Final answer:
Bonds with IR-active stretching frequencies change the dipole moment during vibration. Alkynes and terminal alkenes show characteristic absorbances in IR spectra, aiding in the identification of molecular structures.
Step-by-step explanation:
Bonds that are expected to have infrared (IR)-active stretching frequencies are those that result in a change in the dipole moment of the molecule when they vibrate. In the context of IR spectroscopy, covalent bonds behave like springs that can stretch, bend, and twist. The stretching frequencies are particularly useful for distinguishing compounds.
For instance, the C-C and C-H stretching frequencies can be used to distinguish between the constitutional isomers 1,2-dimethylcyclohexene, 1,3-octadiene, 3-octyne, and 1-octyne. Alkynes, like 3-octyne and 1-octyne, exhibit characteristic IR absorption peaks in the region of 2100-2250 cm¹. Meanwhile, terminal alkenes have absorbances around 3300 cm¹. These stretching vibrations are IR active because they cause periodic changes in molecular dipole moments.
On the other hand, completely symmetrical double and triple bonds may not be IR active if their vibrations do not result in a change in the dipole moment. Thus, while identifying molecular structures using IR spectra, understanding which bonds will show active stretching is crucial.