2. Organic chemist use IR spectroscopy to identify the functional
groups in a compound
C=C , C=C, OH,
C=O, N-H,
IR active : any molecular vibration that leads to
substantial change in dipole moment
IR radiation is the energy source in IR spectroscopy
IR absorption occur 4000 cm-1 – 400 cm-1
Vibrational energy level in
most covalent molecules
3. Absorption of IR light cause changes in the vibrational
motion of molecule (covalent bonds)
Covalent bonds are not static/ flexible (like springs)
Covalent bond can stretch and bend
4.
5. Bending vibrations
A molecule with three or more atoms can experience a bending vibration, a vibrational
mode where the angle between atoms changes
Change in bond angle
Occurs at lower energy: 1400-666 cm-1
There are four types of bend:
Rocking
Scissoring
Wagging
Twisting
https://www.analyticon.eu/tl_files/analyticon/inhalte/Technologien_NIR/schwi
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g_scissor_twisting_rocking_wagging_nir_en.gif
6.
7. Stretching Bending
i)Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a
bond than to stretch or compress it.)
ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms.
iii) Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn
have higher frequencies than single bonds
8. When the frequency of IR light = the frequency of bond stretching
so IR light is absorbed
Different kinds of bonds vibrate at different frequencies so they absorb
different freq of IR
IR spectroscopy distinguishes the different kinds of bonds in a molecule
so possible to determine the functional groups such as
C=O, C=C, N-H, OH and etc
9. Eg – IR spectrum
X axis - wavenumber (decrease from left to right)
Y axis - % transmittance
Each peak corresponds to a particular kind of bond
Each bond type (O-H and C-H) occurs at characteristic freq.
10. IR absorption
Where particular bonds absorp in the IR
depends on
Bond strength Atom mass
Stronger bond
vibrate at higher
freq (υ )
Bond with lighter
mass vibrate at
higher freq (υ )
11. Hooke’s law
f = force constant
the strength of the bond (spring)
f υ
m = mass of atom (weight)
m υ
12. Four region in IR spectrum
Increasing υ and E
Bond to H Triple bonds Double bonds Single bond
C - H C = C C = C C – C
O – H C = N C = O C – O
N - H C = N C – N
C - X
Lighter atom Stronger bonds Finger print region
difficult to analyse
Absorption for bond to H atom – occur on the left side
because H little mass
Bond strength decrease from C=C to C=C to C-C
4000 2500 2000 1500 400
13. For a bond to absorb in the IR
Must be a change in dipole moment during the vibration
Stretching or bending
2- butyne
CH3 –C = C- CH3
Non polar bond so IR inactive
No IR stretching absorption at 2250 cm-1
15. Why do different functional group absorb where they do?
Analogy:
2 weights (atoms) connected by a spring (a bond)
Short, strong bonds vibrates at higher energy and higher freq than do
long and weak bonds
Short, strong spring vibrates faster than long and weak spring
Thus triple bonds > double bonds > single bonds
Absorb higher freq
Spring with small weight vibrate faster than large weight
C-H, O-H and N-H vibrates at higher freq than bonds between heavier C,
O and N atoms
16. Eg 1:
Where might the following compounds have IR absorption?
Identify all the functional groups
i. Alcohol O-H
ii. Alkene C=C
3400 – 3650 cm-1 (O-H)
3020 – 3100 cm-1 (=C-H)
1640 – 1680 cm-1 (C=C)
CH2OH
17. Eg 2:
i. Terminal alkyne triple bond
ii. Saturated ester carbonyl group
3300 cm-1 (=C-H)
2100 -2260 cm-1 (C=C)
1735 cm-1 (C=O)
C=C triple bond) –stretching freq 2100 – 2200 cm-1
Terminal alkynes R-C=C-H give sharp C=C stretching signal
2100- 2200 cm-1
Internal alkynes R-C=C-R’
C=C stretch may be weak or absent
HC C-CH2-CH-CH2-C-OCH3
O
18. a and
b and
c and
d CH3CH2CH2COOH and CH3CH2CH2COOCH3
O
CH3(CH2)3C-H
O
CH3(CH2)2C-CH3
OH O
O
CH3(C-O-CH3
O
CH3(CH = CH C-O-CH3
Describe the characteristic infrared absorption frequencies that would
allow you to distinguished between the following pairs of compounds
