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Chapter 12: Q.9P (page 629)

Show the fragmentations that give rise to the peaks at m/z 43, 57, and 85 in the mass spectrum of 2,4-dimethylpentane (Figure 12-17).

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01

2,4-dimethylpentane

The given compound 2,4-dimethylpentane has has three types of carbons in it. They are primary, secondary, and tertiary. Its structure is shown below.

Structure of the given compound

02

Peat at m/z 43

Solution: fragmentations of 2,4-dimethylpentane

molecular mass(m/z) of 2,4-dimethylpentane is 100.

Formation of m/z 43 ion:

Peak at m/z 43

03

Peat at m/z 57

Formation of m/z 57 ion

Peak at m/z 57

04

Peat at m/z 85

Formation of M/z 85 ion

Peak at m/z 85

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Most popular questions from this chapter

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.

These five structures all have distinguishing absorption in the IR. Match each structure with its characteristic absorption.

Structure 1

Structure 2

Structure 3

Structure 4

Structure 5

(a)sharp, 2254 cm-1 (b) very broad, centered about3330cm-1 (c) strong, slightly broadened, 1645cm-1 (d) broad with spikes at 3367cm-1and 3392cm-1(e) strong, sharp 1717cm-1

Question: Show the fragmentation that accounts for the cation at m/z 57 in the mass spectrum of 2-methylpentane. Explain why this ion is less abundant than those at m/z 71 and 43.


Question:Spectra are given for three compounds. Each compound has one or more of the following functional groups: alcohol, amine, ketone, aldehyde, and carboxylic acid. Determine the functional group(s) in each compound, and assign the major peaks above 1600cm-1.

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?

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