Aromaticity, Reactions of Benzene
Transcript of Aromaticity, Reactions of Benzene
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Chapter 15
Aromaticity.
Reactions of Benzene
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Conjugated System vs. Aromatic System
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Benzene Structure (C6H6)
Br2
Br
Br
Br
Br+
H+/H2O
OH*
Hg(OAc)2, H2O
OH*
1. BH3
OH*2. H2O2, OH-, H2O
CC
CC
C
C
H
H
H
H
H
H
or
Br2
H /H2O+
Hg(OAc)2
BH3
H2O
H2O2, OH-, H2O
no reaction
no reaction
no reaction
no reaction
C-C 0.154 nm C=C 0.134 nmC-C(benzene) 0.139 nm
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What is Benzene’s Real Structure
P. 641
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P. 642
resonance stabilization energy
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Criteria for Aromaticity:Benzene is the typical aromatic compound. It has a VERYhigh resonance stabilization energy (36 kcal/mol). ALL ofthe following three criteria MUST be met:
(1) The compound must have an uninterrupted cycliccloud of -electrons above and below the plane of themolecule. In other words, all the atoms in the ring must beusually sp2 (sometimes sp) hybridized.
(2) In order for the sp2 unhybridized “p” orbitals tooverlap, the molecule must be reasonably planar.
(3) The cloud must have an odd number of pairs of -electrons. Benzene has 6 electrons (equaling 3 pairs).
Aromaticity (15.1 - 15.4)
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Criteria for Aromaticity (15.1-15.4)
Hückel’s rule says an aromatic molecule must have (4n+2) -electrons – this statement is exactly the same as saying an odd number of pairs of -electrons. So aromatic molecules can have a total of 2 or 6 or 10 or 14 etc., -electrons, thus fulfilling Hückel’s rule for “n” = 0, 1, 2, 3, etc. Examples:
n = 0 n = 1 n = 2 n = 3
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A compound is antiaromatic if it is a planar, cyclic compound with an uninterrupted ring of cloud, but it contains 4n+2 number of electrons (15.6)
p 649
> >
anti-aromatic aromatic non-aromatic
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PROBLEM: Which of the following are aromatic, antiaromatic or nonaromatic?
4n+2 or 4n # of fully delocalized electrons in in cyclic plane
14earomatic
18earomatic
6earomatic
10enon-aromatic
4enon-aromatic
2e
aromatic
4e
anti-aromatic
2e
non-aromatic
10
+ : -
4enon-aromatic
4eanti-aromatic
6earomatic
14earomatic
4n+2 or 4n # of fully delocalized electrons in in cyclic plane
10earomatic10e
aromatic
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MO Description of Aromaticity and Antiaromaticity (15.7)
Benzene, an aromatic molecule, has six p atomic orbitals which combine to produce 6 MOs: three bonding MOs and three antibonding MOs. Two of the bonding MOs are degenerate (i.e. of the same energy).
antibonding MOs
bonding MOs
p orbitals
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MO Description of Aromaticity and Antiaromaticity (15.7)
Antiaromatic molecules:
H
bonding
non-bonding
antibonding
antibonding
bonding
bonding
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Aromatic heterocyclic compounds (15.4) Benzene, pyridine, pyrrole, furan and thiophene are examples of aromatic molecules.
NNH
benzene pyrrolepyridine
O
furan
S
thiophene
Except for benzene, these are “heteroaromatic’ compounds because they have a non-carbon atom in the ring
N NH
N
N N
N
H
indole quinoline purine
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P. 646
15NH
NH O S
They are not as good bases as their saturated counterparts
P. 646
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Chemical Consequences of Aromaticity (15.5)
H H H
H
H
resonancehybrid
-H+
P. 647
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Chemical Consequences of Aromaticity (15.5)
P. 648
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Nomenclature of Monosubstituted Benzenes (15.8)
Many monosubstituted benzenes are named simply byusing the name of the substituent, followed by “benzene”:
Br NO2 CH2CH3 Cl
bromobenzene nitrobenzene ethylbenzene chlorobenzene
Because of their historical importance, somemonosubstituted benzenes have trivial names:
CH3 C CH NH2
toluene benzoic acid styrene aniline
OHO CH2
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Nomenclature of Monosubstituted Benzenes (15.8)
When a benzene ring is named as a substituent it is called a PHENYL group. Only when it has an extra methylene is it called a BENZYL group:
CH2
~~~~ ~ ~
phenyl group benzyl group
OO
diphenyl ether dibenzyl ether ARYL is the generic name for a phenyl, or a substituted phenyl group.
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Nomenclature of Monosubstituted Benzenes (15.8)
Alkylbenzenes are named either as alkyl-substituted benzenes OR as phenyl-substituted alkanes:
C
tert-butylbenzene 3-phenylhexane
CH3
CH3
H3C
C6H5 = phenyl (Ph) group
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How Benzene Reacts (15.9)Benzene is electron-rich because of its -electrons: it is nucleophilic so it attacks electrophiles to give initially a carbocation intermediate (“benzenonium ion”) which loses a proton thus regaining its aromatic stability.
EH
EH N
H
EN H
-H+
E
E = electrophile
N = nucleophile
ADDITION PRODUCT
SUBSTITION PRODUCT
This is an electrophilic aromatic substitution reaction
H+
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Reaction Coordinate Diagram
p 654
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General Mechanism - Electrophilic Aromatic Substitution Reactions (15.10)
EE
HH
EE
H
E
H
N
+ N-H
SLOW FAST
Very Important to Know What is the Electrophile!
HE+
E
-H+
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Common Electrophilic Aromatic Substitution Reactions (15.10)
(1) Halogenation (the H on the benzene ring is replaced bya Br, Cl, or I).
(2) Nitration [the H is replaced with a nitro group (NO2)].
(3) Sulfonation [the H is replaced with a sulfonic acidgroup (SO2OH)].
(4) Friedel-Crafts Acylation [the H is replaced with anacyl group (R-C=O)].
(5) Friedel-Crafts Alkylation (the H is replaced with analkyl group).
The two-step mechanism above applies to all five of thesereactions.
Cl
NO
O
SO
OHO
R
O
R
H
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Halogenation of Benzene (15.11)
This reaction requires a catalyst. It is usually a Lewis acid catalyst.
Br+ Br2
FeBr3
bromobenzene
Cl+ Cl2
FeCl3
chlorobenzene
+ HBr
+ HCl
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Halogenation of Benzene (15.11)The Electrophile is “Halonium”
Electrophile (e.g. bromination):
Br Br FeBr3 Br Br FeBr3
Br FeBr4
The “bromonium ion” is the electrophile (or its adduct with FeBr4
-). In the case of iodination, I2 is usually oxidized to I+ using nitric acid.
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p 656
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Nitration of Benzene (15.12)
The catalyst is sulfuric acid.
NO2+ HNO3
H2SO4
nitrobenzene
+ H2O
HO NO2 H OSO3H+ HOH
NO2
+ HSO4
NO2
+ H2O
nitroniumion
The “ nitronium ion” is the electrophile.
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p 658
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Sulfonation of Benzene (15.13)SO3H
+ H2SO4
benzenesulfonic acid
+ H2Oheat
HO-SO3H HO-SO3H HO-SO3HH
O-SO3H+ +
H2O + S OH
O
O The electrophile is SO3H
+ (protonated sulfur trioxide) Sulfonation is reversible – if benzenesulfonic acid is heated in water, de-sulfonation occurs by way of an exact reversal of the sulfonation mechanism. This involves the principle of microscopic reversibility (book, page 659).
Sulfonium ion
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p 659
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Friedel-Crafts Acylation of Benzene (15.14)
+ + HClO
CClR
1. AlCl3
2. H2O
acyl chloride
R
O
+O
COR
1. AlCl3
2. H2O
acid anhydride
R
O
C
O
R
O
COHR
+
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O
CClR
AlCl3+ C OR C OR + AlCl4
acylium ion
+ + HClO
CClR
1. AlCl3
2. H2O
acyl chloride
R
O
p 660
Friedel-Crafts Acylation of Benzene (15.14)The Electrophile is the “Acylium Ion”
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Friedel-Crafts Acylation of Benzene (15.14)The Electrophile is the “Acylium Ion”
The electrophile is an “acylium ion”, [R-C=O]+. The water in the second step is needed to decompose the aluminum salt of the product:
R
O
AlCl3R
OAlCl3
R
O
+ Al(OH)3 + 3 HCl3 H2O
O
CClR
AlCl3+ C OR C OR + AlCl4
acylium ion
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Friedel-Crafts Alkylation of Benzene (15.15)
R+ + HCl
AlCl3R-Cl
The electrophile is a carbocation – WATCH out forrearrangements!!!
R Cl AlCl3+ R + AlCl4
37p 662
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Friedel-Crafts Alkylation of Benzene (15.15)
R+ + HCl
AlCl3R-Cl
The electrophile is a carbocation – WATCH out forrearrangements!!!
R Cl AlCl3+ R + AlCl4
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Friedel-Crafts Alkylation of Benzene (15.15)
All of the previously discussed carbocation rearrangement problems (1,2 alkyl or hydride shifts) apply here. + + HCl
AlCl3CH3CH2Cl
+
+ HCl
AlCl3CH3CH2CH2CH2Cl
+
35% 65%
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Friedel-Crafts Alkylation of Benzene (15.15)
Another example:
p 662
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The electrophile can also be generated by protonation of alkenes (using acids with poor nucleophilic counterions): Or alcohols can be protonated with acid (again, an acid with a poorly nucleophilic anion) and then dehydrated:
Friedel-Crafts Alkylation of Benzene (15.15)
CHH3C CH2 CH3
+HF
CH
CH
CH3H3C
+
OH
H2SO4
OH2+
electrophile
electrophile
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Two Step Alkylation of Benzene: by Acylation and Reduction (15.16)
+
+ HCl
AlCl3CH3CH2CH2CH2Cl
+
35% 65%
+2. H2OCl
OO
H2/Pd-C
1. AlCl3
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Two Step Alkylation of Benzene by Acylation and Reduction (15.16)
O
Zn(Hg), HClheat
H2NNH2, OH-
heat
CLEMMENSEN WOLFF-KISCHNER
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Other Alkylations of Benzene: by Coupling Reactions (15.17)
Recall these reactions from earlier in the course (organometallic coupling reactions, Section 10.13, pp 469-472). Gilman:
+
Br
+ CH3CH2Cu + LiBr(CH3CH2)2CuLi
Stille:
+Br
(CH3CH2CH2)4SnPd(PPh3)4
THF(CH3CH2CH2)3SnBr+
Suzuki:
Cl Pd(PPh3)4
NaOH
OB
O
+ +O
BO
HO
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Cl
O
OAlCl3
Problems: What is the major reaction product?
+OCCl
AlCl3
+ HNO3H2SO4
+CH3
CHCH3
CH2CH2ClAlCl3
O
O
NO2
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Problem: How would you prepare the following molecule from Benzene?
Cl
O
AlCl3
O
Zn(Hg),HCl,or H2NNH2,HO-,
AlCl3
Cl
Cl
AlCl3
Oor anhydride
O
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H E
+ E A catalyst + H A
Br
NO2
R
CR
Cl
SO3HOH2SO4 O
RCCl
AlCl3
AlCl3
RClFeCl3
Cl2Br2 FeBr3
HNO3 H2SO4
chlorination alkylation
sulfonation acylation
bromination
Summary of Electrophilic Substitution Reactions of Benzene
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That’s it!