1. © 2014 Pearson Education, Inc. Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy Paula Yurkanis Bruice University of California, Santa Barbara Chapter 14

2. © 2014 Pearson Education, Inc. Classes of Organic Compounds [Insert Table 14.1]

3. © 2014 Pearson Education, Inc. Mass Spectrometry An electron is ejected from the compound, thereby forming a molecular ion.

4. © 2014 Pearson Education, Inc. A Mass Spectrometer Only positively charged species reach the recorder.

5. © 2014 Pearson Education, Inc. The Mass Spectrum of Pentane m/z = mass-to-charge ratio of the fragment because z = 1

6. © 2014 Pearson Education, Inc. The Molecular Ion Pentane forms a molecular ion with m/z = 72.

7. © 2014 Pearson Education, Inc. Fragmentation of the Molecular Ion The more stable the fragments, the more abundant they will be. C-2—C-3 fragmentation forms more stable fragments.

8. © 2014 Pearson Education, Inc. Loss of H 2 From a Fragment

9. © 2014 Pearson Education, Inc. More Stable Fragments are More Abundant The peak at m/z = 57 is more abundant for isopentane than for pentane because a secondary carbocation is more stable than a primary carbocation.

10. © 2014 Pearson Education, Inc. Secondary Carbocations are More Stable Than Primary Carbocations

11. © 2014 Pearson Education, Inc. Natural Abundance of Isotopes

12. © 2014 Pearson Education, Inc. High Resolution Mass Spectrometry Can Distinguish Between Compound with the Same Molecular Mass Exact Masses of Isotopes

13. © 2014 Pearson Education, Inc. The Carbon—Bromine Bond Breaks Heterolytically

14. © 2014 Pearson Education, Inc. The Carbon—Chlorine Bond Breaks Heterolytically The Carbon—Carbon Bond Breaks Homolytically

15. © 2014 Pearson Education, Inc. α-Cleavage in an Alkyl Chloride The homolytic cleavage of the carbon—carbon bond is called α-cleavage. The bonds that break are the weakest bonds, and the bonds that form the most stable fragments.

16. © 2014 Pearson Education, Inc. The Mass Spectrum of 2-Chloropentane

17. © 2014 Pearson Education, Inc. α-Cleavage Occurs in Alkyl Chlorides but is Less Likely to Occur in Alkyl Bromides The carbon—carbon bond and the carbon—chlorine bond have similar strengths. The carbon—carbon bond is much stronger than the carbon—bromine bond.

18. © 2014 Pearson Education, Inc. The Carbon—Oxygen Bond Breaks Heterolytically

19. © 2014 Pearson Education, Inc. α-Cleavage in an Ether

20. © 2014 Pearson Education, Inc. α-Cleavage in an Alcohol

21. © 2014 Pearson Education, Inc. Loss of a Hydrogen from a γ-Carbon

22. © 2014 Pearson Education, Inc. α-Cleavage in a Ketone

23. © 2014 Pearson Education, Inc. Loss of a Hydrogen from a γ-Carbon

24. © 2014 Pearson Education, Inc. 1. A bond between carbon and a more electronegative atom breaks heterolytically. 2. A bond between carbon and an atom of similar electronegativity breaks homolytically. 3. The bonds most likely to break are the weakest bonds and those that lead to formation of the most stable cation. Common Fragmentation Behavior in Alkyl Halides, Ethers, Alcohols, and Ketones

25. © 2014 Pearson Education, Inc. The Electromagnetic Spectrum high energy low energy high frequency low frequency short wavelengths long wavelengths

26. © 2014 Pearson Education, Inc. The Greater the Energy, the Greater the Frequency The Greater the Energy, the Shorter the Wavelength

27. © 2014 Pearson Education, Inc. Wavelength

28. © 2014 Pearson Education, Inc. Wavenumber

29. © 2014 Pearson Education, Inc. A Stretching Vibration A stretching vibration occurs along the line of the bond.

30. © 2014 Pearson Education, Inc. Stretching and Bending Vibrations

31. © 2014 Pearson Education, Inc. Each Stretching and Bending Vibration Occurs at a Characteristic Wavenumber

32. © 2014 Pearson Education, Inc. The Functional Group Region (4000–1400 cm –1 ) The Fingerprint Region (1400–600 cm –1 ) Functional group regions: Both compounds are alcohols Fingerprint regions: Compounds are different alcohols

33. © 2014 Pearson Education, Inc.

34. The More Polar the Bond, the More Intense the Absorption

35. © 2014 Pearson Education, Inc. Hooke’s Law

36. © 2014 Pearson Education, Inc. The Greater the Bond Order, the Larger the Wavenumber

37. © 2014 Pearson Education, Inc. Electron Delocalization (Resonance) Affects the Frequency of the Absorption The more double bond character, the greater the frequency (wavenumber).

38. © 2014 Pearson Education, Inc. This C═O Bond Is Essentially a Pure Double Bond

39. © 2014 Pearson Education, Inc. This C═O Bond Has Significant Single Bond Character The less double bond character, the lower the frequency.

40. © 2014 Pearson Education, Inc. Resonance Electron Donation Decreases the Frequency Inductive Electron Withdrawal Increases the Frequency

41. © 2014 Pearson Education, Inc. The IR Spectrum of an Ester

42. © 2014 Pearson Education, Inc. The IR Spectrum of an Amide

43. © 2014 Pearson Education, Inc. Carbon—Oxygen Bonds The carbon—oxygen bond in an alcohol is a pure single bond. The carbon—oxygen bond in an ether is a pure single bond. The carbon—oxygen single bond in a carboxylic acid has partial double bond character. One carbon—oxygen single bond in an ester is a pure single bond and one has partial double bond character.

44. © 2014 Pearson Education, Inc. The IR Spectrum of an Alcohol

45. © 2014 Pearson Education, Inc. The IR Spectrum of a Carboxylic Acid

46. © 2014 Pearson Education, Inc. Hydrogen Bonded OH Groups Stretch at a Lower Frequency It is easier to stretch a hydrogen bonded OH group.

47. © 2014 Pearson Education, Inc.

48. The Strength of a Carbon—Hydrogen Bond Depends on the Hybridization of the Carbon An sp 3 -carbon—hydrogen bond is the weakest, so its stretch occurs at the shortest wavenumber (< 3000 cm –1 ).

49. © 2014 Pearson Education, Inc. An sp 3 -carbon—hydrogen stretch occurs at < 3000 cm –1. An sp 2 -carbon—hydrogen stretch occurs at > 3000 cm –1. Where Carbon—Hydrogen Bonds Stretch and Bend Stretching vibrations require more energy than bending vibrations.

50. © 2014 Pearson Education, Inc. Where Carbon—Hydrogen Bonds Bend An sp 3 -carbon—hydrogen bend of a methyl occurs at < 1400 cm –1. An sp 2 -carbon—hydrogen bend of a methyl and/or a methylene occurs at > 1400 cm –1.

51. © 2014 Pearson Education, Inc. The IR Spectrum of an Aldehyde The carbon—hydrogen stretch of an aldehyde hydrogen occurs at 2820 cm –1 and at 2720 cm –1.

52. © 2014 Pearson Education, Inc. The IR Spectrum of an Amine

53. © 2014 Pearson Education, Inc. The IR Spectrum of Diethyl Ether

54. © 2014 Pearson Education, Inc. Some Vibrations are Infrared Inactive A bond absorbs IR radiation only if its dipole moment changes when it vibrates.

55. © 2014 Pearson Education, Inc. The IR Spectrum of 2-Methyl-1-pentene Wavenumber (cm –1 ) Assignment 3075 2950 1650 and 890 absence of bands 1500–1430 and 720 sp 2 CH sp 3 CH A terminal alkene with two substituents has less than four adjacent CH 2 groups.

56. © 2014 Pearson Education, Inc. The IR Spectrum of Benzaldehyde Wavenumber (cm –1 ) Assignment 3050 2810 and 2730 1600 and 1460 1700 sp 2 CH an aldehyde benzene ring a partial single-bond character carbonyl

57. © 2014 Pearson Education, Inc. The IR Spectrum of 2-Propyn-1-ol Wavenumber (cm –1 ) Assignment 3300 2950 2100 OH group sp 3 CH alkyne

58. © 2014 Pearson Education, Inc. The IR spectrum of N-Methylethanamide Wavenumber (cm –1 ) Assignment 3300 2950 1660 1560 N—H sp 3 CH amide carbonyl N—H bend

59. © 2014 Pearson Education, Inc. IR Spectrum of Ethyl Benzyl Ketone Wavenumber (cm –1 ) Assignment >3000 <3000 1605 and 1500 1720 1380 sp 2 CH sp 3 CH a benzene ring a ketone carbonyl a methyl group

60. © 2014 Pearson Education, Inc. U ltraviolet and Visible Spectroscopy Spectroscopy is the study of the interaction between matter and electromagnetic radiation UV/Vis spectroscopy provides information about compounds with conjugated double bonds

61. © 2014 Pearson Education, Inc. An Electronic Transition Only organic compounds with  electrons can produce UV/Vis spectra. A UV spectrum is obtained if UV light is absorbed. A visible spectrum is obtained if visible light is absorbed.

62. © 2014 Pearson Education, Inc. A UV Spectrum

63. © 2014 Pearson Education, Inc. UV/Vis Absorption Bands are Broad UV/Vis absorption bands are broad because an electronic state has vibrational sublevels.

64. © 2014 Pearson Education, Inc. Chromophore A chromophore is that part of a molecule that is responsible for a UV/Vis spectrum.

65. © 2014 Pearson Education, Inc. The Beer–Lambert Law A =  c l A = absorbance of the sample c = concentration of substance in solution l = length of the cell in cm  = molar absorptivity of the sample (a measure of the probability of the transition)

66. © 2014 Pearson Education, Inc. Cells Used for Taking UV/Vis Spectra

67. © 2014 Pearson Education, Inc. The More Conjugated Double Bonds, the Longer the Wavelength

68. © 2014 Pearson Education, Inc. Conjugation Makes the Electronic Transition Easier

69. © 2014 Pearson Education, Inc. Conjugation Makes the Electronic Transition Easier

70. © 2014 Pearson Education, Inc. Colored Compounds Absorb Visible Light (> 400 nm)

71. © 2014 Pearson Education, Inc. Auxochrome An auxochrome is a substituent that alters the position and the intensity of the absorption.

72. © 2014 Pearson Education, Inc. The Color Observed Depends on the Color Absorbed

73. © 2014 Pearson Education, Inc. Common Dyes

74. © 2014 Pearson Education, Inc. Anthocyanins

75. © 2014 Pearson Education, Inc. UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

76. © 2014 Pearson Education, Inc. UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

77. © 2014 Pearson Education, Inc. UV/Vis Spectroscopy Can Be Used to Determine a pK a Value

78. © 2014 Pearson Education, Inc. UV/Vis Spectroscopy Can Be Used to Determine the Melting Temperature of DNA