aromatic compounds aromatic compound: a hydrocarbon that contains one or more benzene-like rings....

Aromatic CompoundsAromatic Compounds

Aromatic compound:Aromatic compound: a hydrocarbon that contains a hydrocarbon that contains one or more benzene-like rings.one or more benzene-like rings.– arene:arene: a term used to describe aromatic compounds. a term used to describe aromatic compounds.– Ar-:Ar-: a symbol for an aromatic group derived by removing a symbol for an aromatic group derived by removing

an -H from an arene.an -H from an arene.– Kekulé structure for benzene (1872).Kekulé structure for benzene (1872).

C

CC

C

CC

H

H

H

H

H

H

A Kekulé structureshowing all atoms

A Kekulé structureas a line-angle formula

BenzeneBenzene Resonance structure for benzene (1930s)Resonance structure for benzene (1930s)

– The theory of resonance, developed by Linus The theory of resonance, developed by Linus Pauling, provided the first adequate description of Pauling, provided the first adequate description of the structure of benzene.the structure of benzene.

– According to the theory of resonance, certain According to the theory of resonance, certain molecules and ions are best described by writing molecules and ions are best described by writing two or more Lewis structures; the real molecule or two or more Lewis structures; the real molecule or ion is a ion is a resonance hybridresonance hybrid of these structures. of these structures.

– Each individual Lewis structure is called a Each individual Lewis structure is called a contributing structure.contributing structure.

– We show that the real molecule is a resonance We show that the real molecule is a resonance hybrid of the two or more Lewis structures by hybrid of the two or more Lewis structures by using a double-headed arrow between them.using a double-headed arrow between them.

BenzeneBenzene– Here are two contributing structures for benzene;Here are two contributing structures for benzene;

– The resonance hybrid has some of the The resonance hybrid has some of the characteristics of each Lewis contributing characteristics of each Lewis contributing structure. structure.

– The length of a carbon-carbon bond in benzene The length of a carbon-carbon bond in benzene (140 pm), for example, is midway between that of (140 pm), for example, is midway between that of a carbon-carbon single bond (146 pm) and a a carbon-carbon single bond (146 pm) and a double bond (134 pm).double bond (134 pm).

CC

CCC

C CCC

CCC

H

HH

H

HH H

H

HH

H

H

BenzeneBenzene

Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.

Bond distances and bond angles of benzeneFigure 11.1

NomenclatureNomenclature– Monosubstituted alkylbenzenes are Monosubstituted alkylbenzenes are

named as derivatives of benzene; for named as derivatives of benzene; for example, ethylbenzene.example, ethylbenzene.

– The IUPAC system retains certain The IUPAC system retains certain common names for several of the simpler common names for several of the simpler monosubstituted alkylbenzenes;monosubstituted alkylbenzenes;

CH2CH3 CH3 CH=CH2

TolueneEthylbenzene Styrene

NomenclatureNomenclature– The common names for these monosubstituted The common names for these monosubstituted

benzenes are also retained.benzenes are also retained.

– phenyl group (Cphenyl group (C66HH55- or Ph-):- or Ph-): the substituent group the substituent group derived by removal of an H from benzene.derived by removal of an H from benzene.

OCH3 C-OHO

NH2OH C-HO

Phenol Aniline Benzoic acidAnisole Benzaldehyde

C6H5

1-Phenylcyclohexene 4-Phenyl-1-butenePhenyl group

124

3

NomenclatureNomenclature


NomenclatureNomenclature When two substituents occur on a benzene ring, When two substituents occur on a benzene ring,

three isomers are possible; they may be located three isomers are possible; they may be located by:by:– numbering the atoms of the ring or numbering the atoms of the ring or – using the locators ortho (o), meta (m), and using the locators ortho (o), meta (m), and

para (p).para (p).

COOHBr

CH3

CH3Cl

CH2CH3

1

2-Bromobenzoic acid(o-Bromobenzoic acid)

1,3-Dimethylbenzene(m-Xylene)

1-Chloro-4-ethylbenzene(p-Chloroethylbenzene)

12

2

23

3

4

1

NomenclatureNomenclature For three or more substituents:For three or more substituents:

– If one of the substituents imparts a special name, If one of the substituents imparts a special name, name the molecule as a derivative of that parent.name the molecule as a derivative of that parent.

– If none of the substituents imparts a special If none of the substituents imparts a special name, number the substituents to give the name, number the substituents to give the smallest set of numbers, and list them in smallest set of numbers, and list them in alphabetical order before the ending "benzene"alphabetical order before the ending "benzene"..

CH3NO2

Cl

OH

Br

BrBrNO2

CH2CH3

Br

43

2

1

5

6

43

21

43

1 2

4-Chloro-2-nitrotoluene

2,4,6-Tribromophenol 2-Bromo-1-ethyl-4-nitrobenzene

PAHsPAHs

Polynuclear aromatic hydrocarbon Polynuclear aromatic hydrocarbon (PAH)(PAH)– A hydrocarbon that contain two or more A hydrocarbon that contain two or more

benzene rings, with each pair of rings benzene rings, with each pair of rings sharing two adjacent carbon atoms.sharing two adjacent carbon atoms.

PhenanthreneAnthraceneNaphthalene Benzo[a]pyrene

Reactions of BenzeneReactions of Benzene



By far the most characteristic reaction of By far the most characteristic reaction of aromatic compounds is substitution at a ring aromatic compounds is substitution at a ring carbon.carbon.– This reaction is called This reaction is called aromatic substitutionaromatic substitution..– Some groups that can be introduced directly Some groups that can be introduced directly

on the ring are the halogens, the nitro (-NOon the ring are the halogens, the nitro (-NO22) ) group, and the sulfonic acid (-SOgroup, and the sulfonic acid (-SO33H) group.H) group.

HalogenationHalogenation

H Cl2FeCl3 Cl HCl+ +

ChlorobenzeneBenzene

General MechanismGeneral Mechanism

The first stageThe first stage– Suppose the electrophile is a positive ion Suppose the electrophile is a positive ion

XX++..– Two of the electrons in the delocalized Two of the electrons in the delocalized

system are attracted towards the Xsystem are attracted towards the X++ and and form a bond with it. This has the effect of form a bond with it. This has the effect of breaking the delocalization, although not breaking the delocalization, although not completely.completely.


The ion formed in this step isn't the final product. It immediately The ion formed in this step isn't the final product. It immediately goes on to react with something else. It is just an goes on to react with something else. It is just an intermediate.intermediate.

There is still delocalization in the intermediate formed, but it only There is still delocalization in the intermediate formed, but it only covers part of the ion. When you write one of these mechanisms, covers part of the ion. When you write one of these mechanisms, draw the partial delocalization to take in all the carbon atoms apart draw the partial delocalization to take in all the carbon atoms apart from the one that the X has become attached to.from the one that the X has become attached to.

The intermediate ion carries a positive charge because you are The intermediate ion carries a positive charge because you are joining together a neutral molecule and a positive ion. This positive joining together a neutral molecule and a positive ion. This positive charge is spread over the delocalized part of the ring. Simply draw charge is spread over the delocalized part of the ring. Simply draw the "+" in the middle of the ring.the "+" in the middle of the ring.

The hydrogen at the top isn't new - it's the hydrogen that was The hydrogen at the top isn't new - it's the hydrogen that was already attached to that carbon. We need to show that it is there already attached to that carbon. We need to show that it is there for the next stage.for the next stage.


Stage TwoStage Two

– Here we've introduced a new ion, YHere we've introduced a new ion, Y--. . Where did this come from? You have to Where did this come from? You have to remember that it is impossible to get a remember that it is impossible to get a positive ion on its own in a chemical positive ion on its own in a chemical system - so Ysystem - so Y-- is simply the negative ion is simply the negative ion


A lone pair of electrons on YA lone pair of electrons on Y-- forms a bond with forms a bond with the hydrogen atom at the top of the ring. That the hydrogen atom at the top of the ring. That means that the pair of electrons joining the means that the pair of electrons joining the hydrogen onto the ring aren't needed any hydrogen onto the ring aren't needed any more. These then move down to plug the gap more. These then move down to plug the gap in the delocalized electrons, so restoring the in the delocalized electrons, so restoring the delocalized ring of electrons which originally delocalized ring of electrons which originally gave the benzene its special stability. gave the benzene its special stability.


NitrationNitration

– A value of nitroarenes is that the nitro A value of nitroarenes is that the nitro group can be reduced to a primary group can be reduced to a primary amino group:amino group:

H HNO3H2SO4

NO2 H2O++

Nitrobenzene

COOHO2N 3H2

NiCOOHH2N 2H2O++

3 atm

4-Nitrobenzoic acid(p-Nitrobenzoic acid)

4-Aminobenzoic acid(p-Aminobenzoic acid, PABA)


SulfonationSulfonation

– An application of sulfonation is in the An application of sulfonation is in the preparation of synthetic detergents.preparation of synthetic detergents.

H H2SO4 SO3H H2O+

Benzenesulfonic acid

+

CH3(CH2)10CH2

1. H2SO4

2. NaOH

CH3(CH2)10CH2 SO3- Na+

Dodecylbenzene

Sodium 4-dodecylbenzenesulfonate, SDS(an anionic detergent)

FRIEDEL-CRAFTS ALKYLATION FRIEDEL-CRAFTS ALKYLATION

Alkylation means substitution of an Alkylation means substitution of an alkyl group. A hydrogen on the ring alkyl group. A hydrogen on the ring is replaced by a group like methyl or is replaced by a group like methyl or ethyl, etc. in the presence of a ethyl, etc. in the presence of a catalyst. catalyst.

FRIEDEL-CRAFTS ALKYLATIONFRIEDEL-CRAFTS ALKYLATION

The formation of the electrophileThe formation of the electrophile– The electrophile is CHThe electrophile is CH33

++. It is formed by . It is formed by reaction between the chloromethane and reaction between the chloromethane and the aluminum chloride catalyst.the aluminum chloride catalyst.

– Step 1Step 1

FRIEDEL-CRAFTS ALKYLATIONFRIEDEL-CRAFTS ALKYLATION

Step 1Step 1– The hydrogen is removed by the AlClThe hydrogen is removed by the AlCl44- -

ion which was formed at the same time ion which was formed at the same time as the CHas the CH33

++ electrophile. The aluminum electrophile. The aluminum chloride catalyst is re-generated in this chloride catalyst is re-generated in this second stage. second stage.

FRIEDEL-CRAFTS ACYLATIONFRIEDEL-CRAFTS ACYLATION

Acylation – Acylation – substitution of an acyl group.substitution of an acyl group.– An acyl group is an alkyl group attached to a An acyl group is an alkyl group attached to a

carbon-oxygen double bond. If "R" represents any carbon-oxygen double bond. If "R" represents any alkyl group, then an acyl group has the formula alkyl group, then an acyl group has the formula RCO-. RCO-.

– The most commonly used acyl group is CHThe most commonly used acyl group is CH33CO-. CO-. This is called the ethanoyl group.This is called the ethanoyl group.

– Benzene is treated with a mixture of ethanoyl Benzene is treated with a mixture of ethanoyl chloride, CH3COCl, and aluminum chloride as the chloride, CH3COCl, and aluminum chloride as the catalyst. A ketone called phenylethanone is formed. catalyst. A ketone called phenylethanone is formed.


The formation of the electrophileThe formation of the electrophile

Step 1Step 1


Step twoStep two– The hydrogen is removed by the AlClThe hydrogen is removed by the AlCl44

-- ion ion which was formed at the same time as the which was formed at the same time as the CHCH33COCO+ + electrophile. The aluminum chloride electrophile. The aluminum chloride catalyst is re-generated in this second stage. catalyst is re-generated in this second stage.

PhenolsPhenols

The functional group of a phenol is a The functional group of a phenol is a hydroxyl group bonded to a benzene hydroxyl group bonded to a benzene ring.ring.– Name substituted phenols either as Name substituted phenols either as

derivatives of phenol or by common derivatives of phenol or by common names.names.

1,2-Benzenediol(Catechol)

1,3-Benzenediol(Resorcinol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OH OH OH OHOH

OHOH

PhenolsPhenols

Some phenols found in natureSome phenols found in nature

2-Isopropyl-5-methylphenol

(Thymol)

4-Hydroxy-3-methoxy-benzaldehyde

(Vanillin)Urushiol

(Poison ivy)

OH

HO

H3CO CHO

CH2(CH2)13CH3

OH

OH

NCO

H

CH3O

HO Capsaicin(from various types of peppers)

Phenols as AntioxidantsPhenols as Antioxidants

Autoxidation:Autoxidation: a reaction that a reaction that converts an R-H group to an R-O-O-H converts an R-H group to an R-O-O-H (hydroperoxide) group.(hydroperoxide) group.

CH2CH=CH-CHH

O2 CH2CH=CH-CH-CH2

O-O-Hlight

or heatSection of a fatty

acid hydrocarbon chain

+Oxygen A hydroperoxide


Autoxidation is a radical chain Autoxidation is a radical chain reaction:reaction:– chain initiation:chain initiation: Formation of a radical Formation of a radical

from a nonradical compound.from a nonradical compound.– radical:radical: An atom of molecule with an An atom of molecule with an

unpaired electron.unpaired electron.

CH2CH=CH-CHH

CH2CH=CH-CHSection of a fatty

acid hydrocarbon chainA carbon radical

lightor heat


– chain propagation:chain propagation: Reaction of a radical Reaction of a radical to form a new radical.to form a new radical.

– propagation step 1:propagation step 1:

– propagation step 2:propagation step 2:

CH2CH=CH-CH O-O CH2CH=CH-CHO-O

A hydroperoxy radical

+Oxygen is adiradical

CH2CH=CH-CH-CH2

O-O-H

A hydroperoxide

CH2CH=CH-CHO-O

CH2CH=CH-CH

H+

+ CH2CH=CH-CH A new carbon radical

Section of a new fattyacid hydrocarbon chain


Hydroperoxides: Hydroperoxides: – are unstable.are unstable.– Under biological conditions, they degrade to short-Under biological conditions, they degrade to short-

chain aldehydes and carboxylic acids with chain aldehydes and carboxylic acids with unpleasant "rancid" smells.unpleasant "rancid" smells.

– Similar formation of hydroperoxides in the low-Similar formation of hydroperoxides in the low-density lipoproteins deposited on the walls of density lipoproteins deposited on the walls of arteries leads to cardiovascular disease in arteries leads to cardiovascular disease in humans.humans.

– In addition, many effects of aging are thought to In addition, many effects of aging are thought to be the result of hydroperoxide formation and their be the result of hydroperoxide formation and their subsequent degradation.subsequent degradation.

Phenols as AntioxidantsPhenols as Antioxidants– Vitamin E is a natural antioxidant.Vitamin E is a natural antioxidant.– BHT and BHA are synthetic antioxidants.BHT and BHA are synthetic antioxidants.

– These compounds are radical scavengers.These compounds are radical scavengers.– They form stable radicals and thus break the cycle They form stable radicals and thus break the cycle

of chain propagation steps; they prevent further of chain propagation steps; they prevent further formation of destructive hydroperoxides.formation of destructive hydroperoxides.

HO

OH

OH OH

OCH3

3

Vitamin E Butylated hydroxy-toluene (BHT)

Butylated hydroxy-anisole (BHA)

aromatic compounds aromatic compound: a hydrocarbon that contains one or more benzene-like rings....

Documents

structure of benzene

benzene rings

carboncarbon bond

ending benzene

derivatives of benzene

aromatic group

ring carbon

carboncarbon single