Transcript Document
Chapter 9: Spectroscopic Identification of Organic Compounds •Infrared (IR) Spectroscopy: used to determine the functional groups present in an organic compound. •Nuclear Magnetic Resonance (NMR) Spectrometry: helps to identify the C-H framework of an organic compound. •Mass Spectrometry (MS): used to determine the molecular mass of organic compounds, and to identify certain structural features of the compound. •Information from all three instrumental techniques is usually required to help identify the structure of an unknown organic compound. Review (Chap 2): Infrared (IR) spectroscopy •Infrared (IR) Spectroscopy: used to identify the functional groups present in an organic compound. •The basic theory depends on the energy associated with vibrating, stretching and bending motions of the functional groups in organic molecules. •If a molecule is subjected to electromagnetic radiation that matches the specific frequency of any vibration, the structure of the functional group can be identified. The absorption bands of different functional groups absorb at different frequencies and intensities. Typical IR spectrum: The absorption bands represent the IR energy absorbed by different functional groups, read as the frequency off of the x-axis (4000 - 600 cm-1) Tool for Unknown Identification: Hydrogen Deficiency • Hydrogen Deficiency – measure of unsaturation, or deficiency in the #H’s vs #C’s. HD = (Max # H’s – Actual # H’s) (Max #H’s = 2n + 2 where n = #carbons) HD for double bond = 2 HD for ring = 2 (cycloalkane) HD for triple bond = 4 HD for benzene ring = 8 Combinations are possible! IR Example An unknown organic compound has a formula of C4H6O. The IR spectrum is shown below: •An unknown organic compound has a formula of C4H6O. 4 •Hydrogen Deficiency = ______ •What do the formula and HD suggest: carbonyl •IR spectrum = _________________ How does this effect your choices? Carbonyl and alkene Carbonyl and ring •(continued) An unknown organic compound has a formula of C4H6O. Hydrogen Deficiency = 4 •There is one 1o carbon and no 2o or 3o carbons. Suggest one or more reasonable structures: Chapter 9. Nuclear Magnetic Resonance (NMR) Spectroscopy •The most important instrumental technique used by organic chemists to determine the structure of organic compounds. •NMR spectroscopy helps to identify functional groups, and the C-H framework of an organic compound. •The technique is based on the spinning nuclei of atoms behaving as if they were tiny magnets. These nuclei include: 1H, 13C, 15N, 19F, 31P.