Q8P - Organic Chemistry (Mcmurry)

Question

How might you use IR spectroscopy to distinguish between the followingpairs of isomers?

 

(a) CH₃ CH₂ OH and CH₃ OCH₃

(b) Cyclohexane and 1 - hexene

(c) CH₃ CH₂ CO₂ H and HOCH₂ CH₂ CHO

Step-by-Step Solution

Answer

a. By comparing absorption band at 3400 to 3650 cm^-1 

b. By comparing bands at 3020-3100 cm^-1  and at 1640-1680 cm^-1 

c. By comparing bands at 2500-3100 cm^-1 and at 3400-3650 cm^-1 

Absorption spectrum

The absorption spectrum shows the presence of dark lines in the spectrum when transmitted through a substance indicating absorption at specific wavelengths of electromagnetic radiation.

Chart for absorption by specific functional group

The given chart shows absorption of particular wavelength radiation by specific functional group.

                                     Functional group absorption band

Part (a) absorption by CH 3 CH 2 OH and CH 3 OCH 3

IR spectroscopy to will distinguish between the following pairs of isomersas hydroxyl group has strong absorption peak at 3400 to 3650 cm^-1. This absorption will only be seen CH₃ CH₂ OH in as it has hydroxyl group while it will be absent in CH₃ OCH₃ as it has an ether group.

Part (b) absorption by Cylohexane and 1- hexene

By using IR spectroscopy it is possible to distinguish between the following pairs of isomersas 1- hexene has an alkene group in them which shows the band at 3020-3100 cm^-1  and at 1640-1680 cm^-1 while cyclohexane has no such band due to the absence of an alkene group.

Part (c) absorption by CH 3 CH 2 CO 2 H and HOCH 2 CH 2 CHO

Using IR spectroscopy the two isomers can be distinguishedas CH₃ CH₂ CO₂ H has a strong broad band at 2500-3100 cm^-1 because carboxylic acid’s hydroxyl group gives absorption in this region and HOCH₂ CH₂ CHO has a strong band at 3400-3650 cm^-1 as it containsan alcohol group.