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Chapter 12: Q.11P (page 632)

Account for the peaks at m/z 87, 111, and 126 in the mass spectrum of 2,6-dimethylheptan-4-ol.

Short Answer

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Answer

2,6-Dimethylheptan-4-ol has molecular weight 144. Highest mass peak at 126 is not the molecular ion peak. It is due to the loss of water molecule from molecular ion.

The peak at m/z = 111 is loss of methyl fragment from fragment of m/z 126. This is known as allylic cleavage which generates secondary allylic carbocation.

Peak at m/z= 87 results from the fragmentation on one side of alcohol.

Step by step solution

01

The peak at m/z 126

2,6-Dimethylheptan-4-ol has molecular weight 144. Highest mass peak at 126 is not the molecular ion peak but it is due to the loss of water molecules from the molecular ion.

Peak at m/z 126

02

The peak at m/z 111

The peak at m/z 111 is formed due to the following fragmentation:

03

The peak at m/z 87

The peak at m/z 87 is formed due to the following fragmentation:

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Most popular questions from this chapter

The mass spectrum of n-octane shows a prominent molecular ion peak (m/z 114). There is also a large peak at m/z 57, but it is not the base peak. The mass spectrum of 3,4-dimethylhexane shows a smaller molecular ion, and the peak at mass 57 is the base peak. Explain these trends in abundance of the molecular ions and the ions at mass 57 and predict the intensities of the peaks at masses 57 and 114 in the spectrum of 2,2,3,3-tetramethylbutane.

Chapter-9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert trans-2,5-dimethylhex-3-yne by adding bromine across the double bond and then doing a double elimination. The infrared and mass spectra of the major product are shown here.

  1. Do the spectra confirm the right product? If not, what is it?
  2. Explain the important peaks in the IR spectrum.

Question: (a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there? (b) Use the IR spectrum to determine the functional group(s), if any. (c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain? (d) Propose structures for the major fragments at 39, 67, 81, and 95 in the mass spectrum.

For each spectrum, interpret all the significant streching frequencies above 1580 cm-10

Question: Which of the bonds shown in red are expected to have IR-active stretching frequencies?
See all solutions

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