Chapter 14 Aromatic Compounds
H H
H
C C
C
C
C C H
H or H
Kekule' structure for benzene, C6H6
!"#$"#"%&"'(#)#*"%+,-.*,.-"%
Bond Order = 1.5 Bond Length = 1.4 Å
/ !"#$"#"%0'%)*,.)112%)%-"'(#)#*"%3 !"#$"#"%0'%)*,.)112%)%-"'(#)#*"%324-05%4",6""#% 24-05%4",6""#%,3"%,6(%7"8.19% ,3"%,6(%7"8.19% ',-.*,.-"':%%% / ;3"%<=<%4(#5%1"#>,3'%0# ;3"%<=<%4(#5%1"#>,3'%0#%4"#$"#"%)-"%'3 %4"#$"#"%)-"%'3(-,"-%,3)#%, (-,"-%,3)#%,2?0*)1%'0#>1"@ 2?0*)1%'0#>1"@ 4(#5%1"#>,3'A%2",%1(#>"-%,3)# 4(#5%1"#>,3'A%2",%1(#>"-%,3)#%,2?0*)1%5(.4 %,2?0*)1%5(.41"@4(#5%1"#>,3 1"@4(#5%1"#>,3'%B4(#5% '%B4(#5% (-5"-%C:DE:% / !"#$"#"F'%-"'(#)#*"%*)#%4"%-"? !"#$"#"F'%-"'(#)#*"%*)#%4"%-"?-"'"#,"5%42%5-"'"#,"5%42%5-)60#>%)%*0-*1"%0# )60#>%)%*0-*1"%0#'05"% '05"% ,3"%'0G@H"H4"-"5%-0#>%)'%)%*( ,3"%'0G@H"H4"-"5%-0#>%)'%)%*(H40#"5%-"?H40#"5%-"?-"'"#,)I(#:% "'"#,)I(#:% 2
!"#$"#"%&"'(#)#*"%+,-.*,.-"%
Bond Order = 1.5 Bond Length = 1.4 Å
/ !"#$"#"%0'%)*,.)112%)%-"'(#)#*"%3 !"#$"#"%0'%)*,.)112%)%-"'(#)#*"%324-05%4",6""#% 24-05%4",6""#%,3"%,6(%7"8.19% ,3"%,6(%7"8.19% ',-.*,.-"':%%% / ;3"%<=<%4(#5%1"#>,3'%0# ;3"%<=<%4(#5%1"#>,3'%0#%4"#$"#"%)-"%'3 %4"#$"#"%)-"%'3(-,"-%,3)#%, (-,"-%,3)#%,2?0*)1%'0#>1"@ 2?0*)1%'0#>1"@ 4(#5%1"#>,3'A%2",%1(#>"-%,3)# 4(#5%1"#>,3'A%2",%1(#>"-%,3)#%,2?0*)1%5(.4 %,2?0*)1%5(.41"@4(#5%1"#>,3 1"@4(#5%1"#>,3'%B4(#5% '%B4(#5% (-5"-%C:DE:% / !"#$"#"F'%-"'(#)#*"%*)#%4"%-"? !"#$"#"F'%-"'(#)#*"%*)#%4"%-"?-"'"#,"5%42%5-"'"#,"5%42%5-)60#>%)%*0-*1"%0# )60#>%)%*0-*1"%0#'05"% '05"% ,3"%'0G@H"H4"-"5%-0#>%)'%)%*( ,3"%'0G@H"H4"-"5%-0#>%)'%)%*(H40#"5%-"?H40#"5%-"?-"'"#,)I(#:% "'"#,)I(#:% 2
J-(H)I*%*(H?(.#5'% – K0',(-0*)112A%4"#$"#"%)#5%0,'%5"-0L)IL"'%3)5%
?1")')#,%)-(H)'%)#5%6"-"%*)11"5%)-(H)I*% *(H?(.#5'% – M"N#0I(#O%P->)#0*%*(H?(.#5'%*(#,)0#0#>%)%
!"#$#%"%&A%!'!()! )--)2%(Q%R@"1"*,-(#'%)--)2%(Q%R@"1"*,-(#'%-"'.1I#>%0#% "'.1I#>%0#% ,3"0-%"*+,-((.&/-0)()1-$"# :%
3
The Special Stability of Benzene Benzene does not show the chemical reactivity of an alkene. The aromatic ring structure shows an unusual stability, a resistance to change.
Comparative Chemical Reactivity of Cyclohexene and Benzene Reagents
cold, dilute KMnO4 Br2 /CCl4
rapid hydroxylation
no reaction
rapid addition
no reaction
HI
rapid addition
no reaction
H2 /Ni
hydrogenation 25oC, 20 psi
very slow hydrogenation >100oC, 1500 psi
Aromatic Substitution Reactions
X
This pattern of reactivity suggests the aromatic ring is unusually stable.
The Resonance Energy of Benzene
1.8 (Resonance Energy)
+ 3H2
36.0 (Resonance Energy)
+ 2H2 + 3H2 y g r e n E l a i t n e t o P
+ H2
-28.6
-55.4
-49.8
The observed and expected heats of hydrogenation for benzene show a remarkable extra stabilization of 85.8 49.8 = 36.0 kcal/mol.
Molecular Orbital Description of the -Electronic Structure of Benzene
*
!antibonding
(out-of-phase) combine
C y g r e n E
C
isolated p-orbitals
! bonding
(in-phase)
The quantity is the stabilization energy of the electrons when a -bond is formed. It measures the stabilization of the two electrons in a bonding molecular orbital relative to two electrons in localized p-orbitals.
Benzene: a 6 -Electron System In a similar way, the energies of the six electrons in the bonding -molecular orbitals of benzene (a regular hexagon) are compared with the energies of six electrons in localized p-orbitals.
!
-anti-bonding
!
-bonding
:
Huckel's Rule: The (4n + 2) -Electron Rule for Predicting Aromaticity
:
The Huckel Formula Prediction
These closed shell -electron systems are predicted to be "aromatic" by the Huckel rule.
2
Total Number of -Electrons 6 10 14
:
bonding MOs
n=
0
1
2
3
A Second Prediction of Aromaticity
Examples Benzene
hexagon
predicted energy levels
a closed shell system "aromatic"
Cyclooctatetraene
octagon
predicted energy levels
an open shell system nonaromatic
S-"50*,%0Q%,3"%Q(11(60#>%)-"%)-(H)I*%(-%#(#@)-(H)I*% <2*1(?-(?"#21%)#0(#%
E
<2*1(?-(?"#21%*)I(#%
E
12
Properties of Cyclooctatetraene In contrast to benzene, this nonaromatic compound is very reactive. It is a yellow liquid, bp 152oC, that is stable at low temperatures, but polymerizes upon heating. It is slowly oxidized by air, and easily undergoes catalytic hydrogenation.
o
o
1.34 A
1.48 A
The different bond lengths indicate localized bonds.
The Annulenes
Annulenes are monocyclic compounds with alternating double and single bonds. The ring size is given by a number inside square brackets before the name "annulene."
benzene [6]annulene
cyclooctatetraene [8]annulene
Prediction of Aromaticity of the Annulenes The Huckel (4n+2) rule predicts whether an annulene is expected to show aromatic properties. Over the past 40 years, many annulenes have been synthesized and studied.
:
Predictions of Aromaticity by the Huckel Rule: 4n + 2
[10]annulene Prediction Observation
[14]annulene
Yes
Yes
No
Yes
[16]annulene Prediction
No
Observation
No
[18]annulene Yes Yes
The Trouble with [10]Annulene
Planar Structures of [10]Annulene
H H
A (all cis) severe angle strain
B
(naphthalene-like) severe steric strain from H-H repulsion
[10]Annulene has a non-planar structure that prevents the continuous overlap of the p-orbitals that is required for an aromatic system.
[4]Annulene (1,3-Cyclobutadiene)
Spectroscopic studies on the matrix trapped 1,3-cyclobutadiene confirm that it is not aromatic. It has a rectangular geometry indicating nonequivalent single and double bonds.
Aromatic, Antiaromatic, and Nonaromatic Compounds
Not the same as aromatic vs. not aromatic 1. Aromatic: cyclic, planar, fully conjugated, (4n +2) electrons (n = 0, 1, 2, ….). Electronic delocalization leads to great stability. 2. Antiaromatic: cyclic, planar, fully conjugated, 4n electrons (n = 1, 2, 3, ……). Electronic delocalization leads to great instability. 3. Nonaromatic: does not meet requirements for aromatic or antiaromatic structure. Energy is not increased or decreased by electronic configuration.
TG)H?1"'% • J-(H)I*%
• J#I)-(H)I*%
• U(#)-(H)I*%
19
Aromatic Ions
Monocyclic species that carry either a positive or negative charge often show unusual stability when they have closed -electron shells of 4n+2 -electrons (Huckel's rule). Such systems are "aromatic." :
Cyclopentadienyl Anion Cyclopentadiene is unusually acidic (pKa = 16) for a hydrocarbon. The acidity is due to the unusual stability of its conjugate base, the cyclopentadienyl anion.
+ B:-
-
+ BH
:
H H cyclopentadiene pKa = 16
H
cyclopentadienyl anion (unusually stable) comparable to water
<2*1(?"#,)50"#21%V(#'%
Resonance Theory: An Inadequate Explanation of Acidity Resonance theory does not adequately explain the unusual stability of the cyclopentadienyl anion. Although a series of resonance structures can be drawn that contribute to and stabilize the hybrid, even more resonance structures can be drawn for the cycloheptatrienyl anion that shows no unusual stability. -
:
+ B:-
-BH
-
etc
:
-
:
H H cyclopentadiene pKa = 16
H
H
H
unusually stable
-
:
-
+
B:-
:
-BH
etc
-
:
H H cycloheptatriene pKa = 36
H
H
H
not unusually stable
Molecular Orbital Theory According to molecular orbital theory, the cyclopentadienyl anion is an aromatic system with unusual stability. It is planar with continuous overlap of 5 p-orbitals with 6 electrons.
also represented as
6
-
electrons
The cyclopentadienyl anion is a 6 -electron system with a closed shell configuration that is aromatic according to the Huckel rule. :
6
a closed shell system "Aromatic"
The Cycloheptatrienyl Anion: A Nonaromatic 4n
System
8
an open shell system with two electrons in antibonding MOs
Cycloheptatrienyl anion is not an aromatic system, and therefore displays no special stability.
Cycloheptatrienyl Cation (Tropylium Ion): A (4n + 2)
System
Removal of a hydride ion (H:-) from cycloheptatriene yields the unusually stable cycloheptatrienyl cation, also called the tropylium ion. -H:-
+ +
HH
H tropylium ion: 6
electrons
Cycloheptatrienyl Cation (Tropylium Ion): A (4n + 2)
System
6
a closed shell system "aromatic"
Successful Predictions of MO Theory
6
8
cyclopentadienyl anion cycloheptatrienyl anion "aromatic"
"nonaromatic"
6 cycloheptatrienyl cation "aromatic"
Two-Electron Reduction and Oxidation of 4n Systems :
Annulenes with 4n electrons are nonaromatic (Huckel's rule). Such compounds typically undergo two-electron redox reactions producing (4n + 2) electron systems that are aromatic. In gaining (reduction) or losing (oxidation) two electrons, closed shell electron systems are formed. An Example: The two-Electron Reduction of Cyclooctatetraene K THF
cyclooctatetraene [8]annulene a 4n system
open shell nonaromatic
--
2 K+
cyclooctatetraene dianion a (4n + 2) system
closed shell aromatic
Polynuclear Benzenoid Aromatic Compounds Many compounds are known with two or more benzene rings fused together. These compounds are called polynuclear aromatic compounds.
naphthalene C10H8
anthracene C14H 10
phenanthrene C14H10
Heterocyclic Aromatic Compounds
N ..
pyridine
..
..
..
N
O
S
..
..
H
pyrrole
The above heterocycles have
furan
thiophene
electronic structures analogous to
previously discussed "aromatics" when the heteroatom is sp2 hybridized.
S2-050#"%S0%+2',"H%
•
S2-050#"%3)'%'0G%5"1(*)10$"5%"1"*,-(#'%0#%0,'%?0%'2',"H:%%%
•
;3"%,6(%#(#@4(#50#>%"1"*,-(#'%(#%#0,-(>"#%)-"%0#%)#%!"W (-40,)1A%)#5%,3"2%5(%#(,%0#,"-)*,%60,3%,3"%?0%"1"*,-(#'%(Q% ,3"%-0#>:% 29
(N,O,S) contributes a nonbonding pair to give a sextet of
electrons.
S2% The "aromaticity" of these five-membered ring heterocycles results
from a closed shell electronic structure as is found in the
S2--(1"A%X.-)#%)#5%;30(?3"#"% cyclopentadiene anion. Note again the heteroatom is sp2 hybridized. ..
..
O:
N-H
..
S:
S :
O :
N H
pyrrole
furan six
thiophene
electron systems
These heterocyclic aromatic compounds are widely found in biological structures.
Nomenclature of Benzene Derivatives
Some of these compounds have two acceptable names.
A systematic naming system has benzene as the parent, and the substitutent as a prefix. NO2
Cl
chlorobenzene
nitrobenzene
For some monosubstituted benzenes, common names are widely used. CH3
toluene
OH
phenol
NH2
aniline
COOH
benzoic acid
Disubstituted Benzenes
The positions of the substituents are indicated relative to each by either of two naming systems. The prefixes ortho (o), meta (m) and para (p) specifically refer to the following relative positions: ortho
meta
para
Br
Br
Br
Br Br
Br o-dibromobenzene
m-dibromobenzene
p-dibromobenzene
The relative positions of two substituents may also be indicated by numbers. When a common name is used for a monosubstituted benzene, that substituent is at position 1. CH3 COOH Cl Cl NO2
NO2 1,2-dichlorobenzene (o-dichlorobenzene)
4-nitrotoluene (p-nitrotoluene)
3-nitrobenzoic acid (m-nitrobenzoic acid)
Benzenes with Three or More Substituents
CH3
NO2
O2N
NO2
1,3,5-trinitrobenzene
Cl
Cl 2,4-dichlorotoluene
The Phenyl Substituent Group The C6H5 group in a structure often is conveniently named as a phenyl group, a substituent group.
CH3CHCH2CH3
2-phenylbutane
CH3CHCH=CH2
3-phenyl-1-butene
