Download - 01 Benzene & Aromaticity
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1Benzene and aromaticity
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2In this section you will learn.
Nomenclature of aromatic compounds Aromaticity consequences on reactivity Hckel rule: aromatic and antiaromatic compounds Aromaticity of some heterocyclic compounds
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3Aromatic Compounds and Sources
Aromatic: used to described some fragrant compounds in 19th century- Not correct
Now: grouped by chemical behavior (undergo substitution)and distinguished from aliphatics by electronic configuration
Obtained from distillation of coal tar or petroleum
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4Naming Aromatic Compounds Many common names: Memorize them to master naming
of aromatic compounds
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5Naming Aromatic Compounds Functional group suffix = benzene Functional group prefix = phenyl
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6The Phenyl Group
When a benzene ring is a substituent, the term phenyl Ph is used (for C6H5)
Benzyl refers to C6H5CH2
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7Disubstituted Benzenes Relative positions on a monosubstituted (X) benzene ring
ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4)
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8Naming Benzenes With More Than Two Substituents
Choose numbers to get lowest possible values List substituents alphabetically with hyphenated
numbers Common names, such as toluene can serve as root
name (as in TNT)
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9Naming Benzenes: Examples
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Benzenes Unusual Structure All its carbon-carbon bonds have same length (1.39
pm): Bond lengths: C-C =154 pm, C=C = 134 pm, CC = 120 pm
Electron density in all six C-C bonds is identical Structure is planar, hexagonal CCC bond angles 120 Each C is sp2 and has a p orbital perpendicular to the
plane of the ring
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Drawing Benzene and Its Derivatives
The two benzene resonance forms can be represented by a single (hybrid) structure with a circle in the center to indicate the equivalence of the carboncarbon bonds
HybridDelocalizedstructure
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Molecular Orbital Description of Benzene The 6 p-orbitals combine to give
Three bonding orbitals with 6 pi electrons, Three antibonding orbitals
Orbitals with the same energy are degenerate
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Reactivity of Benzene
The cyclic array of pi-bonds is a region of high electron density; so arenes are typically nucleophiles (like alkenes and alkynes)
Unlike alkenes and alkynes (which undergo additionreactions), arenes typically undergo substitution reactions in which a group (usually -H) is replaced and the aromatic system is retained
The stability of the aromatic system favors substitution over addition which would destroy the aromatic system
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Aromaticity
In order for a compound to be aromatic, all FOUR of the following criteria must be met:1. Cyclic: linear systems are not aromatic 2. Conjugated: there needs to be one "p" orbital from each
atom in the cycle, so each atom must be either sp2 or sp hybridized
3. Planar (or nearly planar): so there is good overlap / interaction between the "p" orbitals. Delocalization of the pi electrons over the ring must lower the electronic energy
4. Satisfy Hckel Rule: 4n+2 pi electrons
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Antiaromaticity
In order for a compound to be antiaroaromatic, all FOURof the following criteria must be met:1. Cyclic:2. Conjugated:3. Planar (or nearly planar): delocalization of pi
electrons over the ring increases the electronic energy
4. Satisfy: 4 n pi electrons
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Aromatic or Antiaromatic?
To qualify as aromatic or antiaromatic, a cyclic compound must have a continuous ring of overlapping p orbitals, usually in a planar conformation;
Then apply Hckel rule: If the number of pi electrons in the cyclic system is: 4n + 2 the system is aromatic (i.e. 2, 6, 10, 14, ..... ) 4n the system is antiaromatic (i.e. 0, 4, 8, 12, ..... )Where n is an integer, commonly 0, 1, 2, 3, etc.
n = number of filled pairs of degenerate orbitals (Benzene has one pair - see slide 12; Cyclopentadienyl anion also has one pair see slide 20)
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Aromaticity and the 4n + 2 Rule Erich Hckel rule: Planar monocyclic rings containing
(4n+ 2) pi electrons have closed shells of delocalized electrons - just like benzene!
So, planar, rings with 2, 6, 10, 14, 18 delocalized electrons are aromatic
n 4n + 2 pipipipi electrons0 4(0) + 2 = 21 4(1) + 2 = 62 4(2) + 2 = 103 4(3) + 2 = 144 4(4) + 2 = 18
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Aromatic , Antiaromatic or What? 4- and 8-electron compounds are not
delocalized (single and double bonds) Cyclobutadiene C=C bonds are different,
delocalization of pi electrons over the ring increases the electronic energy; dose not satisfy Hckel rule, it is anti-aromatic
Cyclooctatetraene is non-planar and adopts a tub-shaped conformation. C=C bonds are different. It dose not satisfy Hckel rule (not applicable). It is non-aromatic
The compound is readily prepared, and reacts with Br2 as if it were four alkenes
Cyclobutadiene
Cyclooctatetraene
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Aromaticity of the Cyclopentadienyl Anion The 4n + 2 applies to ions
as well as neutral species 1,3-Cyclopentadiene
contains conjugated double bonds joined by a CH2 that blocks delocalization
Removal of H- or Hgenerate nonaromatic 4 and 5 electron systems
Removal of H+ at the CH2produces a cyclic 6-electron system, which is stable
Relatively acidic (pKa = 16) because the anion is stable
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Why 4n +2? It takes two electrons (one pair) to fill the lowest-lying
orbital and four electrons (two pairs) to fill each of nsucceeding energy level a total of 4n + 2
Example: Cyclopentadienyl anion
closedopen
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Remove
H
Aromaticity of the CycloheptatrienylCation Cycloheptatriene has 3 conjugated double bonds joined
by a CH2 Removal of H- leaves the cycloheptatrienyl cation with 6
pi electrons in a ring of continuous p orbitals
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Aromaticity of Heterocycles
Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P
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Pyridine pi electron structure resembles benzene. Pyridine has 6 pi-electrons and 2 non-bonding sp2
electrons The nitrogen lone pair electrons are not part of the
aromatic system (perpendicular orbital). It is in the plane of the ring
Pyridine
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Pyrrole
pi electron system similar to that of cyclopentadienyl anion
Four sp2-hybridized carbons with 4 porbitals perpendicular to the ring and 4 pi electrons
Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a porbital (6 pi electrons)
Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity
sp2
sp3
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Polycyclic Aromatic Compounds
Aromatic compounds can have rings that share a set of carbon atoms (fused rings)
Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic
Three resonance forms for naphthalene
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More Examples
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Planar or Not?
We need: Planar (or near planar) cycle of sp2 hybridized atoms, the p-orbitals of which are oriented parallel to each other
http://www.chemistry.msu.edu/Areas/area_organic.aspNonaromatic