Which one of the following compounds (1 to 4) corresponds to the IR spectrum shown? (you can refer to a table of characteristic absorptions) Spectra reproduced with permission from NIST

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Question restatement:
- The prompt asks which one of the following compounds (1 to 4) corresponds to the IR spectrum shown in the image. The answer choices are labeled a. 1, b. 2, c. 3, d. 4. The actual identities of compounds 1–4 are not provided in the prompt, and the IR spectrum image is given without accompanying functional-group annotations.

Initial observations about the spectrum:
- The IR plot (transmittance vs wavenumber) features a dense pattern of multiple absorption bands across the fingerprint region (roughly 1500–500 cm⁻¹) and several notable bands in the higher-frequency region (around 3200–3600 cm⁻¹ or 2600–3300 cm⁻¹, if present). The spectrum also shows intense absorptions at certain lower wavenumbers (below 1500 cm⁻¹) typical of fingerprint-region vibrations.
- Because the visual alone does not clearly reveal a single dominant peak type (for example, a clean, broad OH around 3200–3500 cm⁻¹ or a sharp carbonyl around ~1700 cm⁻¹), we cannot assign a definite functional group from this image alone without more precise peak positions and intensities.

General approach to match IR spectra to compounds (without knowing 1–4):
- Identify key diagnostic regions and assign possible functional groups based on peak positions and shapes, then compare with the expected features of each candidate compound.
- Common diagnostic signals include:
  - O–H broad stretch: ~3200–3600 cm⁻¹, broad for alcohols; broad and very strong if carboxylic acid O–H (~2500–3300 cm⁻¹ alongside a C=O around 1700 cm⁻¹).
  - N–H stretch: ~3300–3500 cm⁻¹ (sharp or broad depending on h-bonding; one or two bands if imine/amide present).
  - C=O stretch: ~1650–1750 cm⁻¹ (strong and sharp for ketones, aldehydes, esters, carboxylic acids); conjugation lowers the frequency slightly.
  - C≡C and C≡N stretches: ≈2100–2260 cm⁻¹ (usually weaker than carbonyls).
  - C–H stretches: 2850–3100 cm⁻¹ (alkane, alkenyl, arenyl C–H, with slight variations).
- The fingerprint region (below ~1500 cm⁻¹) contains many peaks that help distinguish similar compounds, but it is often specific to a compound’s entire skeleton rather than a single functional group.

Option-by-option reasoning (without compound identities):
- Option a (1): If compound 1 contains a carbonyl group (C=O) and shows a strong absorption near 1700 cm⁻¹, we would expect a clear, intense peak in that region. If the spectrum provided lacks a strong 1700 cm⁻¹ band, this would argue against a carbonyl-containing candidate for that option. Without peak-specific data, we can only note: does the spectrum display a carbonyl signature? If yes, option 1 could be plausible; if not, it is unlikely.
- Option b (2): If compound 2 is expected to have an O–H group (alcohol) or a broad O–H/C–O region around 3200–3600 cm⁻¹, check for a broad band in that region. A sharp, absent, or very weak broad feature would argue against an alcohol. If the spectrum shows a broad strong band there, option 2 becomes more plausible.
- Option c (3): If compound 3 contains an amide (N–H around 3300–3500 cm⁻¹ and C=O near 1650–1700 cm⁻¹), you would look for both a carbonyl signal and an N–H band. If the spectrum lacks these characteristics, this option is less likely. Conversely, presence of both signals would support option 3.
- Option d (4): If compound 4 is characterized by a nitrile (C≡N stretch around 2210–2260 cm⁻¹) or a conjugated system (lowered C=O frequency due to conjugation), you would search for the corresponding absorptions. Absence of a nitrile band would argue against options containing nitrile functionality; presence would support it.

Important caveat:
- Because the actual identities of compounds 1–4 are not provided here, and the IR spectrum image does not come with precise peak assignments, a definitive match cannot be made from the information given alone. In practice, you would:
  - Tabulate the observed strong bands and their approximate wavenumbers from the spectrum image (noting peak positions and shapes).
  - Compare these observed features with the known IR signatures of each candidate compound (1–4) to see which one aligns best.
  - Consider the presence or absence of diagnostic bands (e.g., carbonyl ~1700 cm⁻¹, broad O–H, sharp N–H, nitrile ~2220 cm⁻¹) and the overall pattern in the fingerprint region to rule out incompatible options.

Summary guidance for using this prompt:
- To make a choice, you would need the explicit peak positions from the spectrum and the explicit structures of compounds 1–4. With those, you could justify each option by matching or mismatching key IR features, while noting any uncertainties due to spectral overlap or vibrations in the fingerprint region.
- If you can provide the wavenumber values or a table of observed peaks from the spectrum, along with the identities of compounds 1–4, I can give a precise option-by-option determination.

MUF0042 Chemistry Unit 2 - Semester 2, 2025 8.12 Quiz: IR Spectroscopy

Which one of the following compounds (1 to 4) corresponds to the IR spectrum shown? (you can refer to a table of characteristic absorptions) Spectra reproduced with permission from NIST

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