please help !!!
You are given an organic solid which reacts with most organic solvents. How would you obtain the IR spectrum of it?
Mix the organic compound with KBr and make a KBr pellet. Run the IR spectrum.
Thank you for your answer. Why we do not use mull or thin-solid film method for this one?
I thought of a mull first but since mulls are liquids (nujol mull for example) my guess is it will be soluble in that also although we don't normally think of nujol as a solvent. As for thin films, I don't know how you get a thin film of a solid (powder) although if it were crystalline enough, like NaCl, one might be able to slice off a think film. Many organic compounds don't have that kind of crystal structure. I have always made thin films by dissolving in a solvent and letting a thin film form by evaporation; however, the problem precludes that. Therefore, the KBr pellet is the safest answer, I think, given the parameters.
To obtain the IR spectrum of an organic solid that reacts with most organic solvents, you can follow these steps:
1. Sample Preparation: Start by preparing a representative sample of the organic solid. To do this, take a small amount of the solid and grind it into a fine powder. This increases the surface area of the sample, ensuring better interaction with infrared radiation.
2. Formation of Pellet: To obtain a clear and reliable IR spectrum, it is necessary to convert the powdered sample into a thin and homogeneous solid pellet. This can be done by mixing a small amount of the powdered sample with a suitable non-reactive binder, such as potassium bromide (KBr), which is commonly used.
3. Mixing with KBr: Take a small amount of KBr powder and grind it separately to obtain a fine powder. Then, mix the powdered KBr and the powdered sample in a specific ratio. The optimal ratio varies depending on the sample, but a starting point could be about 1-2% of the sample weight relative to KBr.
4. Pellet Preparation: Now, transfer the mixture into a pellet-making die, which is a small cylindrical mold. Apply pressure to the mixture using a hydraulic press to compact it into a pellet. The pressure applied should be sufficient to form a solid and transparent pellet.
5. IR Measurement: Once the pellet is prepared, it is ready for IR measurement. Place the pellet on an appropriate sample holder or in a specialized cell designed for solid sample analysis. The sample holder should allow infrared radiation to pass through the pellet without significant absorption or interference. Ensure the pellet surface is flat and uniform for accurate spectral analysis.
6. Instrument Setup: Use an infrared spectrometer equipped with a suitable detector, such as a Single Reflection ATR (Attenuated Total Reflectance) or a transmission cell, to analyze the pellet. Configure the spectrometer according to the manufacturer's instructions.
7. Data Acquisition: With the instrument properly set up, acquire the IR spectrum by scanning a specific range of infrared wavelengths. This will provide a plot of infrared absorption intensity versus wavelength (measured in wavenumbers, cm^-1).
8. Data Analysis: Interpret the obtained IR spectrum to identify characteristic peaks and functional groups present in the organic solid. Compare the observed peaks and their intensities with reference spectra or IR databases to help identify the compound.
Remember, these steps are general guidelines, and some modifications may be required based on the exact nature and solubility of the organic solid you are working with. Additionally, consult the instruction manual of your specific IR spectrometer for the best results.