Ir lecture-2010

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  • 1. The scale UNITS:cm-1 unit is thewavenumberand is given by1 / (wavelength in cm). = 1/ Infrared (IR) spectroscopydeals spectrum with the interaction between a molecule and radiation from the IR region of the electromagnetic(IR region = 4000 - 400 cm-1).

2. IR radiation causes the excitation of thevibrations of covalent bondswithin that molecule ( motions between nuclei).These vibrations include thestretchingandbendingmodes 3. 4. How to can we type a Compound by IR?

  • Draw four lines on your IR spectrum
  • 3000 cm -1
  • 2500 cm -1
  • 2000 cm -1
  • 1500 cm -1
  • Get a big picture first!

5. It is important thefrequencyof the energy absorbed by the type of bond (E= hC/ ) and theshape of the peakcorresponding to that absorption A B C D E F

  • broad and strong,typical of alcohols, centered around 3300cm -1, takes 80 % of the chart
  • broad and medium,typical ofcarboxylic acids, is not as smooth as alcohols due to other absorptions in the same region, 50% of the chart.
  • Sharp strong,typical ofcarbonyl group around 1700 cm -1, Usually, 98% of the chart
  • Sharp strong,typical of C-Hcentered around 3300 cm-1, usually 75% of chart

E.Sharp medium,typical of C=C bond and others in the finger print regionF.Sharp weak,typical of amines, and overtones 6. The IR Chart 1800-1600 C=O C=N C=N ~3300 H-EN ~2300 C C C N 7.

    • Compound class Bond Frequency (cm-1)

US 3000 1500 Saturated Sp3 carbon stretching Saturated Sp3 carbon Bending FINGERPRINT

  • Above 3000 cm -1[X-H (X = O, N, sp C,or sp 2= C)]
  • 3000-2500 [C-H (sp 3C H) stretching]
  • 2500-2000 (C C, C N triple bond stretching)
  • 2000-1500 [C=O, appr.1700; C=C and N=O, appr. 1600]
  • Under 1500 (fingerprint region)

8. Single bonds: n-hexane AlkanesC-H2960-2850 (strong, stretching),1470-1350 (bending) US 9. Double bonds/hydrocarbons

  • Alkenes, arenes
  • =C-H
    • Compound Bond Frequency (cm-1)

US 3000 1500 Saturated Sp3 C-H stretching Saturated Sp3 carbon bending Unsaturated Sp2 C-H stretching ~ 1600 cm -1 C=C stretching 10. Double bonds /hydrocarbons Alkenes2-hexeneC=C,1680-1640 (medium, stretching) C-H 3090-3020 (strong, stretching),1000-675 (bending) C=C 11. Methyl benzene Aromatic Rings C=C1600-1500 (stretching)=C-H 3100-3000 (stretching),2000-1600 (phenyl ring substitution overtones) US overtones Double bonds /hydrocarbons 12. Other Double bonds

  • Nitro Compounds
  • N=O1660-1500 (s, asymmetrical s
  • 1390-1260 (s, symmetrical stretching)
  • C=O 1760-1670 (s, stretching)
    • FAMILY A
  • Aldehydes,
  • Ketones,
    • FAMILY B
  • Carboxylic acids,
  • Esters
  • Amides
  • Acyl halides
  • Acid Anhydrides
  • C=O1760-1670 (s, stretching)

13. Acetone:Residues of acetone used during cleaning or in contamination during GC analysis will be observed with a weak peak at 1715 cm-1 Other double bonds 14. 1-HEXYNE Triple bonds /hydrocarbons Alkynes C-H 3333-3267(sharp, strong, stretching),700-610 (bending)C C 2260-2100 (sharp, medium, stretching) C-H C C 15. Internal Alkyne C C overtones C=C 16. Triple bonds

  • Alkynes
  • C-H 3333-3267(sharp, stretching),
  • 700-610 (bending)
  • C C 2260-2100 (sharp, strong, stretching)
  • Nitriles
  • C N 2260-2220 (sharp, strong stretching)
    • Compound Bond Frequency (cm-1)

17. Cyano benzene C=C C N Nitriles C N 2260-2220 (sharp, strong stretching) 18.

  • Alcohols, Phenols
  • O-H(H-bond formation)3640-3160 (s & br, stret.)
  • Carboxylic acids
  • O-H(H-bond formation) 3500-2500 (b, stretching)
  • Amines
  • N-H(H-bond formation)
  • (2 peaks for primary amine, one peak for secondary amine)
  • 3500-3300 (m, stretching)

H-Electronegative atom

    • CompoundBond Frequency (cm-1)

19. 1-pentanol 20. Difference between a carboxylic acid and an alcohol. Hexanoic acid 21. Isopropanol 22. Cyclobutanol 23. cyclohexanol 24. 2-methyl-2-propen-1-ol, and alkene/alcohol 25. Benzyl alcohol 26. Observe C-Ofingerprint region Ethyl-vinyl ether/ether- alkene 27. Diethyl amine N-H 28. Butyl amine N-H H Observe a primary amine shows two absorptions, one for each hydrogen bonded to the electronegative atom Nitrogen 29.

    • CompoundBond Frequency (cm -1 )
  • Ketonesand aldehydes
  • C=O1760-1670 (strong, stretching)
  • Recall that aldehydes also show:
  • CHO two peaks2850 and 2780 medium.

Carbonyl family A 30. 3-pentanone / a ketone 31. 2-butanone-/a ketone 32. (Gas Phase) of Formaldehyde, H(C=O)H 33. 2-methylpropanal: an aldehyde 34. butyraldhyde CHO two peakssharp, medium, around2850and2780cm-1 . 35.

  • Carboxylic acids, Esters
  • C=O1760-1670 (strong, stretching)
  • C-O 1260-1000 ( strong,stret.)
  • Recall that Carboxylic acids also show:
  • O-H(H-bond formation)
  • 3500-2500 (broad, stretching)

Carbonyl/ Family B 36. Carboxylic acid 37. Benzoic acid O-H C=C C=O 38. EsterC=Oand C-OC=O C-O 39. Ethyl acetate C=O C-O 40.

  • amides
  • C=O1760-1670 (strong, stretching)
  • C-O 1260-1000 ( strong,stret.)
  • Recall amide also show:
  • N-H as in amines but strong and broad forH-bond formation)
  • One peak for secondary amide, two peaks for primary amide

Carbonyl family 41. Amide/acetamide C=O N-H H 42. Benzamide C=O C=C N-H N-H