1. Infrared Spectroscopy

3. The IR Spectrum –The detection of different bonds It is important to make note of peak intensities:
•Strong (s) peak is tall, transmittance is low (0-35 %)
•Medium (m) peak is mid-height (75-35%)
•Weak(w) peak is short, transmittance is high (90-75%)

4. The IR Spectrum –The detection of different bonds
It is important to make note of peak shapes:
•Broad (br) strong to medium peak that expands to a range wide wavenumbers (U-shaped)
•Example: O-H bond signal
•Sharp (sr) strong to medium peak that is narrow and is most of the time specific to a single wavenumber
•Example: C=O bond signal
•Multiplet(m) multiple weak peaks that is narrow and cover a range of wavenumbers
•Example: C=C-H (aromatic) bond signal

5. Infrared Group Analysis ,
The four primary regions of the IR spectrum:

6. Characteristic Absorptions (cm-1)
Frequency of functional groups :
Functional Group
Type of Vibration
Characteristic Absorptions (cm-1)
Intensity
Alkane
C-H
stretch
strong
-C-H
bending
variable
Alkene
=C-H
medium
C=C
Alkyne
3300
strong, sharp
variable, not present in symmetrical alkynes
Aromatic
medium-weak, multiple bands

7. Characteristic Absorptions (cm-1)
Functional Group
Type of Vibration
Characteristic Absorptions (cm-1)
Intensity
Alkyl Halide
C-F
stretch
strong
C-Cl
C-Br
C-I
500
Alcohol
O-H
(stretch, H-bonded)
strong, broad
(stretch, free)
strong, sharp
C-O
(stretch)
strong
Amine
N-H
stretch
medium (primary amines have two bands; secondary have one band, often very weak)
C-N
medium-weak
bending
1600
medium

8. Characteristic Absorptions (cm-1)
Functional Group
Type of Vibration
Characteristic Absorptions (cm-1)
Intensity
Carbonyl
C=O
stretch
strong
(conjugation moves absorptions to lower wave numbers)
Acid
C=O
stretch
strong
O-H
strong, very broad
C-O
Aldehyde
=C-H
medium, two peaks
Amide
N-H
unsubstituted have two bands
bending

9. Characteristic Absorptions (cm-1)
Functional Group
Type of Vibration
Characteristic Absorptions (cm-1)
Intensity
 Anhydride
C=O
stretch
&
two bands
Ester
strong
C-O
two bands or more
Ketone
acyclic
α,β-unsaturated
aryl ketone