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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


Question: Four infrared spectra are shown, corresponding to four of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen

Question: (A true story.) While organizing the undergraduate stockroom, a new chemistry professor found a half-gallon jug containing a cloudy liquid (bp 100–105 °C), marked only “STUDENT PREP.” She ran a quick mass spectrum, which is printed below. As soon as she saw the spectrum (without even checking the actual mass numbers), she said, “I know what it is.”

(a) What compound is the “student prep”? Any uncertainty in the structure?

(b) Suggest structures for the fragments at 136, 107, and 93. Why is the base peak (at m/z 57) so strong?

Predict the characteristic infrared absorptions of the functional groups in the following molecules.

(a) cyclopentene

(b) hexan-2-ol

(c) hexan-2-one

(d) hex-1-yne

(e) dimethylamine

(f) hexanoic acid

(g) hexanenitrile

(h) methyl acetate

(i) hexanamide.

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.

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.
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