aromatic compounds. lecture outline aromatic vs aliphatic compounds aromatic vs aliphatic compounds...
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Lecture OutlineLecture Outline
Aromatic vs aliphatic compoundsAromatic vs aliphatic compounds BenzeneBenzene
Structure/ stabilityStructure/ stability Electronic configurationElectronic configuration
The concept of aromaticityThe concept of aromaticity Huckel’s ruleHuckel’s rule Aromatic compounds other than benzeneAromatic compounds other than benzene
Nomenclature of benzene derivativesNomenclature of benzene derivatives
unsaturated cyclic hydrocarbons organic compounds had an aroma
or odourcontain single rings or groups of ringssimplest aromatic compound is benzenearomatic compounds include benzene or
are benzene based and have benzene-like structures and properties.
Aromatic Compounds
♪♫Sweet Dreams are Made of Benzene♪♫
Kekulé The idea of the
benzene structure came to Kekulé in a dream
Benzene, C6H6
6 carbon ring with a hydrogen bonded to each carbon
one electron from each carbon is free to participate in a double bond
Resonance Theory
1.Resonance forms are imaginary benzene has a single hybrid structure which combines the
characteristics of both resonance forms
Resonance forms
Hybrid structure
C
CC
C
CCH
H
H
H
H
H
C
CC
C
CC
H
H
H
H
H
H
C
CC
C
CC
H
H
H
H
H
H
Structure of BenzeneStructure of Benzene Benzene - Kekulé 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
CH
CH
CH
CHC
H
C
H
CC
CC
C
C
H
H
HH
HH
Molecular Orbital
* electron cloud delocalized all over the ring
* the resonance picture this helps to explain lack of reactivity of benzene
* great stability (substitution not addition )
Benzene - Resonance Energy
Resonance energy:Resonance energy: the difference in energy between a resonance hybrid and its most stable 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
Benzene - Resonance Energy
heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated)
these results are shown graphically on the next slide
+ H2Ni
1-2 atmH° = -28.6 kcal/mol (-120 kJ/mol)
+3H2Ni
200-300 atmH° = -49.8 kcal/mol (-208 kJ/mol)
The Concept of AromaticityThe Concept of AromaticityRequirements for aromaticityRequirements for aromaticity
Molecule must be cyclicMolecule must be cyclic Molecule must be planarMolecule must be planar Each atom of the ring (s) must have a p-orbital Each atom of the ring (s) must have a p-orbital
perpendicular to the plane of the ringperpendicular to the plane of the ring Huckel’s rule must be obeyedHuckel’s rule must be obeyed
Planar compounds must have (4n + 2) pi (Planar compounds must have (4n + 2) pi ( electrons, where n is an integerelectrons, where n is an integer
n= integer (1,2,3,…) Ring with 2, 6, 10 or 14 pi electrons Ring with 2, 6, 10 or 14 pi electrons maymay be be
aromaticaromatic Ring with 8, 12 or 16 pi electrons Ring with 8, 12 or 16 pi electrons will notwill not be be
aromaticaromatic
For aromaticity, all pi (For aromaticity, all pi ( electrons must electrons must be paired and all bonding orbitals filledbe paired and all bonding orbitals filled Maximum and complete overlap is required Maximum and complete overlap is required
for stabilizationfor stabilization With unpaired pi (With unpaired pi ( electrons, overlap is electrons, overlap is
not maximizednot maximized
The pi (The pi ( electrons in an aromatic electrons in an aromatic compound are delocalized over the entire compound are delocalized over the entire ring leading to stabilizationring leading to stabilization
02/19/1302/19/13 Farshid ZandFarshid Zand 1515
So what makes a molecule So what makes a molecule aromaticaromatic??
It must be cyclicIt must be cyclic It must be conjugatedIt must be conjugated It must be flat so that the It must be flat so that the pp orbital orbital
overlap can occuroverlap can occur It must also have It must also have
4n + 2 pi electrons…4n + 2 pi electrons…
Six electrons present
n = 1, Aromatic Eight electrons present
Not aromatic The Cyclopentadienyl anionThe Cyclopentadienyl anion
H H
- H+
H Two electronspresent in orbital for anion
H H
- H-
HNo electronspresent in orbital for cation
The Cyclopentadienyl cationThe Cyclopentadienyl cation
Structure
Resonance theory of benzeneResonance theory of benzene All bonds are equivalent! electrons are delocalised around the
ring
Aromaticity
Other Examples?
cyclic
planar conjugated 6 electrons XX
cyclic planar
conjugated 6 electrons XX
© Prentice Hall 2001© Prentice Hall 2001 Chapter 14Chapter 14 2020
AromaticityAromaticity
cyclooctatetraene cyclooctatetraene is is nonnonaromaticaromatic
It is It is notnot planar planar
Physical Properties of Physical Properties of BenzeneBenzene
MPMPbenzenebenzene = 5.5°C, BP = 5.5°C, BPbenzenebenzene=80.1°C=80.1°C
non-polar Moleculenon-polar Molecule
insoluble in waterinsoluble in water
less dense than waterless dense than water
Nomenclature of aromatic compounds
A] Mono substituted benzene:
B] Disubstituted benzene.
*because all C equal no numbers
*Substituent and then benzene
Monosubstituted Benzenes
Monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word):
F NO2 CH2CH3
fluorobenzene nitrobenzene ethylbenzene Chlorobenzene
Cl
2424
II. Nomenclature
A. Single group
OCH3O H O OH
NH2 SO3H
methoxybenzene(anisole)
benzenecarbaldehyde(benzaldehyde)
benzenecarboxylic acid(benzoic acid)
benzenamine(aniline)
benzenesulfonic acid
Common Names of Subs. Benzenes
CH3 OH
OCH3
NH2
CCH2
H
C
O
HC
O
OH
C N
C
O
CH3
CHCH3
CH3
toluene phenol aniline
anisole styrene
benzaldehyde benzoic acid acetophenone
benzonitrile cumene
NH2
orthoposition
orthoposition
NH2
meta position
meta position
NH2
paraposition
When two substituents are present on the When two substituents are present on the benzene ring, the prefixes ortho, meta and benzene ring, the prefixes ortho, meta and para are used.para are used.
NH2
Cl
m-chloroaniline
Cl
Cl
p-dichlorobenzenepara-dichlorobenzene1,4-dichlorobenzene
02/19/1302/19/13 Farshid ZandFarshid Zand 2929
Disubstituted BenzenesDisubstituted Benzenes
Disubstituted benzenes use prefixes Disubstituted benzenes use prefixes ortho-(o), meta(m), or para-(p). These ortho-(o), meta(m), or para-(p). These prefixes are useful when discussing prefixes are useful when discussing reactions.reactions.
Disubstituted Benzenes
ortho-= o-= 1,2-disubstituted
)two groups on adjacent carbons on the ring(
F
F
o-difluorobenzene or1,2-difluorobenzene
CH3
CH3
Cl
Cl
Cl
CH3
1,2-dimethylbenzene 1,3-dichlorobenzene 1-chloro-4-methylbenzene
(ortho-dimethylbenzene) (meta-dichlorobenzene) (para-chloromethylbenzene)
OH
Cl
CH3
NO2
COOH OH
NH2
Br
4-Aminophenolp-
2-Chlorophenol O-
4-Nitrotoluen p-
3-Bromobenzoic acid m -
A. Preparation of Benzene
1. Decarboxylation
COOH1. NaOH
2. NaOH / heat
OH
heat
Zn dust
2. Reduction of phenol
C.Chemical Properties
1. Electrophilic substitution reactiona. Nitration (Forming nitro-compound)b. Sulphonation (Forming sulphonic acid)c. Halogenation (Forming halobenzene)d. Friedal-Craft Alkylation (Forming
alkylbenzene)e. Friedal-Craft Acylation (Forming
carbonyl compound)
Reactions of Benzene
The most characteristic reaction of aromatic compounds is substitution at a ring carbon
H Cl2FeCl3 Cl HCl+ +
Chlorobenzene
Halogenation:
H HNO3H2SO4
NO2 H2O++
Nitrobenzene
Nitration:
Reactions of Benzene
H H2SO4 SO3H H2O+
Benzenesulfonic acid
Sulfonation:
+
H RXAlCl3
R HX++
An alkylbenzene
Alkylation:
H R-C-XO
AlCl3 CR
O
HX++
Acylation:
An acylbenzeneAn acylhalide
Friedel –crafts Alkylation
Friedel –crafts Acylation
C.Chemical Properties
1. Addition Reaction (NOT electrophilic)
a. Catalytic hydrogenation
+ H23
Pt / r.t.
Ni / 150 Co
Some Substitution Reactions of BenzeneSome Substitution Reactions of Benzene
FeCl3
CH3
Cl2
Cl
CH3ClAlCl3
RC
Cl
O AlCl3 C R
O
OH NO
O
H2SO4
N O
O
S
O
OH
OOH S
O
O
OHSO3
+
+
+
+
+
Halogenation
Friedel-CraftsAlkylation
Friedel-CraftsAcylation
Nitration
Sulfonation
+ +
-
-
Summary of
Reactions of TolueneI. At side chain
1. Oxidation of alkyl benzene
With MnO4-/H+
CH3 MnO4 / H+ COOH
CH2CH3 MnO4 / H+ COOH
(white solid)
(white solid)
I. At side chain
2.Substitution at side chain
CH3
Cl2+u.v.
CH2Cl
ClH+
Cl2
CHCl2
u.v.
OH2
CH
OH
OHCO H
OH2-
E.S. Reactions of Toluene
Nitration of toluene (O.S.)
Trinitrotoluene (TNT) is explosive substance.
CH3
conc.HNO3
conc. H2SO4
CH3
NO2
CH3
NO2
major products+
CH3
NO2
NO2
O2N
Trinitrotoluene (TNT)
TolueneCH3
CH3
CH3CH3
CH3
CH3CH3
CH3
CH3
Br
Br
NO2
NO2
SO3H
SO3H
HNO3
H2SO4
SO3
H2SO4
Br2, Fe
+
+
+
faster than the same
reactions with benzene