Quiz - Infra-Red Spectroscopy

Oct 14, 2026

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Infra-Red Spectroscopy quiz questions covering how to use infrared wavenumber ranges to distinguish functional groups and interpret spectra. Problems include assigning characteristic O–H, C=O, C=C, C≡N, and N–H wavenumber bands, matching spectra to named compounds, and discussing limitations such as overlapping peaks and intermolecular interactions. Further sections require empirical and molecular formula calculations from combustion and molar volume data, then deducing possible structures from IR features, including consideration of enantiomers.

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Quiz (Infra-Red Spectroscopy)

Quiz (Infra-Red Spectroscopy) 1. Give the characteristic wavenumber ranges in the infrared spectrum that would enable you to distinguish between each of the following pairs of compounds.’(a) and (b) and (c) and 2. By using the correlation table in notes, interpret the following infrared spectra. (a) Cyclohexane | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2860-2940 | | (b) 3,3-dimethylbut-1-ene | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 1640 | | | 2860 – 3080 | | (c) Butanenitrile | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2250 | | | 2880 – 2980 | | (d) Butan-2-ol | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 3340 | | | 2880 – 2980 | | (e) Butanamine | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2880 – 2960 | | | 3370 | | (f) Butanoic acid | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 1720 | | | 2880 – 2980 | | | 3100 | | 3. Compound A, with a relative molecular mass of 46.0, has the following composition by mass: C: 52. 18%, H: 13.04%, O: 34. 78% (a) Determine the molecular formula of compound A. (b) Based on the above information, deduce the structures of compounds A and B. (Relative atomic masses: H = 1.0; C = 12.0; O = 16.0) 4. Triolein is a triglyceride derived from glycerol and three units of oleic acid. It can be hydrolysed to form oleic acid. Hydrogenation of oleic acid produces stearicacid. The structural formulae of triolein, oleic acid and stearic acid are shown below: Below is the infrared spectrum of one of the above three compounds. (a) To which of the compounds the above infrared spectrum belongs? Explain your answer. (b) State TWO limitations of using infrared spectroscopy in identification of functional groups. 5. The infrared spectrum of compound Y (with the molecular formula of C4 H8O2) is shown below. Deduce the possible structures of compound Y. 6. Compound X contains carbon, hydrogen and oxygen only. 9.20 g of compound X burns completely in air to form 17.6 g of carbon dioxide and 10.8 g of water. On vaporization, 2.30 g of compound X is found to occupy 1200 cm3 at room temperature and pressure. (Relative atomic masses: H = 1.0; C = 12.0; O = 16.0) (Molar volume of gas at room temperature and pressure = 24.0 dm3 mol–1 ) (a) Determine the empirical formula of compound X. (b) Determine the molecular mass of compound X. (c) Determine the molecular formula of compound X. (d) The IR spectrum of compound X is shown below: Deduce the possible structure of compound X. Remark: Do you think you can identify the following enantiomers with IR spectroscopy? Since the atoms in enantiomers are linked together in the same order, their infrared spectra should look the same. Suggested Answer 1. (a) Characteristic wavenumber range 3230 – 3670 cm-1 for O⎯H bond in alcohol; 1680 – 1800 cm-1 for C=O bond. (b) Characteristic wavenumber range 2500 – 3300 cm-1 for O⎯H bond in carboxylic acid; 3230 – 3670 cm-1 for O⎯H bond in alcohol. (c) Characteristic wavenumber range 3350 – 3500 cm-1 for N⎯H bond; 1680 – 1800 cm-1 for C=O bond. 2. (a) Cyclohexane | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2860-2940 | C⎯H bond | (b) 3,3-dimethylbut-1-ene | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 1640 | C=C bond | | 2860 – 3080 | C⎯H bond | (c) Butanenitrile | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2250 | C≡N bond | | 2880 – 2980 | C⎯H bond | (d) Butan-2-ol | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 3340 | O⎯H bond | | 2880 – 2980 | C⎯H bond | (e) Butanamine | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 2880 – 2960 | C⎯H bond | | 3370 | N⎯H bond | (f) Butanoic acid | Wavenumber (cm-1 ) | Interpretation | | --- | --- | | 1720 | C=O bond | | 2880 – 2980 | C⎯H bond | | 3100 | O⎯H bond | 3. (a) Let the mass of compound A be 100 g. Thus, the mass of carbon in the compound = 52.18 g the mass of hy

FAQs

How can infrared characteristic wavenumber ranges help distinguish alcohols, carboxylic acids, and nitriles?

Alcohols are identified by O–H stretching bands (about 3230–3670 cm⁻1). Carboxylic acids also show O–H bands but typically include additional ranges (about 2500–3300 cm⁻1) and C=O bands (about 1680–1800 cm⁻1). Nitriles are indicated by a strong C≡N band near 2250 cm⁻1.

What limitations can make IR spectroscopy difficult for identifying functional groups?

Overlapping absorption peaks can complicate interpretation. Intermolecular interactions can broaden and alter the spectrum, producing patterns that are not straightforward to assign to a single functional group.

How are empirical and molecular formulas of compound X determined from combustion data?

Empirical formula comes from converting masses of CO₂ and H₂O into moles of C and H, then using the given O by difference. The molecular mass is found using gas volume and molar volume (24.0 dm³ mol⁻1), and the molecular formula is then obtained by scaling the empirical formula.

Oct 14, 2026

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