IR Spectroscopy - ORGANIC STRUCTURE DETERMINATION How do we...

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Unformatted text preview: ORGANIC STRUCTURE DETERMINATION How do we know... how atoms are connected together? which bonds are single, double, or triple? what functional groups exist in the molecule? if we have a specific stereoisomer? 1 TOOLS FOR STRUCTURAL ANALYSIS Elemental analysis Chemical tests (see part V of organic lab text) Instrumental methods for organic chemistry o o o o Nuclear Magnetic Resonance (NMR) spectroscopy Infrared (IR) spectroscopy Ultraviolet (UV) spectroscopy Mass spectrometry 2 SPECTROSCOPY - Study of spectral information Physical stimulus Molecule response Detecting instrument Visual (most common) representation, or Spectrum 3 SPECTRUM INTERPRETATION PROCESS Recognize a pattern Associate patterns with physical parameters Identify possible meanings, i.e. propose explanations 4 ELECTROMAGNETIC SPECTRUM Most organic spectroscopy uses electromagnetic energy as the physical stimulus. Electromagnetic energy has no detectable mass component. In other words, it can be referred to as "pure energy." The important parameters associated with electromagnetic radiation are: Energy (E) Frequency () Wavelength (l) E = h 5 EFFECT OF ELECTROMAGNETIC RADIATION ON MOLECULES => Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 6 Infrared radiation is largely thermal energy. It induces stronger molecular vibrations in covalent bonds. Specific bonds respond to (absorb) specific frequencies Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 7 VIBRATIONAL MODES Covalent bonds can vibrate in several modes, including stretching, rocking, and scissoring. Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 8 TRANSMISSION vs. ABSORPTION When a chemical sample is exposed to the action of IR LIGHT, it can absorb some frequencies and transmit the rest. Some of the light can also be reflected back to the source. IR source reflected light Chemical sample sample can absorb specific frequencies transmitted light Detector 9 An IR spectrum in absorption mode 10 An IR spectrum in transmission mode 11 IR bands can be strong, medium, or weak in intensity. 12 The intensity of the bands depends on the magnitude of the dipole moment associated with the induced vibration. Strongly polar bonds such as carbonyl groups (C=O) produce strong bands. Medium polarity bonds and asymmetric bonds produce medium bands. Weakly polar bond and symmetric bonds produce weak or non observable bands. 13 Functional groups capable of forming hydrogen bonds such as the hydroxyl group (O-H) show broad, intense IR bands. Typical O-H band 14 INFORMATION OBTAINED FROM IR SPECTRA IR is most useful in providing information about the presence or absence of specific functional groups. IR can provide a molecular fingerprint that can be used when comparing samples. IR does not provide detailed information or proof of molecular structure. It provides information on molecular fragments, specifically functional groups. 15 The typical IR absorption range for organic molecules is 600 - 4000 cm-1 Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 16 IR SPECTRUM OF A REGULAR ALKANE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 17 IR SPECTRUM OF A REGULAR ALKENE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 18 AN ASYMMETRICAL AND A SYMMETRICAL ALKYNE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 19 IR SPECTRUM OF A NITRILE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 20 IR SPECTRUM OF AN ALCOHOL Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 21 IR SPECTRUM OF AN AMINE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 22 IR SPECTRUM OF AN AMIDE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 23 IR SPECTRUM OF A KETONE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 24 IR SPECTRUM OF AN ALDEHYDE Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 25 IR SPECTRUM OF A CARBOXYLIC ACID Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003 26 A PowerPoint presentation of ch. 12 in Wade's Organic Chemistry is available at the Prentice Hall website at http://www.prenhall.com/wade Click on the 5th edition icon. Under Instructor Resources follow the links to chapter 12. HAVE FUN ! 27 ...
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This note was uploaded on 05/02/2011 for the course CH 310 N taught by Professor Blocknack during the Spring '08 term at University of Texas at Austin.

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