Chapter 9: Spectroscopic Identification of Organic Compounds CH 9-1: Review of IR and Hydrogen Deficiency 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 IR is an instrumental method used to identify some of the functional groups present in an organic compound. Basic Theory: A molecule is subjected to electromagnetic radiation (EMR) at a frequency in the IR region of the electromagnetic spectrum. When the IR frequency matches the vibrational frequency of a specific functional group, we observe an energy absorbance band in the IR spectrum.

Example IR spectrum of organic compound: The absorption bands represent IR energy absorbed by the covalent bonds of different functional groups, read as frequency units off the x-axis (4000 - 600 cm-1). Most of these bands are due to C-C and C-H bonds and can be ignored.

The absorption bands of different functional groups absorb at different frequencies and intensities. Keep it simple! (~3300 if terminal)

Tool for Unknown Identification: Hydrogen Deficiency • Hydrogen Deficiency – measure of unsaturation, or deficiency in the #H’s you would expect vs #C’s. • A “deficiency” in the number of hydrogens indicates the presence of pi bonds or rings. 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!