Infrared Spectroscopy

                                  Infrared Spectroscopy helps us to identify Organic molecules

·         In IR (Infrared Spectroscopy), a beam of IR radiation passes through the sample.

·         The IR energy is absorbed by the bonds in the molecules, increasing their vibrational energy

·         Different bonds absorb different wavelengths.

The table below shows the frequencies different groups absorb ---

   Functional group

 

   Where it’s found



       Frequency/ Wavelength (cm^-1)

 Type of absorption

            O-H

         alcohols



         3200 – 3750

       strong, broad

            O-H

carboxylic acids



         2500 – 3300

  medium, very broad

            C-O

alcohols, carboxylic acids & esters

         1100 – 1310

       strong, sharp

            C=O

 

aldehydes, ketones, carboxylic acids &
          esters

         1680 - 1800

         strong sharp

O-H groups in alcohol and carboxylic acids tend to have broad absorptions due to their hydrogen bonding.

C=O groups in esters and carboxylic acids tend to have strong and sharp absorptions at around 1740cm^-1.

An example of the Ethanol Infrared Spectrum is shown below. Please have a look.

                                                     

http://www.google.co.uk/imgresimgurl=http://wetche.cmbi.ru.nl/vwo/spplus/ir/spectra/ethanol3.gif&imgrefurl=http
://wetche.cmbi.ru.nl/vwo/spplus/ir/spectra/ethol.html&usg=__US46PPQYb0IEJettv43FO5uhZRE=&h=449&w=702&sz
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/search%3Fq%3Dinfrared%2Bspectrum%2Bof%2Bethanol%26um%3D1%26hl%3Den%26safe%3Dactive%26sa%3DN26biw%3D1259%26bih3D83926tbm3Disch&um=1&itbs=1&iact=rc&dur=140&page=4&ndsp=23&ved=1t:429,r:15,s:70&tx=115&ty=55

Infrared Spectroscopy is a very useful.  It is used in:

·         breath analyser

·         forensic analysis

·         identification of polymer degradation etc.