Question

Question: Which of the following possible structures for X can be excluded on the basis of its IR spectrum?

Short answer

Answer

Structures (b) and (d) can be excluded from the possible structure for X.

Step-by-step solution

IR spectrum of molecules

Bonds in a molecule vibrate with a frequency. This causes the formation of a varying electromagnetic field.

When IR radiation encounters the bond, the 2 waves couple, and IR light is absorbed. Absorption occurs when the vibrational frequency of the bond is equal to the frequency of radiation.

This causes an increase in the amplitude of oscillation of the bond. Excitation occurs between vibrational energy levels.

Identifying functional groups from the IR spectrum

An IR spectrum is a plot of percentage transmission of IR radiation through the molecule versus the wavenumber of radiation used.

Each peak in the spectrum corresponds to the wavenumber of radiation absorbed by the molecule.

Each bond vibrates with a different vibrational frequency, and therefore, the wavenumber of radiation absorbed by each bond will be different.

This can be used to identify the functional groups in a compound.

Identifying possible structures for X

Hydroxyl groups have a strong peak at approximately 3600 ${\mathrm{cm}}^{-1}$in the IR spectrum.

The IR spectrum of X does not have a peak at 3600 ${\mathrm{cm}}^{-1}$. Therefore, it does not have a hydroxyl group in its structure.

Possible structures for compound X are:

(a) ${\mathrm{CH}}_3{\mathrm{COOCH}}_2{\mathrm{CH}}_3$

(c) ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{COOCH}}_3$

Therefore, from the given structures, structures (b) and (d) can be excluded from possible structures for compound X.