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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

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.

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?

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

A laboratory student added 1-bromobutane to a flask containing dry ether and magnesium turnings. An exothermic reaction resulted, and the ether boiled vigorously for several minutes. Then she added acetone to the reaction mixture and the ether boiled even more vigorously. She added dilute acid to the mixture and separated the layers. She evaporated the ether layer, and distilled a liquid that boiled at 143C∘. role="math" localid="1649504068494" GC-MSanalysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.

(a) Draw out the reactions that took place and show the product that was formed.

(b) Explain why the molecular ion is not visible in the mass spectrum, and show what ions are likely to be responsible for the strong peaks at m/z, 59 and 101.


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.
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