How do you rationalize an infrared spectrum of salicylic acid and aspirin? How do you know which is which?

To rationalize an infrared spectrum and determine which peaks correspond to salicylic acid and aspirin, you need to understand the characteristic functional groups present in each compound. Infrared (IR) spectroscopy is a technique that measures the absorption of infrared radiation by different functional groups in a molecule.

Here are the steps to rationalize the IR spectra and distinguish between salicylic acid and aspirin:

1. Collect the IR spectra: Acquire the infrared spectra of both salicylic acid and aspirin using an infrared spectrometer. The spectra will display the intensity of absorption as a function of wavelength or wavenumber.

2. Identify typical functional group vibrations: The key to distinguishing between the two compounds lies in understanding the characteristic vibrational frequencies associated with the functional groups present in their structures. Some common functional groups and their corresponding IR absorption frequency ranges include:

- Carbonyl (C=O) stretch: Typically appears around 1700-1750 cm^-1.
- Hydroxyl (O-H) stretch: Generally observed between 3200-3600 cm^-1.
- Aromatic C-H stretch: Often occurs between 3000-3100 cm^-1.

3. Compare the spectra: Analyze the IR spectra of salicylic acid and aspirin side-by-side, paying attention to the presence and position of peaks in the spectra.

- Salicylic acid: This compound contains a carboxylic acid (-COOH) and an aromatic ring. Look for a strong peak in the range of 1700-1750 cm^-1, indicating the presence of a carbonyl group. Additionally, there should be a broad peak in the range of 3200-3600 cm^-1, indicating the presence of the hydroxyl group. Aromatic C-H stretching peaks should appear in the 3000-3100 cm^-1 range.

- Aspirin: Aspirin contains an ester functional group (-COO-). Look for a peak around 1750 cm^-1, corresponding to the carbonyl stretch of the ester. Aromatic C-H stretching peaks should also be present in the 3000-3100 cm^-1 range. The absence of a hydroxyl peak in the 3200-3600 cm^-1 range, which is present in salicylic acid, helps distinguish the two compounds.

4. Interpret other peaks: Besides the characteristic functional groups, there may be additional peaks in the spectra corresponding to other vibrations present in the molecules. However, the focus should primarily be on the characteristic peaks associated with the functional groups mentioned above.

By considering the specific functional groups and their expected vibrational frequencies, you can compare the observed peaks in the IR spectra of salicylic acid and aspirin. This allows you to differentiate between the two compounds based on their distinct infrared absorption patterns.