To Do s. Read Chapter 3. Complete the end-of-chapter problems, 3-1, 3-3, 3-4, 3-6 and 3-7. Answer Keys are available in CHB204H

Transcription

1 Read Chapter 3. To Do s Complete the end-of-chapter problems, 3-1, 3-3, 3-4, 3-6 and 3-7 Answer Keys are available in CB204

2 NMR Chemical Shifts Further Discussion A set of spectral data is reported when a new molecule is synthesized or isolated.

3 Always Do Peak Assignment! D B, C A E F NMR peak assignments

4 Typical 1 Chemical Shift Ranges

5 Why Different Chemical Shifts Are Observed? Shielding by bonding electrons Magnetic anisotropy

6 Shielding by Bonding Electrons β B 0 ΔE B i B T B 0 α B B T < B 0 because of B i Electron density B i B T Resonates at smaller ν ν = γb T 2π B T = B 0 (1 - σ), σ : shielding constant

7 TMS and Shielding C 3 3 C Si C 3 C 3 ighly shielded TMS, a reference compound δ = 0 ppm More shielded Electronegativity Si = 1.9 C = 2.55 Less shielded ppm TMS 0

8 1-Chloro-2,2-dimethylpropane Less shielded Electronegative element Cl C 2 CC 3 C 3 More shielded C PPM 1 0

9 Shielding and Electronegative Elements C 3 -F C 3 -Cl C 3 -Br C 3 -I C 3 - δ(c 3 ) Electronegativity

10 O is more electronegative than Br X-C 2 -C 2 -Y

11 Silicon-containing Compound, Dow 704 Pump Oil

12 Dow 704 Pump Oil, 13 C-NMR

13 ow Far does the Effect Transmit? C 3 -R R-C 2 -R R-C-R δ (ppm) R C 3 -C 2 -C 2 -C 2 -O O O O

14 Alkenic Protons (δ 4.5 ~ 7.7) cyclohexene: δ 5.66ppm Angle strain Cyclopentene: 5.73 Cyclobutene: 5.95 More s-charactor angle strain More shielding Less shielding

15 Trans-2-pentenal Less shielded B c

16 C 6 6 : δ 7.27 ppm Aromatics Less shielding More shielding electronegative

17 o-nitrophenol D & E + + B C Less shielded

18 Aromatic Ions δ (ppm) /7-1/5 electron withdrawing electron donating

19 Beyond Shielding More s-charactor (more electronegative carbon) C C sp3 C C sp2 sp C C δ (ppm) ppm 9.3ppm - 3.0ppm!!

20 Magnetic Anisotropy B 0 When B i = B i magnetically isotropic A A When B i = B i B i B i magnetically anisotropic Decreased B T Increased B T

21 Benzene Ring B 0 B i << B i B i B i Benzene protons are deshielded, and show up at more down field positions than vinylic protons. shielded deshielded 9.3ppm deshielded - 3.0ppm shielded

22 Other Benzene Derivatives ppm C 3 C 3 : ppm ~ 7ppm C 3 ~ 8 ppm

23 1,3-Dimethylnaphthalene 2.574ppm 2.391ppm

24 Cyclophane 0.51 C C 2 C 2 C C ChemDraw says PPM Softwares are useful, but don t trust them too much.

25 Alkynes shielded + ~ 9ppm ~ 7ppm ~ 2ppm deshielded - C C - deshielded + shielded

26 Nitriles

27 Alkenes - C + C ppm 1.42 ppm 1.44 ppm 2.82 ppm ppm O O 3.75 ppm

28 Alkanes O O O O C 3 O O 3.78 ppm 5.18 ppm O O O O 4.69 ppm O C 3 O 3.97 ppm

29 Stereochemistry of Aldol Products O + O O O O O C 3

30 Predicting the Chemical Shifts C 3 -R R-C 2 -R R-C-R δ (ppm) R O C 3 -C-C 3 δ (ppm) β (O) = 0.28 α (O) = 2.1

31 Alkane Parameters

32 Alkanes - Examples O a b c a b c C=C O-R δ (obsd) a b c C=O CO 2 δ (obsd) O O O a b c d d a b c O-R Brδ (obsd) O Br a b c

33 Which one is it? 2.3 ppm 4.3 ppm O O or O O

34 Alkene Parameters

35 Aromatic Parameters

36 Examples - Alkenes and Aromatics δ = (-0.26) = 6.37 δ = = C N 3 C δ = (-0.60) + (-0.06) = 6.78 δ = (-0.04) + (-0.10) = 8.18 NO 2 Cl δ = (-0.10) = 8.14

37 13 C-NMR 12 C does not have nuclear spin (I = 0) 13 C has a nuclear spin of 1/2. 13 C presents only 1.1%, and has low gyromagnetic ratio. 13 C-NMR is about 6000 times less sensitive than 1 -NMR. It was not common until FT-NMR was invented.

38 Broadband Decoupling The sample is irradiated with 1 -frequency during the experiment to eliminate all the C- spin-spin coupling. Broadband decoupling increases the sensitivity of 13 C-NMR by converting multiplets to a singlet and by nuclear Overhauser effect (NOE). C C C C C C C C C C C C C C more NOE and higher peak intensity 13 C relaxation time is usually longer than 1, and vary significantly, depending on types of carbon. Peak area is NOT proportional to # of carbons

39 Spin-spin Coupling in 13 C-NMR 13 C- 13 C couplings are not usually observed due to the low natural abundance. 13 C- 1 couplings are usually eliminated experimentally. One sharp peak is observed for each chemically non-equivalent carbon.

40 Dichlorobenzenes Cl Cl Cl Cl Cl X 4 peaks Cl Y X X Y, Y 6 peaks

41 Both s and p Electrons Contribute 13 C Chemical Shifts p-electrons are promoted to the exited state by external magnetic field to produce a large shielding effect (paramagnetic shielding or σ p ). Paramagnetic shielding(σ p ) >> shielding by s-electrons (diamagnetic shielding or σ d ) σ p shows opposite effects to σ d.

42 Karplus-Pople equation A measure of multiple bonding Energy difference between ground state and excited states Bond order σ p ΔE LUMO Distance of p-electrons from nucleus ΔE σ p OMO e - density repulsion r σ p

43 3-chloro-4'-fluoropropiophenone Aromatic carbons Alkyl carbons single bonds C4 Low C=O electron dencity small ΔE C2 C5 C3

44 1-Chlorohexane

45 13 C-NMR Chemical Shift Range Both s- and p-electrons are responsible for shielding

46 Empirical Calculation of 13 C Chemical Shifts - Alkanes

47 Functionalized Alkanes C 4 C 1 C 2 C 3 C 6 Cl C 5 δ(c1) = = 18.5 ppm δ(c2) = = 68.7 ppm

48 An Example O O Cl?

49 Calculating 13 C Chemical Shifts

50 Peak Assignment Completed O O Cl

51 More Examples

52 The γ-effect (Steric Compression Effect) R C 3 R R C 3 R R R 8-11 C 3 Steric compression O

53 Axial and Equatorial Methyls O O

54 Empirical Calculation - Alkenes C C δ = ppm δ = x x 1 + (-7.9 x 2) + (-1.8 x 2) + (-1.5 x 1) = ppm 3 C C 2 C 3 C C 3 C C 2 C 2 C 2 C 3 Cγ Cβ Cα Cα' Cβ' Cγ' C δ = x x 1 + (- 7.9 x 2) + (-1.8 x 2) + (-1.8 x 1) = ppm

55 Alkenes-Electronic Effects 120 ~ 140 ppm O O e-density σp δ

56 Empirical Calculation - Alkenes

57 Alkynes and Nitriles β N N α

58 Aromatic Carbons (110 ~ 160 ppm) More shielded Less shielded

59 Empirical Calculation - Aromatics δ = ppm O OC 3 δ = = ppm δ = = ppm δ = = ppm δ = = ppm O δ = = ppm NO 2 δ = = ppm

60 Carbonyl Carbons ( ppm) Aldehydes ( ppm) Ketones ( ppm) Carboxylic acid derivatives ( ppm) O CO C OC

61 Problem, C 5 9 OCl

62 Problem, C 5 9 OCl, - cont. Solvent (CDCl 3 )

63 A Carbonyl! C 5 9 OCl, ID = 1 What kind of carbonyl?! Solvent (CDCl 3 )

64 1 -NMR Integrals and Splitting triplet triplet quartet triplet

65 Draw Structures and Eliminate Wrong Ones! O O Cl Cl O Cl Cl Cl O O

66 Answer O O Cl Cl O Cl Cl Cl O O

67 DEPT (Distortionless Enhancement by Polarization Transfer) DEPT sequence is designed to enhance 13 C signal, depending on the number of protons attached. DEPT45 DEPT90 DEPT C-signal intensity θ C No signal

68 Typical DEPT Experiment This is also called Spectral Editing

69 DEPT - Alkyne

70 Ibuprofen, DEPT C C

71 DEPT - Lactam A peak Is hidden N C 3 O CO 2

72 DEPT - Lactam N C 3 O CO 2 C C 2 C=O C 3

73 DEPT of Codeine C & C 3 C 2 C only C, C 2 and C 3