Aromatic compounds | Practice Problems, Examples & Solutions

Introduction: The Discovery of Benzene

The Structure and Properties of Benzene

Organic Chemistry

Draw structures of the following compounds:
a. para-toluidine
b. meta-cresol
c. para-xylene
d. ortho-chlorobenzenesulfonic acid

Reactions of Substituted Benzenes

01:55

Introductory Chemistry: An Active Learning Approach

The boiling point of benzene, $\mathbf{C}_{6} \mathbf{H}_{6},$ is $80.10^{\circ} \mathrm{C}$ at 1 atmosphere. $\mathrm{K}_{\mathrm{b}}$ for benzene is $2.53^{\circ} \mathrm{C} / \mathrm{m}$. In a laboratory experiment, students synthesized a new compound and found that when 11.5 grams of the compound were dissolved in 246 grams of benzene, the solution began to boil at $80.43^{\circ} \mathrm{C}$. The compound was also found to be a nonvolatile molecular compound. What is the molecular mass that they determined for this compound?

Solutions

01:16

Introductory Chemistry: An Active Learning Approach

Calculate the molal concentration of a solution of 44.9 g of naphthalene, $C_{10} \mathrm{H}_{8},$ in $175 \mathrm{g}$ of benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$

Solutions

The Molecular Orbitals of Benzene

1 Practice Problems

06:00

Organic Chemistry

Answer the following questions for the $\pi$ molecular orbitals of 1,3,5,7 -octatetraene:
a. How many $\pi$ MOs does the compound have?
b. Which are the bonding MOs and which are the antibonding MOs?
c. Which MOs are symmetric and which are asymmetric?
d. Which MO is the HOMO and which is the LUMO in the ground state?
e. Which MO is the HOMO and which is the LUMO in the excited state?
f. What is the relationship between HOMO and LUMO and symmetric and asymmetric orbitals?
g. How many nodes does the highest-energy $\pi$ molecular orbital of 1,3,5,7 -octatetraene have between the nuclei?

Electron Delocalization and Resonance • More About Molecular Orbital Theory

The Molecular Orbital Picture of Cyclobutadiene

0 Practice Problems

Aromatic, Antiaromatic, and Nonaromatic Compounds

19 Practice Problems

00:34

Introduction to General, Organic and Biochemistry

(Chemical Connections 13 A) One of the degradation products of DDT is dichlorodiphenyldichloroethylene (DDE), which is formed by the loss of HCl from adjacent carbons of DDT. DDE inhibits the enzyme responsible for the incorporation of calcium ion into bird eggshells. Draw a structural formula for DDE.

Benzene and Its Derivatives

06:06

Introduction to General, Organic and Biochemistry

Draw structural formulas for these compounds.
(a) $1-$ Bromo- 2 -chloro- 4 -ethylbenzene
(b) 4 -Bromo-1,2-dimethylbenzene
(c) 2,4,6 -Trinitrotoluene
(d) 4 -Phenyl-1-pentene
(e) $p$ -Cresol
(f) 2.4 -Dichlorophenol

Benzene and Its Derivatives

01:32

Organic Chemistry

a. What is the value of $n$ in Huckel's rule when a compound has nine pairs of $\pi$ electrons?
b. Is such a compound aromatic?

Aromaticity • Reactions of Benzene

Hückel’s Rule

4 Practice Problems

02:34

Organic Chemistry

To be aromatic, a molecule must have $4 n+2 \pi$ electrons and must have cyclic conjugation. $1,3,5,7,9-$ Cyclodecapentaene fulfilis one of these criteria but not the other and has resisted all attempts at synthesis. Explain.

Benzene and Aromaticity

01:52

Organic Chemistry

Classify each compound as aromatic, antiaromatic, or not aromatic.

Benzene and Aromatic Compounds

02:26

Organic Chemistry

Estimate where the protons bonded to the $sp^2$ hybridized carbons will absorb in the $^1$H NMR
spectrum of each compound.

Benzene and Aromatic Compounds

The Criteria for Aromaticity-Huckel's Rule

Molecular Orbital Derivation of Hückel’s Rule

1 Practice Problems

06:43

Organic Chemistry

(a) In 1960 T. Katz (Columbia University) showed that cyclooctatetraene adds two electrons when treated with potassium metal and forms a stable, planar dianion, $\mathrm{C}_{8} \mathrm{H}_{8}^{2-}$ (as the dipotassium salt):
(Check your book to see figure)
Use the molecular orbital diagram given in Fig. 14.7 and explain this result.
(b) In 1964 Katz also showed that removing two protons from the compound below (using butyllithium as the base) leads to the formation of a stable dianion with the formula $\mathrm{C}_{8} \mathrm{H}_{6}^{2-}$ (as the dilithium salt).
(Check your book to see figure)
Propose a reasonable structure for the product and explain why it is stable.

Aromatic Compounds

Aromatic Ions

8 Practice Problems

10:33

Organic Chemistry

The $\mathrm{p} K_{\mathrm{a}}$ values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:
The relative $\mathrm{p} K_{\mathrm{a}}$ values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic. Explain these relative acidities.
a. Cl: ortho > meta > para
b. $\mathrm{NO}_{2}$ : ortho $>$ para $>$ meta
c. $\mathrm{NH}_{2}:$ meta $>$ para $>$ ortho

Reactions of Benzene and Substituted Benzenes

03:10

Organic Chemistry

Benzene underwent a Friedel-Crafts acylation followed by a Wolff-Kishner reduction. The product gave the following $^{1} \mathrm{H}$ NMR spectrum. What acyl chloride was used in the Friedel-Crafts acylation?

Reactions of Benzene and Substituted Benzenes

04:23

Organic Chemistry

Quinolines, heterocyclic compounds that contain a pyridine ring fused to a benzene ring, are commonly synthesized by a method known as the $\mathrm{Skraup \space synthesis}$, in which aniline reacts with glycerol under acidic conditions. Nitrobenzene is added to the reaction mixture to serve as an oxidizing agent. The first step in the synthesis is the dehydration of glycerol to propenal.
a. What product would be obtained if para-ethylaniline were used instead of aniline?
b. What product would be obtained if 3-hexen-2-one were used instead of glycerol?
c. What starting materials are needed for the synthesis of 2,7 -diethyl- 3 -methylquinoline?

More About Amines • Reactions of Heterocyclic Compounds

Heterocyclic Aromatic Compounds

9 Practice Problems

01:35

Introduction to General, Organic and Biochemistry

Draw the structure of proline. To which class of heterocyclic compounds does this molecule belong? (See Section $16-1 .)$

Proteins

08:08

Organic Chemistry

Rank each group of compounds from most reactive to least reactive toward electrophilic aromatic substitution:
a. benzene, ethylbenzene, chlorobenzene, nitrobenzene, anisole
b. 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, 1-methyl-2,4-dinitrobenzene
c. toluene, $p$ -cresol, benzene, $p$ -xylene
d. benzene, benzoic acid, phenol, propylbenzene
e. $p$ -methylnitrobenzene, 2 -chloro- 1 -methyl- 4 -nitrobenzene, 1 -methyl- 2,4 -dinitrobenzene, $p$ -chloromethylbenzene
f. bromobenzene, chlorobenzene, fluorobenzene, iodobenzene

Reactions of Benzene and Substituted Benzenes

21:42

Organic Chemistry

Show how each of the following compounds can be synthesized from benzene:
a. $o$ -bromopropylbenzene
b. $m$ -nitrobenzoic acid
c. $p$ -nitrobenzoic acid
d. $p$ -chloroaniline
e. $m$ -chloroaniline
f. $m$ -xylene
g. 2 -phenylpropene
h. $m$ -bromopropylbenzene
i. 1-phenyl-2-propanol

Reactions of Benzene and Substituted Benzenes

Polynuclear Aromatic Hydrocarbons

3 Practice Problems

00:52

Chemistry: An Atoms-Focused Approach

Benzene rings with two methyl groups are called xylenes.
1,4-Dimethylbenzene (para-xylene or $p$ -xylene) and
1,3-dimethylbenzene (ortbo-xylene or o-xylene) have very similar boiling points $\left(138^{\circ} \mathrm{C} \text { and } 139^{\circ} \mathrm{C},$ respectively) \right. but very different melting points $\left(13^{\circ} \mathrm{C} \text { and }-48^{\circ} \mathrm{C}\right.$ respectively). Explain this observation.

Organic Chemistry: Fuels, Pharmaceuticals, and Modern Materials

01:22

CHEMISTRY: The Molecular Nature of Matter and Change

How does an aromatic hydrocarbon differ from a cycloalkane in terms of its bonding? How does this difference affect structure?

Organic Compounds and the Atomic Properties of Carbon

01:31

Chemistry Matter and Change

Explain how aromatic hydrocarbons differ from aliphatic hydrocarbons.

Hydrocarbons

Aromatic Allotropes of Carbon

4 Practice Problems

01:33

Chemistry: An Atoms-Focused Approach

Diamond and the fullerenes are allotropes of carbon. On the basis of their different structures and properties, predict which has the higher standard molar entropy.

Thermodynamics: Why Chemical Reactions Happen

09:28

Holt: Modern Chemistry

a. What do the terms saturated and unsaturated mean when applied to hydrocarbons?
b. What other meanings do these terms have in chemistry?
c. Classify alkenes, alkanes, alkynes, and aromatic hydrocarbons as either saturated or unsaturated.

Orgamic Chemistry

00:24

Chemistry A Molecular Approach

What is graphene? Why is graphene unique?

Solids and Modern Materials

Fused Heterocyclic Compounds

1 Practice Problems

Nomenclature of Benzene Derivatives

9 Practice Problems

04:39

Organic Chemistry

Give IUPAC names for the following substances (red = O, blue = N):

Benzene and Aromaticity

06:09

Organic Chemistry

Provide a name for each of the following compounds.
(Check your book to see figure)

Aromatic Compounds

03:42

Chemistry : An Introduction to General, Organic, and Biological Chemistry

Consider the compound ethylcyclopentane. $(7.4,7.7,8.6,11.2,$ 11.4)
a. Draw the skeletal formula for ethylcyclopentane.
b. Write the balanced chemical equation for the complete combustion of ethylcyclopentane.
c. Calculate the grams of $\mathrm{O}_{2}$ required for the combustion of $25.0 \mathrm{g}$ of ethylcyclopentane.
d. How many liters of $\mathrm{CO}_{2}$ would be produced at STP from the reaction in part $\mathbf{c} ?$

Introduction to Organic Chemistry: Hydrocarbons

Aromatic Compounds

Physical Properties of Benzene and Its Derivatives

0 Practice Problems

Spectroscopy of Aromatic Compounds

3 Practice Problems

04:06

Organic Chemistry

Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent? How many absorption lines would you expect to see in the 'H NMR and '3 C NMR spectra of the anion?

Benzene and Aromaticity

05:14

Organic Chemistry

What is the structure of a compound of molecular formula C$_{10}$H$_{14}$O$_2$ that shows a strong IR absorption at 3150$-$2850 cm$^{-1}$ and gives the following $^1$H NMR absorptions: 1.4 (triplet, 6 H), 4.0 (quartet, 4 H), and 6.8 (singlet, 4 H) ppm?

Benzene and Aromatic Compounds

Spectroscopic Properties