10IR- Empirical Comparisons Identifying functional groups in organic molecules
11Infrared Spectroscopy Region of infrared that is most useful lies between mm ( cm-1)depends on transitions between vibrational energy statesStretching: higher energy / higher wave number (cm-1)Bending: lower energy / lower wave number (cm-1)2
12A 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 distancebetween 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.
13Functions & Infrared Spectra Analyzing Structure:Functions & Infrared SpectraThe 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 notdetected.
14Structural/Functional Components Figure: UNCaption:The IR spectrum has distinct regions. The left of the spectrum shows the C-H, O-H, and N-H stretches. The triple bonds absorb around 2200 cm-1 followed by the double bonds to the right at around 1700 cm-1. The region below 1400 cm-1 is called the fingerprint region.
40Infrared Spectroscopy Common Functional Groups Questions 1
412,4,6-cycloheptaheptatrien-1-one Question #1C7H6OIdentify the compound from the IR above.benzyl alcohol2,4,6-cycloheptaheptatrien-1-oneacetophenonebenzaldehydephenylacetic acid
422,4,5-trimethylbenzaldehyde Question #2C10H12OIdentify the compound from the IR above.2,4,5-trimethylbenzaldehydep-allylanisole2-allyl-4methylphenol1-phenyl-2-butanone
43Question #3 Identify the compound from the IR above. cyclopropanone C3H4OIdentify the compound from the IR above.cyclopropanonepropynolacrylaldehydepropenoic acid
44Question #4 Identify the compound from the IR above. methylbenzoate C8H8O2Identify the compound from the IR above.methylbenzoateo-hydroxyacetophenoneo-toluic acidp-anisaldehyde (p-methoxybenzaldehyde)
45Question #5 Identify the compound from the IR above. benzylformate C8H8O2Identify the compound from the IR above.benzylformateo-hydroxyacetophenone2-methoxytroponep-toluic acido-anisaldehyde (p-methoxybenzaldehyde)