UNIVERSITY OF DAR ES SALAAM

1. IR SPECTRA OF DIFFERENT FUNCTIONAL
GROUPS
Alkanes: e.g Hexane
• Only C−C single bonds and sp3 hybridized C atoms.
• Therefore, it has only one major absorption at 3000–2850 cm−1.

2. ALKENES
e.g 1-Hexene
• 1-Hexene has a C=C and Csp2−H, in addition to sp3 hybridized
C atoms.
• Therefore, there are three major absorptions:
Csp2−H at 3150−3000 cm−1;
Csp3−H at 3000−2850 cm−1;
C=C at 1650 cm−1.

3. ALKYNES
Absorptions from nitriles are generally of medium intensity and are clearly defined.
Alkynes absorb weakly in this region unless they are highly asymmetric;
NOTE: symmetrical alkynes do not show absorption bands.
e.g.1-Hexyne
• has a CC and Csp−H, in addition to sp3 hybridized C atoms.
• Therefore, there are three major absorptions: Csp−H at
3300 cm−1; Csp3−H at 3000−2850 cm−1; CC at 2250 cm−1.

4. C
N
C≡C and C≡N Absorption
The CC and CN triple bond absorptions at 2100-
2260 cm-1 are small but exposed/unique.

5. AROMATIC RINGS

6. 6
Aromatic C-H and C=C
O
• Aromatic C-H around 3050 cm-1 and C=C two
medium bands at about 1500 and 1600 cm-1.

7. ALCOHOLS

8. 8
Carboxylic Group O-H
• Characteristic broad band (3400 – 2500 cm-1)
due to H-bonding.
• Identifies/confirms presence of a carboxylic acid.

9. ALDEHYDES

10. KETONES

11. AMINES

12. 12

13. 13
Base Values for IR Absorptions
Bond type Wavenumbers
O-H 3400
N-H 3400
C-H 3000
C≡N 2250
C≡C 2150
C=O 1715
C=C 1650
C-O 1100

14. IR-spec Exercises

15. IR-spec Problem

16. More Problems