMR spectrum
| Technique / position | Observation |
|---|---|
| IR ~1740 cm⁻¹ | very strong |
| IR ~1240/1040 cm⁻¹ | strong |
| ¹H δ 4.12 | quartet, 2H |
| ¹H δ 2.05 | singlet, 3H |
| ¹H δ 1.26 | triplet, 3H |
Show a hint
A 4.12 ppm quartet and 1.26 ppm triplet represent an OCH₂CH₃ group.
Show worked explanation
Correct answer: D. Ethyl acetate
Ester IR bands near 1740, 1240 and 1040 cm⁻¹ combine with ethyl acetate's OCH₂ quartet (4.12), CH₃C(O) singlet (2.05), and terminal ethyl CH₃ triplet (1.26).
Molecular ion at m/z 58
Provided molecular formula: C₃H₆O
Electron ionization mass spectrum
| m/z | Relative intensity |
|---|---|
| m/z 58 | molecular ion, 39% |
| m/z 43 | base peak, 100% |
| m/z 15 | minor fragment, 20% |
Show a hint
Calculate the molecular mass of each option before interpreting the fragment.
Show worked explanation
Correct answer: C. Propanone (M=58)
Acetone has nominal molecular mass 58 and can form an acylium fragment at m/z 43 in electron ionization. Mass spectra alone do not always uniquely prove structure.
Molecular ion at m/z 88
Provided molecular formula: C₄H₈O₂
Electron ionization mass spectrum
| m/z | Relative intensity |
|---|---|
| m/z 88 | molecular ion, 18% |
| m/z 43 | base peak, 100% |
| m/z 61 | fragment, 26% |
Show a hint
The molecular ion gives nominal mass; look for an acetyl-containing candidate.
Show worked explanation
Correct answer: A. Ethyl acetate (M=88)
Ethyl acetate has nominal mass 88 and can produce an abundant m/z 43 acetyl fragment. The simulated EI spectrum shows selected ions only, not a complete experimental measurement.
About the spectra: These are original, calculated schematic plots for teaching, not experimental measurements or copies of published spectra. Chemical shifts and IR positions are representative approximations; peak widths, intensities, NMR multiplet line separation and selected mass fragments are intentionally simplified. The accessible data table provides the interpreted input values independently of the graphic. Always evaluate all evidence.
More interactive spectroscopy practice
For a separate question bank with practice-mode explanations, use the existing IntoChemistry exam practice below. The IR/GC Evidence Studio also teaches how to combine chromatographic and spectroscopic evidence.
Interactive questions load when JavaScript is available. Download the printable study sets for offline review.
Study references and sources
Reference frequency and chemical-shift ranges are checked against OpenStax: Interpreting Infrared Spectra, OpenStax: ¹H NMR Chemical Shifts and OpenStax: Spin–Spin Splitting. These are source references, not reused spectrum artwork.