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Benzene and and the Concept of Aromaticity. Chapter 21. 21.1 A. Benzene - Kekulé . Discovered by Michael Faraday in 1825. (#C = #H) The first structure for benzene was proposed by August Kekulé in 1872.

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benzene and and the concept of aromaticity
Benzene and

and the Concept of

Aromaticity

Chapter 21

21 1 a benzene kekul
21.1 A.Benzene - Kekulé
  • Discovered by Michael Faraday in 1825. (#C = #H)
  • The first structure for benzene was proposed by August Kekulé in 1872.
  • This structure, however, did not account for the unusual chemical reactivity of benzene.
benzene
Benzene
  • 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
    • all C-C-C and H-C-C bond angles 120°.
b benzene molecular orbital model
B. Benzene - Molecular Orbital Model
  • The linear combination of six overlapping p orbitals must form six molecular orbitals.
  • Three will be bonding, three antibonding.
  • Lowest energy MO will have all bonding interactions, no nodes.
  • As energy of MO increases, the number of nodes increases.
benzene molecular orbital model
Benzene - Molecular Orbital Model

Orbitals of equal energy are degenerate.

Antibonding orbitals are starred (*).

Electrons in the orbitals of lowest energy are in the “ground state”.

benzene molecular orbital model6
Benzene - Molecular Orbital Model
  • The pi system of benzene:
    • (a) the carbon framework with the six 2p orbitals.
    • (b) overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another. below it.
    • this orbital represents the lowest-lying pi-bonding molecular orbital.

Figure 21.2

benzene molecular orbitals
Benzene - Molecular Orbitals

Viewed from the top

of each carbon atom

c benzene resonance model
C. Benzene - Resonance Model
  • We often represent benzene as a hybrid of two equivalent Kekulé structures.
    • each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between.
benzene resonance
Benzene - Resonance
  • Resonance energy:the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds.
    • one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene.
21 2 a concept of aromaticity
21.2 A.Concept of Aromaticity
  • The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations.
  • To be aromatic, a compound must:

1. be cyclic.

2. have one p orbital on each atom of the ring (sp2).

3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring.

4. have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

frost circles polygon rule
Frost Circles (Polygon Rule)
  • Frost circle: a graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds.
    • inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring.
    • the relative energies of the MOs in the ring are given by where the vertices touch the circle
  • Those MOs:
    • below the horizontal line through the center of the ring are bonding MOs.
    • on the horizontal line are nonbonding MOs.
    • above the horizontal line are antibonding MOs.
frost circles fig 21 4
Frost Circles, Fig 21.4
  • following are Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.
b aromatic hydrocarbons
B. Aromatic Hydrocarbons
  • Annulene:a cyclic hydrocarbon with a continuous alternation of single and double bonds.
    • [14]annulene is aromatic according to Hückel’s criteria.
aromatic hydrocarbons
Aromatic Hydrocarbons
  • [18]annulene is also aromatic.
aromatic hydrocarbons16
Aromatic Hydrocarbons
  • according to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not.
  • nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.
aromatic hydrocarbons17
Aromatic Hydrocarbons
  • what is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH2) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.
c antiaromatic hydrocarbons
C. Antiaromatic Hydrocarbons
  • Antiaromatic hydrocarbon:a monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...), it does not obey Huckel’s rule.
    • an antiaromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms.
  • Cyclobutadiene is antiaromatic.
    • in the ground-state electron configuration of this molecule, two electrons fill the 1 bonding MO.
    • the remaining two electrons lie in the 2 and 3 nonbonding MOs.
cyclobutadiene fig 21 5
Cyclobutadiene, Fig. 21.5
  • planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.
cyclooctatetraene
Cyclooctatetraene
  • cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes.
  • x-ray studies show that the most stable conformation is a nonplanar “tub’ conformation.
cyclooctatetraene21
Cyclooctatetraene

2p orbital overlap forms each pi bond, there is essentially no overlap between adjacent alkenes.

cyclooctatetraene fig 21 6
Cyclooctatetraene, Fig. 21.6
  • planar cyclooctatetraene, if it existed, would be antiaromatic.
  • it would have unpaired electrons in the 4 and 5 nonbonding MOs.
d heterocyclic aromatics
D. Heterocyclic Aromatics
  • Heterocyclic compound: a compound that contains more than one kind of atom in a ring.
    • in organic chemistry, the term refers to a ring with one or more atoms are other than carbon.
  • Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.
pyridine
Pyridine
  • the nitrogen atom of pyridine is sp2 hybridized.
  • the unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic system.
  • pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene.
furan fig 21 7
Furan, Fig. 21.7
  • the oxygen atom of furan is sp2 hybridized.
  • one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet.
  • the other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system.
  • the resonance energy of furan is 67 kJ (16 kcal)/mol.
other heterocyclics27

C

H

C

H

N

H

2

2

2

H

O

N

N

H

H

Serotonin

Indole

(a neurotransmitter)

O

C

H

3

N

H

C

N

N

3

N

N

N

O

N

N

H

C

H

3

Purine

Caffeine

Other Heterocyclics
e aromatic hydrocarbon ions
E. Aromatic Hydrocarbon Ions
  • Any neutral, monocyclic unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic.
    • cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.
cyclopropenyl cation
Cyclopropenyl Cation
  • if, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.
cyclopropenyl cation30
Cyclopropenyl Cation
  • when 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt.
  • this chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.
cyclopentadienyl cation
Cyclopentadienyl Cation
  • if planar cyclopentadienyl cation existed, it would have 4 pi electrons and be antiaromatic.
  • note that we can draw five equivalent contributing structures for the cyclopentadienyl cation; yet this cation is not aromatic because it has only 4 pi electrons.
cyclopentadienyl anion
Cyclopentadienyl Anion
  • To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital.

pKa = ~16

cyclopentadienyl anion33
Cyclopentadienyl Anion
  • as seen in the Frost circle, the six pi electrons occupy the p1, p2, and p3 molecular orbitals, all of which are bonding.
cyclopentadienyl anion34
Cyclopentadienyl Anion
  • The pKa of cyclopentadiene is 16.
    • in aqueous NaOH, it is in equilibrium with its sodium salt.
    • it is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.
mos of aromatic ions
MOs of Aromatic Ions
  • Cyclopropenyl cation and cyclopentadienyl anion.
cycloheptatrienyl cation
Cycloheptatrienyl Cation
  • Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.
21 3 a nomenclature

T

oluene

E

thylbenzene

C

umene

S

tyrene

O

H

N

H

C

H

O

C

O

O

H

O

C

H

2

3

21.3 A.Nomenclature
  • Monosubstituted alkylbenzenes are named as derivatives of benzene.
    • many common names are retained.

Phenol

Aniline

Benzaldehyde

Benzoic acid

Anisole

nomenclature

C

H

C

H

-

3

2

P

henyl group, Ph-

B

enzyl group, Bn-

H

C

O

3

P

h

4-(3-Methoxyphenyl)-

(Z)-2-Phenyl-

2-butanone

2-butene

Nomenclature
  • Benzyl and phenyl groups.

Benzene

Toluene

O

O

1-Phenyl-1pentanone

b disubstituted benzenes
B. Disubstituted Benzenes
  • Locate two groups by numbers or by 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.
disubstituted benzenes
Disubstituted Benzenes
  • where neither group imparts a special name, locate the groups and list them in alphabetical order.
c polysubstituted derivatives

N

O

O

H

C

H

2

3

B

r

N

O

B

r

2

B

r

C

l

B

r

C

H

C

H

2

3

2,4,6-Tribromo-

4-Chloro-2-nitro-

2-Bromo-1-ethyl-4-

phenol

toluene

nitrobenzene

C. Polysubstituted Derivatives
  • if one group imparts a special name, name the molecule as a derivative of that compound.
  • if no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.

1

1

4

2

6

2

2

4

4

1

21 4 a phenols

O

H

O

H

O

H

O

H

O

H

C

H

3

O

H

3-Methylphenol

1,2-Benzenediol

1,4-Benzenediol

(

m-

Cresol)

(Catechol)

(Hydroquinone)

21.4 A.Phenols
  • The functional group of a phenol is an -OH group bonded to a benzene ring.

Phenol

1,3-Benzenediol is resorcinol

phenols
Phenols
  • hexylresorcinol is a mild antiseptic and disinfectant.
  • eugenol is used as a dental antiseptic and analgesic.
  • urushiol is the main component of the oil of poison ivy.
b acidity of phenols
B. Acidity of Phenols
  • Phenols are significantly more acidic than alcohols, compounds that also contain the OH group.
acidity of phenols
Acidity of Phenols
  • the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.
acidity of phenols47
Acidity of Phenols
  • Alkyl and halogen substituents effect acidities by inductive effects.
    • alkyl groups are electron-releasing.
    • halogens are electron-withdrawing.
acidity of phenols48
Acidity of Phenols
  • nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.
acidity of phenols49
Acidity of Phenols
  • part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect.
  • in addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.
c acid base reactions of phenols
C. Acid-Base Reactions of Phenols
  • Phenols are weak acids and react with strong bases to form water-soluble salts.
    • water-insoluble phenols dissolve in NaOH(aq).
acid base reactions of phenols
Acid-Base Reactions of Phenols
  • most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3

No

reaction

d alkyl aryl ethers

-

+

R

O

N

a

D. Alkyl-Aryl Ethers
  • Alkyl-aryl ethers can be prepared by the Williamson ether synthesis.
    • but only using phenoxide salts and haloalkanes.
    • haloarenes are unreactive to SN2 reactions.
  • The following two examples illustrate:
    • the use of a phase-transfer catalyst.
    • the use of dimethyl sulfate as a methylating agent.

+

X

no reaction

e kolbe carboxylation
E. Kolbe Carboxylation
  • Phenoxide ions react with carbon dioxide to give a carboxylic salt.
kolbe carboxylation
Kolbe Carboxylation
  • the mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2.
f quinones

O

H

H

C

r

O

2

4

1,4-Benzoquinone

P

henol

(

p-

Quinone)

F. Quinones
  • Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.
  • Quinonesare six-membered rings with two C=O.

O

O

quinones58
Quinones
  • Perhaps the most important chemical property of quinones is that they are readily reduced to hydroquinones by sodium hydrosulfite.
coenzyme q
Coenzyme Q
  • Coenzyme Q is a carrier of electrons in the respiratory chain.
vitamin k
Vitamin K
  • both natural and synthetic vitamin K (menadione) are 1,4-naphthoquinones.
21 5 a benzylic oxidation
21.5 A.Benzylic Oxidation
  • Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4.
    • halogen and nitro substituents are also unaffected by these reagents.
    • an alkyl group with at least one hydrogen on its benzyliccarbon is oxidized to a carboxyl group.
benzylic oxidation

K

C

r

O

2

2

7

H

C

C

H

H

O

C

C

O

H

3

3

H

S

O

2

4

1,4-Dimethylbenzene

1,4-Benzenedicarboxylic acid

(

p-

xylene)

(terephthalic acid)

Benzylic Oxidation
  • if there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group.
  • an aryl-COOH is the oxidation product regardless of the alkyl group that was attached to the aromatic ring (may be Me, Et, Pr, Bu, vinyl, etc.).

O

O

b benzylic chlorination
B. Benzylic Chlorination
  • Chlorination (and bromination) occurs by a radical mechanism.
benzylic reactions
Benzylic Reactions
  • Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates.
    • the benzyl radical is a hybrid of five contributing structures.
benzylic halogenation
Benzylic Halogenation
  • benzylic bromination is highly regioselective.
  • benzylic chlorination is less regioselective.
c hydrogenolysis
C. Hydrogenolysis
  • Hydrogenolysis:

Cleavage of a single bond by H2:

    • among ethers, benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.
benzyl ethers
Benzyl Ethers
  • Benzyl ethers are used as protecting groups for the OH groups of alcohols and phenols.
    • to carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent.
slide68

Benzene and

and the Concept of

Aromaticity

End of Chapter 21