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Chapter 20: Q10P c. (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鈥檛 the mass spectrum of 2-methylpentanoic acid show a large peak at m/z 60 ?

Short Answer

Expert verified

Answer

(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

01

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.

02

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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Most popular questions from this chapter

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.

The 2000 Nobel Prize in Chemistry was awarded for work on polyacetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cis-Polyacetylene has a copper color, and trans-polyacetylene is silver.

(a) Draw the structures of cis- and trans-polyacetylene.

(b) Use your structures to show why these polymers conduct electricity.

(c) It is possible to prepare polyacetylene films whose electrical conductivity is anisotropic. That is, the conductivity is higher in some directions than in others. Explain how this unusual behavior is possible.

Question: The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.

Show how you would synthesize the following compounds from the appropriate carboxylic acids or acid derivatives.

a)


b)



c)

Question: Show the products you expect when each compound reacts with NBS with light shining on the reaction.

c)
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