Problem 2
What evidence makes us believe that it is incorrect to consider benzene as a triene?
Problem 4
Describe the bonding in benzene in terms of an appropriate hybridization scheme for the \(C\) atoms and account for the equivalence of the \(\mathrm{C}-\mathrm{C}\) bonds.
Problem 5
Which of the following are aromatic: (a) cyclopropenyl cation, \(\mathrm{C}_{3} \mathrm{H}_{3}^{+} ;\) (b) cyclopropenyl anion, \(\mathrm{C}_{3} \mathrm{H}_{3}^{-} ;\) (c) cyclobutadiene, \(\mathrm{C}_{4} \mathrm{H}_{4}\) (d) cycloheptatriene, \(C_{7} \mathrm{H}_{8} ;\) (e) cycloheptatrienyl cation, \(\mathrm{C}_{7} \mathrm{H}_{7}^{+} ;\) (f) cyclooctatetraene, \(\mathrm{C}_{8} \mathrm{H}_{8} ?\)
Problem 7
Draw structural formulae for the following compounds: (a) \(\mathrm{PhOH}\) (b) \(\operatorname{Ph}_{2} \mathrm{CH}_{2},\) (c) \(\mathrm{Ph}_{2} \mathrm{NH}\) (d) \( P h N O_{2}\) and (e) PhSMe.
Problem 17
Suggest ways of making the following compounds starting from phenol, toluene or nitrobenzene.
Problem 18
How would you differentiate between the following pairs of isomers using \(^{13} \mathrm{C}\left\\{^{1} \mathrm{H}\right\\} \mathrm{NMR}\) spectroscopy? (a) 1,2 -dimethylbenzene and ethylbenzene (b) 1,3 -dichlorobenzene and 1,4 -dichlorobenzene (c) 1,3,5 -trimethylbenzene and 1,2,3 trimethylbenzene (d) \(n\) -propylbenzene and isopropylbenzene (e) cyclooctatetraene and styrene
Problem 20
Suggest mechanisms for the conversions of (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\) to \(\mathrm{PhCl} ;(\mathrm{b}) \mathrm{C}_{6} \mathrm{H}_{6}\) to \(\mathrm{PhSO}_{3} \mathrm{H} ;(\mathrm{c}) \mathrm{PhNO}_{2}\) to \(m\) -bromonitrobenzene. Describe the role of catalysts where appropriate.
Problem 23
When PhMeNH reacts with \(\mathrm{PhN}_{2}+\) in aqueous solution, two azo- compounds are obtained. In one, the substituent in one aromatic ring para to the azo-group is MeNH, while in the second, the parasubstituent is OH. Draw the structures of the products and suggest an explanation for these observations.
Problem 24
(a) Account for the fact that the \(^{1} \mathrm{H}\) NMR spectrum of benzene contains a singlet at \(\delta 7.2 \mathrm{ppm},\) but the spectrum of buta-1,3-diene shows lower frequency multiplets \((\delta 5.0,5.1\) and \(6.3 \mathrm{ppm}\) ). (b) An aromatic compound A shows a parent ion in its mass spectrum at \(m / z=104 .\) The results of an elemental analysis are: \(\mathrm{C}, 92.3 \% ; \mathrm{H}\) \(7.7 \% .\) Compound A reacts readily with \(\mathrm{Br}_{2}\) to give an addition product, B. The \(^{13} \mathrm{C} \mathrm{NMR}\) spectrum of A exhibits signals at \(\delta 137.6,137.0\) 128.5,127.8,126.2 and 113.7 ppm. In the \(600 \mathrm{MHz}^{1} \mathrm{H}\) NMR spectrum, overlapping signals appear in the range \(\delta 7.5-7.1 \mathrm{ppm},\) and there are three doublets of doublets at \(\delta 6.7\) \((J 18 \mathrm{Hz}, 11 \mathrm{Hz}), 5.7(\mathrm{J} 18 \mathrm{Hz}, 2 \mathrm{Hz})\) and \(5.2(J 11 \mathrm{Hz}, 2 \mathrm{Hz})\) ppm. Suggest identities for \(\mathbf{A}\) and \(\mathbf{B}\).
