Chapter 18 Aromatic Compounds

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Organic Chemistry. Second Edition. David Klein. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.. Klein, Organic Chemistry 2e . 18.1 Introduction to Aromatic Compounds. Aromatic. compounds or . ID: 641200 Download Presentation

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Chapter 18 Aromatic Compounds




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Presentations text content in Chapter 18 Aromatic Compounds

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Chapter 18Aromatic Compounds

Organic ChemistrySecond Edition

David Klein

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Klein, Organic Chemistry 2e

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18.1 Introduction to Aromatic CompoundsAromatic

compounds or arenes include benzene and benzene derivatives

Many aromatic compounds were originally isolated from fragrant oilsHowever, many aromatic compounds are odorless

Aromatic compounds are quite common

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Klein, Organic Chemistry 2e

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18.1 Introduction to Aromatic Compounds8 of the 10 best-selling drugs have aromatic moieties

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-3Klein, Organic Chemistry 2e

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18.1 Introduction to Aromatic CompoundsCoal contains aromatic rings fused together and joined by nonromantic moieties

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-4

Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene DerivativesBenzene is generally the parent name for

monosubstituted derivativesCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-5Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene Derivatives

Many benzene derivatives have common names.For some compounds, the common name becomes the parent name.Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-6Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene DerivativesIf 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

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-7Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene DerivativesThe common name for

dimethyl benzene derivatives is xylene

What do

ortho

, meta, and

para

mean?

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Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene DerivativesIdentify the parent chain (the longest consecutive chain of carbons)

Identify and Name the substituentsNumber the parent chain and assign a locant (and prefix if necessary) to each substituentGive the first substituent the lowest number possible

List the numbered substituents before the parent name in alphabetical orderIgnore prefixes (except iso) when ordering alphabetically

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-9

Klein, Organic Chemistry 2e

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18.2 Nomenclature of Benzene DerivativesLocants

are required for rings with more than 2 substituentsIdentify the parent chain (generally the aromatic ring)Often a common name can be the parent chain

Identify and Name the substituents

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Number the parent chain and assign a locant (and prefix if necessary) to each substituent

A substituent that is part of the parent name must be assigned locant NUMBER 1

List the numbered substituents before the parent name in alphabetical orderIgnore prefixes (except iso) when ordering alphabetically

Complete the name for the molecule abovePractice with

SkillBuilder 18.1

18.2 Nomenclature of Benzene DerivativesCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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18.2 Nomenclature of Benzene DerivativesName the following molecules

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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18.3 Structure of BenzeneIn 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 processCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-13Klein, Organic Chemistry 2e

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18.3 Structure of Benzene

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

HOW is resonance different from equilibrium?

Sometimes the ring is represented with a circle in itWHY?

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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18.4 Stability of BenzeneThe stability that results from a ring being aromatic is striking

Recall that in general, alkenes readily undergo addition reactionsAromatic rings are stable enough that they do not undergo such reactions

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Does every fully conjugated cyclic compound have aromatic stability? NO

Some fully conjugated cyclic compounds are reactive rather than being stable like benzene18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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AROMATIC compounds fulfill two criteriaA fully conjugated ring with overlapping p-orbitalsMeets Hückel’s rule:

an ODD number of e- pairs or 4n+2 total π electrons where n=0, 1, 2, 3, 4, etc. Show how the molecules below do NOT meet the criteria18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-17

Klein, Organic Chemistry 2e

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We can explain Hückel’s rule using MO theoryLet’s consider the MOs for cyclobutadiene

The instability of the unpaired electrons (similar to free radicals) makes this antiaromatic 18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-18

Klein, Organic Chemistry 2e

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A similar MO analysis for cyclooctatetraene suggests that it is also

antiaromatic18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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However, if the structure adopts a tub-shaped conformation, it can avoid the antiaromatic instability

The conjugation does not extend around the entire ring, so the system is neither aromatic nor

antiaromatic

18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Is the compound below aromatic or antiaromatic? HOW?

Practice with conceptual checkpoint 18.8

18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

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Predicting the shapes and energies of MOs requires sophisticated mathematics, but we can use Frost circles to predict the relative MO energies

18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-23

Klein, Organic Chemistry 2e

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Use the Frost circles below to explain the 4

n+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

Practice with conceptual checkpoint 18.9

18.4 Stability of Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

Slide26

AROMATIC compounds fulfill two criteriaA fully conjugated ring with overlapping p-orbitals

Meets Hückel’s rule: an ODD number of e- pairs or 4n+2 total π electrons where n=0, 1, 2, 3, 4, etc. ANTIAROMATIC compounds fulfill two criteria

A fully conjugated ring with overlapping p-orbitalsAn EVEN number of electron pairs or 4n total π

electrons where n=0, 1, 2, 3, 4, etc.

18.5 Aromatic Compounds Other Than BenzeneCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Annulenes are rings that are fully conjugated

Some

annulenes are aromatic, while others are

antiaromatic[10]Annulene is neither. WHY?

Practice with conceptual checkpoint 18.10

18.5 Aromatic Compounds Other Than Benzene

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Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

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Some rings must carry a formal charge to be aromaticConsider a 5-membered ring

If 6 pi electrons are present, draw the resonance contributors for the structure

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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

vs.18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-30Klein, Organic Chemistry 2e

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Consider a 7-membered ring

If 6 pi electrons are present, what charge will be necessary?

Draw the resonance contributors for the structure

Practice with SkillBuilder 18.2

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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If the heteroatom’s lone pair is necessary, it will be included in the Hückel number of pi electrons18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-33Klein, Organic Chemistry 2e

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If the lone pair is necessary to make it aromatic, the electrons will not be as basic18.5 Aromatic Compounds Other Than Benzene

pKa=5.2

pK

a=0.4

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-

34Klein, Organic Chemistry 2e

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

Practice with

SkillBuilder

18.3

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

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

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Many polycyclic compounds are also aromatic

Such compounds are shown to be aromatic using heats of hydrogenation. HOW?

18.5 Aromatic Compounds Other Than Benzene

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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38Klein, Organic Chemistry 2e

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Show that the molecules below meet the criteria for aromaticityA fully conjugated ring with overlapping p-orbitalsMeets Hückel’s rule:

an ODD number of e- pairs or 4n+2 total π electrons where n=0, 1, 2, 3, 4, etc. 18.5 Aromatic Compounds Other Than BenzeneCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-39Klein, Organic Chemistry 2e

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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 PositionCopyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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In general, benzylic positions can readily be fully oxidized

The benzylic position needs to have at least 1 proton attached to undergo oxidation

18.6 Reactions at the Benzylic Position

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18-41

Klein, Organic Chemistry 2e

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Permanganate can also be used as an oxidizing reagent

Practice with conceptual checkpoint 18.19

18.6 Reactions at the Benzylic Position

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-42

Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

Slide44

Benzylic positions have similar reactivity to allylic positions. WHY?

18.6 Reactions at the Benzylic Position

Benzylic positions readily undergo free radical

bromination

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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

18.6 Reactions at the Benzylic Position

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-45

Klein, Organic Chemistry 2e

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

18.6 Reactions at the Benzylic Position

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-

46Klein, Organic Chemistry 2e

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18.6 Reactions at the Benzylic Position

Practice with SkillBuilder 18.4Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

18-47Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

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Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.Klein, Organic Chemistry 2e

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18.6 Reactions at the Benzylic PositionGive necessary reagents for the reactions below

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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18.7 Reduction of the Aromatic MoietyUnder forceful conditions, benzene can be reduced to cyclohexane

Is the process endothermic or exothermic? WHY?

WHY are forceful conditions required?

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18.7 Reduction of the Aromatic MoietyVinyl side groups can be selectively reduced

ΔH is just slightly less than the expected -120 kJ/mol expected for a C=C  C-C conversionWHY are less forceful conditions required?

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-52

Klein, Organic Chemistry 2e

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Graphite, Buckyballs, and Nanotubes

Graphite consists of layers of sheets of fused aromatic rings

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-

53Klein, Organic Chemistry 2e

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Graphite, Buckyballs, and Nanotubes

Buckyballs are C60 spheres made of interlocking aromatic rings

Fullerenes come in other sizes such as C

70How are Buckyballs aromatic when they are not FLAT?

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-

54Klein, Organic Chemistry 2e

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Graphite, Buckyballs, and Nanotubes

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

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Additional Practice ProblemsGive the names for the structures below.

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Klein, Organic Chemistry 2e

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Additional Practice ProblemsExplain how we know that

aromaticity is stabilizing experimentally and how MO theory rationalizes that stabilization.Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.18-57

Klein, Organic Chemistry 2e

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Additional Practice ProblemsLabel each molecule below as aromatic,

antiaromatic, or nonaromatic

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Klein, Organic Chemistry 2e

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Additional Practice ProblemsPredict the major product(s) for the reaction below

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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Klein, Organic Chemistry 2e

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Additional Practice ProblemsGiven NMR data, predict the structure of an aromatic compound

1H NMR: a) triplet at 1.1 ppm integrates to 3 b) singlet at 2.0 integrates to 3

c) quartet at 2.3 integrates to 2 d) overlapping signals between 7-7.5 ppm

integrating to 813C NMR: signals at 12, 23, and 26

ppm and 8 signals between 100 and 150 ppm

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

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