Group+1+IR+and+NMR

Return to Preparation of Aspirin Data




 * Figure 1. ** Structure of aspirin.


 * Figure 2. ** IR spectrum of our aspirin sample using a KBR pellet, completed at Northeastern Illinois University using FT-IR.
 * Analysis: ** Aspirin has an ester ortho to the carboxylic acid on the benzene ring. The broad peak that spans from approximately 3550 to 2500 is representative of a carboxylic acid -OH stretch. The peak at 1687.25 is most likely representative of the carboxylic acid carbonyl stretching. This peak's frequency is lowered because of resonance delocalization of electron density in the carbonyl bond due to its attachment to the aromatic ring. The delocalization lowers the stretching frequency from the typical 1710 to approximately 1685 cm-1. The peak at 1754.64 is most likely representative of the carbonyl stretch of the ester. The conjugated ester carbonyl is usually greater than the typical 1710 cm-1. The two peaks at 1577 and 1581 are most likely the aromatic C=C stretching. The peaks at either 1220.79 or at 1306.88 may be representative of the acid C-O stretching, but it is not readily apparent. These and other peaks ranging from 1000-1300 cm-1 could also represent the C-O stretching of the ester. Finally, the single peak at 755.23 may be representative of the ortho-substituted aromatic ring C-H oop bend.


 * Figure 3. ** IR spectrum for aspirin using a KBR pellet from the SDBS database for comparison.


 * Figure 4. ** 1H NMR spectrum of aspirin product completed at Northeastern Illinois University.
 * Analysis: ** The NMR spectrum was much more difficult to obtain, and it took several trials before an acceptable spectrum of our aspirin product was produced. The peaks that range from just above 7 until just above 8 represent the four hydrogens of the aromatic ring. These hydrogens are nonequivalent, so they split one another into four doublets and absorb at different chemical shifts. The ring hydrogen that absorbs the most downfield is most likely ortho to the carboxylic acid, since it deshielded by both the aromatic ring and the carboxylic acid. The SDBS 1H NMR spectrum for aspirin shows the four hydrogen doublets very clearly. However, our spectrum shows at least 7 doublets. Based on the melting point data, our aspirin was not 100% pure. These extra peaks may be representative of salicylic acid aromatic protons or other impurities in our product. The singlet at 4.5 ppm was not integrated and is not visible on the SDBS 1H NMR spectrum of aspirin, so I am assuming that it represents the single -OH hydrogen of the carboxylic acid in our sample (it rather rather represents a LOT of water in the sample). Finally, the singlet at 2.25 represents the three hydrogens attached to the carbonyl carbon atom of the ester. These hydrogens appear as a singlet since there are no neighboring hydrogens that split them. They are more downfield due to less deshielding.


 * Figure 5. ** 1H NMR of aspirin using CDCl3 from SDBS (for comparison).