Benzene and Aromaticity

Draw structures corresponding to the following names:

(a) 3-Methyl-1,2-benzenediamine 

(b) 1,3,5-Benzenetriol 

(c) 3-Methyl-2-phenylhexane 

(d) o-Aminobenzoic acid

(e) m-Bromophenol 

(f) 2,4,6-Trinitrophenol (picric acid)

Draw and name all possible isomers of the following: 

1. Dinitrobenzene
2. Bromo dimethyl benzene 
3. Trinitrophenol

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: $\mathbf{7}\mathbf{.}\mathbf{18}\mathbf{\delta }$ (4 H, broad singlet); $\mathbf{2}\mathbf{.}\mathbf{70}\mathbf{\delta }$(4 H, quartet, J = 7 Hz);  $\mathbf{1}\mathbf{.}\mathbf{20}\mathbf{\delta }$(6 H, triplet, J =7 Hz) IR:$\mathbf{745}{\mathbf{cm}}^{-1}$ .

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

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 ${\mathbf{\lambda }}_{\max }\mathbf{=}\mathbf{208}\mathbf{nm}$  ( $\mathbf{\epsilon }\mathbf{=}\mathbf{8000}$). On treatment with a small amount of strong acid, isomerization occurs and a new substance with ${\mathbf{\lambda }}_{\max }\mathbf{=}\mathbf{250}\mathbf{nm}$ ($\mathbf{\epsilon }\mathbf{=}\mathbf{15}\mathbf{,}\mathbf{8000}$) 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)  ${\mathbf{C}}_8{\mathbf{H}}_9\mathbf{Br}\mathbf{ }\mathbf{IR}\mathbf{=}\mathbf{820}{\mathbf{cm}}^{-1}$

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

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

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 $\mathbf{890}{\mathbf{cm}}^{-1}$ :

The proton NMR spectrum for a compound with formula ${\mathbf{C}}_{10}{\mathbf{H}}_{12}{\mathbf{O}}_2$  is shown below. The infrared spectrum has a strong band at $\mathbf{1711}{\mathbf{cm}}^{-1}$ . The normal carbon-13 NMR spectral results are tabulated along with the DEPT-135 and DEPT-90 information. Draw the structure of this compound. 

The proton NMR spectrum of a compound with formula ${\mathbf{C}}_6{\mathbf{H}}_5{\mathbf{NCl}}_2$  is shown. The normal carbon-13 and DEPT experimental results are tabulated. The infrared spectrum shows peaks at 3432 and $\mathbf{3313}{\mathbf{cm}}^{-1}$ and a series of medium-sized peaks between 1618 and $\mathbf{1466}{\mathbf{cm}}^{-1}$ . Draw the structure of this compound.

Aromatic substitution reactions occur by the addition of an electrophile such as ${\mathbf{Br}}^{+}$  to an aromatic ring to yield an allylic carbocation intermediate, followed by loss of ${\mathbf{H}}^{+}$. Show the structure of the intermediate formed by the reaction of benzene with ${\mathbf{Br}}^{+}$ .

Azo dyes are the major source of artificial color in textiles and food. Part of the reason for their intense coloring is the conjugation from an electron-donating group through the diazo bridge $\left(-N=N-\right)$ to an electron-withdrawing group on the other side. For the azo dyes below, draw a resonance form that shows how the electron-donating group is related to the electron-withdrawing group on the other side of the diazo bridge. Used curved arrows to show how the electrons are reorganized.

a.

Methyl orange

b.

C.I. Acid Red 74