Introduction

Copyright© 2005, Michael J. Wovkulich. All rights reserved.

IntroductionIntroduction

1. Benzene has the formula _____ and was discovered in 1825 by _______ _______. This compound and others like it were called ____________ because of its pleasant odor. Today, the term aromatic refers to the unusual ____________ these compounds possess.

C6H6

Michael Faraday

aromatic

stability

The Structure & Properties of BenzeneThe Structure & Properties of Benzene

1. Draw the structure of benzene Kekulé proposed in 1866.

C

CC

C

CC

H

H

HH

H H

2. Experimental evidence shows that all the carbon-carbon bonds in benzene have the same bond length. Why is this in disagreement with Kekulé's structure?

Double bonds are expected to be shorter than single

bonds.Kekulé's structure should have alternating short and

long bonds.

Draw a better representation of the benzene structure.

The bond order between carbon atoms is _______, the bond angles are _______, and the carbon atoms are _____-hybridized.

=

1½

120°sp2

3. The benzene molecule is flat, with the unhybridized _____ orbitals extending above and below the plane of carbon atoms. Draw the benzene ring showing the p orbitals overlapping to form a continuous ring of orbitals above and below the plane of carbon atoms.

p

C

C

C C

C

C

sp2 framewo

rk

p orbital

p orbital

circle of delocalize

d electrons

4. The double bonds in benzene are said to be _______________, which means that they are separated by one single bond and overlap their p orbitals in the _____ bonds. Draw examples of conjugated and non-conjugated double bonds.

conjugated

conjugated

non-conjugated

5. Based upon what you've learned about benzene, are aromatic compounds cyclic or acyclic? Are they planar or nonplanar? Do they contain conjugated or non-conjugated double bonds? Do they have or lack resonance stability?

6. Draw the structure of cyclooctatetraene.

Using the information in number 5, is this compound expected to be aromatic?

Yes, it’s expected to be aromatic because it appears to...

be cyclic be planar be conjugated have resonance

stability

Construct a model of cyclooctatetraene. Is this consistent with your prediction of whether or not cyclooctatetraene is aromatic?

No Why or why not?

In this view, cyclooctatetraene

appears to be planar, and could

be aromatic.

This view shows that

cyclooctatetraene is nonplanar,

therefore it cannot be aromatic.

Cyclooctatetraene... is not planar.

is non-conjugated.

has no resonance structures. is not aromatic.

We need a better set of requirements for a molecule to be aromatic.

Aromatic, Antiaromatic, & Nonaromatic CompoundsAromatic, Antiaromatic, & Nonaromatic Compounds

1. A compound is Aromatic if…

It is _______ with _________ pi bonds. All the atoms in the ring are _____- or _____-hybridized. It is _________ (or nearly) so that it forms a continuous, overlapping ring

of parallel _____ orbitals. It has _______ energy due to delocalization of _____ electrons.

cyclic conjugatedsp

sp2

planar

plower

2. A compound is Antiaromatic if…

Numbers , , and are True, but is False. Cyclobutadiene is antiaromatic. Why? Remember what the polygon rule told us about the stability of benzene vs. cyclobutadiene.

The polygon rule shows that cyclobutadiene is expected to be unstable.

There is no resonance stability.

3. A compound is Nonaromatic if…

Neither the criteria described in number 1 nor number 2 apply. Cyclooctatetraene is nonaromatic. Why? Consider whether the four criteria in number 1 are true or false.

cyclic, but non-conjugated

sp2-hybridized carbon atoms

nonplanar (poor overlap of p orbitals)

higher energy (polygon rule)

Hückel’s RuleHückel’s Rule

1. Another way to predict whether a compound is aromatic or antiaromatic is Hückel's Rule. Before we can apply this rule, the compound must have a _______________ ring of overlapping _____ orbitals, usually in a _________ arrangement. Once these conditions are met, we can use Hückel's Rule.

continuous

pplanar

2. Hückel's Rule says: If the number of pi electrons in the cyclic system is… 4N + 2, the system is _________________. 4N, the system is _________________.

Where N is an integer.

aromaticantiaromat

ic

3. Fill in the table below.

2

4 6

8 10

12

14

4. Fill in the table below.

6 1 4N + 2 Aromatic

4 1 4NAnti-

aromatic

8 2 4NAnti-

aromatic if

planar**COT is nonplanar = nonaromatic

(Hückel’s rule doesn’t apply).

5. Fill in the table below.

10

2 4N + 2 Aromatic

8 2 4NAnti-

aromatic

8 2 4NNon-

aromatic*

*There is not a continuous ring of overlapping p orbitals (Hückel’s rule doesn’t apply).

Nomenclature of Benzene DerivativesNomenclature of Benzene Derivatives

1. Although benzene is a stable aromatic compound, it doesn't mean it's unreactive. Benzene undergoes many reactions to form useful derivatives. Some of the more commonly used derivatives, shown below, use common names instead of IUPAC names. For each compound below, give the common name and the IUPAC name.

OH CH3 NH2 OCH3

O

H

O

OH

O

phenol

benzenol

toluene

methylbenzene

aniline

benzenamine

anisole

methoxybenzene

styrene

vinylbenzene

acetophenonemethyl

phenyl ketone

benzaldehyde

benzoic acid

2. Some benzene derivatives are named by combining the substituent name with the word "benzene." Name the following compounds.

NO2

ethylbenzene

isopropylbenzene

t-butylbenzene

nitrobenzene

3. When benzene loses one H atom and becomes a substituent, it forms the C6H5— group and is named _________. When toluene loses one methyl H atom and becomes a substituent, it forms the C6H5CH2— group and is named _________. Name the following compounds.

phenyl

benzyl

O CH2Cl CH2OH

diphenyl ether

benzyl chloride

benzyl alcohol

4. Disubstituted benzenes are given common names by using the prefixes ortho-, meta-, and para- (abbreviated _____, _____, and _____) to show the relative position of the two substituents on the benzene ring. Numbers are used by the IUPAC system. For example, the numbers _____ indicate ortho-, _____ indicate meta-, and _____ indicate para-.

o- m- p-

1,2-1,3-

1,4-

Identify the substitution pattern in each example below.

X

Y

X

Y

X

Y

ortho- or o- meta- or m- para- or p-

1,2- 1,3- 1,4-

5. When there are two methyl groups on the benzene ring, the molecule is given the common name xylene. For each compound below, give the common name and the IUPAC name.

o-xylene m-xylene p-xylene

1,2-xylene 1,3-xylene 1,4-xylene

CH3

CH3

CH3

CH3

CH3

CH3

6. For each compound below, give the common name and the IUPAC name.

Br

Br

F

CH3

Cl

OH

Cl

Br

NO2

F

Cl

CH2CH3

o-dibromobenzene

1,2-dibromobenzene

m-bromochlorobenzene

1-bromo-3-chlorobenzene

P-fluoronitrobenze

ne1-fluoro-4-nitrobenzene

o-chloroethylbenzen

e1-chloro-2-ethylbenzene

m-fluorotoluene

3-fluorotoluene

p-chlorophenol

4-chlorophenol

7. When there are three or more substituents on the benzene ring, numbers are used to show their relative positions. One substituent is often used to form the base name and this substituent is assigned number 1. The other substituents are numbered in the direction that gives the lowest possible set of numbers. Name the following compounds.

Br

BrBr

NO2

Br

Cl

Br

Cl

NO2

1,3,5-tribromobenzene

2-bromo-4-chloro-1-nitrobenzene

4-bromo-1-chloro-2-

nitrobenzene

1

35

1

2

4 1

2

4

OH

Cl

Cl

COOH

FF

CH3

NO2

NO2

O2N

Br CH3

NO2HO Br

Cl

CH2CH3

I

NH2

2,4-dichlorophenol3-bromo-

4-chlorophenol

5-bromo- 2-

nitrotoluene

1

2

4

1 3

4 1

2

5

2-ethyl-4-iodoaniline

3,5-difluorobenzoic

acid

2,4,6-trinitrotoluene

12

4

1

35

1 2

4

6