The structures and IR spectra for 1-butanol, 2-butanol and isobutyl alcohol are given below. As you can see, the three spectra look very similar in the area above the fingerprint region (above 1500 cm-1). Explain why the IR spectra for these 3 different molecules are so similar.
The Correct Answer and Explanation is :
The IR spectra for 1-butanol, 2-butanol, and isobutyl alcohol are quite similar in the region above 1500 cm-1 primarily because they all share the same functional group – the hydroxyl group (-OH), which dominates their spectral characteristics in this region. Specifically, the key features that contribute to the similarity in their spectra include the broad, intense O-H stretching absorption and the C-H stretching and bending vibrations that occur in this region.
Key factors explaining the similarities:
- Hydroxyl Group (O-H) Stretching:
All three alcohols have the hydroxyl group, which produces a broad, strong absorption peak typically around 3200-3550 cm-1. This broadness is due to hydrogen bonding between alcohol molecules. Since all three compounds feature an -OH group, they will exhibit a similar broad absorption band in this region, making their spectra appear nearly identical in the O-H stretch region. - C-H Stretching:
The C-H stretching vibrations of the alkyl groups in these alcohols appear in the region between 2850-2960 cm-1. Despite differences in branching (e.g., 1-butanol is a straight-chain alcohol, while 2-butanol and isobutyl alcohol are branched), the C-H stretching frequencies of the alkyl groups do not shift drastically. Thus, these alkyl stretches will be similar in all three spectra, contributing to the overall similarity in the IR patterns. - Fingerprint Region:
The fingerprint region (below 1500 cm-1) is typically where the unique structural features of each molecule manifest. However, in the region above 1500 cm-1, the spectra are dominated by the O-H and C-H stretches, which are consistent across these three compounds, making the spectra appear quite similar. - Molecular Size and Shape Effects:
While there are subtle differences due to molecular structure (such as the positioning of the hydroxyl group), these differences do not significantly alter the major vibrational frequencies seen in the spectra above 1500 cm-1, which are primarily influenced by the functional group rather than the specific position or branching of the alkyl chain.
Thus, the similarity in the IR spectra of 1-butanol, 2-butanol, and isobutyl alcohol can be attributed to their common functional group (the hydroxyl group), which is responsible for the characteristic absorptions in the higher wavenumber region.