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Chem 140A Lecture Notes Chapter 11 _partial_ Winter 2011

Chem 140A Lecture Notes Chapter 11 _partial_ Winter 2011 -...

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Chemistry 140A Winter 2011 (K. Albizati) 1 Structure Elucidation in Organic Chemistry (Chapter 11-8 through 11-10) An organic chemist is commonly faced with the problem of identifying the full structure of a molecule/compound of unknown structure. The compound may be the product of a chemical reaction or it could be a compound derived from a natural source like a plant or elsewhere. Key pieces of desired structural information (and how to get them): What is the compound’s molecular weight? (mass spectrometry) What is the compound’s molecular formula? (mass spectrometry) What functional groups are present? (IR Spectroscopy and 13 C NMR) What is the carbon skeleton? ( 1 H and 13 C NMR; mass spectrometry) What is the relative stereochemistry in the molecule? ( 1 H and 13 C NMR; x-ray crystallography) What is the absolute stereochemistry? (Polarimetry, x-ray crystallography, NMR and chemical interconversion) There are several technologies and instruments we can use to determine the structure of an organic compound. Some of the technologies you will learn in Chem 140 and Chem 143 are:
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Chemistry 140A Winter 2011 (K. Albizati) 2 Mass Spectrometry We have already seen Polarimetry, which gives information regarding absolute (R or S) stereochemical information. Unless you know a lot about where the molecule came from, the most important piece of structural information is the molecular formula and/or molecular weight of the molecule. Mass Spectrometry gives us this information. Key Information Obtainable: molecular weight, molecular formula, gross connectivity of large parts of the molecule (via fragmentation study). Only a very small sample of the compound in question is needed (less than a microgram) and the sample is destroyed during analysis. This technique requires a fairly pure sample of the compound for unambiguous analysis. The Mass Spectrometer A mass spectrometer (called an MS) is generally a $100K and greater instrument that requires special handling and training to use. Usually one or two people in a Chemistry Department are trained to use the high-end instruments. The instruments become more expensive as you require greater accuracy in the measurement of molecular mass. Instruments come in 2 categories – low resolution and high resolution.
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