Question

Question: What are the major IR absorptions in the functional group region for oleic acid, a common unsaturated fatty acid (Section 10.6A)?

Short answer

Answer

The IR absorption peaks in the compound are at:

• 2850-3000 ${\mathrm{cm}}^{-1}$
• 1650 ${\mathrm{cm}}^{-1}$
• Approximately 1700 ${\mathrm{cm}}^{-1}$
• 2500-3500 ${\mathrm{cm}}^{-1}$

Step-by-step solution

IR spectroscopy

IR spectroscopy is used to identify the functional groups present in a compound.

In IR spectroscopy, the compound to be analyzed is taken in liquid film or gaseous form.

The sample is exposed to IRradiations; each functional group absorbs a particular wavelength and starts vibrating. Those vibrations associated with the change in dipole moment will absorb the IR region.

The IR spectrum has a wavenumber of IR radiation on the x-axis and percentage transmittance on the y-axis.

Peaks in the IR spectrum

Each peak in the IR spectrum gives the wavenumber of IR radiation absorbed by the functional groups present in the compound.

These wave number values depend on the vibrational frequency of the bonds.

Therefore, the wavenumber corresponding to each bond will be a constant independent of the compound it is present.

If the functional groups present in a molecule are known, the IR peaks can be predicted.

IR peaks of oleic acid

Oleic acid is an unsaturated fatty acid.

The functional groups in oleic acid are therefore an unsaturation and a carboxylic acid group.

The region of the IR spectrum below 1500 ${\mathrm{cm}}^{-1}$is considered a fingerprint region and is very complex with overlapping peaks. Therefore, peaks above 1500 ${\mathrm{cm}}^{-1}$ are generally considered functional group regions.

The 2D structure of oleic acid can be given as:

Oleic acid

The IR absorption peaks in the compound are:

• At 2850-3000 ${\mathrm{cm}}^{-1}$ due to absorption of ${\mathrm{sp}}^3$ hybridized carbon-carbon single bond.
• At 1650 ${\mathrm{cm}}^{-1}$due to absorption of ${\mathrm{sp}}^2$hybridized carbon-carbon double bond.
• At approximately 1700 ${\mathrm{cm}}^{-1}$ for $\mathrm{C}=\mathrm{O}$carbonyl bond vibrations.
• At 2500-3500 ${\mathrm{cm}}^{-1}$due to absorption of the$\mathrm{O}-\mathrm{H}$ group.