Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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the expected B, but also 10% of C, suggest-
ing the operation of a competing pathway to the normal mechanism. Propose such a pathway.

A

OH
D

D

B

D

D

C

D

D

BF3
�

712 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

15-12 Limitations of Friedel-Crafts Alkylations
The alkylation of benzenes under Friedel-Crafts conditions is accompanied by two important
side reactions: One is polyalkylation; the other, carbocation rearrangement. Both cause the
yield of the desired products to diminish and lead to mixtures that may be diffi cult to
separate.

Consider fi rst polyalkylation. Benzene reacts with 2-bromopropane in the presence of
FeBr3 as a catalyst to give products of both single and double substitution. The yields are
low because of the formation of many by-products.

H

(CH3)2CHBr

CH(CH3)2

H H
25%

(1-Methylethyl)benzene
(Isopropylbenzene)

CH(CH3)2

CH(CH3)2
15%

1,4-Bis(1-methylethyl)benzene
(p-Diisopropylbenzene)

� �
FeBr3

�HBr
Overalkylation

The electrophilic aromatic substitutions that we studied in Sections 15-9 and 15-10 can
be stopped at the monosubstitution stage. Why do Friedel-Crafts alkylations have the prob-
lem of multiple electrophilic substitution? It is because the substituents differ in electronic
structure (a subject discussed in more detail in Chapter 16). Bromination, nitration, and
sulfonation introduce an electron-withdrawing group into the benzene ring, which renders
the product less susceptible than the starting material to electrophilic attack. In contrast, an
alkylated benzene is more electron rich than unsubstituted benzene and thus more suscep-
tible to electrophilic attack.

Exercise 15-30

Treatment of benzene with chloromethane in the presence of aluminum chloride results in a complex
mixture of tri-, tetra-, and pentamethylbenzenes. One of the components in this mixture crystallizes
out selectively: m.p. 5 808C; molecular formula 5 C10H14;

1H NMR d 5 2.27 (s, 12 H) and 7.15
(s, 2 H) ppm; 13C NMR d 5 19.2, 131.2, and 133.8 ppm. Draw a structure for this product.

The second side reaction in aromatic alkylation is skeletal rearrangement (Section 9-3).
For example, the attempted propylation of benzene with 1-bromopropane and AlCl3 pro-
duces (1-methylethyl)benzene.

H

CH3CH2CH2Br

CH(CH3)2

�
AlCl3
�HBr

Rearranged
alkyl group

The starting haloalkane rearranges by a hydride shift to the thermodynamically favored
1-methylethyl (isopropyl) cation in the presence of the Lewis acid.

CH3CH

H

Rearrangement of 1-Bromopropane
to 1-Methylethyl (Isopropyl) Cation

CH2 AlCl3 CH3CHCH3
1-Methylethyl

(isopropyl) cation

Br � BrAlCl3�
� �

O O

REACTION

MECHANISM

 C h a p t e r 1 5 713

Exercise 15-31

Working with the Concepts: Rearrangements in Friedel-Crafts Alkylations

Attempted alkylation of benzene with 1-chlorobutane in the presence of AlCl3 gave not only the
expected butylbenzene, but also, as a major product, (1-methylpropyl)benzene. Write a mechanism
for this reaction.
Strategy
First write an equation for the described transformation:

Cl� �
AlCl3
�HCl

Butylbenzene (1-Methylpropyl)-
benzene

Consider the mechanism for each product separately.
Solution
• The fi rst product is derived readily by a normal Friedel-Crafts alkylation:

�
 AlCl3

�
�

Cl�š

AlCl3�HCl�

O �

H
�

�

 AlCl3Clð�š O

• The second product contains a rearranged butyl group, the result of electrophilic aromatic sub-
stitution by a secondary butyl cation. The required rearrangement is brought about by a Lewis
acid – catalyzed hydride shift:

H��

CH3CH2CH

H

CH2 AlCl3 CH3CH2CH� �AlCl4�

�

�
O CH3O

CH3CH2CHCH3
�

OClð�š

H�

Exercise 15-32

Try It Yourself

Write a mechanism for the following reaction:

�
AlCl3Cl

1 5 - 1 2 L i m i t a t i o n s o f F r i e d e l - C r a f t s A l k y l a t i o n s

714 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

Because of these limitations, Friedel-Crafts alkylations are rarely used in synthetic
chemistry. Can we improve this process? A more useful reaction would require an electro-
philic carbon species that could not rearrange and that would, moreover, deactivate the ring
to prevent further substitution. There is such a species — an acylium cation — and it is used
in the second Friedel-Crafts reaction, the topic of the next section.

In Summary Friedel-Crafts alkylation suffers from overalkylation and skeletal rearrange-
ments by both hydrogen and alkyl shifts.

15-13 Friedel-Crafts Acylation (Alkanoylation)
The second electrophilic aromatic substitution that forms carbon – carbon bonds is Friedel-
Crafts alkanoylation (as in butanoylation, pentanoylation, and so on). A more popular alter-
native systematic name for this process is acylation, which is the term we shall use. IUPAC
 O O
 B B
retains the common naming of formyl for HC– and acetyl for CH3C– (Section 17-1), hence
the corresponding acylations are called formylation and acetylation. Acylation proceeds
through the intermediacy of acylium cations, with the general structure RCqO:1. This
 section describes how these ions readily attack benzene to form ketones.

Friedel-Crafts Acylation

RCX, AlX3
�HX

C

O

R

B
O
B

Friedel-Crafts acylation employs acyl chlorides
Benzene reacts with acyl halides in the presence of an aluminum halide to give
1-phenylalkanones (phenyl ketones). An example is the preparation of 1-phenylethanone
(acetophenone) from benzene and acetyl chloride, by using aluminum chloride as the
 Lewis acid.

Friedel-Crafts Acylation of Benzene
with Acetyl Chloride

1. AlCl3
2. H2O, H�

C

O

CH3

61%
1-Phenylethanone
(Acetophenone)

B
O
BH

CH3CCl� � HCl

Acyl chlorides are reactive derivatives of carboxylic acids. They are readily formed from car-
boxylic acids by reaction with thionyl chloride, SOCl2. (We shall explore this process in detail
in Chapter 19.)

SOCl2RCOH

O

� SO2RCCl � HCl�
B

O

Preparation of an Acyl Chloride

B

Acyl halides react with Lewis acids to produce acylium ions
The key reactive intermediates in Friedel-Crafts acylations are acylium cations. These species
can be formed by the reaction of acyl halides with aluminum chloride. The Lewis acid initially

REACTION

Even though it is unstable and
 reactive, the formyl cation,
H –C q O1, is a fundamental small
organic molecule that is (relatively)
abundant in such diverse environ-
ments as hot fl ames and cold
 interstellar space. It has been
 detected in the gas surrounding
the comet Hale-Bopp, a spectacu-
lar visitor to Earth’s skies in 1997.

 C h a p t e r 1 5 715

coordinates to the carbonyl oxygen because of resonance (see Exercise 2 -11). This complex is
in equilibrium with an isomer in which the aluminum chloride is bound to the halogen. Dis-
sociation then produces the acylium ion, which is stabilized by resonance and, unlike alkyl
cations, is not prone to rearrangements. As shown in the electrostatic potential map of the
acetyl cation in the margin, most of the positive charge (blue) resides on the carbonyl carbon.

Acylium Ions from Acyl Halides

X AlCl3RC

O

�
B
O

ðð

ðš� AlCl3Xš�RC

O
B
O O

ðð
�

�

AlCl3

�

�

Xš�RC

O
A
P

ð ð
D

AlCl3
�

�

Xðš�RC

O
B
O

ð
D

� �
� Xðš�[RC RCOð AlCl3

�
O]

Acylium ion
š�q P

Sometimes carboxylic anhydrides are used in acylation in place of the halides. These
molecules react with Lewis acids in a similar way.

Acylium Ions from Carboxylic Anhydrides

O CR AlCl3
�AlCl3

RC

O

�
�B

O O

ðð O
B
ðð

š� O CRRC

O
B
O O

ðð O
B
ðð

š
A

AlCl3
�

�

O CRRC

O
B
O O

ð O
B
ðð

š�

D
AlCl3

�

�

O CRRC

O
A
P O

ð ð O
B
ðð