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Lecture 9 - Determining molecular structure(What is that...

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Determining molecular structure (What is that white powder?) Some important techniques (5): 1. Elemental Analysis Used to identify the empirical formula 2. Mass spectroscopy Used to identify the molecular weight (MW) of a molecule Can use isotopic ratios to identify certain atoms 1 3. Degree of Unsaturation Used to identify the number of double bonds, triple bonds, and ring in a molecule 4. Infrared spectroscopy (IR) Used to identify key functional groups in a molecule (ketone, alkyne, alcohol, etc) 5. Nuclear Magnetic Resonance Spectroscopy (NMR) Analysis results in precise determination of molecular structure The sample is placed in a superconducting magnet and is irradiated with radiofrequency waves *Apply these in order when given an unknown compound to identify*
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Degree of Unsaturation (DU) •D U = ( 2 C + 2 + N H X ) / 2 –Igno re O and S 2
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4- Infrared Spectroscopy (IR) 3 Each type of bond (C—H vs C—O) vibrate at specific frequencies ( ν = c/ λ ) Reported in wavenumbers (cm -1 )
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IR – cont. Key absorptions Key Absorptions (cm -1 ): C-H Alkyl C-H 2850-2960 m, sharp C-H sp 2 C-H just >3000 m, sharp Alcohol R O-H 3200-3650 s, broad 4 Carboxylic acid RC(=O) O-H 2500-3300 s, broad Amine R2 N-H 3300-3500 s, broad *Carbonyl R2 C=O 1650-1780 s, sharp Nitrile R C≡N 2220-2260 v, sharp Alkynyl C≡ C-H ~3300 m, sharp Alkynyl C≡C 2100-2260 v, sharp
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IR – Identify the key absorptions: 5
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IR – Identify the key absorptions: 6 OH
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IR – Identify the key absorptions: 7 NH 2
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IR – Identify the key absorptions: 8
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IR – terminal alkyne 9
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