1. SURVEY OF SPECTRA HYDROCARBONS (C-H ABSORPTIONS) ALCOHOLS ACIDS
N-H 3400
C-H 3000
C=N 2250
C=C 2150
C=O 1715
C=C 1650
C-O =
HYDROCARBONS
(C-H ABSORPTIONS)
ALCOHOLS
ACIDS
(O-H ABSORPTIONS)
AMINES
(N-H ABSORPTIONS)
SURVEY OF SPECTRA

2. Typical Infrared Absorption Regions
WAVELENGTH (mm)
2.5 4 5
5.5
6.1
6.5
15.4
C=O
C=N
C-Cl
O-H
C-H
C N
Very
few
bands
C-O
N-H
C C
C=C
C-N
X=C=Y
C-C
N=O N=O *
(C,O,N,S)
4000
2500
2000
1800
1650
1550
650
FREQUENCY (cm-1)

3. BASE VALUES
Guideposts for you to memorize.

4. BASE VALUES O-H 3600 N-H 3400 C-H 3000 C N 2250 C C 2150 C=O 1715
These are
the minimum
number of
values to
memorize.
(+/- 10 cm-1)
O-H 3600
N-H 3400
C-H 3000
C N 2250
C C 2150
C=O 1715
C=C 1650
C O ~1100
large range

5. C-H STRETCH

6. Typical Infrared Absorption Regions
WAVELENGTH (mm)
C-H
2.5 4 5
5.5
6.1
6.5
15.4
C=O
C=N
C-Cl
O-H
C-H
C N
Very
few
bands
C-O
N-H
C C
C=C
C-N
X=C=Y
C-C *
(C,O,N,S)
N=O N=O
4000
2500
2000
1800
1650
1550
650
FREQUENCY (cm-1)
We will look at
this area first

7. The C-H stretching region
BASE VALUE = 3000 cm-1
C-H sp stretch ~ 3300 cm-1
UNSATURATED
C-H sp2 stretch > 3000 cm-1
3000 divides
C-H sp3 stretch < 3000 cm-1
SATURATED
C-H aldehyde, two peaks (both weak)
~ 2850 and 2750 cm-1

8. STRONGER BONDS HAVE LARGER FORCE CONSTANTS
AND ABSORB AT HIGHER FREQUENCIES
increasing frequency (cm-1)
3300
3100
3000
2900 =
=C-H
=C-H
-C-H
-CH=O
(weak)
sp-1s
sp2-1s
sp3-1s
aldehyde
increasing s character in bond
increasing CH Bond Strength
increasing force constant K
CH BASE VALUE = 3000 cm-1

9. METHYLENE GROUP STRETCHING VIBRATIONS
Two C-H bonds share a central carbon
(hydrogens attached to the same carbon)
in-phase
Symmetric Stretch
~2853 cm-1
out-of-phase
Asymmetric Stretch
~2926 cm-1
Any time you have two or more of the same kind of bond sharing
a central atom you will have symmetric and asymmetric modes.

10. METHYL GROUP STRETCHING VIBRATIONS
Three C-H bonds share a central carbon
(hydrogens attached to the same carbon)
in-phase
Symmetric Stretch
~2872 cm-1
out-of-phase
Asymmetric Stretch
~2962 cm-1

11. Hexane CH stretching vibrations ALKANE CH bending vibrations
discussed shortly CH
stretching
vibrations
includes
CH3 sym and asym
CH2 sym and asym

12. C-H BENDING

13. THE C-H BENDING REGION CH2 bending ~ 1465 cm-1
CH3 bending (asym) appears near
the CH2 value ~ 1460 cm-1
CH3 bending (sym) ~ 1375 cm-1

14. METHYLENE GROUP BENDING VIBRATIONS
Scissoring
Wagging
~1465 cm-1
~1250 cm-1
~720 cm-1
~1250 cm-1
Rocking
Twisting
in-plane
out-of-plane
Bending
Vibrations

15. METHYLENE AND METHYL BENDING VIBRATIONS
CH2
CH3
C-H Bending, look near
1465 and 1375 cm-1
asym
sym
1465
1460
1375
these two peaks
frequently overlap
and are not resolved

16. METHYLENE AND METHYL BENDING VIBRATIONS
ADDITIONAL DETAILS FOR SYM CH3
The sym methyl peak
splits when you have
more than one CH3
attached to a carbon.
CH2
CH3
asym
sym
one peak
1465
1460
1375
geminal dimethyl
1380
1370
two peaks
(isopropyl)
t-butyl
1390
1370
two peaks

17. Hexane
ALKANE
CH2
rocking
> 4C
CH3
bend
CH2
bend CH
stretch

18. ALKENE
1-Hexene
=CH
C=C
CH2
CH3
bend
CH oops CH

19. AROMATIC
Toluene
CH3
Ar-H
C=C
benzene
Ar-H oops

20. ALKYNE
1-Hexyne
C=C =
CH2, CH3
C-H
=C-H =

21. O-H STRETCH

22. Typical Infrared Absorption Regions
O-H
WAVELENGTH (mm)
2.5 4 5
5.5
6.1
6.5
15.4
C=O
C=N
C-Cl
O-H
C-H
C N
Very
few
bands
C-O
N-H
C C
C=C
C-N
X=C=Y
C-C
N=O N=O *
(C,O,N,S)
4000
2500
2000
1800
1650
1550
650
FREQUENCY (cm-1)

23. The O-H stretching region
O-H cm-1 (alcohol, free)
O-H cm-1 (alcohols & acids,
H-bonding)
3600
3300
H-BONDED
FREE
broadens
shifts

24. Effect of Hydrogen-Bonding on O-H Stretching
Free OH
Free OH
C-H
H-bonded OH
H-bonded OH
C-H
C-H
Pure Liquid
Dilute Solution
Very Dilute Solution
(a)
(b)
(c)
“neat”
1-Butanol

25. HYDROGEN-BONDED HYDROXYL
Many kinds of OH
bonds of different
lengths and strengths
This leads to a broad
absorption.
“Neat” solution.
Longer bonds are weaker and lead to
lower frequency.
Hydrogen bonding occurs in concentrated solutions
( for instance, undiluted alcohol ).

26. “FREE” HYDROXYL
The “free” hydroxyl vibrates without interference from any
other molecule.
Distinct bond has a
well-defined length
and strength.
Solvent molecules
surround but do not
hydrogen bond.
Occurs in dilute solutions of alcohol in an “inert” solvent like CCl4.

27. ALCOHOL
Cyclohexanol
neat solution
O-H
H-bond
CH2
C-O
C-H

28. CARBOXYLIC ACID
Butanoic Acid
neat solution
O-H
H-bond
C-O
CH2
C-H
C=O

29. CARBOXYLIC ACID DIMER
Strong hydrogen bonding in the dimer weakens the OH
bond and leads to a broad peak at lower frequency.

30. N-H STRETCH

31. Typical Infrared Absorption Regions
N-H
WAVELENGTH (mm)
2.5 4 5
5.5
6.1
6.5
15.4
C=O
C=N
C-Cl
O-H
C-H
C N
Very
few
bands
C-O
N-H
C C
C=C
C-N
X=C=Y
C-C
N=O N=O *
(C,O,N,S)
4000
2500
2000
1800
1650
1550
650
FREQUENCY (cm-1)

32. The N-H stretching region
N-H cm-1
Primary amines give two peaks
Secondary amines give one peak
Tertiary amines give no peak
symmetric
asymmetric

33. PRIMARY AMINE
aliphatic
1-Butanamine
NH2
scissor
CH3
CH2
NH2

34. 3-Methylbenzenamine PRIMARY AMINE aromatic -CH3 Ar-H NH2 benzene Ar-H
oops

35. SECONDARY AMINE
N -Ethylbenzenamine NH
CH3
Ar-H
oops
benzene

36. N,N -Dimethylaniline TERTIARY AMINE Ar-H -CH3 no N-H CH3 benzene Ar-H
oops