18.1 intro to aromatic compounds - organic chemistry … · klein, organic chemistry 1e section:...

• 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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Klein, Organic Chemistry 1e 18-1

• 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 φ (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 Kekulé proposed that benzene is

a ring comprised of alternating double and single

bonds.

– Kekulé 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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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 heteroatom’s lone pair is necessary, it will be

included in the HÜCKEL 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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• Give necessary reagents for the reactions below.

18.6 Reactions at the Benzylic

Position

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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.

– ΔH is just slightly less than the expected –120 kJ/mol

expected for a C=C C–C 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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Section: 18.7 What is the product(s) of the following

reaction.

A. C.

B. D.

NO2

Na, CH3OH NH3

NO2

NO2

NO2

NO2


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

_______ cm–1.

a. 3–4, 100–150, 1450–1600

b. 6.5-8.5, 100–150, 3000-3100

c. 3–4, 100–150, 2250

d. 6.5-8.5, 100–150, 1450–1600

18.8 Spectroscopy of Aromatic

Compounds

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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

18.1 intro to aromatic compounds - organic chemistry … · klein, organic chemistry 1e section:...

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