Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy

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

2. Classes of Organic Compounds [Insert Table 14.1]

3. Mass Spectrometry An electron is ejected from the compound, thereby forming a molecular ion.

4. A Mass Spectrometer Only positively charged species reach the recorder.

5. The Mass Spectrum of Pentane m/z = mass-to-charge ratio of the fragment because z = 1

6. The Molecular Ion Pentane forms a molecular ion with m/z = 72.

7. 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. Loss of H2 From a Fragment

9. 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. Secondary Carbocations are More Stable Than Primary Carbocations

11. Natural Abundance of Isotopes

12. High Resolution Mass Spectrometry Can Distinguish Between Compound with the Same Molecular Mass Exact Masses of Isotopes

13. The Carbon—Bromine Bond Breaks Heterolytically

14. The Carbon—Chlorine Bond Breaks Heterolytically The Carbon—Carbon Bond Breaks Homolytically

15. 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. The Mass Spectrum of 2-Chloropentane

17. 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. The Carbon—Oxygen Bond Breaks Heterolytically

19. α-Cleavage in an Ether

20. α-Cleavage in an Alcohol

21. Loss of a Hydrogen from a γ-Carbon

22. α-Cleavage in a Ketone

23. Loss of a Hydrogen from a γ-Carbon

24. Common Fragmentation Behavior in Alkyl Halides, Ethers, Alcohols, and Ketones 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.

25. The Electromagnetic Spectrum high energy low energy high frequency low frequency short wavelengths long wavelengths

26. The Greater the Energy, the Greater the Frequency The Greater the Energy, the Shorter the Wavelength

27. Wavelength

28. Wavenumber

29. A Stretching Vibration A stretching vibration occurs along the line of the bond.

30. Stretching and Bending Vibrations

31. Each Stretching and Bending Vibration Occurs at a Characteristic Wavenumber

32. 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

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

35. Hooke’s Law

36. The Greater the Bond Order, the Larger the Wavenumber

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

38. This C═O Bond Is Essentially a Pure Double Bond

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

40. Resonance Electron Donation Decreases the Frequency Inductive Electron Withdrawal Increases the Frequency

41. The IR Spectrum of an Ester

42. The IR Spectrum of an Amide

43. 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. The IR Spectrum of an Alcohol

45. The IR Spectrum of a Carboxylic Acid

46. Hydrogen Bonded OH Groups Stretch at a Lower Frequency It is easier to stretch a hydrogen bonded OH group.

48. The Strength of a Carbon—Hydrogen Bond Depends on the Hybridization of the Carbon An sp3-carbon—hydrogen bond is the weakest, so its stretch occurs at the shortest wavenumber (< 3000 cm–1).

49. Where Carbon—Hydrogen Bonds Stretch and Bend Stretching vibrations require more energy than bending vibrations. An sp3-carbon—hydrogen stretch occurs at < 3000 cm–1. An sp2-carbon—hydrogen stretch occurs at > 3000 cm–1.

50. Where Carbon—Hydrogen Bonds Bend An sp3-carbon—hydrogen bend of a methyl occurs at < 1400 cm–1. An sp2-carbon—hydrogen bend of a methyl and/or a methylene occurs at > 1400 cm–1.