Q12-44E - Organic Chemistry (Mcmurry)

Question

4-Methyl-2-pentanone and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy.

Step-by-Step Solution

Answer

The simplest route to distinguish between the isomers is by taking the IR spectra. The aldehyde carbonyl group absorbs at ${\text{1730 cm}}^{\text{ - 1}}$, and the ketone carbonyl group absorbs at ${\text{1715 cm}}^{\text{ - 1}}$.

Step 1: Determining the structures from infrared and mass spectra

The infrared spectra can be used to identify the functional group of each compound as each functional group has its characteristic absorption. The fragmentation of different molecules can be understood by McLafferty rearrangement in mass spectroscopy.

Step 2: Differentiating the given isomers from mass spectrometry and IR spectra

The simplest route to distinguish between the isomers is by taking the IR spectra. The aldehyde carbonyl group absorbs at ${\text{1730 cm}}^{\text{ - 1}}$, and the ketone carbonyl group absorbs at  ${\text{1715 cm}}^{\text{ - 1}}$.

The mass spectra of both the compounds are different. Like ketones, aldehydes undergo alpha cleavage and McLafferty rearrangement.

The McLafferty rearrangement can be given as:

                                     McLafferty rearrrangment

 

 

The fragments from McLafferty rearrangement differ in the values of m/z.

The alpha cleavage can be given as:

                                               Alpha cleavage

 

The fragments from alpha cleavage also differ in the m/z values.