Predicting IR Absorptions of Compounds Where might the following compounds have IR absorptions?

Question

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 ([latex]cm^{-1}[/latex])	Intensity	Functional group	Absorption ([latex]cm^{-1}[/latex])	Intensity
Alkane			Amine
C-H	2850–2960	Medium	N-H	3300–3500	Medium
Alkene			C-N	1030–1230	Medium
[latex]=C-H[/latex]	3020–3100	Medium	Carbonyl
C=C	1640–1680	Medium	compound
Alkyne			C=O	1670–1780	Strong
[latex]\equiv C-H[/latex]	3300	Strong	Carboxylic acid
[latex]C\equiv C [/latex]	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			[latex]NO_{2} [/latex]	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

Accepted Answer

(a) This molecule has an alcohol O–H group and an alkene double bond. 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).
(b) This molecule has a terminal alkyne triple bond and a saturated ester carbonyl group. 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).

Question 10.5: Predicting IR Absorptions of Compounds Where might the follo...

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