CH 106 - Lesson 2
Home Table of Contents Preliminary Information Bonding Alkenes Polymers Alkynes Aromatic Compounds Physical Properties Infrared Spectra Wrap-Up


Infrared Spectra

The infrared spectra for unsaturated compounds have some things in them that are different from what you looked at for saturated compounds in the previous lesson.


Here is the infrared spectrum for 1-octene. (It is also shown as spectrum #5 in your workbook.) The C-H bonds show up on both sides of 3000. That means that hydrogen is bonded to carbon atoms that have single bonds and multiple bonds. In the middle section there is an absorption and it is between 1500 and 2000. That corresponds to a C-to-C double bond. Again we will ignore the absorptions in the fingerprint region to the right of 1500.

IR spectrum of 1-octene. [62irsp05.JPG]

What can we conclude about this compound? It is an alkene because alkenes will have an absorption to the left of 3000 and another one in the double bond region between 1500 and 2000.



Next, here is the infrared spectrum of benzene, (also shown in spectrum #6 in your workbook). The C-H bond absorptions are all to the left of 3000. All of the hydrogen atoms are attached to carbon atoms with multiple bonds. In the middle section of the spectrum there are two absorptions in the multiple bond region between 1500 and 2000.

IR spectrum of benzene. [62irsp06.JPG]

I mentioned earlier that benzene does not really have double bonds. A skilled interpreter of these spectra can use the number and location of the absorptions to distinguish between alkene double bonds and benzene pi bonds, but it is not something that is particularly appropriate for this course.

However, you should know these generalities.

An alkene will generally have one absorption between 2500 and 1500 and it will be to the right of 2000.
An alkyne, of which I have no examples, will generally have one absorption between 2500 and 1500 and it will be to the left of 2000.
Aromatic compounds will generally have more than one absorption between 2500 and 1500 and they could be anywhere in that region.



The spectrum for polystyrene (also shown as spectrum #7 in your workbook) shows lots of absorptions. In the-left-of-the-2500 section there are absorptions to the left and right of 3000. That means there are hydrogen atoms bonded to carbon atoms with just single bonds and also to carbon atoms with some multiple bonds. In the multiple bond section from 1500 to 2500 the large number of absorptions between 1500 and 2000 indicates C-to-C aromatic bonds (benzene ring) or possibly some double bonds also.

IR spectrum for polystyrene. [62irsp07.JPG]



Lab Work

Much of this information about IR spectra is summarized for you in the table below (and also in Example 19 in your workbook).

Bond Type Wavenumber Region

general              [specific]

3      3       2      2       1
5      0      5       0      5
0      0      0       0      0
0      0      0       0      0

(C to C bonds)
C - C
C = C
(1500-2500 range)
not in this range
1500-2000 [1620-1680]
2000-2500 [2100-2260]
(C to H bonds)
. \   
. /   

(2500-up range)

right of 3000


left of 3000


left of 3000

[ 3030]

further left
of 3000
[ 3300]





(C to halogen bonds) not in this range

You should now complete your lab work by answering the questions about IR spectra in parts I.D and III.D and work on part IV of your experiment. Also, be sure to answer the review questions and turn them in as part of your lab report.


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