Using IR Spectroscopy to Distinguish between Two Compounds

IR spectroscopy can be used to differentiate between two compounds. This technique is often extremely helpful when performing a reaction in which a functional group is transformed into a different functional group. For example, consider the following reaction. 

After running the reaction, it is possible to verify formation of the product by looking for the absence of an O—H signal and the presence of a C=0 signal. In order to distinguish two compounds using IR spectroscopy, you must know what to look for in the spectrum. 

Carefully inspect both compounds (reactant and product), and think about what signals each compound would produce in its IR spectrum. It might be helpful to work your way through the various regions of the spectmm and determine if there would be a difference in each region. 

Let s begin with the double-bond region (1600-1850 cm ). Each compound has a C=C bond, but consider the intensity of the expected signals. The reactant has an unsym-metrical C=C bond, while the product has a symmetrical C=C bond. 

The reactant has a weak dipole moment because each vinylic carbon atom is in a slightly different electronic environment. A weak signal is therefore expected at 1650 cm . In contrast, each vinylic position of the product is connected to two methyl groups, so there is no dipole moment associated with this bond. It is completely inefficient at absorbing IR radiation, and therefore, it will not produce a signal at 1650 cm . 

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