Q12-12-8 - Organic Chemistry (Mcmurry)

Question

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

 

(a) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH and C}}{{\rm{H}}_3}{\rm{OC}}{{\rm{H}}_3}{\rm{ }}\) 

(b) \({\rm{Cyclohexane and 1 - hexene}}\)

(c) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{O}}_2}{\rm{H and HOC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CHO}}\)

Step-by-Step Solution

Answer

a. By comparing absorption band at 3400 to 3650 \(c{m^{ - 1}}\).

b. By comparing bands at 3020-3100 \(c{m^{ - 1}}\) and at 1640-1680 \(c{m^{ - 1}}\)

c. By comparing bands at 2500-3100 \(c{m^{ - 1}}\) and at 3400-3650 \(c{m^{ - 1}}\)

Step 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.

Step 2: Chart for absorption by specific functional group

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

                                         Functional group absorption band

Step 3: Part (a) absorption by \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH and C}}{{\rm{H}}_3}{\rm{OC}}{{\rm{H}}_3}{\rm{ }}\)

IR spectroscopy to will distinguish between the following pairs of isomers as hydroxyl group has strong absorption peak at 3400 to 3650\(c{m^{ - 1}}\). This absorption will only be seen in \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\) as it has hydroxyl group while it will be absent in \({\rm{C}}{{\rm{H}}_3}{\rm{OC}}{{\rm{H}}_3}{\rm{ }}\)as it has an ether group.

 

Step 4: Part (b) absorption by \({\rm{Cyclohexane and 1 - hexene}}\)

By using IR spectroscopy it is possible to distinguish between the following pairs of isomers as \({\rm{1 - hexene}}\) has an alkene group in them which shows the band at 3020-3100 \(c{m^{ - 1}}\) and at 1640-1680 \(c{m^{ - 1}}\) while cyclohexane has no such band due to the absence of an alkene group.

Step 5: Part (c) absorption by \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{O}}_2}{\rm{H and HOC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CHO}}\)

Using IR spectroscopy the two isomers can be distinguished as \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{O}}_2}{\rm{H}}\)has a strong broad band at 2500-3100 \(c{m^{ - 1}}\)because carboxylic acid’s hydroxyl group gives absorption in this region and \({\rm{HOC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CHO}}\)has a strong band at 3400-3650 \(c{m^{ - 1}}\)as it contains an alcohol group.