OrganicChemistry-CityUniversityofNewYork.pptVIP

12-* 12-* William H. Brown ? Beloit College William H. Brown Christopher S. Foote Brent L. Iverson Eric Anslyn /chemistry/brown Chapter 12 Infrared Spectroscopy Molecular Spectroscopy Molecular spectroscopy The study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure. we study three types of molecular spectroscopy Molecular vibrations Fundamental stretching and bending vibrations for a methylene group. Molecular Vibrations For a molecule to absorb IR radiation the bond undergoing vibration must be polar and its vibration must cause a periodic change in the bond dipole moment. Covalent bonds which do not meet these criteria are said to be IR inactive. The C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds. Infrared Spectroscopy Infrared spectrum of 3-methyl-2-butanone. Fingerprint Region: Highly Complex and Unique for Every Molecule C-H Stretching Correlation Tables Infrared stretching frequencies of selected functional groups. Potentially difficult to distinguish between one-another, but good indication of heteroatom-hydrogen bond C=C often too weak or encroaching on fingerprint region More Valuable Less Valuable Buried in fingerprint region Almost all organic molecules have C-H bonds Signature Stretch. Very strong, very identifiable Carboxylic acids Infrared spectrum of pentanoic acid. Amines Infrared spectrum of 1-butanamine, a 1° amine. Hydrocarbons-Table 12.5 Difficult to distinguish between one-another Aromatics Infrared spectrum of toluene. Hydrocarbons-Table 12.5 Can be useful. In fairly distinct regions Alkynes Infrared spectrum of 1-octyne. * 12-* 12-* William H. Brown ? Beloit College William H. Brown Christopher S. Foote Brent L. Iverson Eric Anslyn /chemistry/brown Chapter 12 Infrared Spectroscopy