Chapter 18 Aromatic Compounds - PowerPoint Presentation

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Chapter 18 Aromatic Compounds - PPT Presentation

Organic Chemistry Second Edition David Klein Copyright 2015 John Wiley amp Sons Inc All rights reserved Klein Organic Chemistry 2e 181 Introduction to Aromatic Compounds Aromatic compounds or ID: 641200

sons organic chemistry reserved organic sons reserved chemistry 2015 john wiley amp rights copyright aromatic klein benzene compounds benzylic ring practice number

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Slide1

Chapter 18Aromatic Compounds

Organic ChemistrySecond Edition

David Klein

Klein, Organic Chemistry 2e Slide2

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

18-

Klein, Organic Chemistry 2e Slide3

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

18-3Klein, Organic Chemistry 2e Slide4

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

Klein, Organic Chemistry 2e Slide5

18.2 Nomenclature of Benzene DerivativesBenzene is generally the parent name for

18-5Klein, Organic Chemistry 2e Slide6

18.2 Nomenclature of Benzene Derivatives

18-6Klein, Organic Chemistry 2e Slide7

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

18-7Klein, Organic Chemistry 2e Slide8

18.2 Nomenclature of Benzene DerivativesThe common name for

dimethyl benzene derivatives is xylene

What do

ortho

, meta

, and para

mean?

18-

Klein, Organic Chemistry 2e Slide9

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

Klein, Organic Chemistry 2e Slide10

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

18-

Klein, Organic Chemistry 2e Slide11

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-

Klein, Organic Chemistry 2e Slide12

18.2 Nomenclature of Benzene DerivativesName the following molecules

18-12

Klein, Organic Chemistry 2e Slide13

18.3 Structure of BenzeneIn 1866, August Kekulé proposed that benzene is a ring comprised of alternating double and single bonds

18-13Klein, Organic Chemistry 2e Slide14

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?

18-

Klein, Organic Chemistry 2e Slide15

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

18-

Klein, Organic Chemistry 2e Slide16

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

18-

Klein, Organic Chemistry 2e Slide17

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

Klein, Organic Chemistry 2e Slide18

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

Klein, Organic Chemistry 2e Slide19

A similar MO analysis for cyclooctatetraene suggests that it is also

antiaromatic18.4 Stability of Benzene

18-19

Klein, Organic Chemistry 2e Slide20

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

18-

Klein, Organic Chemistry 2e Slide21

Is the compound below aromatic or antiaromatic? HOW?

Practice with conceptual checkpoint 18.8

18.4 Stability of Benzene

18-

Klein, Organic Chemistry 2e Slide22

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

Klein, Organic Chemistry 2e Slide24

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

18-

Klein, Organic Chemistry 2e Slide25

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-

Klein, Organic Chemistry 2e Slide27

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

18-

Klein, Organic Chemistry 2e Slide28

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

18-

Klein, Organic Chemistry 2e Slide30

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

vs.18.5 Aromatic Compounds Other Than Benzene

18-30Klein, Organic Chemistry 2e Slide31

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

18-

Klein, Organic Chemistry 2e Slide32

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

18.5 Aromatic Compounds Other Than Benzene

18-32

Klein, Organic Chemistry 2e Slide33

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

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

a=0.4

34Klein, Organic Chemistry 2e Slide35

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

18-

Klein, Organic Chemistry 2e Slide36

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

18.5 Aromatic Compounds Other Than Benzene

18-

Klein, Organic Chemistry 2e Slide38

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

18-

38Klein, Organic Chemistry 2e Slide39

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

18-39Klein, Organic Chemistry 2e Slide40

A carbon that is attached to a benzene ring is benzylic

Recall that aromatic rings and alkyl groups are not easily oxidized

18-40

Klein, Organic Chemistry 2e Slide41

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

18-41

Klein, Organic Chemistry 2e Slide42

Permanganate can also be used as an oxidizing reagent

Practice with conceptual checkpoint 18.19

18.6 Reactions at the Benzylic Position

18-42

Klein, Organic Chemistry 2e Slide43

Benzylic positions have similar reactivity to allylic positions. WHY?

18.6 Reactions at the Benzylic Position

Benzylic positions readily undergo free radical

bromination

18-

Klein, Organic Chemistry 2e Slide45

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

Klein, Organic Chemistry 2e Slide46

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

46Klein, Organic Chemistry 2e Slide47

18.6 Reactions at the Benzylic Position

18-47Klein, Organic Chemistry 2e Slide48

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

18-

Klein, Organic Chemistry 2e Slide51

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?

18-

51Klein, Organic Chemistry 2e Slide52

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?

Klein, Organic Chemistry 2e Slide53

Graphite, Buckyballs, and Nanotubes

Graphite consists of layers of sheets of fused aromatic rings

53Klein, Organic Chemistry 2e Slide54

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?

Klein, Organic Chemistry 2e Slide55

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

18-

55Klein, Organic Chemistry 2e Slide56

Additional Practice ProblemsGive the names for the structures below.

18-

Klein, Organic Chemistry 2e Slide57

Additional Practice ProblemsExplain how we know that

Klein, Organic Chemistry 2e Slide58

Additional Practice ProblemsLabel each molecule below as aromatic,

antiaromatic, or nonaromatic

18-

Klein, Organic Chemistry 2e Slide59

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