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Structural Analysis 1

Structural Analysis 1. Gordon Watson Chemistry Department, Kelso High School. Adv Higher Unit 3 Topic 4. Introduction. This topic explores various methods used in the Structural Analysis of organic molecules starting with Elemental Analysis and Mass Spectrometry. Elemental Analysis 1.

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Structural Analysis 1

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  1. StructuralAnalysis 1 Gordon Watson Chemistry Department, Kelso High School Adv Higher Unit 3 Topic 4 Unit 3.4 Structural Analysis

  2. Introduction This topic explores various methods used in the Structural Analysis of organic molecules starting with Elemental Analysis and Mass Spectrometry. Unit 3.4 Structural Analysis

  3. Elemental Analysis 1 Empirical formulas are determined by combustion analysis: Carbon, Hydrogen, Sulphur & Nitrogen can all be determined by combustion analysis. Other elements can be determined by other methods. Unit 3.4 Structural Analysis

  4. Elemental Analysis 2 Calculation is same as taught at Standard Grade. Unit 3.4 Structural Analysis

  5. Empirical Formula 1 A substance was analysed and gave these results: C 47.6% ; H 4.76% ; N 22.2% and O 25.4% by mass. What is the empirical formula? In 100 g there will be 47.6 / 12 moles of C = 3.97 4.76 / 1 moles of H = 4.76 22.2 / 14 moles of N = 1.59 25.4 / 16 moles of O = 1.59 Unit 3.4 Structural Analysis

  6. Empirical Formula 2 Ratio of C:H:N:O is 3.97: 4.76: 1.59: 1.59 Divide by smallest number to simplify Simplifying ratio: 2.5: 2.99: 1: 1 Whole number ratio is 5: 6: 2: 2 Empirical formula is C5H6N2O2 Unit 3.4 Structural Analysis

  7. Mass Spectrometry • A mass spectrometer does three things • vaporises a minute sample of compound (10-10 g) • ionises the vaporised molecules • separates and analyses the ions, produced when the molecules fall apart, according to their mass/charge ratio, giving a mass spectrum Unit 3.4 Structural Analysis

  8. Mass Spectrometer Unit 3.4 Structural Analysis

  9. Molecular Ion A high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e-). Molecular ion m1+ + m2. MM+. m1.+ m2+ m1+ + m2 Fragmentation then occurs. Unit 3.4 Structural Analysis

  10. Common Fragments Some fragments are more stable and, therefore, more likely Fragmentm or m/z H2O 18 CH3+ 15 CH3CH2+ 29 CH3CO+ 43 91 C7H7+ + Unit 3.4 Structural Analysis

  11. Tropylium Ion 91 C7H7+ + This ion is a particularly stable ion and is often the most abundant peak (Base Peak). Its presence indicates that the original molecule contained a benzene ring with a carbon attached - often a very useful aid to the identification of the overall molecule. Unit 3.4 Structural Analysis

  12. The Mass Spectrum Masses are graphed or tabulated according to their relative abundance. Unit 3.4 Structural Analysis

  13. Example 1a 100 CH3 80 60 H3C CH CH2 CH2 CH3 rel abundance 40 2-METHYLPENTANE 20 20 40 60 80 100 m/z Unit 3.4 Structural Analysis

  14. Example 1b M+. 100 80 CH3 60 rel abundance H3C CH CH2 CH2 CH3 40 86 20 20 40 60 80 100 m/z The main use of a mass spectrum (at Advanced Higher ) is to identify the Molecular ion. Unit 3.4 Structural Analysis

  15. Example 1c 100 CH3+ C5H11+ 80 CH3 60 rel abundance 71 H3C CH CH2 CH2 CH3 40 15 20 20 40 60 80 100 m/z Occasionally you can be asked to identify possible fragments. Unit 3.4 Structural Analysis

  16. Example 1d 100 C4H9+ C2H5+ 80 CH3 60 rel abundance 27 40 H3C CH CH2 CH2 CH3 29 57 20 CH2 CH C2H3+ 20 40 60 80 100 m/z Unstable fragments will often rearrange themselves to form more stable molecules. Unit 3.4 Structural Analysis

  17. Example 1e The detailed analysis of a mass spectrum is best left to experienced operators with years of experience. Unit 3.4 Structural Analysis

  18. Use of Mass Spectra M+. 100 80 CH3 60 rel abundance H3C CH CH2 CH2 CH3 40 86 20 20 40 60 80 100 m/z At Advanced Higher, the main (only?) use of mass spectra is to determine the molecular formula of the molecule. IR Spectroscopy and NMR Spectroscopy will be used to identify the actual structure of the molecule. Unit 3.4 Structural Analysis

  19. High Resolution Mass Spectrometer A more accurate measurement of mass from the spectrometer could be 44.029 C3H8 C2H4OCO2 CN2H4 44.06260 44.02620 43.98983 44.03740 Some modern Mass Spectrometers can now determine masses to 5 or 6 decimal places. This is accurate enough to differentiate between molecules of the ‘same mass’. Eg molecules with a mass of ‘44’ could be: C3H8 C2H4O CO2 or CN2H4 Unit 3.4 Structural Analysis

  20. StructuralAnalysis 1 End of Topic 4 Unit 3.4 Structural Analysis

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