34 E - Organic Chemistry (Mcmurry)

Question

Question: How would you use infrared spectroscopy to distinguish between the following pairs of constitutional isomers?

(a)

(b)

(c)

$$\begin{gathered} C{H}_3\equiv CC{H}_{3 } AND C{H}_{3}COC{H}_{2}C\equiv CH \\ C{H}_{3}COCH\equiv CHC{H}_3 AND C{H}_{3}COC{H}_{2}CH=C{H}_2 \\ C{H}_{3}C{H}_{2}CHO AND H_{2}C=CHOC{H}_3 \end{gathered}$$

Step-by-Step Solution

Answer

(a) $C{H}_{3}C\equiv CC{H}_3$exhibits no terminal $\equiv C-H$ stretching vibration at 3300 $c{m}^{-1}$, as $C{H}_{3}C{H}_{2}C\equiv CH$ does.

(b) $C{H}_{3}COCH=CHC{H}_3$ a ketone next to a double bond, shows a strong ketone absorption at 1690$c{m}^{-1}$;$C{H}_{3}COC{H}_{2}CH=C{H}_2$shows a ketone absorption at 1715 and monosubstituted alkene absorption at 910$c{m}^{-1}$.

(c) $C{H}_{3}C{H}_{2}CHO$exhibits an aldehyde band at 1730$c{m}^{-1}$; $H_{2}C=CHOC{H}_3$ shows characteristic monosubstituted alkene absorption at 910 and 990$c{m}^{-1}$.

Step-by-step Solution Step 1: Importance of Infrared spectroscopy in structure determination

Infrared spectroscopy is the most widely used tool for detection of functional groups in pure compounds, mixtures, and for compound comparison.

• Infrared spectra may be obtained from samples in all phases (solid, liquid, and gaseous).
• The spectrum is obtained in a minute.
• It provides a variety of information about structure, symmetry, purity structural, and geometrical isomers and H-bonding.

Step 2: IR absorption

In general, IR absorption iscaused by the interaction between the IR electric field vector and molecular dipole transition related to the molecular vibrations. Absorption is at maximum when the electric field vector and dipole transition moment are parallel to each other.

step 3 : constitutional isomers

Isomers are the molecules that have the same molecular formula, but have a different arrangement of the atoms in space.

Constitutional isomers are compounds that differ in connectivity, i.e., in the way in which constituent’s atom are connected to one another. For example,

$C{H}_{3}C\equiv CC{H}_3$ and $C{H}_{3}C{H}_{2}C\equiv CH$

Step 4: distinguish between the following pairs of constitutional isomers

(a) $C{H}_{3}C\equiv CC{H}_3$exhibits no terminal $\equiv C-H$ stretching vibration at 3300 $c{m}^{-1}$,as$C{H}_{3}C{H}_{2}C\equiv CH$ does .

(b) $C{H}_{3}COCH=CHC{H}_3$a ketone next to a double bond, shows a strong ketone absorption at 1690$c{m}^{-1}$;$C{H}_{3}COC{H}_{2}CH=C{H}_2$shows a ketone absorption at 1715$c{m}^{-1}$ and monosubstituted alkene absorption at 910$c{m}^{-1}$.

(c) $C{H}_{3}C{H}_{2}CHO$exhibits an aldehyde band at 1730$c{m}^{-1}$;$H_{2}C=CHOC{H}_3$shows characteristic monosubstituted alkene absorption at 910$c{m}^{-1}$ and 990$c{m}^{-1}$.