Jones M., Fleming S.A. Organic Chemistry

Подождите немного. Документ загружается.

13.6 A Generalization of Aromaticity: Hückel’s 4

ⴙ 2 Rule 589

= 43

~ 50–60

–

FIGURE 13.28 Resonance stabilization of its conjugate base makes propene far more acidic than propane.

–

base, B

Energy

Antibonding

molecular

orbitals

Bonding

molecular

orbitals

Nonbonding

A pentagon inscribed in a circle (vertex down)

= 15

The cyclopentadienyl

anion is easily formed

–

FIGURE 13.29 Cyclopentadiene is

quite a strong acid.The cyclopen-

tadienyl anion is an aromatic species.

Notice the filled degenerate orbitals.

For an anion, it is extremely stable.

six  (4n  2), n  1.The value of n is simply the number of filled degenerate pairs.

Hückel’s rule is 4n  2 because aromaticity requires a filled degenerate pair of orbitals

(the 4n part) and the lowest single orbital is filled first (the 2 part).

There is an anionic counterpart to the stable tropylium cation, the cyclopenta-

dienyl anion.The pK

of propene is 43,and the pK

of propane is 50–60.The added

delocalization in the allyl anion makes propene a stronger acid than propane by a

factor of about 10

(Fig. 13.28). But the pK

of 1,3-cyclopentadiene is 15!

Cyclopentadiene is a much stronger acid than propene. The difference in acidity is

enormous. Look carefully at the structure of the cyclopentadienyl anion. Here too,

we have a planar, cyclic, and fully conjugated system.The molecular orbitals can be

derived from a Frost circle (Fig. 13.29).There are six electrons to put into the molec-

ular orbitals, and, as in the tropylium ion or benzene, they fully occupy the lowest

molecular orbital and the set of degenerate bonding molecular orbitals.The cyclopen-

tadienyl anion can be described as aromatic, and for an anion, this species is remark-

ably stable. Do not fall into the trap of expecting this anion to be as stable as benzene.

Benzene is a neutral molecule in which all of carbon’s valences are satisfied.The anion

is charged and contains unsatisfied valence. It is very stable, but only in comparison

with the rest of the family of carbon anions.

But wait. We have been at pains to equate stability with delocalization of

electrons, which results in a distribution of charge to several atoms in the molecule.

590 CHAPTER 13 Conjugation and Aromaticity

Both the tropylium ion and the cyclopentadienyl anion can be described by a most

impressive array of resonance forms (Fig. 13.30).

Named for the philosopher William of Ockham (1285–1349; he died during the bubonic plague), who put

it better: “What can be done with fewer is done in vain with more.”

–

Cyclopentadienyl

ion

Cycloheptatrienylium

ion

(tropylium ion)

–

FIGURE 13.30 Resonance descriptions

of the tropylium (cycloheptatrienylium)

cation and the cyclopentadienyl anion.

Perhaps the aromaticity concept is unnecessary. Why attribute the stability of

these species to aromaticity when simple delocalization of electrons may do as well?

That question is a good one, and needs an answer. It is not necessary to invoke spe-

cial effects if simpler concepts will suffice. The principle of “Ockham’s razor”

says

that when faced with a variety of equally attractive explanations, pick the simplest.

Wielding Ockham’s razor, we might say that the cyclopentadienyl anion is very

stable because of the five resonance forms so apparent in Figure 13.30. What then

would we expect of the cycloheptatrienyl anion, whose seven resonance forms are

shown in Figure 13.31? If an ion with five resonance forms is stable, surely one

base

–

Cycloheptatrienyl

anion

–

FIGURE 13.31 The seven resonance forms for the cycloheptatrienyl anion.

13.6 A Generalization of Aromaticity: Hückel’s 4

ⴙ 2 Rule 591

with seven forms will be even more stable. Our explanation predicts a formidable

acidity for 1,3,5-cycloheptatriene; in particular, it must be more acidic than

cyclopentadiene. Its pK

must be . However, the pK

of cycloheptatriene is

about 39. Cycloheptatriene is a weaker acid than cyclopentadiene by a factor of

(Fig. 13.32)!

615

= 15

–

= 39

Five resonance forms

Seven resonance forms

–

base

–

base

–

FIGURE 13.32 Cycloheptatriene is

a weaker acid than cyclopentadiene

by a factor of 10

So,the number of resonance forms is not always a predictor of stability. No longer

are we faced with equally attractive explanations.The aromaticity concept is neces-

sary to make sense of the data. Aromaticity trumps resonance. A look at the molec-

ular orbitals of cycloheptatrienyl anion shows the source of the instability. The

cycloheptatrienyl anion must contain unpaired electrons in degenerate antibonding

orbitals (Fig. 13.33)! It is not aromatic, it is a diradical.

–

Energy

Antibonding

molecular orbitals

Bonding

molecular orbitals

Nonbonding

Cycloheptatrienyl anion

FIGURE 13.33 In the cyclohepta-

trienyl anion, two electrons must

occupy degenerate antibonding

orbitals.

PROBLEM 13.9 How many signals do the

C NMR spectra of the cyclopentadienyl

anion and the tropylium cation show?

Now let’s search for other neutral compounds exhibiting aromaticity. Of course,

all manner of substituted benzenes exist, but such aromatic compounds hardly con-

stitute exciting new examples. It is possible to combine benzene rings so that they

share an edge in what is called a fused relationship (p. 211). These molecules are

also aromatic.Atoms other than carbon can be introduced into the ring while main-

taining the sextet of electrons to produce heterobenzenes, which are fully conju-

gated, six-membered rings containing one or more noncarbon atoms (N, O, S, etc.).

Some smaller rings also contain a sextet of π electrons; pyrrole is an example.

Molecules of these various types are shown in Figure 13.34, and will be considered

further in later sections.

592 CHAPTER 13 Conjugation and Aromaticity

Toluene

(a simple substituted

benzene)

Naphthalene

(a fused, two-ring

aromatic compound)

Pyridine

(a six-membered

heterobenzene)

Pyrrole

(a five-membered

aromatic compound)

WEB 3D

WEB 3D WEB 3D

WEB 3D

FIGURE 13.34 Some simple exten-

sions of aromaticity: substituted,

fused, and heterobenzene compounds,

as well as a five-membered ring

aromatic compound.

PROBLEM 13.10 It is startlingly easy to form the cyclopropenyl cation. Explain

why. For this system, what is the value of n in Hückel’s formula?

A cyclopropenium ion

)

–

What we are looking for here are not examples of relatively straightforward

extensions, but new 4n  2 molecules. You have learned that n can be any integer.

We have seen many examples of n  1. In Problems 13.7 and 13.10 we have seen

n  0, 2, 3, and 4.

Let’s consider the cyclodecapentaenes, cyclic molecules with 10 π electrons

(4n  2, n  2) (Fig. 13.35). Are they as stable as the six-electron molecules we

have been examining?

cis,cis,cis,cis,cis-

Cyclodecapentaene

trans,cis,cis,cis,cis-

Cyclodecapentaene

trans,cis,trans,cis,cis-

Cyclodecapentaene

WEB 3D

FIGURE 13.35 Some

cyclodecapentaenes.These molecules

have 10 π electrons (4n  2  10,

n  2).There are two pairs of

degenerate orbitals that are filled.

PROBLEM 13.11 Do you expect the corresponding cyclopropenyl anion to be stable?

Explain.

PROBLEM 13.12 The three isomeric cyclodecapentaenes of Figure 13.35 differ in

the stereochemistries of the double bonds. Why isn’t there more than one isomer

of benzene?

PROBLEM 13.13 How many signals would be seen in the

C NMR spectra of the

three cyclodecapentaenes in Figure 13.35?

13.6 A Generalization of Aromaticity: Hückel’s 4

ⴙ 2 Rule 593

A remarkable amount of effort was expended on the synthesis of the cyclodec-

apentaenes before they were successfully made.The magnitude of this effort should

make us suspicious; aromatic compounds are everywhere and extraordinarily easy to

make. Moreover, the known cyclodecapentaenes are not only not especially stable,

they are instead strikingly unstable. So, we have some explaining to do.We have been

relying on a theory that predicts special stabilization conferred by the aromatic 4n  2

number of π electrons, and the cyclodecapentaenes are members of the 4n  2 club.

If we cannot explain why these molecules are unstable, Hückel’s rule must be aban-

doned. It turns out to be not too difficult to resolve the problem. Aromatic stability

is not some magical force. It is just one kind of stabilizing effect, which in the face

of stronger destabilizing effects will not prevail.Put simply, the planar cyclodecapen-

taenes are destabilized by an amount greater than aromaticity can overcome. What

might those destabilizing effects be? Look first at the molecule on the right in

Figure 13.35, the trans,cis,trans,cis,cis isomer. For the molecule to be flat, the two

hydrogens inside the ring must occupy the same space.This crowding induces extraor-

dinary strain that is greater than any resonance stabilization. The molecule cannot

be flat. Aromaticity cannot compensate for this strong destabilizing effect.

The trans,cis,cis,cis,cis isomer, the middle molecule in Figure 13.35, has some

of the same problems, although here only one hydrogen is inside the ring. In this

molecule,however, there is severe angle strain,and the planar form, required for aro-

maticity, is badly destabilized.

The all-cis isomer, the molecule on the left in Figure 13.35, has even more angle

strain, and it too can be planar, and therefore aromatic, only at a prohibitively high

energy cost. The benefits of delocalization are overwhelmed by the angle strain.

A good set of models will show this strain easily. Construct the planar, all-cis

cyclodecapentaene and the strain will be apparent. If you are lucky, the plastic will

mimic the molecule and the model will snap into a pretzel shape, which approxi-

mates the energy minimum form of the molecule.These molecules are best described

as strained polyenes. They fail the test of planarity for aromatic molecules because

the planar structures are much more strained than the nonplanar forms. In this case,

strain trumps aromaticity.

PROBLEM 13.14 The all-cis cyclodecapentaene forms a new, cross-ring bond (see

below). As the molecule warms up, a bicyclo[4.4.0]decatetraene is formed. Write

an arrow formalism for this reaction.

cis,cis,cis,cis,cis-

Cyclodecapentaene

Bicyclo[4.4.0]deca-

2,4,7,9-tetraene

So we haven’t really judged the limits of the theory of aromaticity by using the

cyclodecapentaenes. We might be able to if the offending strain could somehow be

removed. On paper, this transfomation is simple. For example, we might just erase

the two inside hydrogens in one of the molecules (Fig. 13.36). Unfortunately, this

erasure doesn’t give a real molecule because this structure has two trivalent carbons

at the positions marked with dots in Figure 13.36.

trans,cis,trans,cis,cis-

Cyclodecapentaene

(strain caused by the

inside red hydrogens)

magic

hydrogen

eraser

(much less strain! But…

what is going on at the

red-dot positions?)

FIGURE 13.36 The strain caused

by two inside hydrogens in one

cyclodecapentaene can be eliminated

by removing the offending

hydrogens.

594 CHAPTER 13 Conjugation and Aromaticity

A particularly clever German chemist, Emanuel Vogel (b. 1927), found a way

to do this erasure chemically. He used a remarkable synthesis to replace the offend-

ing inside hydrogens with a bridging methylene group (Fig. 13.37). The removal

of the offending hydrogens eliminates the strain that comes from their “bumping”

into each other.

Vogel’s clever

synthesis

FIGURE 13.37 Vogel’s bridged

cyclodecapentaene.

The resulting molecule, a bridged system containing 10 π electrons, isn’t a per-

fect cyclodecapentaene. The overlap between the 2p orbitals at the bridgehead and

adjacent positions isn’t optimal, for example (Fig. 13.38). Still, the molecule main-

tains enough orbital overlap to remain fully conjugated and satisfy the criteria for

aromaticity (cyclic, flat, conjugated, and 4n  2 π electrons). Both its chemical and

physical properties allow it to be classified as aromatic.

Bridgehead position

Overlap here is not ideal (make a model!), but it is good

enough to allow substantial electron delocalization

FIGURE 13.38 Vogel’s molecule is

fully conjugated, but orbital overlap is

not optimal, as it is, for example, in

benzene. Nonetheless, this molecule

is aromatic.

The generic term for monocyclic, fully conjugated molecules is annulene.

Benzene could be called [6]annulene, square cyclobutadiene is [4]annulene, and

planar cyclooctatetraene is [8]annulene (Fig. 13.39).

[6]Annulene

[4]Annulene [8]Annulene etc.

FIGURE 13.39 Some simple annulenes.

Some other annulenes, besides [6]annulene,appear to be aromatic.These annu-

lenes are large enough so that the destabilizing steric effects present in the cyclodec-

apentaenes ([10]annulenes) are diminished, and severe angle strain is avoided.

A nice example is [18]annulene,a planar molecule in which all carbon–carbon bond

lengths are very similar. It contains 4n  2 π electrons, where n  4 (Fig. 13.40).

The description of this molecule as aromatic derives largely from an examination of

its nuclear magnetic resonance (NMR) spectrum, and that must wait for Chapter 15,

but [18]annulene does appear to be aromatic.

[18]Annulene

FIGURE 13.40 [18]Annulene.

Summary

Planar, cyclic, fully conjugated molecules containing 4n  2 π electrons are espe-

cially stable and are called “aromatic.” The “4n  2 rule” works because such mol-

ecules have a set of filled degenerate molecular orbitals, in a sense a closed shell.

13.7 Substituted Benzenes 595

We have concentrated on the phenomenon of aromaticity as applied to benzene

and larger and smaller polyenes. We saw two experimental methods of evaluat-

ing the stabilization of aromaticity quantitatively, and with the aid of Hückel’s

insight, we found a generalization of the phenomenon.Now we move on to ways

of varying the structure of our prototypal example, benzene, and then take a quick

look at reactivity.

Generic name

Phenyl R

CH(CH

)

Toluene

(methylbenzene or

phenylmethane)

OCH

Anisole

(methyl phenyl ether or

methoxybenzene)

Phenol

(benzenol or

hydroxybenzene)

Aniline

(benzenamine or

aminobenzene)

Cumene

(isopropylbenzene or

2-phenylpropane)

Ethylbenzene

(phenylethane)

COOH

CHO

Benzyl X

Generic name

Biphenyl

(phenylbenzene)

Benzaldehyde

(benzenecarboxaldehyde)

Benzoic acid

(benzenecarboxylic

acid)

Styrene

(vinylbenzene or

ethenylbenzene)

Benzyl chloride

(chlorophenylmethane)

Benzyl alcohol

(phenylmethanol)

Bromobenzene

Cl CH

FIGURE 13.41 Some simple

monosubstituted benzenes. Common

names are widely used (boldface).

13.7 Substituted Benzenes

13.7a Mono- and Disubstitution Monosubstituted benzenes are usually named

as derivatives of benzene, and the names are written as single words.There are numer-

ous common names that have survived a number of attempts to systemize them out

of existence. You really do have to know a number of these names in order to be lit-

erate in organic chemistry.We don’t think the systematic “benzenol”will replace “phe-

nol”in your lifetime,for example.At least we hope it doesn’t. Figure 13.41 shows some

simple benzenes with both the systematic and common names.

596 CHAPTER 13 Conjugation and Aromaticity

Two of the most often used common names are phenyl (sometimes even Ph),

which stands for a benzene ring with an open valence (the ring is attached to an R

group, for example), and benzyl, which stands for the group that has an

open valence. Both are shown in Figure 13.41.

When one Kekulé structure is drawn, you know that you must (mentally or

physically) draw the other form as well for a full picture.

There are three possible isomers of disubstituted benzenes (Fig. 13.42).

1,2-Substitution is known as ortho (o-), 1,3-substitution as meta (m-), and

1,4-substitution as para (p-).

FIGURE 13.42 The three possible substitution patterns for disubstituted benzenes.

ortho (o-)

1,2-Disubstituted

meta (m-)

1,3-Disubstituted

para (

p-)

1,4-Disubstituted

CONVENTION ALERT

In practice, either the numbers or the “ortho, meta, para” designations are used.

If there is a widely used common name, it is used as a base for the full name. For

example, the first and third molecules in Figure 13.43 are quite properly named as

o-bromotoluene and p-nitrophenol, respectively.

2-Bromotoluene

(o-bromotoluene)

1,3-Dichlorobenzene

(m-dichlorobenzene)

4-Nitrophenol

(

p-nitrophenol)

1-Ethyl-4-fluorobenzene

(

p-ethylfluorobenzene)

FIGURE 13.43 Four disubstituted

benzenes.

PROBLEM 13.15 The isolation of three, and only three isomers of disubstituted

benzenes was a crucial factor in the assignment of the Kekulé form as the correct

structure of benzene. Use the fact that there are three and only three achiral

isomers of a disubstituted benzene (C

) to criticize the suggestions that

benzene might have the following structures:

13.7 Substituted Benzenes 597

13.7b Polysubstitution There are disubstituted benzenes that are also known

by common names. Figure 13.44 shows a sampling of these molecules.

o -Xylene

(1,2 -dimethylbenzene)

m-Xylene

(1,3 -dimethylbenzene)

p-Xylene

(1,4 -dimethylbenzene)

Catechol

(o-dihydroxybenzene)

Resorcinol

(m-dihydroxybenzene)

Hydroquinone

(p-dihydroxybenzene)

WEB 3DWEB 3D WEB 3D

FIGURE 13.44 More common names

for disubstituted benzenes.

PROBLEM 13.16 How many signals will appear in the

C NMR spectra of the

three dihydroxybenzenes at the bottom of Figure 13.44?

1-Bromo-3-ethyl-2-nitrobenzene

1,3,5-Trichlorobenzene

ClCl

2-Bromo-4-chloro-6-fluoroaniline

FBr

Hexachlorobenzene

ClCl

4-Bromo-2,5-dichlorophenol

ClBr

OHCl

4-Bromo-2-chlorotoluene

FIGURE 13.45 Some named polysubstituted benzenes.

The substituents on polysubstituted benzenes are numbered and named alpha-

betically (Fig. 13.45). Notice that a benzene with a methyl-,a hydroxy-,or an amino-

group will have the root word of toluene, phenol, or aniline respectively. Because it

is the root word, the carbon attached to that group is automatically C(1).

598 CHAPTER 13 Conjugation and Aromaticity

13.8 Physical Properties of Substituted Benzenes

The physical properties of substituted benzenes resemble those of alkanes and alkenes

of similar shape and molecular weight. Table 13.1 collects some physical properties

for a number of common substituted benzenes. Notice the effects of symmetry.

p-Xylene melts at a much higher temperature than the ortho or meta isomer, for

example. Many para isomers have high melting points,and crystallization can some-

times be used as a means of separating the para regioisomer from the others.

TABLE 13.1 Physical Properties of Some Substituted Benzenes

Name bp (°C) mp (°C) Density (g/mL)

Cyclohexane 80.7 6.5 0.78

Benzene 80.1 5.5 0.88

Methylcyclohexane 100.9 126.6 0.77

Toluene 110.6 95 0.87

o-Xylene 144.4 25.2 0.88

m-Xylene 139.1 47.9 0.86

p-Xylene 138.3 13.3 0.86

Aniline 184.7 6.3 1.02

Phenol 181.7 43 1.06

Anisole 155 37.5 1.0

Bromobenzene 156.4 30.8 1.5

Styrene 145.2 30.6 0.91

Benzoic acid 249.1

122.1 1.1

Benzaldehyde 178.6 26 1.0

Nitrobenzene 210.8 5.7 1.2

Biphenyl 255.9 71 0.87

At 10 mm pressure.

You have learned that aromatic compounds are particularly stable. This stabili-

ty is reflected in the chemical and physical properties of substances that contain aro-

matic compounds, such as graphite and coal. Aromatic molecules are hardy.

Another important physical property of aromatic compounds is their involvement

in π stacking. These flat molecules lie easily on top of each other, and this property

is a critical feature of enzyme structure–activity and in DNA double helix formation.

Aromatic compounds are called arenes and the term for an arene with an open

valence is aryl. A general abbreviation for aryl is Ar, in analogy to the R used for a

general alkyl species.

CONVENTION ALERT

Pyridine

Benzene

FIGURE 13.46 Pyridine, a heterobenzene.

13.9 Heterobenzenes and Other Heterocyclic

Aromatic Compounds

13.9a Contrasting Structures of Pyridine and the Five-Membered

Rings Pyrrole, Furan, and Thiophene

If a CH unit of benzene is

replaced with a nitrogen, the result is pyridine, another aromatic six-membered

ring (Fig. 13.46). From the provincial view of organic chemistry all atoms except