AROMATIC compounds or ARENES include benzene and benzene derivatives.

Aromatic compounds are quite common.

Many aromatic compounds were originally isolated from fragrant oils.

However, many aromatic compounds are odorless.

18.1 Intro to Aromatic Compounds

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8 of the 10 best-selling drugs have aromatic moieties.

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Coal contains aromatic rings fused together and joined by nonromantic moieties.

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Benzene is generally the parent name for monosubstituted derivatives.

18.2 Nomenclature of Benzene

Derivatives

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Many benzene derivatives have common names that become the parent name. (memorize these)

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If the substituent is larger than the ring, the substituent becomes the parent chain.

Aromatic rings are often represented with a Ph (for phenyl) or with a (phi) symbol.

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The common name for dimethyl benzene derivatives is XYLENE.

What do ORTHO, META, and PARA mean?

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Locants are required for rings with more than 2 substituents.

1. Identify the parent chain (generally the aromatic

ring):

Often a common name can be the parent chain.

2. Identify and name the substituents.

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3. Number the parent chain

A substituent that is part of the parent name must be assigned locant

NUMBER 1.

4. List the numbered substituents

before the parent name in

alphabetical order:

Ignore prefixes (except iso) when ordering alphabetically.

Complete the name for the molecule above.

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Klein, Organic Chemistry 1e

Section: 18.2 What is the correct name for the following?

a. o-bromoaniline

b. 2-bromoaniline

c. 1-amino-2-bromobenzene

d. all of the above

NH2

Br

In 1866, August Kekul proposed that benzene is a ring comprised of alternating double and single

bonds.

Kekul suggested that the exchange of double and single bonds was an equilibrium process.

18.3 Structure of Benzene

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We now know that the aromatic structures are resonance contributors rather than in equilibrium.

Sometimes the ring is represented with a circle in it

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The stability that results from a ring being aromatic is striking.

Recall that in general, alkenes readily undergo addition reactions.

Aromatic rings are stable enough that they do not undergo such reactions.

18.4 Stability of Benzene

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Heats of hydrogenation can be used to quantify aromatic stability.

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Molecular orbital (MO) theory can help us explain aromatic stability.

The six atomic p-orbitals of benzene overlap to make six MOs.

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Summary of Critical Points of MO Theory

MO Theory is good for describing excited states, ions, exotic species that defy Lewis structures.

Mix n atomic orbitals to form n molecular orbitals

each orbital has max occupation = 2 electrons

Fill from lowest energy MO

An MO has NO EFFECT until it contains an electron

For each bonding MO, there exists an anti-bonding MO that can exactly cancel the bonding MO

Most likely excitation moves one electron from HOMO to LUMO

we can use FROST CIRCLES to predict the relative MO energies.

18.4 Stability of Benzene

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Use the FROST CIRCLES below to explain the 4n+2 rule.

Note that the number of bonding orbitals is always an odd number; aromatic compounds will always have an

odd number of electron pairs.

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Some rings must carry a formal charge to be aromatic.

Consider a 5-membered ring.

18.5 Aromatic Compounds Other

Than Benzene

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The pKa value for cyclopentadiene is much lower than typical C-H bonds. WHY?

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vs.

Consider a 7-membered ring.

If six pi electrons are present, what charge will be necessary?

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Klein, Organic Chemistry 1e

Which of the following are aromatic?

A. B. C.

a. A

b. B

c. C

d. A and B

e. B and C

f. A and C

g. none of the above

h. all of the above

Heteroatoms (atoms other than C or H) can also be part of an aromatic ring.

If the heteroatoms lone pair is necessary, it will be included in the HCKEL number of pi electrons.

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If the lone pair is necessary to make it aromatic, the electrons will not be as basic.

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pKa=5.2

pKa=0.4

The difference in electron density can also be observed by viewing the electrostatic potential

maps.

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Will the compounds below be aromatic, antiaromatic, or non aromatic?

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A carbon that is attached to a benzene ring is BENZYLIC.

Recall that aromatic rings and alkyl groups are not easily oxidized.

18.6 Reactions at the Benzylic

Position

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HOWEVER, benzylic positions can readily be fully oxidized.

The benzylic position needs to have at least one proton attached to undergo oxidation.

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Permanganate can also be used as an oxidizing reagent.

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BENZYLIC positions have similar reactivity to allylic positions.

Benzylic positions readily undergo free radical bromination.

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Once the benzylic position is substituted with a bromine atom, a range of functional group

transformations are possible.

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Once the benzylic position is substituted with a bromine atom, a range of functional group

transformations are possible.

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Give necessary reagents for the reactions below.

18.6 Reactions at the Benzylic

Position

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Klein, Organic Chemistry 1e

Section: 18.6 What is the correct order of reagents to

achieve the following synthesis?

a. 1) CH3CH2CH2MgBr 2) H2O

b. NBS, light, CCl4

c. HBr

d. NaBr

e. 1) LAH 2) H2O

O Br

1) A

2) E, B

3) E, C

4) E, D

5) E, B, C

Under forceful conditions, benzene can be reduced to cyclohexane.

Is the process endothermic or exothermic? WHY?

WHY are forceful conditions required?

18.7 Reduction of the Aromatic

Moiety

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Vinyl side groups can be selectively reduced.

H is just slightly less than the expected 120 kJ/mol expected for a C=C CC conversion.

WHY are less forceful conditions required?

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Note that the BIRCH reduction product has sp3 hybridized carbons on opposite ends of the ring.

18.7 Reduction of the Aromatic

Moiety

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Like alkenes, benzene can undergo the BIRCH reduction.

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Like alkenes, benzene can undergo the BIRCH reduction.

Draw the final product.

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The presence of an electron donating alkyl side group or EWD groups provides different regioselectivity. Why?

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Klein, Organic Chemistry 1e

Section: 18.7 What is the product(s) of the following

reaction.

A. C.

B. D.

NO2

Na, CH3OH NH3

NO2

NO2

NO2

NO2

Klein, Organic Chemistry 1e

Section: 18.8

In 1H NMR spectrometry, aromatic C-H bonds show peaks

at _______ ppm.

In C NMR spectrometry, aromatic carbons show peaks at

_______ ppm.

In IR spectroscopy, aromatic C-H bonds show peaks at

_______ cm1.

a. 34, 100150, 14501600 b. 6.5-8.5, 100150, 3000-3100 c. 34, 100150, 2250 d. 6.5-8.5, 100150, 14501600

18.8 Spectroscopy of Aromatic

Compounds

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IR spectra for ethylbenzene:

NMR of Aromatics

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Recall from Section 16.5 how the anisotropic effects of an aromatic ring affect NMR shifts.

NMR of Aromatics

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The integration and splitting of protons in the aromatic region of the 1H NMR (7 ppm) in often very useful.

NMR of Aromatic Compounds

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Because of possible ring symmetry, the number of signals in the 13C NMR (100-150 ppm) generally provides structural information.

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For the molecule below, predict the shift for the 13C signals, and predict the shift, integration,

and multiplicity for the 1H NMR signals.

Graphite, Buckyballs, and

Nanotubes

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Graphite consists of layers of sheets of fused aromatic rings.

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Buckyballs are C60 spheres made of interlocking aromatic rings.

Fullerenes come in other sizes such as C70.

How are Buckyballs aromatic when they are not FLAT?

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Fullerenes can also be made into tubes (cylinders).

Single, double, and multi-walled carbon nanotubes have many applications:

Conductive Plastics, Energy Storage, Conductive Adhesives, Molecular Electronics, Thermal Materials,

Fibres and Fabrics, Catalyst Supports, Biomedical

Applications