Question 10.5: Predicting IR Absorptions of Compounds Where might the follo...
Predicting IR Absorptions of Compounds
Where might the following compounds have IR absorptions?
Strategy
Identify the functional groups in each molecule, and check Table 10.1 to see where those groups absorb.
| TABLE 10.1 | |||||
| Characteristic IR Absorptions of Some Functional Groups | |||||
| Functional group | Absorption (cm^{-1}) | Intensity | Functional group | Absorption (cm^{-1}) | Intensity |
| Alkane | Amine | ||||
| C-H | 2850–2960 | Medium | N-H | 3300–3500 | Medium |
| Alkene | C-N | 1030–1230 | Medium | ||
| =C-H | 3020–3100 | Medium | Carbonyl | ||
| C=C | 1640–1680 | Medium | compound | ||
| Alkyne | C=O | 1670–1780 | Strong | ||
| \equiv C-H | 3300 | Strong | Carboxylic acid | ||
| C\equiv C | 2100–2260 | Medium | O-H | 2500–3100 | Strong, broad |
| Alkyl halide | Nitrile | ||||
| C-Cl | 600–800 | Strong | C=N | 2210–2260 | Medium |
| C-Br | 500–600 | Strong | Nitro | ||
| Alcohol | NO_{2} | 1540 | Strong | ||
| O-H | 3400–3650 | Strong, broad | |||
| C-O | 1050–1150 | Strong | |||
| Arene | |||||
| C-H | 3030 | Weak | |||
| Aromatic ring | 1660–2000 | Weak | |||
| 1450–1600 | Medium | ||||
(a) This molecule has an alcohol O–H group and an alkene double bond. Absorptions: 3400–3650 cm^{-1} (O–H), 3020–3100 cm^{-1} (=C–H), 1640–1680 cm^{-1} (C=C).
(b) This molecule has a terminal alkyne triple bond and a saturated ester carbonyl group. Absorptions: 3300 cm^{-1} (\equiv C–H), 2100–2260 cm^{-1} (C\equiv C), 1735 cm^{-1} (C=O).
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