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Chapter 20: Q10P b. (page 1055)

Question:

  1. Why do most long-chain fatty acids show a large peak in the mass spectrum at m/z 60 ?
  2. Use equations to explain the prominent peaks at m/z 74 and m/z 87 in the mass spectrum of 2-methylpentanoic acid.
  3. Why doesn’t the mass spectrum of 2-methylpentanoic acid show a large peak at m/z 60 ?

Short Answer

Expert verified

Answer

b. The prominent peaks at m/z 74 and m/z 87 in the mass spectrum:

Step by step solution

01

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.

02

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

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Most popular questions from this chapter

Question: Show how the following ketones might be synthesized from the indicated acids, using any necessary reagents.

(a)propiophenone from propionic acid (two ways, using alkylation of the acid and using Friedel-Crafts acylation)

(b)methyl cyclohexyl ketone from cyclohexanecarboxylic acid

Question:

  1. Why do most long-chain fatty acids show a large peak in the mass spectrum at m/z 60?
  2. Use equations to explain the prominent peaks at m/z 74 and m/z 87 in the mass spectrum of 2-methylpentanoic acid.
  3. Why doesn’t the mass spectrum of 2-methylpentanoic acid show a large peak at m/z 60?

Show how Fischer esterification might be used to form the following esters. In each case, suggest a method for driving the reaction to completion.

(a)methyl salicylate

(b) methyl formate (32°C)

(c) ethyl phenylacetate

Grignard reagents add to carbonate esters as they add to other esters.

(a) Predict the major product of the following reaction.

(b) Show how you would synthesize 3-ethylpentan-3-ol using diethyl carbonate and ethyl bromide as your only organic reagents.

(c) Diethyl carbonate is a liquid reagent that is easy to handle. In contrast, phosgene is a highly toxic and corrosive gas. Show how you might use diethyl carbonate instead of phosgene to make Lexan®. Also, show how you might use diethyl carbonate instead of methyl isocyanate to make Sevin® insecticide.

Question: A carboxylic acid has two oxygen atoms, each with two nonbonding pairs of electrons.

  1. Draw the resonance forms of a carboxylic acid that is protonated on the hydroxy oxygen atom.
  2. Compare the resonance forms with those given previously for an acid protonated on the carbonyl oxygen atom.
  3. Explain why the carbonyl oxygen atom of a carboxylic acid is more basic than the hydroxy oxygen.
See all solutions

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