1. SPECTROSCOPY SL Chemistry Topic 11.3

2. IHD – Index of hydrogen deficiency (degree of unsaturation) ■Used to determine from a molecular formula the number of rings or multiple bonds in a molecule. ■IHD is used to calculate number of rings or π bonds in a structure, where: –A double bond is one degree of unsaturation –A triple bond is two degrees of unsaturation –A ring is counted as one degree of unsaturation –An aromatic ring is counted as four degrees of unsaturation

3. What is the IHD for ecstasy, shown below?

4. IHD from molecular formula ■IHD from a generic molecular formula C c H h N n O o X x : IHD = (0.5)(2c+2-h-x+n) ■Example: C 4 H 8 O 2 : –c= 4 –h= 8 –n= 0 –o=2 –x=0 ■IHD = (0.5)(8+2-8-0+0)=1 Draw two isomers for this molecule.

5. IHD from formula -- Determine the IHD for these molecules: 1.C 17 H 21 NO 4 (cocaine) 2.C 27 H 46 O (cholesterol) 3.C 6 H 7 N (aniline) 4.C 15 H 10 ClN 3 O 3 (clonazepam) Using the ChemSpider RSC database () look at the structure of these molecules and check to see if you calculations are correct. Using the ChemSpider RSC database (www.chemspider.com) look at the structure of these molecules and check to see if you calculations are correct.www.chemspider.com

6. ELECTROMAGNETIC SPECTRUM SL Chemistry Topic 11.3

7. **h = Planck’s constant = 6.63 x 10 -34 J s E = energy of radiation (measured in J) **c = speed of light = 3.00 x 10 8 m s -1 λ = wavelength (measured in m)

8. Spectroscopy – the study of the way matter interacts with radiation ■X-rays: –High energy leads to removal of electrons from inner energy levels leading to diffraction patterns –Diffraction patterns lead to information about bond length and angles –Basis of X-ray crystallography X-ray crystallography of DNA by Rosalind Franklin in 1953. Led to discovery of helical structure.

9. Spectroscopy ■Visible and ultraviolet (UV) light: –Electronic transitions between energy levels –Gives information on the energy levels in an atom or molecule –Basis for UV-vis spectroscopy

10. Spectroscopy ■Infrared spectroscopy: –Causes certain bonds to vibrate (stretch and bend) –Provides information on the functional groups present –Basis for IR spectroscopy

11. Spectroscopy ■Microwaves: –Cause molecular rotations –Gives information on bond lengths

12. Spectroscopy ■Radiowaves: –Cause nuclear transitions in a strong magnetic field because radiowaves can be absorbed by certain nuclei –Information on different chemical environments of atoms and connectivity of the atoms present –Basis for Nuclear magnetic resonance (NMR) spectroscopy

13. INFRARED SPECTROSCOPY SL Chemistry Topic 11.3

14. IR Spectroscopy Infrared radiation causes molecular vibrations which result from vibration of certain groups of molecules about their bonds, leading to deduction of functional groups. Spring Model for covalent bonds treats them as if they are springs which can be stretched (symmetrically and asymetrically), bent, or twisted, giving rise to a distortion. Lighter atoms will vibrate at a higher frequencies, v, and heavier atoms will vibrate at lower frequencies, v. (Hooke’s Law) For multiple bonds, the stronger the bond, the more energy required to vibrate.

15. IR Spectroscopy For diatomic molecules (like HCl), there is only one kind of vibration (stretching). When looking the frequencies of vibration of HCl, HBr, and HI, HCl has the smallest mass and greatest bond enthalpy which will mean the greatest frequency. Different molecules absorb at different frequencies because the energy required depends on bond enthalpy. IR absorption is typically cited as inverse wavelength (or wavenumber) and has units cm -1.

16. IR Spectroscopy For polyatomic species (like H 2 O), there may be several different modes of vibration –A symmetric stretch (3652 cm -1 ) –An asymmetric stretch (3756 cm -1 ) –A symmetric bend (1595 cm -1 ) However, for a covalent bond to absorb IR radiation, there needs to be a change in the molecular dipole moment associated with the vibration mode (IR active). ■Is there a change in the dipole moments for H 2 O?

17. IR Spectroscopy

18. Absorbance, A, of a sample can be related to transmittance by A = -log 10 T An IR spectrum is a plot of %transmittance versus wavenumber. This spectrum can be used to identify functional groups. There are characteristic ranges of wavenumbers for certain functional groups. DB Section 26

19. ■Strong, broad peak in the range 2500-3000 cm -1 characteristic of O-H ■Strong peak in the range 1700-1750 cm -1 characteristic of C=O ■Peak in the range 2850-3090 cm -1 characteristic of C-H

20. 1 H NMR SPECTROSCOPY SL Chemisry Topic 11.3

21. 1 H NMR Spectroscopy ■Gives information on the chemical environment of hydrogen atoms in a molecule. ■Chemical environments change depending on proximity to other groups, especially electronegative groups (O, N, Hal, etc). ■Each signal on the spectrum indicates a different chemical environment –E.g.: spectrum with 3 peaks has 3 different chemical environments ■Integration trace on each peaks gives the relative number of H within that environment DB Section 27 and pg 284

22. 1 H NMR Spectroscopy

23. MASS SPECTROSCOPY SL Chemistry Topic 11.3

24. Mass Spectroscopy ■We used MS earlier in Topic 2 to look at isotopes. In OChem, it is used to supplement the identification of molecules. ■The molecular ion peak (M + ) in a mass spectrum corresponds to the molecular mass of the compound. ■The high energy involved in ionization also means that fragments of the molecule can split off. This fragmentation pattern can be used to help identify molecules. (M r -15) + results from loss of –CH 3 (M r -17) + results from the loss of –OH (M r -29) + results from the loss of – CHO or –CH 2 CH 3 (M r -31) + results from the loss of – OCH 3 (M r -45) + results from the loss of -- COOH

25. Mass Spectroscopy Some of the peaks for propan-1-ol:

26. ANALYTICAL ORGANIC CHEMISTRY PUTTING IT ALL TOGETHER!!!

27. When deducing a chemical structure: 1.Determine the empirical formula from elemental analysis. 2.Determine the molecular formula from MS. 3.Determine IHD for types of bonds. 4.Determine the possible functional groups from IR. 5.Determine the possible H environments from 1 H NMR. –Watch for upfield shifting! 6.Compare #3, #4, and #5 for consistency. 7.Draw structure and name. Worked example in textbook on page 286-288.