Problem 2
Draw the structures of (a) pyridine, (b) indole, (c) the pyrylium cation and (d) imidazole.
Problem 3
Draw the structures of (a) 4-methylpyridine, (b) 2 -chloropyrrole, (c) 1,4 -dihydropyridine, (d) tetrabromopyrrole, (e) nicotinic acid.
Problem 4
(a) How many isomers of dimethylpyridine do you expect? Draw their structures and give each a systematic name. (b) Draw diagrams to show the dipole moments in furan and pyrrole.
Problem 5
(a) Explain how thiophene achieves a \(6 \pi\) -aromatic system. (b) What experimental evidence is there for the aromatic character of thiophene? (c) Does thiophene possess more or less aromatic character than furan? Rationalize your answer.
Problem 6
(a) Using a disconnection approach, suggest suitable precursors for the synthesis of 2,5 -dimethylpyrrole. (b) Propose a mechanism for the formation of 2,5 -dimethylpyrrole from the starting materials you have suggested.
Problem 9
Explain why protonation of thiophene by a strong acid leads to the formation of a polymer.
Problem 16
(a) Why is the acylation of pyrrole not carried out under Friedel-Crafts conditions? (b) With reference to the nitration of pyrrole, show how acetyl nitrate behaves as a nitrating agent. Which site in pyrrole is preferentially nitrated?
Problem 17
Propose mechanisms for the reaction of 2 -pyridone with a general electrophile, \(\mathrm{E}^{+},\) to give (a) the 3-substituted and (b) the 5-substituted products.
Problem 18
Explain why nucleophilic substitution occurs more readily in 4-chloropyridine than in 3-chloropyridine.
Problem 20
Three isomeric chloro-derivatives of pyridine \((\mathbf{A}, \mathbf{B}\) and \(\mathbf{C}\) ) analyse as containing \(40.58 \%\) C \(, 2.04 \%\) H and \(9.46 \%\) N. The \(^{1}\) H NMR spectroscopic data for the compounds are as follows where \(\mathrm{d}=\) doublet, \(\mathrm{d} \mathrm{d}=\) doublet of doublets and \(\mathrm{t}=\) triplet: $$\begin{array}{ll} \hline \text { Compound } & ^{1} \mathbf{H} \text { NMR } \delta / \text { ppm } \\ \text { A } & 7.66(\mathrm{t}, J=7.6 \mathrm{Hz}) \\ & 7.31(\mathrm{d}, J=7.6 \mathrm{Hz}) \\ \text { B } & 8.64(\mathrm{d}, J=2.1 \mathrm{Hz}) \\ & 8.25(\mathrm{t}, J=2.1 \mathrm{Hz}) \\ \text { C } & 8.70(\mathrm{dd}, J=3.0 \text { and } 0.3 \mathrm{Hz}) \\ & 8.13(\mathrm{dd}, J=9.0 \text { and } 3.0 \mathrm{Hz}) \\ & 7.68(\mathrm{dd}, J=9.0 \text { and } 0.3 \mathrm{Hz}) \\ & \\ \hline \end{array}$$ In each isomer, \(\mathrm{Cl}\) atoms are in either the 2 - or \(3-\) position with respect to the \(\mathrm{N}\) atom. Suggest structures for \(\mathbf{A}, \mathbf{B}\) and \(\mathbf{C}\).
