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Organic Chemistry and Structural Properties

In the mass spectrum of butylbenzene, the base peak occurs at m/z
a. 134
b. 119
c. 105
d. 91

The number of mono-bromination products of p-xylene is
a. 1
b. 2
c. 3
d. 4

Which of the following compounds has the lowest boiling point?
a. p-dichlorobenzene
b. o-dichlorobenzene
c. m-dichlorobenzene
d. all boiling points are the same

The carbanion of cyclopentadiene would be classified as
a. aromatic
b. non-aromatic
c. anti-aromatic
d. none of the above

Consider the possible isomers of chlorotoluene. Which of the possibilities will result in only two types of aromatic protons by NMR?
a. o-chlorotoluene
b. m-chlorotoluene
c. p-chlorotoluene
d. all of the above

Solution Preview

In the mass spectrum of butylbenzene, the base peak occurs at m/z
a. 134
b. 119
c. 105
d. 91

The molecular mass of butylbenzene is 134; therefore, we might surmise that 134 is the base peak. Let's look at the possible fragmentation pattern of butylbenzene. The place where it would be expected to fragment easiest would be at the junction between the benzene ring and the n-butyl group. If it fragments there, what would be those m/z peaks? The C6H5 group would be at 77 and the butyl group would be 57, neither of which appears as an option in the multiple choice question. Therefore, we would surmise that 134, the molecular ion peak is probably the base peak as well. Remember, base peak is the tallest peak.

In fact 119 is the peak resulting from loss of a methyl group, hardly likely to be the main peak. The 105 peak results from loss of ethyl group, hardly like to occur readily. And the 91 peak is the one resulting from loss of propyl group, again, hardly likely.

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Solution Summary

This solution identifies the correct answer to the question set, accompanied with full explanations and structural forms.

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