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Magazine Chapter 20: Q10P a (page 1055)
Question:
- Why do most long-chain fatty acids show a large peak in the mass spectrum at m/z 60?
- Use equations to explain the prominent peaks at m/z 74 and m/z 87 in the mass spectrum of 2-methylpentanoic acid.
- Why doesn鈥檛 the mass spectrum of 2-methylpentanoic acid show a large peak at m/z 60?
Short Answer
Answer
- The carboxylic acids, which are linear, show a McLafferty peak at m/z 60.
- The prominent peaks at m/z 74 and m/z 87 in the mass spectrum:
c. The large peak at m/z 60 shows up if there is a linear carboxylic acid and no substituents present at alpha or beta positions. So, 2-methypentanoic acid has methyl substituent, so it doesn鈥檛 show a large peak at m/z 60.
Step by step solution
Long-chain fatty acids
Long-chain fatty acids are those which contain 14 or more than 14 carbons. These acids are primarily present in soyabean oil, fish, and olive oil.Some are saturated, and some are unsaturated fatty acids.
Most long-chain fatty acids show a large peak at m/z 60
- The linear carboxylic acids show a McLafferty peak at m/z 60.
McLafferty Rearrangement
McLafferty rearrangement is a 1,4 migration of hydrogen, and this reaction is observed in mass spectroscopy when organic molecules are fragmented.The molecule contains at least one gamma hydrogen.
Prominent peaks at m/z 74 and m/z 87
b. By using McLafferty rearrangement, a molecule of molar mass 42 is cleaved, and m/z of 74 is formed.
Prominent peaks at m/z 74
The total molar mass of the molecule is 116, and the molecule with a mass of 29 is cleaved. Then, the molecule with m/z 87 along with four resonance is formed.
Prominent peaks at m/z 87
Fragmentation of carboxylic acid
The molecular ion is usually weak in straight chain carboxylic acids, but they are present in the mass spectrum.With the McLafferty rearrangement, the prominent and often base peak was observed.
Prominent peaks at m/z 74 and m/z 87
(c) The large peak at m/z 60 can also show up if there is a linear carboxylic acid and no substituents present at alpha or beta positions. The mass of the fragment increases when any substituent alpha or beta to the COOH group is present in the McLafferty fragment.
The lowest mass possible for the McLafferty peak is m/z 60.
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