Q29E - Organic Chemistry (Mcmurry)

Question

Would you expect two diastereomers such as meso-2, 3dibromobutane

and (2R, 3R)-dibromobutane to have identical or different IR spectra? Explain.

Step-by-Step Solution

Answer

Since diastereomers have different physical properties and chemical behavior, their IR spectra are also different.

Step 1: Infrared (IR) spectroscopy

The Infrared spectroscopy is the study of the interaction of the sample with IR radiation brings about change in vibrational-rotational level. Therefore it is also called rotational-vibrational spectroscopy.

• It is a technique based on the vibrations of the atoms of a molecule.
• The waves are electromagnetic in nature and interact with polarity of the chemical bonds of the molecule.

If there is no polarity in the molecule then the infrared interaction is inactive and the molecule does not produce any IR spectrum

 

Step 2: Diastereomerism

Diastereomers are the stereoisomers that are non-identical, do not have mirror images, and hence are non-superimposable on each other. Examples of diastereomers include cis and trans-2-butene, D-threose and D-erythrose, 2-chloro,3-bromobutane, 

Step 3: Infrared (IR) spectra of given diastereomers

Since diastereomers have different physical properties and chemical behavior, their IR spectra are also different.

The spectra would be different for two diastereomers such as meso-2, 3dibromobutane

and (2R, 3R)-dibromobutane

The reason for this being, diastereomers are not mirror image compounds, so every bond of one compound cant found another bond of the other compound that has identical circumstance, and the absorption of the two compounds are different as a result.