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CHEM 281
John Carran

NMR Spectroscopy → before used to have to use x-ray crystallography (make a crystal and then shine x-rays through) but now NMR: very sensitive type of question: Introduction Classic methods for organic structure determination • Boiling point • Refractive index • Solubility tests • Functional group tests • Derivative preparation • Sodium fusion (to identify N, Cl, Br, I & S) • Mixture melting point • Combustion analysis: burning material and seeing what molecules are formed • Degradation: heating up material and looking at fragments that come off → classic methods for organic structure determination require large quantities of sample and are time consuming Spectroscopic methods for organic structure determination a) mass spectroscopy (MS) • molecular mass & characteristic fragmentation pattern b) infrared spectroscopy (IR) • characteristic functional groups c) ultraviolet spectroscopy (UV) • characteristic chromophore d) nuclear magnetic resonance (NMR) Don't need to know Spectroscopic methods for organic structure determination • combination of these spectroscopic techniques provides a rapid, accurate and powerful tool for identification and structure • elucidation of organic compounds • rapid • effective in mg and microgram quantities General steps for structure elucidation 1. Elemental analysis • empirical formula • e.g. C 2 4 2. Mass spectroscopy • molecular weight • molecular formula • e.g. C 4 8 ,2C H6O 12e3c. • characteristic fragmentation pattern for certain functional groups 3. From molecular formula • double bond equivalent (DBE) 4. Infrared spectroscopy (IR) • identify some specific functional groups • e.g. C=O, C–O, O–H, COOH, NH … etc. 2 5. UV • sometimes useful especially for conjugated systems • e.g. dienes, aromatics, enones 6. H, C NMR and other advanced NMR techniques • full structure determination Electromagnetic spectrum END:ofdon't need to know Nuclear Magnetic Resonance (NMR) Spectroscopy • a graph that shows the characteristic energy absorption frequencies and intensities for a sample in a magnetic field is called a nuclear magnetic resonance (NMR) spectrum Formpartterns:IMPORTANT -quartet, triplet Groups oflines refer to Why do these patterns occur? -protons that make that patterare next to groups ofprotons anothergroup ofpatterns → change inproton -only when have neighbouring protons in different environment get peaks need to remember: - 3 mainregions for NMR - 0:alkanes → alkenes → aromatics:8 closer get to zero: “more boring” → gives # of peaks: n+1 (three protons → quartet) CH h2s a Bromide group attached to it → pull signal away from zero, towards higher numbers 1. The number of signals in the spectrum tells us how many different sets of protons there are in the molecule (2 signals: 2 different groups of protons in compound) 2. The position of the signals in the spectrum along the x-axis tells us about the magnetic environment of each set of protons arising largely from the electron density in their environmnt 3. The area under the signal tells us about how many protons there are in the set being measured 4. The multiplicity (or splitting pattern) of each signal tells us about the number of protons on atoms
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