Q 8 P - Organic Chemistry (Mcmurry)

Question

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

$$\begin{gathered} \mathbf{a}\mathbf{)}\mathbf{ }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{ }\mathbf{and}\mathbf{ }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{OCH}}_{\mathbf{3}} \\ \mathbf{b}\mathbf{)}\mathbf{ }\mathbf{cyclohexane}\mathbf{ }\mathbf{and}\mathbf{ }\mathbf{1}\mathbf{-}\mathbf{hexene} \\ \mathbf{c}\mathbf{)}\mathbf{ }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\mathbf{ }\mathbf{and}\mathbf{ }{\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{CHO} \end{gathered}$$

Step-by-Step Solution

Answer

a. By comparing absorption band at 3400 to 3650 ${\mathrm{cm}}^{-1}$.

b. By comparing bands at 3020-3100 ${\mathrm{cm}}^{-1}$  and at 1640-1680 ${\mathrm{cm}}^{-1}$

c. By comparing bands at 2500-3100 ${\mathrm{cm}}^{-1}$ and at 3400-3650 ${\mathrm{cm}}^{-1}$

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.

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

IR spectroscopy to will distinguish between the following pairs of isomers as hydroxyl group has strong absorption peak at 3400 to 3650${\mathrm{cm}}^{-1}$. This absorption will only be seen in ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$ as it has hydroxyl group while it will be absent in ${\mathrm{CH}}_3{\mathrm{OCH}}_3$ as it has an ether group.

Step 4: Part (b) absorption by Cyclohexane and 1- hexane

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

Step 5: 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 distinguished as  ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CO}}_2\mathrm{H}$ has a strong broad band at 2500-3100 ${\mathrm{cm}}^{-1}$ because carboxylic acid’s hydroxyl group gives absorption in this region and ${\mathrm{HOCH}}_2{\mathrm{CH}}_2\mathrm{CHO}$ has a strong band at 3400-3650 ${\mathrm{cm}}^{-1}$ as it contains an alcohol group.