91Ó°ÊÓ

How might you use IR spectroscopy to distinguish between the following pairs of isomers? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (b) Cyclohexane and hex-1-ene (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\) and \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\)

The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogens has an odd-numbered molecular ion. Conversely, a compound containing an even number of nitrogens has an even-numbered \(\mathrm{M}^{+}\) peak. Explain.

Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as \({ }^{35} \mathrm{Cl}(75.8 \%)\) and \({ }^{37} \mathrm{Cl}(24.2 \%)\); bromine occurs as \({ }^{79} \mathrm{Br}(50.7 \%)\) and \({ }^{81} \mathrm{Br}(49.3 \%)\). At what masses do the molecular ions occur for the following formulas? What are the relative percentages of each molecular ion? (a) Bromomethane, \(\mathrm{CH}_{3} \mathrm{Br}\) (b) 1-Chlorohexane, \(\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Cl}\)

Propose structures for compounds that fit the following data: (a) \(\mathrm{A}\) ketone with \(\mathrm{M}^{+}=86\) and fragments at \(\mathrm{m} / \mathrm{z}=71\) and \(\mathrm{m} / \mathrm{z}=43\) (b) An alcohol with \(\mathrm{M}^{+}=88\) and fragments at \(\mathrm{m} / \mathrm{z}=73, \mathrm{~m} / \mathrm{z}=70\), and \(m / z=59\)

How might you use IR spectroscopy to distinguish among the three isomers but-1-yne, buta-1,3-diene, and but-2-yne?

4-Methylpentan-2-one and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy.

Organomagnesium halides (R-Mg-X), called Grignard reagents, undergo a general and very useful reaction with ketones. Methylmagnesium bromide, for example, reacts with cyclohexanone to yield a product with the formula \(\mathrm{C}_{7} \mathrm{H}_{14} \mathrm{O}\). What is the structure of this product if it has an IR absorption at \(3400 \mathrm{~cm}^{-1}\) ?

Benzene has an ultraviolet absorption at \(\lambda_{\max }=204 \mathrm{~nm}\), and \(p\) -toluidine has \(\lambda_{\max }=235 \mathrm{~nm}\). How do you account for this difference?