CHM1022 Week 3/Lecture 1: Chemical Detectives PDF

Week 3/Lecture 1: Chemical Detectives Weekly objectives 1. Calculate a molecular formula from its microanalysis and mass spectrum result 2. Determine the Index of Hydrogen Deficiency from a molecular formula 3. Use IR spectroscopy to identify functional groups 4. Apply the basic principles of 1H a...

Week 3/Lecture 1: Chemical Detectives Weekly objectives 1. Calculate a molecular formula from its microanalysis and mass spectrum result 2. Determine the Index of Hydrogen Deficiency from a molecular formula 3. Use IR spectroscopy to identify functional groups 4. Apply the basic principles of 1H and 13C NMR spectroscopy to analyse NMR spectra 5. Apply the spectroscopy techniques of NMR spectroscopy, IR spectroscopy and Mass Spectrometry to solve chemical structures  Pre-workshop recap: Why structure determination matters. Question: What is the Functional Connectivity? If you don’t know a molecule’s structure you can’t molecular groups? make it! formula? Technique: MS IR NMR Micro- X-ray analysis analysis Data m/z 267.0742 3100- Only 3375 cm-1 Molecular formula: matching N-H This week! formula is: C21H30O2 C16H10N2O2 1675 cm-1 C=O O Indigofera blue tinctoria H N N H Q. What is the chemical in Indigofera blue O tinctoria flowers? A. Indigo’s structure determined in 1883. Used in dyeing for more than 1 500 years. First commercial synthesis 1901. High value commodity, called “blue gold.” 50 000 tonnes manufactured in 2011. Electron Impact Ionisation (EI) Mass Spectrometry 12C Molecular ion (M+) For ion with z = 1, the m/z value corresponds to the molecular mass, Mr Most intense signal is assigned 100% e.g. Low-resolution CO m/z = 28 N2 m/z = 28 High-resolution CO m/z = 27.99491 N2 m/z = 28.00614 High-resolution MS provides compound compositions from molecules with similar masses.  Fundamentals of IR Spectroscopy The frequency absorbed indicates the presence of certain functional groups. Most useful for identifying C=O (carbonyl) O−H (alcohol) Information which summarises key absorption frequencies in IR will be given in the exam. ACTIVITY 1: Index of Hydrogen Deficiency 1. What is the IHD for the following molecule? NH A) 0 B) 1 C) 2 D) 3 2. What does this tell you about its structure? 5 mins. ACTIVITY 1: Feedback and discussion 1. What is the IHD for the following molecule? NH A) 0 B) 1 C) 2 D) 3 IHD = 1 2. What does this tell you about its structure? The molecule has one unsaturated bond, or contains one cycle 5 mins. Nuclear Magnetic Resonance How does NMR work? Atoms with spin ≠ 0 are NMR active 1H and 13C have a spin = ½ (I = ½) and 2I + 1 orientations in a magnetic field. The more stable is aligned with the magnetic field Radio-frequency energy flips spin to the less stable state. How much depends on (1) the type of atom and (2) the strength of the magnetic field. What does NMR tell us about…. functional groups? H H H δ δ C OCH3 C CH3 C Si(CH 3)3 H H H H H H + = + = + = External C−H bond local External C−H bond local External C−H bond local (B ext) (B int) (BL) (B ext) (B int) (BL) (B ext) (B int) (BL) Right to left: Increasing BL → Increasing Energy to flip spin → larger number in NMR spectra What does NMR tell us about…. functional groups? 5.5 3.4 0 Right to left: Increasing BL → Increasing Energy to flip spin → larger number in NMR spectra  What does NMR tell us about…. functional groups? H H O H H H N H 1H-NMR vinylic X H allylic saturated aromatic X=halide 8 7 6 5 4 3 2 1 0 Chemical Shift (δ) O H H O H H H C C C C C C 13C-NMR N H vinylic C allylic saturated carbonyls aromatic X H C X=halide 240 210 180 150 120 90 60 30 0 Chemical Shift (δ) Information which summarises chemical shift ranges in NMR will be given in the exam. What does NMR tell us about….. symmetry? If a molecule has symmetry, then protons and carbons can be observed at the same chemical shift. Number of Number of 13C- Number of 1H-NMR Size of signals in 1H-NMR Symmetry axis? NMR signals? signals? (Integration)? H 3C O CH3 1 2 2 2 X CH2 : 2 X CH3 = 4:6 O 1 4 3 2 X CH2 : 2 X CH2 : CH2 = 4:4:2 2 2 1 4 X CH2 = 8 O O 1 4 4 OH : 2 X CH : 2 X CH : CH OH = 1:2:2:1 What does NMR tell us about….. symmetry? For example: Tert-butyl acetate (C6H12O2) has two signals in the 1H-NMR spectrum with different integrations. Why are there two signals? Are the chemical shifts consistent with the structure? Are the integrations consistent with the structure? O CH3 H 3C O CH3 CH3 ACTIVITY 2 1. Solve the molecular structure of C5H12O by: i. Calculating the Index of Hydrogen Deficiency. ii. Based on the number of 1H-NMR signals and integration does the molecule have symmetry? iii. What “substructures” do the two signals correspond to? iv. Draw your predicted structure. Are the chemical shifts consistent with this structure? NB. The “3H” and “9H” indicate the integration of the signal 15 mins. ACTIVITY 2: Discussion and feedback 1. Solve the molecular structure of C5H12O by: i. Calculating the Index of Hydrogen Deficiency. IHD = 0 ii. Based on the number of 1H-NMR signals and integration does the molecule have symmetry? Yes iii. What “substructures” do the two signals correspond to? CH3 O H 3C CH3 CH3 iv. Draw your predicted structure. Are the chemical shifts consistent with this structure? O CH3 H 3C CH3 CH3 NB. The “3H” and “9H” indicate the integration of the signal 15 mins. ACTIVITY 2 13 2. Assign each carbon in your proposed structure to the C-NMR spectrum? 15 mins. ACTIVITY 2: Discussion and feedback 1 13C NMR spectrum of C5H12O H NMR spectrum of C5H12O ANSWER – Methyl tert-butyl ether O CH3 H 3C CH3 CH3 15 mins.  Summary Today we have: Introduced the fundamentals of NMR spectroscopy Discussed how symmetry can be identified using NMR spectroscopy Discussed how integration and chemical shift can be used to solve unknown structures Solved the structures for some simple unknown molecules