1. Che 440/540 Infrared (IR) Spectroscopy

2. Some Major IR Absorption Bands
cm-1
N–H stretch
Amines
cm-1
O–H stretch
alcohols, a broad, strong band
cm-1
C–H stretch
Alkenes
cm-1
Alkanes
cm-1
C=O stretch
ketones, aldehydes, esters
cm-1
C=C stretch
alkenes

3. How to Analyze an IR Spectrum
2. If a C::O is present you want to determine if it is part of an acid, an ester, or an aldehyde or ketone. At this time you may not be able to distinguish aldehyde from ketone and you will not be asked to do so.
How to Analyze an IR Spectrum
Look first for the carbonyl C=O band. Look for a strong band at cm-1.
This band is usually the most intense absorption band in a spectrum. It will have a medium width. If you see the carbonyl band, look for other bands associated with functional groups that contain the carbonyl by going to step 2. If no C=O band is present, go to step 3 and check for alcohols.
2. If a C=O is present determine if it is part of an acid, an ester, aldehyde, ketone or anhydride anhydrides show two C=O absorptions near 1810 and 1760 cm-1
ACID
Look for indications that an O-H is also present. It has a broad absorption near cm-1. This actually will overlap the C-H stretch. There will also be a C-O single bond band near cm-1. Look for the carbonyl band near cm-1.
ESTER
Look for C-O absorption of medium intensity near cm-1. There will be no O-H band.
ALDEHYDE
Look for aldehyde type C-H absorption bands. These are two weak absorptions to the right of the C-H stretch near 2850 cm-1 and 2750 cm-1 and are caused by the C-H bond that is part of the CHO aldehyde functional group. Look for the carbonyl band around cm-1.
KETONE
AMIDE
The weak aldehyde CH absorption bands will be absent. Look for the carbonyl C=O band around cm-1
Carbonyl C=O band from In addition, NH stretch(es) will be present from cm-1; primary amides show two bands, secondary amides one band, and tertiary amides no bands in this region

4. 3. If the C=O band is absent, look for the following:
ALCOHOLS, PHENOLS Broad absorption near cm-1.
Confirm by finding C-O stretch near cm-1
AMINES Check for N-H stretch. Medium absorption(s)
Near 3400 cm-1.
ETHERS Check for a C-O stretch near cm-1 (and an
absence of O-H near 3400 cm-1).
If no functionalities in step 3 are present, check for double bonds and/or aromatic rings.
C=C is a weak absorption near 1650 cm-1.
Aromatic rings show medium to strong absorptions from cm-1.
Consult the C-H region to confirm. Aromatic and vinyl
C-H absorptions occur to the left of 3000 cm Aliphatic C-H occurs to the right of 300 cm-1.

5. 5. Triple bonds
CN shows a medium, sharp absorption near 2250 cm-1.
CC shows a weak, sharp absorption near 2150 cm-1.
6. Nitro groups
Shows two strong absorptions at cm-1 and
cm-1.
7. Hydrocarbons
This may be indicated if none of the other functional groups
are present
Major absorptions are found to the right of 3000 cm to C-H.
Hydrocarbons show very simple spectra. The only other
absorptions appear near 1460 and 1375 cm-1.

6. C–H stretch from 3000–2850 cm-1
C–H bend or scissoring from cm-1
C–H rock, methyl from cm-1
C–H rock, methyl, seen only in long chain alkanes, from cm-1

7. C=C stretch from cm-1
=C–H stretch from cm-1
=C–H bend from cm-1

9. –C≡C– stretch from 2260-2100 cm-1
–C≡C–H: C–H stretch from cm-1
–C≡C–H: C–H bend from cm-1

10. C–H wag (-CH2X) from 1300-1150 cm-1
C–X stretches (general) from cm-1
C–Cl stretch cm-1
C–Br stretch cm-1

11. C–H stretch from cm-1
overtones, weak, from cm-1
C–C stretch (in-ring) from cm-1
C–C stretch (in-ring) from cm-1
C–H "oop" from cm-1

12. O–H stretch, hydrogen bonded 3500-3200 cm-1
C–O stretch cm-1 (s)

13. C=O stretch:
aliphatic ketones 1715 cm-1
α, β-unsaturated ketones cm-1

14. H–C=O stretch cm-1
C=O stretch:
aliphatic aldehydes cm-1
alpha, beta-unsaturated aldehydes cm-1

15. (aromatic)

16. Asymmetric C=O
stretch
Symmetric C=O
stretch

17. O–H stretch from cm-1
C=O stretch from cm-1
C–O stretch from cm-1
O–H bend from and cm-1

18. C=O stretch
aliphatic from cm-1
α, β-unsaturated from cm-1
C–O stretch from cm-1

20. N–H stretch cm-1
1° amine: two bands from and cm-1
2° amine: one band from cm-1
3° amine: no bands in this region
N–H bend (primary amines only) from cm-1
C–N stretch (aromatic amines) from cm-1
C–N stretch (aliphatic amines) from 1250–1020 cm-1
N–H wag (primary and secondary amines only) from cm-1

23. N–O asymmetric stretch from 1550-1475 cm-1
N–O symmetric stretch from cm-1

25. Effect of Conjugation on C=O absorptions
Conjugation moves the C=O absorption to a smaller wave number.
Non-conjugated Compound
Frequency
cm-1
Conjugated Compound
butanal
1725
2-butenal
1691
benzaldehyde
1702
2-butanone
1717
methyl vinyl ketone
1700 (s-cis)
1681
(s-trans)
acetophenone
1685
propanoic acid
1715
propenoic acid
benzoic acid
1688
ethyl propionate
1740
ethyl acrylate
1727
ethyl benzoate
1718
butanoic anhydride
1819,
1750
2-butenoic
anhydride
1782,
1722
benzoic
1786,
1726

26. Match the following spectra with the structures shown below.
Support your answers with specific approximate frequencies.

31. aliphatic C-H
stretch
arom C-H
stretch
C-O
stretch
arom C=C
O-H stretch

32. Propose a structure for a compound with molecular formula C7H12O,
whose IR spectrum appears below.

33. Propose a structure for a compound with molecular formula C6H12O,
with the following IR spectrum:

34. Propose a structure for a compound with molecular formula C6H13N,
whose IR spectrum appears below.

35. Alkenes (out-of-plane C-H bending)
Frequency (cm-1)
RCH=CH2
(mono-substituted)
R2C=CH2
(gem-di-substituted)
RCH=CHR
(trans-di-substituted)
(cis-di-substituted)

36. 993 cm-1
912 cm-1

37. 890 cm-1

38. 965 cm-1
(trans)

39. 698 cm-1
(cis)

40. Limitations of IR in Identifying Alkenes
What would be a better
way to identify these?

41. Benzenoid Aromatics monosubstituted: strong absorption near 690 cm-1
Combination and
overtone bands
C-H out-of-plane
bending
C=C out-of-plane
ring bending
monosubstituted: strong absorption near 690 cm-1
ortho-disubstituted: strong absorption near 750 cm-1
meta-disubstituted: the 690 cm-1 band, one near 780 cm-1, and often one near 880 cm-1
para-disubstituted: strong absorption in the region cm-1

42. monosub-
stitution
oop
aromatic
C=C stretch

43. ortho-di-
substitution
aromatic
C=C stretch
oop

44. meta-di-
substitution
aromatic
C=C stretch
oop

45. para-di-
substitution
aromatic
C=C stretch
oop

46. Propose a structure for an aromatic compound with the molecular
formula C9H12 which gives the following IR spectrum. Justify your answer.

47. Propose a structure for an aromatic compound with the molecular
formula C10H14 which gives the following IR spectrum. Justify your answer.

48. C-O stretch

49. overlap
N-H bend
C=O stretch
NH2 stretch
(primary amide)
C-N stretch

50. Overtone of 1540 band
(3080 cm-1)
NH stretch
(secondary amide)
N-H
Bend
(1540 cm-1)
C=O
stretch

51. C-N stretch C=O stretch Notice that there is no N-H stretch
in the cm-1
region in a tertiary amide.
C-N
stretch
C=O
stretch

52. Propose a structure for a compound with the molecular formula C8H14O2 which gives the following IR spectrum. Justify your answer.

53. Propose a structure for a compound with the molecular formula C8H14O2 whose IR spectrum appears below. Justify your answer.

54. Propose a structure for a compound with the molecular formula C8H9NO which gives the following IR spectrum. Justify your answer.

55. Propose a structure for a compound with the molecular formula C10H12O2 whose IR spectrum appears below. Justify your answer.

56. An unknown compound has the mass spectrum shown below. The IR spectrum
shows peaks in the and the cm-1 ranges and a strong
absorption at 1688 cm-1. Suggest a structure consistent with this data.

57. An unknown compound has the mass spectrum shown below
An unknown compound has the mass spectrum shown below. The IR spectrum shows peaks in the cm-1 range and a strong absorption at 1718 cm-1. Suggest a structure consistent with this data.