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Spectra
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All electromagnetic radiation travels in waves at the
same velocity, commonly known as the speed of
light.
In a vacuum this value is known to be 2.998 x 108
meters per second.
Electromagnetic radiation differs in wavelength (l)
and frequency (n).
The product of the wavelength and frequency is
equal to the speed of light.
C = nl..
The energy of a wave is proportional to its
frequency:
E = hn,
where h = Planck's constant or 6.626 x 10-34 J s.
Spectra and Organic
Molecules
• When light strikes an organic molecule,
some of its energy is absorbed.
• Molecules are affected in various ways
depending on the energy of the light.
• If the light has a wavelength in the range
from about 400 to 700 nm, it falls in the
visible region.
• When light of a particular wavelength is
absorbed, the compound is seen as having
a color. This is especially true of organic
dyes,
Absorption Spectra
• If electromagnetic radiation with wavelengths
outside of the visible region is absorbed, the
results cannot be observed directly.
•H owever the absorption can be detected using a
spectrophotometer.
Absorption Spectra and
Molecular Characteristics
• Many of the functional groups that are found
in organic molecules absorb infrared at
characteristic frequencies.
• This energy that is absorbed is converted to
various kinds of molecular vibrations
including the rhythmic stretching of chemical
bonds,
• Each type of chemical bond has a
characteristic absorbance in the infrared
region.
Absorption depends on the
Molecular Environment
• The exact frequency of absorption depends
on the specific environment of the bond.
• Some kinds of bonds appear in more than
one in of compound.
Absorption peaks depends
on functional groups
•For example, the absorption frequencies for
a hydroxyl group, -OH, appear in
approximately the same location whether it
was in an alcohol or a carboxylic acid, but
obviously it would not appear in a alkanone
or ester. Both acids and alkanones show a
strong absorption frequency for the carbonyl
group C=O.
Typical Infrared Absorption
frequencies
Class of
compound
Frequency of
absorption in cm-1
Intensity of
the absorption
OH
Alcohols, phenols
3500-4650
OH
Carboxylic Acids
2500-3000
OH
Hydrogen bonded
alcohols and phenols
3200-3400
Variable,
sharp
Variable,
broad
Strong, broad
C-O
1080-1300
Strong
1690-1750
Strong
C-H
Alcohols, ethers,
esters, and carboxylic
acids
Alkanones, alkanals,
esters, and carboxylic
acids
Alkanes
2850-2950
Strong
N-H
amines
3300-3500
Medium
Functional
group
C=O
Typical Infrared Spectra
-- Cyclohexanol
Typical Infrared Spectra
-- 1 Octanol
Typical Infrared Spectra
-- 2 bromobutanoic acid
Typical Infrared Spectra
-- 2 hydroxybenzaldehyde
Typical Infrared Spectra
-- 2 aminomethylbenzene
1 Propanol
1 propanol
2 Butanol
2 butanol
Ethyl Methanoate
Benzaldehyde
3 Pentanone
Acetophenone