1. Infrared Spectroscopy1

2. Infrared Absorbance

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

4. Infrared Spectroscopy
Region of infrared that is most useful lies between mm ( 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) 2

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. Functions & Infrared Spectra
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
Figure: UN
Caption:
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.

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

11. The functional group
stretching region
The fingerprint
region

12. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols)
O—H (carboxylic acids)
N—H
First examine the absorption bands in the vicinity of
cm–1 8

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

15. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H
sp2 C—H
sp3 C—H
sp2 C—O 1200
sp3 C—O 8

16. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H
sp2 C—H
sp3 C—H
sp2 C—O 1200
sp3 C—O 8

17. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (multiple bonds) C —C C— —C N 8

18. Some hydrocarbon absorption bands

19. 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 ( cm-1) and the ester ( cm-1) is shorter (stronger).

20. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (carbonyl groups)
Aldehydes and ketones
Carboxylic acids
Acid anhydrides and
Esters
Amides 8

21. Cyclic aliphatic ketone

22. Mono substituted aromatic methyl ketone

23. Mono substituted aromatic ketone

24. Aliphatic ester I

25. Aliphatic ester II

26. Aliphatic ester III

27. Mono substituted aromatic ester

28. Mono substituted aromatic conjugated ester

29. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp2 C—O 1200
sp3 C—O 8

30. Dihexyl Ether
~1100 cm-1
cm-1 8

31. ~1200 cm-1

32. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Bending vibrations of alkenes
CH2
RCH
CH2
R2C
890
CHR'
cis-RCH
CHR'
trans-RCH
CHR'
R2C 8

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

34. cis
trans

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

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

37. Aliphatic aldehyde

38. Mono-substituted aromatic aldehyde

39. Mono-substituted aromatic conjugated aldehyde

40. Mono substituted aromatic ester

41. Para di-substituted aromatic ether & aldehyde

42. Infrared Spectroscopy Common Functional Groups Handout 1

43. Aromatic Absorption Frequencies
Structural unit Frequency, cm-1
Bending vibrations of derivatives of benzene
Monosubstituted and
Ortho-disubstituted
Meta-disubstituted and
Para-disubstituted 8

45. Infrared Spectrum of tert-butylbenzene
C6H5C(CH3)3
Ar—H
H—C
Monsubstituted benzene
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1 8

46. Chemical Communication: Smell / Pheromones
Pheromone Synthesis
[20:40-23:51]

47. Infrared Spectroscopy Common Functional Groups QUIZ1

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

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

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

51. Question Identify the compound from the IR above. cyclopropanone
C3H4O
Identify the compound from the IR above.
cyclopropanone
propynol
2-cyclopropenol
1,2-propadienol

52. Question Identify the compound from the IR above. N-methylacetamide
C3H7NO
Identify the compound from the IR above.
N-methylacetamide
N,N-dimethylformamide
3-aminopropanal
N-methylamino-ethanal

53. Question I II III Match the ortho, meta and para isomers of xylene.B) C) D)

54. Question Identify the compound from the IR above. methylbenzoate
C8H8O2
Identify the compound from the IR above.
methylbenzoate
phenylacetate
p-anisaldehyde
o-anisaldehyde