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Infrared Spectroscopy

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  1. Infrared Spectroscopy

  2. Infrared Absorbance

  3. IR- Empirical Comparisons Identifying functional groups in organic molecules

  4. Infrared Spectroscopy Region of infrared that is most useful lies between2.5-16 mm (4000-625 cm-1) depends on transitions between vibrational energy states Stretching: higher energy / higher wave number (cm-1) Bending: lower energy / lower wave number (cm-1)

  5. A bond must have a dipole or an induced dipole in order to have an absorbance in the IR spectrum. When the bond stretches, the increasing distance between the atoms increases the dipole moment. Therefore, the greater the dipole, the more intense the absorption. (i.e., The greater the molar extinction coefficient () in Beer’s law, A = bc.

  6. The energy (IR frequency/ wave number) and the intensity of the absorption band also depends on the concentration of solution from Beer’s law, A = bc. It is easier to stretch an O–H bond if it is hydrogen bonded

  7. Analyzing Structure: Functions & Infrared Spectra The molecular formula is a critical piece of information, which limits the functional possibilities. The presence & absence of absorption bands must be considered in identifying a possible structure in IR spectroscopy. Empiricism is critical to successful identification. NOTE: Bonds which lack dipole moments are not detected.

  8. Structural/Functional Components

  9. An Infrared Spectrum The peaks are quantized absorption bands corresponding to molecular stretching and bending vibrations

  10. The fingerprint region The functional group stretching region

  11. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 N—H 3350-3500 First examine the absorption bands in the vicinity of 4000-3000 cm–1

  12. IR Spectrum of a Primary Amine(1o) The N–H bending vibration occurs at ~1600 cm–1

  13. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200

  14. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200

  15. C C —C C— —C N Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (multiple bonds) 1620-1680 2100-2200 2240-2280

  16. Some hydrocarbon absorption bands

  17. Structural Components & Functional Differences: The nitrogen of an amide is less electronegative than the oxygen of an ester. Therefore the amide has a longer (weaker) C=O bond (1680-1700 cm-1) and the ester (1730-1750 cm-1) is shorter (stronger).

  18. C O Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (carbonyl groups) Aldehydes and ketones 1710-1750 Carboxylic acids 1700-1725 Acid anhydrides 1800-1850 and 1740-1790 Esters 1730-1750 Amides 1680-1700

  19. Cyclic aliphatic ketone

  20. Mono substituted aromatic methyl ketone

  21. Mono substituted aromatic ketone

  22. Aliphatic ester I

  23. Aliphatic ester II

  24. Aliphatic ester III

  25. Mono substituted aromatic ester

  26. Mono substituted aromatic conjugated ester

  27. Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp2 C—O 1200 sp3 C—O 1025-1200

  28. Dihexyl Ether ~1100 cm-1 1025-1200 cm-1

  29. ~1200 cm-1

  30. RCH CH2 R2C CH2 cis-RCH CHR' trans-RCH CHR' R2C CHR' Infrared Absorption Frequencies Structural unit Frequency, cm-1 Bending vibrations of alkenes 910-990 890 665-730 960-980 790-840

  31. wavenumber (cm–1) assignment 3075 2950 1650 and 890 ? ? ?

  32. cis- 665-730 trans- 960-980

  33. Summary: C–H bond absorption and hybridization of the carbon atom

  34. Distinctive Stretch of C–H Bond in an Aldehyde (the “waggle” vibration)

  35. Aliphatic aldehyde

  36. Mono-substituted aromatic aldehyde

  37. Mono-substituted aromatic conjugated aldehyde

  38. Mono substituted aromatic ester

  39. Para di-substituted aromatic ether & aldehyde

  40. Infrared SpectroscopyCommon Functional Groups Handout http://chemconnections.org/general/chem121/Spectroscopy/IR-handout-11.htm

  41. Aromatic Absorption Frequencies Structural unit Frequency, cm-1 Bending vibrations of derivatives of benzene Monosubstituted 730-770 and 690-710 Ortho-disubstituted 735-770 Meta-disubstituted 750-810 and 680-730 Para-disubstituted 790-840

  42. Ar—H H—C 3500 3000 2500 2000 1500 1000 500 Infrared Spectrum of tert-butylbenzene C6H5C(CH3)3 Monsubstitutedbenzene Wave number, cm-1

  43. Chemical Communication: Smell / Pheromones http://chemconnections.org/COT/COT-chemcomm-eg.html Pheromone Synthesis [20:40-23:51] http://www.learner.org/resources/series61.html

  44. Infrared SpectroscopyCommon Functional Groups QUIZ

  45. Question Is the following IR of cis or trans 2-pentene? • cis B) trans

  46. Question C7H6O Identify the compound from the IR above. • benzyl alcohol • 1,4,6-heptatrien-3-one • 2,4,6-heptatrienaldehyde • benzaldehyde

  47. Question C10H12O Identify the compound from the IR above. • 4-phenylbutanaldehyde • phenylpropyl ketone • meta-isopropylbenzaldehyde • 1-phenyl-2-butanone