KHS ChemistryUnit 3.4 Structural Analysis1 Structural Analysis 1 Adv Higher Unit 3 Topic 4 Gordon Watson Chemistry Department, Kelso High School

KHS ChemistryUnit 3.4 Structural Analysis2 Introduction Structural Analysis Elemental AnalysisMass Spectrometry This topic explores various methods used in the Structural Analysis of organic molecules starting with Elemental Analysis and Mass Spectrometry.

KHS ChemistryUnit 3.4 Structural Analysis3 Elemental Analysis 1 combustion analysis Empirical formulas are determined by combustion analysis: CarbonHydrogen combustion analysis Carbon, Hydrogen, Sulphur & Nitrogen can all be determined by combustion analysis. Other elements can be determined by other methods.

KHS ChemistryUnit 3.4 Structural Analysis4 Elemental Analysis 2 Calculation is same as taught at Standard Grade.

KHS ChemistryUnit 3.4 Structural Analysis5 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 = / 1 moles of H = / 14 moles of N = / 16 moles of O = 1.59

KHS ChemistryUnit 3.4 Structural Analysis6 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 Whole number ratio is 5: 6: 2: 2 Empirical formula Empirical formula is C 5 H 6 N 2 O 2

KHS ChemistryUnit 3.4 Structural Analysis7 Mass Spectrometry A mass spectrometer does three things ¬vaporisesminute sample ¬vaporises a minute sample of compound ( g) ­ionises ­ionises the vaporised molecules ®separatesanalysesmolecules fall apart ®separates and analyses the ions, produced when the molecules fall apart, according to their mass/charge ratio, giving a mass spectrum

KHS ChemistryUnit 3.4 Structural Analysis8 Mass Spectrometer

KHS ChemistryUnit 3.4 Structural Analysis9 Molecular Ion A high-energy electron can dislodge an electron from a bond, positive ion creating a radical cation (a positive ion with an unpaired e - ). Molecular ion m m 2. M M +. m 1. + m 2 + m m 2 Fragmentation Fragmentation then occurs.

KHS ChemistryUnit 3.4 Structural Analysis10 Common Fragments Fragmentm or m/z H2OH2O18 CH CH 3 CH CH 3 CO C7H7+C7H7+ + more stablemore likely Some fragments are more stable and, therefore, more likely

KHS ChemistryUnit 3.4 Structural Analysis11 Tropylium Ion 91 C7H7+C7H7+ + particularly stable Base Peak This ion is a particularly stable ion and is often the most abundant peak (Base Peak). benzene ring with a carbon 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.

KHS ChemistryUnit 3.4 Structural Analysis12 The Mass Spectrum Masses are graphed or tabulated according to their relative abundance.

KHS ChemistryUnit 3.4 Structural Analysis13 Example 1a m/z rel abundance H3CH3CCH CH 2 CH 3 CH 2 CH 3 2-METHYLPENTANE

KHS ChemistryUnit 3.4 Structural Analysis14 Example 1b m/z rel abundance H3CH3CCH CH 2 CH 3 CH 2 CH 3 86 M+.M+. Molecular ion The main use of a mass spectrum (at Advanced Higher ) is to identify the Molecular ion.

KHS ChemistryUnit 3.4 Structural Analysis15 Example 1c m/z rel abundance 71 H3CH3CCH CH 2 CH 3 CH 2 CH 3 15 CH 3 + C 5 H 11 + fragments Occasionally you can be asked to identify possible fragments.

KHS ChemistryUnit 3.4 Structural Analysis16 Example 1d rearrange Unstable fragments will often rearrange themselves to form more stable molecules. m/z rel abundance H3CH3CCH CH 2 CH 3 CH 2 CH C4H9+C4H9+ C2H5+C2H5+ CH CH 2 C2H3+C2H3+ 27

KHS ChemistryUnit 3.4 Structural Analysis17 Example 1e The detailed analysis of a mass spectrum is best left to experienced operators with years of experience.

KHS ChemistryUnit 3.4 Structural Analysis18 Use of Mass Spectra molecular formula At Advanced Higher, the main (only?) use of mass spectra is to determine the molecular formula of the molecule. IR Spectroscopy NMR Spectroscopy IR Spectroscopy and NMR Spectroscopy will be used to identify the actual structure of the molecule. m/z rel abundance H3CH3CCH CH 2 CH 3 CH 2 CH 3 86 M+.M+.

KHS ChemistryUnit 3.4 Structural Analysis19 High Resolution Mass Spectrometer Some modern Mass Spectrometers can now determine masses to 5 or 6 decimal places. ‘same mass’ This is accurate enough to differentiate between molecules of the ‘same mass’. Eg molecules with a mass of ‘44’ could be: C 3 H 8 C 2 H 4 O CO 2 or CN 2 H 4 A more accurate measurement of mass from the spectrometer could be C 3 H 8 C 2 H 4 O CO 2 CN 2 H

KHS ChemistryUnit 3.4 Structural Analysis20 End of Topic 4 Structural Analysis 1