Question 8 of 13: IR-Spectrum Matching for Six Isomeric/Related C–H, O, and Functional Classes

Question 8: IR-Spectrum Matching for Six Isomeric/Related C–H, O, and Functional Classes

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. Sort by which diagnostic group-frequency region is (or is not) present: O–H stretch (broad, 2500–3300 for an acid vs. 3200–3550 for an alcohol), C=O stretch (1700–1750), aromatic ring overtone/fingerprint bands (weak combination bands 1660–2000, plus sharp fingerprint peaks 1600–1450 and 900–690), and a strong C–O stretch region (1050–1300) for ethers/esters. Compounds lacking any O–H, C=O, or aromatic ring (the plain alkene B) show only C–H stretches plus a comparatively simple fingerprint.

1. Spectrum [5] → A (pentanoic acid). A very broad, deep absorption spanning roughly 3600 down through 2500 cm⁻¹ (the classic carboxylic-acid O–H, broadened far beyond an alcohol's O–H by strong hydrogen-bonded dimerisation) overlapping the C–H stretches, plus a strong, sharp carbonyl dip near 1710 cm⁻¹, is diagnostic for –COOH and matches no other candidate.
2. Spectrum [2] → F (3-ethyl-3-pentanol). A broad but comparatively narrower O–H hump centred near 3200–3400 cm⁻¹ (alcohol, not acid), with no carbonyl band, plus strong C–O fingerprint absorption — the only other O–H-bearing candidate, and F has no carbonyl to confuse it with A.
3. Spectrum [6] → E (tert-butyl acetate). A strong, sharp carbonyl near 1740 cm⁻¹ (ester, slightly higher than a ketone/acid) together with strong, often doubled C–O stretching bands in the 1150–1250 cm⁻¹ region (especially intense for a tert-butyl ester) and no O–H at all identifies the ester E.
4. Spectrum [4] → C (cumene). No O–H, no carbonyl, but a dense cluster of sharp fingerprint bands through 1600–1450 cm⁻¹ (aromatic ring C=C stretches) plus out-of-plane bending bands below 900 cm⁻¹ mark the monosubstituted benzene ring — this pattern is unique to the one aromatic compound in the set.
5. Spectrum [3] → B (2-methyl-1-hexene). No O–H, no carbonyl, no aromatic ring bands; instead a simpler but still busy fingerprint with a moderate C=C stretch (~1650 cm⁻¹, often weak) and a strong 1,1-disubstituted-alkene out-of-plane =CH₂ wag near 890 cm⁻¹ — the terminal-alkene signature that distinguishes it from the fully saturated ether.
6. Spectrum [1] → D (diisopropyl ether). By elimination: no O–H, no carbonyl, no aromatic bands, no alkene C=C/oop bend — only C–H stretches and a strong, simple C–O–C stretch near 1100 cm⁻¹ plus the isopropyl CH₃ doublet bend near 1380/1365 cm⁻¹, giving the least cluttered fingerprint of the whole set.