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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: A C-D (carbon鈥揹euterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however.

(a) The infrared absorption frequency is approximately proportional tokm , when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use as a typical C-H absorption frequency.

(b) A chemist dissolves a sample in deuterochloroform (CDCl3) and then decides to take the IR spectrum and simply evaporates most of theCDCl3 . What functional group will appear to be present in this IR spectrum as a result of theCDCl3 impurity?

Question: Consider the following four structures, followed by mass spectral data. Match each structure with its characteristic molecular ion or fragment. In each case, give a likely structure of the ion responsible for the base peak

(a) base peak at 105

(b) base peak at 72

(c) doublet at 198 and 200, base peak at 91

(d) base peak at 91, large peak at 43

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?
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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鈥�105 掳C), marked only 鈥淪TUDENT PREP.鈥� 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, 鈥淚 know what it is.鈥�

(a) What compound is the 鈥渟tudent prep鈥�? Any uncertainty in the structure?

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