Q25P - Organic Chemistry (Mcmurry)

Question

How might you use mass spectrometry to distinguish between the following pairs of isomers?

(a) 3-Methyl-2-hexanone and 4-methyl-2-hexanone

(b) 3-Heptanone and 4-heptanone

(c) 2-Methylpentanal and 3-methylpentanal

Step-by-Step Solution

Answer

1. Isomer 3-Methyl-2-hexanone and 4-methyl-2-hexanone are distinguished by Mc Lafferty rearrangement.
2. Isomer3-Heptanone and 4-heptanone can be distinguished by alpha cleavage and Mc Lafferty rearrangement.
3. Isomer2-Methylpentanal and 3-methylpentanalare distinguished by Mc Lafferty rearrangement.

Step-by-Step Solution Step 1: Mass spectrometry

Mass spectroscopy detects "charged particles" fragment ions from McLafferty rearrangement and alpha-cleavagevisible in the mass spectrum of aliphatic aldehydes and ketones.

 

• Aliphatic aldehydes and ketones with hydrogens on their gamma-carbon atoms undergo a characteristic mass spectral cleavage called the McLafferty rearrangement.
• Aldehydes and ketones also undergo cleavage of the bond between the carbonyl groupand the carbon, called an alpha cleavage.

Step 2: Distinguishing between 3-Methyl-2-hexanone and 4-methyl-2-hexanone

Both isomer 3-Methyl-2-hexanone and 4-methyl-2-hexanone exhibit a peak at m/z =43 due to alpha cleavage.

3-Methyl-2-hexanone product of Mc Lafferty rearrangement occurs at m/z =72, and for 4-Methyl-2-hexanone, it occurs at m/z = 43.

 

Therefore, Mc Lafferty's rearrangement product is used to distinguish between the isomer products of 3-Methyl-2-hexanone and 4-methyl-2-hexanone.

 

 

The m/z values due to Mc Lafferty rearrangement product and alpha cleavage for isomer 3-Methyl-2-hexanone and 4-methyl-2-hexanone are shown below

3-Methyl-2-hexanone and 4-methyl-2-hexanone m/z values

Step 3: Distinguishing between 3-Heptanone and 4-heptanone

3-Heptanone and 4-Heptanone show different m/z values due to Mc Lafferty rearrangement and alpha-cleavage.

 

Mc Lafferty rearrangement product of 3-Heptanone shows m/z value = 72 and 

4-Heptanone shows m/z value = 86.

 

Alpha-cleavage of 3-Heptanone shows m/z value = 57 and 4-Heptanone shows m/z value = 71.

 

 Therefore, both Mc Lafferty's rearrangement product and alpha cleavage are used to distinguish between the isomer products of 3-Heptanone and 4-Heptanone.

 

 The m/z values due to Mc Lafferty rearrangement product and alpha cleavage for isomer Heptanone and 4-Heptanoneare are shown below.

3-Heptanone and 4-heptanone m/z values

Step 4: Distinguishing between 2-Methylpentanal and 3-methylpentanal

Due to alpha cleavage, both isomer 2-Methylpentanal and 3-methylpentanal exhibit a peak at m/z =29.

 

2-Methylpentanal product of Mc Lafferty rearrangement occurs at m/z = 58, and for 3-Methylpentanal, it occurs at m/z = 44.

 

Therefore, Mc Lafferty's rearrangement product is used to distinguish between the isomer products of 2-Methylpentanal and 3-Methylpentanal.

 

 

The m/z values due to Mc Lafferty rearrangement product and alpha cleavage for isomer 2-Methylpentanal and 3-Methylpentanal are shown below.

 

2-Methylpentanal and 3-methylpentanal m/z values