Vivian Marlowe
The OH bond of an alcohol group usually absorbs in the range of 3200–3600 cm-1, while vinyl sp2 C-H bonds absorb at about 3000-3100 cm-1. The absorption bands in IR spectra have different intensities, which can be referred to as strong, medium, weak, broad, and sharp. The intensity of an absorption band depends on the polarity of the bond, with a higher polarity resulting in a more intense absorption band. The O-H bond in alcohol is polar and usually shows strong and broad absorption bands, making it easy to identify. On the other hand, the N-H stretch of vinyl groups falls in the same wavenumber range as O-H stretches but is less broad and less intense due to the lower polarity of N-H bonds.
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What You'll Learn
The OH bond of an alcohol group absorbs in the range of 3200–3600 cm-1
The OH bond of an alcohol group usually absorbs in the range of 3200–3600 cm-1. This is a characteristic absorption band that is easy to identify. The broad shape of the absorption band results from the hydrogen bonding of the OH groups between molecules. The O-H bond in alcohol is polar and usually shows strong and broad absorption bands. The intensity of an absorption band depends on the polarity of the bond, and a bond with a higher polarity will show a more intense absorption band.
The OH bond of the carboxylic acid group occurs at about 2500–3300 cm-1. The polarity of the N-H bond (in amine and amide) is weaker than the OH bond, so the absorption band of N-H is not as intense or as broad as O-H, and the position is in the 3300–3500 cm-1 region.
The C-H bond stretching of all hydrocarbons occurs in the range of 2800–3300 cm-1, and the exact location can be used to distinguish between alkane, alkene, and alkyne. The =C-H (sp2 C-H) bond of alkene gives absorption at about 3000–3100 cm-1, while the ≡C-H (sp C-H) bond of terminal alkyne gives absorption at about 3300 cm-1.
The N-H stretch (or two) above 3200 cm-1 has a significantly different shape than the alcohol O-H stretch. N-H stretches fall in the same wavenumber range as O-H stretches, but they are less broad and less intense because N-H bonds are less polar than O-H bonds.
The IR spectrum of a compound usually shows multiple absorption bands, and each bond has a different vibration mode. The wavenumber is defined as the reciprocal of wavelength, and the wavenumbers of infrared radiation are normally in the range of 4000 cm-1 to 600 cm-1. The region of the spectrum that is mostly used to identify functional groups is between 1500 and 3500 cm-1, as most functional groups give characteristic absorptions in this region.
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The N-H stretch differs in shape and is less broad and intense
The N-H stretch is also distinct from the vinyl sp2 C-H stretch. The vinyl sp2 C-H stretch is found in alkenes and has an absorption range of approximately 3000-3100 cm-1. The N-H stretch, as mentioned, typically falls within a broader range of 3200 to 3600 cm-1 but is positioned in the 3300-3500 cm-1 region. The vinyl sp2 C-H stretch is characteristic of alkenes, while the N-H stretch is commonly associated with amines and amides.
The sharpness or broadness of a stretch in IR spectra is influenced by the extent of hydrogen bonding in the molecule. When a molecule undergoes significant intermolecular hydrogen bonding, the peaks tend to be broader. Conversely, when the hydrogen bonding is weaker, the peaks tend to be sharper. This is because the strength of hydrogen bonding depends on the electronegativity of the atom attached to the hydrogen. The more electronegative the atom, the stronger the hydrogen bonding and the broader the peaks.
In the case of N-H stretches, the attached atom is nitrogen (N), which has a lower electronegativity compared to oxygen (O). As a result, the extent of hydrogen bonding is lesser with N than with O. This leads to sharper peaks for N-H stretches compared to the broader peaks of O-H stretches in alcohols.
Additionally, the intensity of an absorption band is influenced by the number of bonds responsible for the absorption. A higher number of bonds involved results in a stronger absorption band. In the context of N-H stretches, the C-N stretches are not very polar, leading to weaker peak intensities. This further contributes to the N-H stretch being less intense compared to other stretches with more polar bonds.
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The vinyl sp2 C-H stretch absorbs at 3000-3100 cm-1
In contrast, the O-H bond of an alcohol group usually has absorption in the range of 3200–3600 cm-1. The hydroxyl group peak, typical of an alcohol, can be observed around 3300 cm-1. The broad shape of the absorption band in this range is due to the hydrogen bonding of the OH groups between molecules.
The absorption bands in IR spectra have different intensities, which can be strong (s), medium (m), weak (w), broad, or sharp. The intensity of an absorption band depends on the polarity of the bond, with higher polarity bonds showing more intense absorption bands. The O-H bond in alcohols is polar, which results in strong and broad absorption bands that are easy to identify.
The N-H stretch, on the other hand, falls in the same wavenumber range as the O-H stretch but is less broad and less intense due to the lower polarity of the N-H bond compared to the O-H bond. Therefore, the key distinguishing factor between the vinyl sp2 C-H stretch and the alcohol stretch is the position of their absorption bands, with the vinyl group absorbing at a lower wavenumber range of 3000-3100 cm-1 and the alcohol group absorbing at a higher range of 3200-3600 cm-1.
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The vinyl C=C stretch absorbs at ~1600 cm-1
The vinyl C=C stretch absorbs at around 1600 cm^-1. This absorption is characteristic of alkenes, with the C=C stretch appearing as a single band at ~1600 cm^-1. This is in contrast to the benzene ring, which exhibits two sharp absorption bands, one of which is at ~1600 cm^-1 and the other at 1500-1430 cm^-1.
The vinyl C=C stretch is a useful indicator of the presence of alkenes, and it is important to distinguish it from other stretches. The hydroxyl group peak of an alcohol, for example, typically occurs at around 3300 cm^-1, with a sharp peak at 3600 cm^-1 indicating non-hydrogen bonding. The OH bond of an alcohol group usually absorbs in the range of 3200-3600 cm^-1, while the OH bond of a carboxylic acid group absorbs at 2500-3300 cm^-1.
The vinyl C=C stretch is also distinct from the C=O stretch of carbonyl groups, which appears at 1650-1750 cm^-1. Within this range, carboxylic acids, esters, ketones, and aldehydes tend to absorb at the higher end (1700-1750 cm^-1), while conjugated unsaturated ketones and amides tend to absorb at the lower end (1650-1700 cm^-1).
Additionally, the vinyl C=C stretch is separate from the C-H stretch of alkanes, alkenes, and alkynes, which occurs in the range of 2800-3300 cm^-1. Specifically, the =C-H (sp2 C-H) bond of an alkene absorbs at 3000-3100 cm^-1, while the ≡C-H (sp C-H) bond of a terminal alkene absorbs at about 3300 cm^-1.
The vinyl C=C stretch at ~1600 cm^-1 is, therefore, a distinctive feature in IR spectroscopy, helping to identify the presence of alkenes and differentiate them from other functional groups.
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The alcohol O-H stretch is strong and broad, vinyl C-H is weaker
The O-H bond in an alcohol group usually absorbs in the range of 3200–3600 cm-1. This is a polar bond, and its absorption band is strong and broad. The intensity of an absorption band depends on the polarity of the bond, and a bond with a higher polarity will show a more intense absorption band. The broad shape of the absorption band results from the hydrogen bonding of the OH groups between molecules.
On the other hand, the =C-H (sp2 C-H) bond of an alkene gives absorption at about 3000-3100 cm-1. The C-H bond stretching of all hydrocarbons occurs in the range of 2800–3300 cm-1, and the exact location can be used to distinguish between alkane, alkene, and alkyne. The =C-H (sp2 C-H) bond of an alkene is weaker than the O-H bond of an alcohol group because it is less polar. The polarity of the N-H bond (in amine and amide) is weaker than the OH bond, so the absorption band of N-H is not as intense or as broad as O-H.
The OH bond of an alcohol group can be distinguished from the OH bond of a carboxylic acid group, which occurs at about 2500–3300 cm-1. Hydroxyl groups in carboxylic acids are considerably broader than in alcohols. The OH bond of an alcohol group can also be distinguished from the N-H stretch, which falls in the same wavenumber range but is less broad and less intense because the N-H bond is less polar.
The C=O stretch, which occurs in aldehydes, carboxylic acids, esters, ketones, and aldehydes, can also be distinguished from the O-H stretch of an alcohol group. The C=O stretch appears in the 1650-1750 cm-1 region, while the O-H stretch of an alcohol group is typically above 3200 cm-1. The C=O stretch is strong, but it is not as broad as the O-H stretch of an alcohol group.
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Frequently asked questions
The OH bond of an alcohol group usually absorbs in the range of 3200–3600 cm-1, while the =C-H (sp2 C-H) bond of alkene absorbs at about 3000-3100 cm-1.
The OH bond of an alcohol group is polar and usually shows strong and broad absorption bands that are easy to identify.
You will never see a C=O stretch below 1630.
N-H stretches fall in the same wavenumber range as O-H stretches, but N-H stretches are less broad and less intense.
The polarity of the N-H bond is weaker than the OH bond, so the absorption band of N-H is not as intense or as broad as O-H, and the position is in the 3300–3500 cm-1 region.
