Where might the following compounds have IR absorptions?

Question

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 ([latex]cm^{-1}[/latex])	Intensity	Functional Group	Absorption ([latex]cm^{-1}[/latex])	Intensity
Alkane C–H	2850–2960	Medium	Amine N–H C–N	3300–3500 1030–1230	Medium Medium
Alkene =C–H C=C	3020–3100 1640–1680	Medium Medium	Carbonyl compound C=O Aldehyde Ketone Ester Amide Carboxylic acid	1670–1780 1730 1715 1735 1690 1710	Strong Strong Strong Strong Strong Strong
Alkyne C–H C[latex]\equiv[/latex] C	3300 2100–2260	Strong Medium	Carboxylic acid O–H	2500–3100	Strong, broad
Alkyl halide C–Cl C–Br	600–800 500–600	Strong Strong	Nitrile C[latex]\equiv[/latex] N	2210–2260	Medium
Alcohol O–H C–O	3400–3650 1050–1150	Strong, broad Strong	Nitro NO2	1540	Strong
Arene C–H Aromatic ring	3030 1660–2000 1450–1600	Weak Weak Medium

Accepted Answer

(a) Absorptions: 3400–3650 [latex]cm^{-1}[/latex] (O–H), 3020–3100 [latex]cm^{-1}[/latex] (=C–H), 1640–1680 [latex]cm^{-1}[/latex] (C=C). This molecule has an alcohol O–H group and an alkene double bond.

(b) Absorptions: 3300 [latex]cm^{-1}[/latex] ([latex]\equiv C-H [/latex]) 2100–2260 [latex]cm^{-1}[/latex] ([latex]C \equiv C [/latex]), 1735 [latex]cm^{-1}[/latex] (C=O). This molecule has a terminal alkyne triple bond and a saturated ester carbonyl group.

Question 10.5: Where might the following compounds have IR absorptions?

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