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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To determine which compound (1–4) corresponds to the given infrared (IR) spectrum, we first restate what the spectrum typically reveals about functional groups and how to compare that to candidate structures.

Key approach:
- Identify broad, strong absorptions indicative of O–H groups (usually around 3200–3600 cm−1, often broad for carboxylic acids) or N–H stretches (about 3300–3500 cm−1, sometimes with fine structure).
- Look for a sharp, strong peak around 1650–1750 cm−1 for carbonyl groups (C=O) such as ketones, aldehydes, esters, acids, or amides (the exact position and shape can help distinguish among them).
- Observe C–H stretching regions (approximately 2850–2960 cm−1) for aliphatic CH, and around 3000 cm−1 for sp2 C–H in alkenes/aromatics.
- Examine the fingerprint region (roughly 600–1500 cm−1) for characteristic patterns that can differentiate among similar functionalities, such as esters vs. ethers, or aromatic substitution patterns.

Option-by-option reasoning framework (without the explicit options at hand):
- If the spectrum shows a broad, intense band spanning roughly 2500–3300 cm−1, this strongly suggests a carboxylic acid O–H in addition to a C=O around 1700 cm−1. In such a case, the correct structure would need to contain a carboxyl group, and other peaks should align with typical acid derivatives (e.g., broad O–H plus C=O). If a candidate lacks a carboxyl moiety, that candidate can be ruled out.
- If there is a distinct sharp band near 1715–1750 cm−1 without a broad O–H, a carbonyl-containing compound (like a ketone, aldehyde, ester, or acid chloride) is implicated. The presence or absence of other bands (e.g., C–O stretches around 1050–1300 cm−1 for esters; aldehyde C–H stretches near 2720 cm−1) helps distinguish among carbonyl subclasses.
- The presence of strong bands near 1600 cm−1 and 1500 cm−1 could indicate aromatic rings (benzene C=C stretches). If an aromatic pattern is evident, candidates containing aromatic rings must be favored, while aliphatic-only structures would be disfavored unless other evidence supports them.
- If the spectrum lacks broad O–H and carbonyl signals, the compound might be an alcohol, amine, ether, alkane, or alkene. In such cases, use the combination of C–O stretches (~1050–1300 cm−1), absence/presence of N–H stretches (~3200–3500 cm−1), and C=C stretches (~1640–1680 cm−1 for alkenes) to narrow down.

Common pitfalls to avoid:
- Assuming a molecule has a carbonyl merely because there is a peak near 1700 cm−1; verify that the peak is not overlapped with other absorptions and that the overall pattern matches a carbonyl-containing compound (e.g., look for accompanying C–O or C–N bands).
- Overlooking broad O–H signals that can be masked by sample conditions (e.g., hydrogen bonding) or concentration effects; always correlate with the rest of the spectrum.
- Ignoring the fingerprint region; sometimes two very similar-looking spectra diverge in this region due to subtle differences in functional groups or substitution patterns.

In a typical multiple-choice setting, you would compare the actual peaks for each candidate structure to the observed spectrum. The correct choice would be the one whose characteristic absorptions (O–H, C=O, C–O, C–N, aromatic C=C, C≡C, etc.) align with the observed features, while all other options would fail to account for one or more prominent bands.

If you can share the specific options (1–4) or their structural formulas, I can map each key absorption to the corresponding functional group and explain exactly why the spectrum supports one particular option over the others, step by step.

MUF0042 Chemistry Unit 2 - Semester 1, 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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