Organic Chemistry

1. Organic Chemistry William H. Brown & Christopher S. Foote

2. Infrared Spectroscopy Chapter 12

3. Electromagnetic Radiation • Electromagnetic radiation:light and other forms of radiant energy • Wavelength ():the distance between consecutive identical points on a wave • Frequency ():the number of full cycles of a wave that pass a point in a second • Hertz (Hz):the unit in which radiation frequency is reported; s-1 (read “per second”)

4. Electromagnetic Radiation • Wavelength

5. Molecular Spectroscopy • Molecular spectroscopy:the study of which frequencies of electromagnetic radiation are absorbed or emitted by substances and the correlation between these frequencies and specific types of molecular structure • we study three types of molecular spectroscopy

6. Infrared Spectroscopy • The vibrational IR extends from 2.5 x 10-6 m (2.5 m) to 2.5 x 10-5 m (25 m) • the frequency of IR radiation is commonly expressed in wavenumbers • wavenumber:the number of waves per centimeter, cm-1 (read reciprocal centimeters) • expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1

7. Infrared Spectroscopy • IR spectrum of 3-methyl-2-butanone

8. Molecular Vibrations • atoms joined by covalent bonds undergo continual vibrations relative to each other • the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed • the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm-1

9. Molecular Vibrations • For a molecule to absorb IR radiation • the bond undergoing vibration must be polar and • its vibration must cause a periodic change in the bond dipole moment • Covalent bonds which do not meet these criteria are said to be IR inactive • the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds

10. Molecular Vibrations • For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibrations • For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex • The simplest vibrational motions are bending and stretching

11. Molecular Vibrations

12. Molecular Vibrations • Consider two covalently bonded atoms as two vibrating masses connected by a spring • as the bond vibrates, its energy continually changes from kinetic to potential and vice versa • the total energy is proportional to the frequency of vibration

13. Molecular Vibrations • For a simple harmonic oscillator, the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring K = a force constant, which is a measure of the bonds’ strength. Force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm m = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams

14. Molecular Vibrations • From this equation, we see that the position of a stretching vibration depends on • the strength of the vibrating bond and • the masses of the atoms connected by the bond • The intensity of absorption depends primarily on the polarity of the vibrating bond

15. Correlation Tables • Table 12.4Characteristic IR absorptions for the types of bonds and functional groups we deal with most often

16. Hydrocarbons-Table 12.5

17. Alkanes • IR spectrum of decane (Fig 12.4)

18. Alkenes • IR spectrum of cyclohexene (Fig 12.5)

19. Alkynes • IR spectrum of 1-octyne (Fig 12.6)

20. Aromatics • IR spectrum of toluene (Fig 12.7)

21. Other Functional Groups • see also Appendix 6

22. Alcohols • IR spectrum of 1-hexanol (Fig 12.8)

23. Ethers • IR spectrum of dibutyl ether (Fig 12.9)

24. Ethers • IR spectrum of anisole (Fig 12.10)

25. Amines • IR spectrum of 1-butanamine (Fig 12.11)

26. Aldehydes and Ketones • IR spectrum of menthone (Fig 12.12)

27. Carbonyl groups • The position of C=O stretching vibration is sensitive to its molecular environment • as ring size decreases and angle strain increases, absorption shifts to a higher frequency • conjugation shifts the C=O absorption to lower frequency

28. Carboxylic acids • IR spectrum of pentanoic acid (Fig 12.13)

29. Esters • IR of ethyl butanoate (Fig 12.14)

30. Prob 12.13 Show how IR spectroscopy can be used to distinguish between the compound in each set.

31. Prob 12.13 (cont’d) Show how IR spectroscopy can be used to distinguish between the compound in each set.

32. Infrared Spectroscopy End Chapter 12