9-1 © 2005 John Wiley & Sons, Inc. All rights reserved Chapter 9: Benzene and its Derivatives.

9-1© 2005 John Wiley & Sons, Inc.All rights reserved

Chapter 9: Benzene and its Derivatives


Benzene and Its Derivatives


Benzene - Kekulé• The first structure for benzene was proposed by

August Kekulé

• this structure, however, did not account for the unusual chemical reactivity of benzene

CH

CH

CH

CHC

H

C

H

CC

CC

C

C

H

H

HH

HH


Benzene - Orbital Overlap Model

• The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure• the carbon skeleton is a regular hexagon, with all

C-C-C and H-C-C bond angles 120°

sp2-sp2

sp2-1s1.09 Å

120°

120°120°

1.39 Å

C

C

C

C

C CH

H H

H

H H


Benzene - Resonance Model• We often represent benzene as a hybrid of two

equivalent Kekulé structures• each Kekulé structure makes an equal contribution to

the hybrid• the C-C bonds are neither double nor single but

something in between

Benzene as a hybrid of two equivalent contributing structures


Benzene

Alas, benzene is both toxic and carcinogenic. In fact, it might

be considered "the mother of all carcinogens," as a large

number of carcinogens have structures that include benzene

rings. My mentor’s Undergraduate Organic Chemistry

professor joked about how he used to "practically bathe in

benzene up to the elbows" when he would use it in his research

(presumably before it was identified as a carcinogen). He

predicted that this would probably lead to his demise. He was

right--he died due to leukemia several years ago.


Aromatic Compounds and the Structure of Benzene

• In the early days the word aromatics was used to described many fragrant molecules isolated from natural sources. Today the term aromatic is used to describe benzene like molecules.

• Benzene is a flat, symmetrical molecule with the molecular formula C6H6.

• It has alternating three carbon-carbon double and three single bonds.

• Benzene’s relatively lack of chemical reactivity is due to its structure.


• Experimental evidence suggest that all six carbon-carbon bonds in benzene are identical.

• The properties, including the above one, of benzene can only be explained by assuming that the actual structure of benzene is an average of the above two possible equivalent structures-known as resonance.

• Simple aromatic compounds like benzene are non-polar, insoluble in water, volatile, and flammable.

• Unlike alkenes, several aromatic hydrocarbons are toxic. Benzene itself is implicated as a cancer causing chemical.


Concept of Aromaticity• The underlying criteria for aromaticity were

recognized in the early 1930s by Erich Hückel• To be aromatic, a ring must

• have one 2p orbital on each atom of the ring• be planar or nearly planar, so that overlap of all 2p

orbitals of the ring is continuous or nearly continuous• have 2, 6, 10, 14, 18, and so forth pi electrons in the

cyclic arrangement of 2p orbitals

• Benzene meets these criteria• it is cyclic, planar, has one 2p orbital on each atom of

the ring, and has 6 pi electrons (the aromatic sextet) in the cyclic arrangement of its 2p orbitals


Aromaticity and the 4n + 2 Rule

• Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2 electrons (n is 0,1,2,3,4)

• For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized


Aromatic Ions

• The 4n + 2 rule applies to ions as well as neutral species• Both the cyclopentadienyl anion and the

cycloheptatrienyl cation are aromatic • The key feature of both is that they contain 6 electrons

in a ring of continuous p orbitals


Compounds With 4n Electrons Are Not Aromatic (May be Antiaromatic)

• Planar, cyclic molecules with 4 n electrons are much less stable than expected (anti-aromatic)

• They will distort out of plane and behave like ordinary alkenes

• 4- and 8-electron

cyclobutadiene

cyclooctatetraene


Aromatic Heterocycles: Pyridine and Pyrrole

• Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P

• Aromatic compounds can have elements other than carbon in the ring

• There are many heterocyclic aromatic compounds and many are very common

• Cyclic compounds that contain only carbon are called carbocycles (not homocycles)

• Nomenclature is specialized


Heterocyclic Aromatics• Heterocyclic compound: a compound that

contains one or more atoms other than carbon in its ring

• Heterocyclic aromatic compound: a heterocyclic compound whose ring is aromatic

• pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic

Pyridine Pyrimidine

N

N

N


Pyridine

N•

•

•

•

•

•

this orbital is perpendicularto the six 2p orbitals of thepi system

this pair of electronsis not a part of the aromatic sextet


Concept of Aromaticity• the five-membered-ring compounds furan, pyrrole, and

imidazole are also aromatic

Furan Pyrrole Imidazole

N

N

NOH H


Concept of Aromaticity• Nature abounds in compounds with a

heterocyclic aromatic ring fused to another aromatic ring

Indole

N

N

NN

N

HH

N

H Serotonin(a neurotransmitter) AdeninePurine

N

NN

N

H

NH2HO NH2


Nomenclature• Monosubstituted alkylbenzenes are named as

derivatives of benzene• many common names are retained

Toluene CumeneEthylbenzene Styrene

Phenol Aniline Benzoic acid Anisole

COOHNH2 OCH3OH

Benzaldehyde

CHO


Aromatic Compounds in Nature and Health

Many aromatic compounds are common in nature and in medicine.

COOH

COOCH3

CHO

OCH3

OH

Aspirin Vanillin

CHCOOH

CH3

CH3CHCH2

CH3

Ibuprofen


Naming Aromatic CompoundsAromatic compounds are named with benzene as the

parent chain. One side group is named in front of the name benzene. - No number is needed for mono-substituted

benzene since all the ring positions are identical.

methylbenzene chlorobenzene (toluene)

CH3 Cl


21

Disubstituted Benzenes

• Relative positions on a benzene ring• ortho- (o) on adjacent carbons (1,2)• meta- (m) separated by one carbon (1,3)• para- (p) separated by two carbons (1,4)

• Describes reaction patterns (“occurs at the para position”)

Ortho

Para

Meta

Ortho

Meta

First Position


Nomenclature• Disubstituted benzenes

• locate substituents by numbering or• use the locators ortho (1,2-), meta (1,3-), and para (1,4-)

• Where one group imparts a special name, name the compound as a derivative of that molecule

CH3

Br

COOHNO2

Cl

NH2

3-Chloroaniline(m-Chloroaniline)

4-Bromotoluene(p-Bromotoluene)

2-Nitrobenzoic acid(o-Nitrobenzoic acid)


Nomenclature• Polysubstituted benzenes

• with three or more substituents, number the atoms of the ring

• if one group imparts a special name, it becomes the parent name

• if no group imparts a special name, number to give the smallest set of numbers, and then list alphabetically

CH3

Cl

NO2

OH

Br

BrBr

NO2

CH2CH3

Br6

436

43

21

5

5

2

15

6

43

1 2

4-Chloro-2-nitro-toluene

2,4,6-Tribromo-phenol

2-Bromo-1-ethyl-4-nitrobenzene


CH3

O2N NO2

NO2 Br Br

1,3-dibromobenzene1,3,5-trinitro-toluene

Br

Cl

2-chloro3-bromotoluene


Learning Check

CH3

O2N NO2

NO2 Br Br

1,3-dibromobenzene1,3,5-trinitro-toluene

Br

Cl

2-chloro3-bromotoluene


Learning Check

Cl

CH3

CH3


Learning Check

Cl

CH3

CH3

Chlorobenzene

Meta-dimethylbenzene


PAHs• Polynuclear aromatic hydrocarbon (PAH)

• a hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

PhenanthreneAnthraceneNaphthalene Benzo[a]pyrene


Reactions of Benzene• The most characteristic reaction of aromatic

compounds is substitution at a ring carbon

H Cl2FeCl3 Cl HCl+ +

Chlorobenzene

Halogenation:

H HNO3H2SO4

NO2 H2O++

Nitrobenzene

Nitration:


Reactions of Benzene

H H2SO4 SO3H H2O+

Benzenesulfonic acid

Sulfonation:

+

H RXAlCl3

R HX++

An alkylbenzene

Alkylation:

H R-C-XO

AlCl3 CR

O

HX++

Acylation:

An acylbenzeneAn acylhalide


Phenols• The functional group of a phenol is a hydroxyl

group bonded to a benzene ring• name substituted phenols either as derivatives of

phenol or by common names

1,2-Benzenediol(Catechol)

1,3-Benzenediol(Resorcinol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OH OH OH OHOH

OHOH


Phenols

• Some phenols found in nature

2-Isopropyl-5-methylphenol

(Thymol)

4-Hydroxy-3-methoxy-benzaldehyde

(Vanillin)Urushiol

(Poison ivy)

OH

HO

H3CO CHO

CH2(CH2)13CH3OH

OH

NCO

H

CH3O

HO Capsaicin(from various types of peppers)


Phenols as Antioxidants• Vitamin E is a natural antioxidant

• BHT and BHA are synthetic antioxidants

HO

OH

3Vitamin E

OH OH

OCH3

Butylated hydroxytoluene(BHT)

Butylated hydroxyanisole(BHA)

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