31 E - Organic Chemistry (Mcmurry)

Question

Question: How could you use infrared spectroscopy to distinguish between the

following pairs of isomers?

(a) $\mathbf{HC}\equiv {\mathbf{CCH}}_{\mathbf{2}}{\mathbf{NH}}_{\mathbf{2}}{\mathbf{andCH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{C}\equiv \mathbf{N}$

(b) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{COCH}}_{\mathbf{3}}{\mathbf{andCH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{CHO}$

Step-by-Step Solution

Answer

(a) Alkyne absorptions at 3300 , 2100-2600, Amine absorption at 3300-3500 andNitrile absorption at 2210-2260

(b) Strong ketone (CO) absorption at 1715and 

Strong aldehyde (CHO) absorption at 1730

 

Step-by-step Solution Step 1: IR (Infrared spectroscopy)

Infrared spectroscopy is the study of the interaction betweenthe sample andIR radiation which 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: Importance of Infrared spectroscopy in structure determination

Infrared spectroscopy is 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 3: Distinguish compounds of pair (a) and (b) by Infrared spectroscopy

(a) $HC\equiv CC{H}_{2}N{H}_2$Alkyne$\left(C\equiv C\right)$ absorptions at 3300 , 2100-2600, Amine .

$\left(N{H}_2\right)$ absorption at 3300-3500.

$C{H}_{3}C{H}_{2}C\equiv N$ Nitrile $C\equiv N$ absorption at 2210-2260$c{m}^{-1}$

$C{H}_{3}COC{H}_3$