91Ó°ÊÓ

Draw and name all possible aromatic compounds with the formula C₇H₇Cl.

Draw and name all possible aromatic compounds with the formula C₈H₉Br.

Propose structures for compounds that fit the following descriptions:

(a) ${\mathbf{C}}_{10}{\mathbf{H}}_{14}$${}^1\mathbf{H}$NMR: $\mathbf{7}\mathbf{.}\mathbf{18}\text{ }\mathbf{δ}$(4 H, broad singlet);$\mathbf{2}\mathbf{.}\mathbf{70}\text{ }\mathbf{δ}$(4 H, quartet, J = 7 Hz); $\mathbf{1}\mathbf{.}\mathbf{20}\text{ }\mathbf{δ}$(6 H, triplet, J =7 Hz) IR:$\mathbf{745}\text{ }{\mathbf{cm}}^{-1}$.

(b)${\mathbf{C}}_{10}{\mathbf{H}}_{14}$${}^1\mathbf{H}$NMR:$\mathbf{7}\mathbf{.}\mathbf{0}\text{ }\mathbf{δ}$(4 H, broad singlet);$\mathbf{2}\mathbf{.}\mathbf{85}\text{ â¶Ä‰}\mathbf{δ}$(1 H, septet, J = 8 Hz);$\mathbf{2}\mathbf{.}\mathbf{28}\text{ â¶Ä‰}\mathbf{δ}$(3 H, singlet);$\mathbf{1}\mathbf{.}\mathbf{20}\text{ â¶Ä‰}\mathbf{δ}$(6 H, doublet, J = 8 Hz) IR:$\mathbf{825}\text{ }{\mathbf{cm}}^{-1}$.

On reaction with acid, 4-pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hückel’s $\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2}$rule, explain why the protonated product is so stable.

4-Pyrone

Bextra, a COX-2 inhibitor once used in the treatment of arthritis, contains an isoxazole ring. Why is the ring aromatic?

Bextra

N-Phenylsydnone, so-named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain, using the Hückel’s $\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2}$rule .

N-phenylsydnone

Show the relative energy levels of the seven p molecular orbitals of the cycloheptatrienyl system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation.

1-Phenyl-2-butene has an ultraviolet absorption ${\mathbf{λ}}_{\max }\mathbf{=}\mathbf{208}\text{ }\mathbf{nm}$($\mathbf{Î\mu }\mathbf{=}\mathbf{8000}$). On treatment with a small amount of strong acid, isomerization occurs and a new substance with ${\mathbf{λ}}_{\max }\mathbf{=}\mathbf{250}\text{ }\mathbf{nm}$($\mathbf{Î\mu }\mathbf{=}\mathbf{15}\mathbf{,}\mathbf{8000}$) is formed. Propose a structure for this isomer, and suggest a mechanism for its formation.

7 Propose structures for aromatic compounds that have the following ${}^1\mathbf{H}$NMR spectra:

(a) ${\mathbf{C}}_8{\mathbf{H}}_9\mathbf{Br}\mathbf{}\mathbf{IR}\mathbf{=}\mathbf{820}\mathbf{\text{ }}{\mathbf{cm}}^{-1}$

b) ${\mathbf{C}}_9{\mathbf{H}}_{12}\mathbf{}\mathbf{}\mathbf{IR}\mathbf{:}\mathbf{750}\mathbf{\text{ }}{\mathbf{cm}}^{-1}$

c) ${\mathbf{C}}_{11}{\mathbf{H}}_{16}\mathbf{}\mathbf{}\mathbf{IR}\mathbf{:}\mathbf{820}\mathbf{\text{ }}{\mathbf{cm}}^{-1}$

Propose a structure for a molecule ${\mathbf{C}}_{14}{\mathbf{H}}_{12}$ that has the following ${}^1\mathbf{H}$NMR spectrum and has IR absorptions at 700, 740, and $\mathbf{890}\text{ }{\mathbf{cm}}^{-1}$: