1 / 62

12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy

12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy. Based on McMurry’s Organic Chemistry , 7 th edition. Determining the Structure of an Organic Compound.

rob
Télécharger la présentation

12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy Based on McMurry’sOrganic Chemistry, 7th edition

  2. Determining the Structure of an Organic Compound • The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants • In the 19th and early 20th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other

  3. Determining the Structure of an Organic Compound • Physical methods now permit structures to be determined directly. We will examine: • mass spectrometry (MS)—this chapter • infrared (IR) spectroscopy—this chapter • nuclear magnetic resonance spectroscopy (NMR)—Chapter 13 • ultraviolet-visible spectroscopy (VIS)—Chapter 14

  4. 12.1 Mass Spectrometry (MS) • Sample vaporized and bombarded by energetic electrons that remove an electron, creating a cation-radical • Bonds in cation radicals begin to break (fragment)

  5. Mass Spectrometer

  6. Mass Spectrometer

  7. The Mass Spectrum • Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (corresponding to the number of ions) (y-axis) • Tallest peak is base peak (100%) • Other peaks listed as the % of that peak • Peak that corresponds to the unfragmented radical cation is parent peak or molecular ion (M+)

  8. MS Examples: Methane and Propane • Methane produces a parent peak (m/z = 16) and fragments of 15 and 14

  9. MS Examples: Methane and Propane • The Mass Spectrum of propane is more complex (Figure 12-2 ) since the molecule can break down in several ways

  10. Mass spectrum of propane

  11. 12.2 Interpreting Mass Spectra • Molecular weight from the mass of the molecular ion • Double-focusing instruments provide high-resolution “exact mass” • 0.0001 atomic mass units – distinguishing specific atoms • Example MW “72” is ambiguous: C5H12 and C4H8O but: • C5H12 72.0939 amu exact mass C4H8O 72.0575 amu exact mass • Result from fractional mass differences of atoms 16O = 15.99491, 12C = 12.0000, 1H = 1.00783

  12. Other Mass Spectral Features • If parent ion not present due to electron bombardment causing breakdown, “softer” methods such as chemical ionization are used • Peaks above the molecular weight appear as a result of naturally occurring heavier isotopes in the sample • (M+1) from 13C that is randomly present

  13. Interpreting Mass-Spectral Fragmentation Patterns • The way molecular ions break down can produce characteristic fragments that help in identification • Serves as a “fingerprint” for comparison with known materials in analysis (used in forensics) • Positive charge goes to fragments that best can stabilize it

  14. 2,2-Dimethylpropane: MM = 72 (C5H12)

  15. Mass Spectral Fragmentation of Hexane Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57, 43, 29

  16. Hexane

  17. Worked example 12.1: methylcyclohexane or ethylcyclopentane?

  18. Mass Spectral Cleavage Reactions of Alcohols • Alcohols undergo -cleavage (at the bond next to the C-OH) as well as loss of H-OH to give C=C

  19. Mass Spectral Cleavage of Amines • Amines undergo -cleavage, generating radicals

  20. Fragmentation of Ketones and Aldehydes • A C-H that is three atoms away leads to an internal transfer of a proton to the C=O, called the McLafferty rearrangement • Carbonyl compounds can also undergo  cleavage

  21. Fragmentation of Ketones and Aldehydes

  22. 12.4 Mass Spec. in Biochemistry: TOF • ESI and MALDI are techniques to produce charged molecules at relatively low energy, to minimize fragmentation. • The large biological molecules are separated by Time of Flight analysis (TOF) in a drift tube without a magnetic field imposed.

  23. MALDI-TOF spectrum of chicken egg-white lysozyme

  24. 12.5 The Electromagnetic Spectrum

  25. Wavelength and Frequency

  26. Absorption Spectra • Organic compounds exposed to electromagnetic radiation can absorb photons of specific energies (wavelengths or frequencies) • Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum • Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11)

  27. Infrared Absorption Spectrum of Ethanol

  28. 12.6 Infrared Spectroscopy of Organic Molecules • IR region is lower in photon energy than visible light (below red – produces heating as with a heat lamp) • 2.5  106 m to 2.5  105 m region used by organic chemists for structural analysis • IR energy in a spectrum is usually measured as wavenumber (cm-1), the inverse of wavelength and proportional to frequency: • Wavenumber (cm-1) = 1/l(cm) • Specific IR absorbed by organic molecule is related to its structure

  29. IR region and vicinity

  30. Infrared Energy Modes • IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes” • Energy is characteristic of the atoms in the group and their bonding • Corresponds to molecular vibrations

  31. Infrared Energy Modes

  32. 12.7 Interpreting Infrared Spectra • Most functional groups absorb at about the same energy and intensity independent of the molecule they are in • Characteristic IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule • IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region)

  33. 4000-2500 cm-1 N-H, C-H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm-1 CºC and C º N (stretching) 2000-1500 cm-1 double bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1 Below 1500 cm-1 “fingerprint” region Regions of the Infrared Spectrum

  34. Regions of the Infrared Spectrum

  35. Differences in Infrared Absorptions • Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants) • Bond stretching dominates higher energy (frequency) modes

  36. Differences in Infrared Absorptions • Light objects connected to heavy objects vibrate fastest (at higher frequencies): C-H, N-H, O-H • For two heavy atoms, stronger bond requires more energy (higher frequency): C º C, C º N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

  37. C-H, C-C, C=C, C º C have characteristic peaks 12.8 Infrared Spectra of Hydrocarbons

  38. Hexane

  39. Alkenes

  40. 1-Hexene

  41. Alkynes

  42. 12.8 Infrared Spectra of Some Common Functional Groups • Spectroscopic behavior of functional groups is discussed in later chapters • Brief summaries presented here

  43. Aromatic compounds:

  44. Phenylacetylene

  45. IR: Alcohols Cyclohexanol

  46. Amines

  47. IR: Carbonyl Compounds • Strong, sharp C=O peak 1670 to 1780 cm1 • Exact absorption characteristic of type of carbonyl compound • 1730 cm1 in saturated aldehydes • 1705 cm1 in aldehydes next to double bond or aromatic ring

More Related