I just finished an organic chemistry lab experiment on separating and analyzing an unknown acid/neutral compound mixture using extraction, recrystallization, TLC, melting point analysis, and IR spectrum.
In the instructions, "Obtain an IR spectrum for each of your recrystallized compounds. Analyze each spectrum, and assign the principal vibrational bands in each." I identified the unknowns and obtained their recrystallized IR spectrums, but I don't know what how I am suppose to assign the principal vibrational bands.
Thank you in advance!
C-H stretching
C-H bending
O-H stretchning
etc.
Assigning the principal vibrational bands in an IR spectrum involves analyzing the peaks observed and relating them to the different functional groups present in the compound. Here's how you can approach this task:
1. Familiarize yourself with common functional groups: Start by reviewing the characteristic vibrational frequencies of common functional groups in organic chemistry. Some examples include carbonyl groups (C=O), hydroxyl groups (OH), amino groups (NH2), and alkenes (C=C).
2. Interpret the spectrum: Look for peaks in the IR spectrum and identify their location on the wavelength (or wavenumber) axis. IR spectra are typically plotted with wavenumbers (cm-1) on the x-axis, which are directly proportional to the frequency of absorption. The y-axis represents the percent transmittance or absorbance.
3. Identify the major functional groups: The presence of certain functional groups will produce distinct peaks at specific wavenumbers. For example, the presence of a carbonyl group (C=O) typically leads to a strong, sharp peak around 1700-1750 cm-1. Similarly, an alcohol (OH) group often produces a broad peak around 3200-3600 cm-1.
4. Compare with reference data: To confirm the assignment of functional groups, you can compare the observed peaks in your IR spectrum to reference data in spectroscopy databases, textbooks, or online resources. These references provide typical ranges of absorption frequencies for various functional groups.
5. Consider other factors: Keep in mind that the specific environment around a functional group can influence the vibrational frequency. For example, conjugation and bond strength can affect the position of peaks. Additionally, some functional groups may exhibit multiple peaks due to stretching or bending vibrations.
By following these steps and utilizing reference materials, you should be able to assign the principal vibrational bands in your recrystallized compounds' IR spectra. Remember that practice and familiarity with different functional groups will enhance your ability to interpret IR spectra accurately.