Organic Chemistry Laboratory
Infrared Spectroscopy
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(Hertz (Hz)) |
(meters (m)) |
Structural Information |
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(infrared region) |
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(radio wave region) |
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2.5 X 1016 - 2 X 1015
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1 X 10-8 - 3.8 X 10-7 | Pi and Conjugated System |
Table 6.2: Regions of the Electromagnetic Spectrum Used in Spectroscopy
In IR spectroscopy, a sample molecule is exposed to radiation with a
frequency in the infrared region of the electromagnetic spectrum.
Only a portion of the infrared region of the electromagnetioc
spectrum(2.5
X 10-6 - 2.5 X 10-5m) is used in conventional IR
spectroscopy. Specific frequencies of radiation within the
infrared
region of the spectrum are absorbed by individual bonds in the
molecule,
causing the bonds to vibrate. The frequency of the radiation that
is absorbed by the molecule is recorded as a peak or absorbance in the
spectrum. These peaks are characteristic of certain types of
bonds.
Table 6.3 lists various functional group classes and specific bonds
associated
with that functional group, and the characteristic position of the peak
corresponding to that bond. Click on the link to view a sample
spectrum
containing peaks corresponding to the bond type indicated.
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2850-2960 cm-1 |
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medium-strong |
3020-3100 cm-1 |
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medium-strong |
1640-1680 cm-1 |
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medium |
3300 cm-1 |
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strong |
2100-2260 cm-1 |
CºC of nitriles |
weak-medium |
500-800 cm-1 |
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strong |
3200-3650 cm-1 |
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strong, broad |
1660-2000 cm-1 1450-1600 cm-1 |
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weak medium |
1680-1850 cm-1 |
ketones, aldehydes, esters, carboxylic acids, amides See more specific ranges |
strong |
3300-3500 cm-1 1030-1230 cm-1 |
C-N of amines |
medium medium |
1540 cm-1 |
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strong |
Table 6.3: Characterisic IR Absorbances Ranges for Various Bond Types
Sample Preparation and the Infrared Spectrophotometer
There are a variety of methods for preparing samples for IR
analysis.
The most common methods of sample prepartion for IR analysis are listed
in Table 6.4. Liquid samples will typically be run in this course
using sodium chloride plates (i.e. salt plates). The sample is
prepared
by placing a few drops of the liquid compound on one salt plate and
placing
a second salt plate on top of the first. Solid samples will be
prepared
for IR analysis using potassium bromide pellets. Pellets are
prepared
by mixing the solid sample with KBr, placing the mixture in a
mini-press
and compressing the mixture for form a small disc or pellets.
Other
sampel prepartion methods are available but will not be used in this
course.
These include mulls and solutions. A mull is prepared by grinding
a solid sample with mineral oil to form a paste which is then placed
between
salt plates for analysis. The solutions of samples, either
liquids
or solids can analyzed using special solution cells.
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Table 6.4: Sample Preparation Methods for IR Spectroscopy
Interpretation of the IR Spectrum
Interpretation of IR spectra involved correlating peaks in an
experimentally
generated spectrum with known ranges for specific bond or functional
group
types. There are some general and more specific guidelines that
can
be used to interpret an IR spectrum. Refer to Table 6.3 (click on
links) to view spectra of common organic functional groups/bonds.
General Guidelines for Simple Organic Functional Groups
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Specific Guidelines for Simple Organic Functinal Groups
Step 1 | Identify all the major peaks in the spectrum between 3600cm-1 and 1500cm-1 (+/- 100cm-1). Label the peaks 1,2,3 etc and prepare a table with four columns with the following headings: Peak Number, Absorbance Range, Absorbance Intensity, Bond Type. Enter all peak numbers in the "Peak Number" column, their corresponding absorbance range in the "Absorbance Range column, and the absorbance intensity (Strong =50% of scale or more; Medium 25-505 of scale; Weak = less than 25% of scale) in the "Absorbance Intensity" column. |
Step 2 | Is there a peak between 1650-1850cm-1 AND 3600-3200cm-1? If yes go to Step 3. If no go to Step 5. |
Step 3 | Is the peak between 3600-3200cm-1 broad and
intense?
If no, go to Step 4. If yes, the compound probably contains one
of the following functional groups or functional group combinations. 1) Carboxylic acid 2) Ketone and alcohol or phenol 3) Aldehyde and alcohol or phenol 4) Ester and alcohol or phenol 4) Alkene and Alcohol or phenol 5) Phenol List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table. |
Step 4 | Is the peak between 3600-3200cm-1 sharp and
medium to weak
in intensity? If yes, the compound probably contains one of
the
following functional groups or functional group combinations: 1) Secondary amide (if only one peak between 3600-3200cm-1) primary amide (if two peaks between 3600-3200cm-1) or 2) Ketone and amine or 3) Aldehyde and Amine List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table. |
Step 5 | Is there a peak between 1650-1850cm-1 ? If
no, go
to Step 6. If yes, the compound contains one of the following
carbonyl
or C=C containing functional groups: 1) ester 2) ketone 3) aldehyde 4) tertiary amide 5) alkene (~1660cm-1) 6) aromatic (~1450 and 1600cm-1) List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table. |
Step 6 | Is there a peak between 3600-3200cm-1
? If no,
go to Step 7. If yes, the compound contains one of the following
functional groups: 1) Alcohol (if the peak is broad and strong in intensity) 2) Amine (if the peak is sharp and medium in intensity) List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table. |
Step 7 | The compound contains one or more of the following functional
groups: 1) Alkane 2) Ether 3) Alkyl Halide List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table. |