Final answer:
In IR spectroscopy, different bonds absorb at different wavenumbers. Absorbance just below 3000 cm−1 often indicates sp2 C-H stretching in alkanes. Benzene, which exhibits such absorbance due to its aromatic C-H stretch and lacks additional functional groups with strong absorbance outside this range, is the correct answer (a).
Step-by-step explanation:
Infrared Spectroscopy and IR Absorbance
He infrared (IR) absorbance of a compound can give valuable insights into which functional groups are present within a molecule. Specifically, different types of bonds absorb IR radiation at different characteristic frequencies, and these can be used to identify the underlying functional groups. For compounds like alkenes, alkynes, and those with various other functional groups, the IR spectrum will have distinctive strong peaks at certain wavenumbers.
An IR absorbance just below 3000 cm−1 would typically indicate the presence of C-H stretch vibrations of alkanes, which often appear as medium to strong bands. Considering the molecular structures and functional groups present in the answer choices, we can determine which compound is likely to show an IR absorbance in this range, exclusively.
Compounds such as benzene (option a) might exhibit absorbance just below 3000 cm−1 due to C-H stretch from the aromatic ring. However, benzene also has an aromatic C=C stretch that could give rise to additional IR peaks. Compounds with multiple bond types, like carbon-carbon double bonds (alkenes) or carbon-carbon triple bonds (alkynes), will show additional absorbance peaks outside of the C-H stretching range, often with alkynes showing characteristic peaks in the region of 2100-2250 cm−1.
Looking at the options given in the question and understanding that alkyne groups' C-H stretching occurs near 3300 cm−1, we can eliminate hex-1-yne, hex-3-yne, and benzonitrile because they contain triple bonds, which produce additional peaks in our specified region.
Similarly, compounds with ethers, like methyl phenyl ether, might show distinct peaks due to additional C-O stretching. In contrast, aromatic compounds without any additional functional groups that would lead to absorbance peaks outside of the specified range may only have absorbance just below 3000 cm−1 due to the C-H bonds in the aromatic ring. Hence, option (a) benzene is the correct answer as it is the only compound listed that will display a strong IR absorbance just below 3000 cm−1, confined to the sp2 C-H stretch vibrations of the aromatic ring, excluding the fingerprint region.