Infrared Spectra
Home Table of Contents Preliminary Information Carboxylic Acids Carboxylate Salts Esters Ethers Infrared Spectra Reaction Summary Wrap-Up

 

Infrared Spectra

Next, let's take a look at the IR Spectra for this lesson. Printed copies are in your workbook at the end of the examples right before the experiments. Use them to get a better look at the spectra and to have something to take notes on.

Carboxylic Acids

In this spectrum (Spectrum #10) for butanoic acid, a number of different absorptions show up. There are several absorptions to the right of 3000 cm-1 which correspond to carbon-hydrogen bonds, but also notice that those are superimposed on a very broad absorption that sort of starts all the way up at about 3600 cm-1 and continues all the way down to about 2600 cm-1. That very broad absorption corresponds to the O-H from the carboxylic group. Also, there is the absorption at about 1700 cm-1 in the double bond region which corresponds to the carbonyl group. As usual, there are absorptions down in the fingerprint region that we won't attempt to analyze!

Infrared spectrum for butanoic acid. [64irsp10.JPG]

 

Esters

In this spectrum of pentyl acetate (Spectrum #11), an ester, you can see that the carbonyl absorption is still there at about 1700 cm-1 in the double bond region. Up near 3000 cm-1 again are the C-H bonds. Notice that the big broad O-H absorption seen with the butanoic acid is not there. That is because the O-H group is not there. All in all it is very similar to the IR spectrum of a ketone.

Infrared spectrum for pentyl acetate. [64irsp11.JPG]

 

Ethers

In the spectrum of this ether (Spectrum #12), diethyl ether, again there are several absorptions in the fingerprint region. In the diagnostic region all we have are some absorptions around 3000 cm-1 corresponding to carbon-hydrogen bonds. I notice that those are a little bit different than most of the ones that we have looked at earlier. Probably the presence of the oxygen has caused some shifting in the absorption of energy of those particular bonds. For our purposes, there isn't really anything that would distinguish between an ether and an alkane.

Infrared spectrum of diethyl ether. [64irsp12.JPG]

 

Again, we won't go into the details of how to do a complete interpretation of these spectra, but you can see that there are characteristic absorptions for different kinds of bonding combinations.

 

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