CHM 223 Organic Chemistry I Prof. Chad Landrie. Lecture 10: September 20, 2018 Ch. 12: Spectroscopy mass spectrometry infrared spectroscopy

Transcription

1 M 223 Organic hemistry I Prof. had Landrie Lecture 10: September 20, 2018 h. 12: Spectroscopy mass spectrometry infrared spectroscopy

2 i>licker Question onsider a solution that contains 65g R enantiomer and 35g S enantiomer. If the observed specific optical rotation for the mixture is 74.0º, what is the specific optical rotation of the pure R isomer? A. 247 B D E Slide 2

3 Spectroscopy vs. Spectrometry Spectroscopy = study of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation Spectrometry = evaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation Slide 3 3

4 Spectroscopic and Spectrometric Methods Method Mass Spectrometry (MS) Infrared (IR) Spectroscopy Nuclear Magnetic Resonance (NMR) Spectroscopy Ultraviolet-Visible (UV-vis )Spectroscopy Measurement/Application Used to determine the molecular mass and the molecular formula. reates radical cations and measures their movement through a magnetic field. Used to determine the functional groups in a substance Measures the stretching and bending frequencies of covalent bonds that contain a dipole moment. Used to determine the connectivity (, framework) of molecule Measures the energy associated with spin states of nuclei in the presence of a magnetic field Used to determine the nature of conjugated p-systems (e.g. adjacent double bonds, aromatic rings) Measures the energy associated with promotion of an electron in a ground state to an exited state Slide 4 4

5 Spectroscopic and Spectrometric Methods Method Mass Spectrometry (MS) Measurement/Application Used to determine the molecular mass and the molecular formula. reates radical cations and measures their movement through a magnetic field. eroin Slide 5 5

6 Spectroscopic and Spectrometric Methods Method Infrared (IR) Spectroscopy Measurement/Application Used to determine the functional groups in a substance Measures the stretching and bending frequencies of covalent bonds that contain a dipole moment. eroin Slide 6 6

7 Spectroscopic and Spectrometric Methods Method Nuclear Magnetic Resonance (NMR) Spectroscopy Measurement/Application Used to determine the connectivity (, framework) of molecule Measures the energy associated with spin states of nuclei in the presence of a magnetic field Slide 7 7

8 Spectroscopic and Spectrometric Methods Method Ultraviolet-Visible (UV-vis )Spectroscopy Measurement/Application Used to determine the nature of conjugated p-systems (e.g. adjacent double bonds, aromatic rings) Measures the energy associated with promotion of an electron in a ground state to an exited state tetraphenylcyclopentadienone O Slide 8 8

9 M 223 Organic hemistry I Prof. had Landrie Mass Spectrometry Sections

10 Mass Spectrometry Taxol Primary Applications: 1. Determine molecular mass. 2. Establish fragmentation patterns (mass of molecular fragments formed when covalent bonds are broken), which can be indexed in a database. 3. Determine presence of some heteroatoms (e.g., l, Br). 4. Determine the exact mass of molecules. Slide 10

11 Mass Spectrometer Schematic creates charged molecules called radical cations radical cations are accelerated by negatively charged plates magnetic fields exert forces on moving charges Slide 11

12 Formation of Radical ations radical = unpaired electron organic molecules are bombarded with 70-eV electrons causes organic molecule to lose one electron from a covalent bond or electron lone-pair organic molecule is then charged the mass of charged species is determined by a mass spectrometer Slide 12

13 Lorentz Force Like electric currents, moving charges create magnetic fields. These magnetic fields experience Lorentz forces when passed perpendicularly past an external magnetic field. The magnitude depends on the size, mass and velocity of the charge. Slide 13

14 Lorentz Force Slide 14

15 Mass Spectrometer Schematic creates charged molecules called radical cations radical cations are accelerated by negatively charged plates magnetic fields exert forces on moving charges Slide 15

16 Molecular Ion Peaks : 6 x 12 = 72 : 6 x 1 = 6 Total = 78 molecular ion peak = highest m/z (mass/charge) peak since charge (z) is usually 1, molecular ion peak = molecular mass molecular ion peak does not have to have relative intensity of 100% most intense peak = base peak relative intensity = height of peak base peak Slide 16

17 Fragmentation heterolysis radical cation fragments (heterolysis) to give a neutral radical species and a cation fragment contains less mass than parent ion relative intensity of the fragment depends on its concentration (likelihood of occurring) more stable cations are more likely; give more intense peaks Slide 17

18 Fragmentation ommon Fragmentation Pattern for Alkanes Slide 18

19 Fragmentation ommon Fragmentation Pattern for Alkyl Benzenes benzylic carbon benzylic carbocation benzylic carbocation stabilized by resonance = common fragment in MS Slide 19

20 Fragmentation Slide 20

21 Fragmentation Since fragmentation patterns should be the same for identical molecules, they can be saved in a database and matched to unknowns later. SI anyone? Slide 21

22 Isotopic lusters: arbon and ydrogen 14 M + 1 M + 1 Slide 22

23 Isotopic lusters: arbon and ydrogen Probability of M+1 6 x 1.1% = 6.6% of 13 6 x 0.015% = 0.1% of 2 M + 1 M + 1 Total Probablility = 6.7% Natural Abundance of Isotopes Isotope Abundance % % 2 (D) 0.015% If there are n carbon atoms in a molecular ion, then the ratio of M+ to [M+1]+ is 100:(1.10 x n) % Slide 23

24 Isotopic lusters: hlorine & Bromine Natural Abundance of Isotopes Isotope Abundance 35 l 75.77% 37 l 24.23% 79 Br 50.69% hlorine: M + :[M+2] + ~ 3:1 Bromine: M + :[M+2] + ~ 1:1 81 Br 49.31% Slide 24

25 Isotopic lusters: hlorine Duclofenac: analgesic behaves similarly to aspirin more soluble/bioavailable l l N M + = O O (295, 40) (297, 26.6) = = 7.0 Natural Abundance of Isotopes Isotope Abundance 35 l 75.77% 37 l 24.23% Predicting Ratio of Relative Intensity for 2 l Atoms Molecular Ion hlorine ombination Math % Ratio [M] + 35 l x 35 l x (299, 3.8) [M+2] + 37 l x 35 l x l x 37 l x [M+4] + 37 l x 37 l x Slide 25

26 i>licker Question Duclofenac is an analgesic that behaves similarly to aspirin except that it is more soluble and thus bioavailable. What atom is most likely represented by X based on the mass spectrum? Natural Abundance of Isotopes Isotope Abundance 35 l 75.77% 37 l 24.23% 79 Br 50.69% X X N O A. l B. Br 81 Br 49.31% % % 2 (D) 0.015% % O. D. Slide 26

27 i>licker Question Which peak in the mass spectrum represents the molecular ion peak and gives the molecular weight? A. 295 l 214 B. 278 l N O O D. 214 E. 179 Slide 27

28 i>licker Question Which peak in the mass spectrum represents the base peak? A. 295 l 214 B. 278 l N O O D. 214 E. 179 Slide 28

29 i>licker Question Which peak in the mass spectrum represents the molecular ion formed by loss of a hydroxyl group (-O) from this molecule? A. 295 l l N O B O D. 214 E. 179 Slide 29

30 i>licker Question Which peak in the mass spectrum represents the molecular ion formed by loss of a carboxylic acid group (-O2) and a chlorine atom (-l) from this molecule? A. 295 l 215 B. 278 l N O O D. 214 E. 179 Slide 30

31 M 223 Organic hemistry I Prof. had Landrie Infrared Spectroscopy Sections

32 Electromagnetic Spectrum shorter wavelength (λ) higher frequency (ν) higher energy (E) longer wavelength (λ) lower frequency (ν) lower energy (E) Electromagnetic Radiation propagated at the speed of light (3 x10 8 m/s) has properties of particles and waves energy is directly proportional to frequency energy is indirectly proportional to wavelength E = hν c = νλ Slide 32

33 Review: Principles of Infrared Spectroscopy IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment (µ). Stretching δ + δ + δ + δ δ δ δ Bending δ δ δ + δ + δ + Slide 33

34 Stretching & Bending Vibrations Slide 34

35 Dipole Moment more electronegative atom covalent 2 electron bond dipole arrow less electronegative atom δ (partially negatively charged) δ (partially positively charged) In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ). Slide 35

36 ooke s Law: Bonds are Like Springs Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached Trends: ~ ν = vibrational frequency in wavenumbers (cm -1 ) k = force constant; strength of bond (spring) m* = reduced mass ma = mass of 1 atom of a bond strength = frequency mass = frequency Slide 36

37 Spring Analogy smaller mass = higher frequency = higher energy stronger spring (bond) = higher frequency = higher energy Slide 37

38 i>licker Question Which covalent bond, highlighted in bold (red) in the molecules below, would not be expected to exhibit an IR stretching band? O O 3 N 3 N A B D E 3 3 Slide 38

39 Wavenumber (ῡ) and Infrared Scale ῡ (cm -1 ) = 1 λ (cm) higher wavenumber (ῡ) = higher frequency (υ) = lower wavelength (λ) = higher energy (E) N- O- (sp)- (sp2)- (sp3)- O 2 (2380) N O N lower wavenumber (ῡ) = lower frequency (υ) = longer wavelength (λ) = lower energy (E) fingerprint region O wavenumber (cm -1 ) wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency Slide 39

40 General Need-to-Know IR Frequencies Slide 41

41 Infrared Spectrum % Transmission Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations Frequency: typically measured in wavenumbers; higher wavenumber = higher frequency = higher energy vibration Bands: frequency of vibration absorbed by molecules; can be broad or narrow; number of bands does not equal number of bonds Wavenumbers Slide 42

42 Example: Alkanes Wavenumbers = sp 3 - bond stretching motion; general absorb around cm -1 1 = - rocking motion when atom is part of a methyl group (- 3); cm -1 3 = scissor motion of -3 hydrogen atoms; cm cm -1 = fingerprint region for organic molecules; typically complex and unhelpful hexane 3 2 Slide 43

43 Example: Alkenes : notice sp2 - (~3100 cm -1 ) at higher frequency than sp3 - (~2950 cm -1 ) 1-hexene Wavenumbers more s-character = stronger bond = higher frequency 4: also, = bond at higher frequency than - bond; ~1600 cm -1 Slide 44

44 Example: Alkynes hexyne 7: notice sp - (~3300 cm -1 ) at higher frequency than sp2 - (~3100 cm -1 ), which was higher than sp3 - (~2950 cm -1 ) 6: stretch is very weak because carbons have almost identical electronegativities = small dipole moment Slide 45

45 Example: Alcohols 5 9 O 4 prop-2-en-1-ol (allyl alcohol) 9: hydroxyl groups (-O) exhibit strong broad bands; ~3300 cm -1 broad peak is a result of hydrogen bonding; width depends on solution concentration lower concentration = less hydrogen bonding = more narrow -O band Slide 46

46 Example: Nitriles O N hydroxy-propionitrile 8: nitriles ~2200 cm nitriles ( N) absorb a greater magnitude of energy than alkynes ( ) because they have a larger dipole moment larger dipole moment = more intense peak Wavenumbers size of the dipole does NOT affect frequency of vibration Slide 47

47 Example: Ester, Amine, Benzene 100 O N O Wavenumbers amino-benzoic acid butyl ester 10: strong carbonyl (=O) band ~1700 cm -1 11: amines; secondary amines (- N) give one band; primary amines (-N2) gives two bands 4: several alkene bands ~1600 cm -1 for benzene ring = double bonds Slide 48

48 Example: arboxylic Acid O 10 O 9 60 cyclohex-2-enecarboxylic acid 40 10: strong carbonyl (=O) band ~1700 cm : hydroxyl band (-O) can be less intense and sharper in carboxylic acids Wavenumbers : weak alkene band (=) since small dipole moment Slide 49

49 Example: Aldehyde 100 O hept-2-enal 12: usually two bands for - of aldehydes; may overlap with sp3 - bands (Fermi doublet) Wavenumbers Slide 50

50 M 223 Organic hemistry I Prof. had Landrie Next Lecture... hapter 12