Final answer:
Benzonitrile is the compound with an IR absorbance just below 3,000 cm−1, which corresponds to the C-H stretching vibrations, excluding the fingerprint region and revealing the presence of its nitrile group.
Step-by-step explanation:
The compound that will only have an IR absorbance (strong) just below 3,000 cm−1, excluding the fingerprint region is benzonitrile. A key feature in infrared (IR) spectroscopy is that different functional groups have characteristic absorption frequencies. For example, the carbon-carbon triple bond of an alkyne absorbs in the range of 2100-2250 cm−1 due to stretching of the bond. This bond, however, is much less polar compared to carbonyl bonds, resulting in a weaker absorption.
In contrast, carbonyl groups are highly polar and exhibit strong IR absorption in the range of 1650-1750 cm−1. Therefore, to determine the functional group present in a molecule, one could examine the IR spectrum and look for these distinctive peaks. Benzonitrile, with its distinctive nitrile group, would have a strong absorbance just below 3000 cm−1 because of its C-H stretching vibrations close to the aromatic ring, and a weaker peak around 2200 cm−1 corresponding to the C≡N stretching vibration, excluding absorbances in the fingerprint region.