1Benzene and aromaticity

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

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

4Naming Aromatic Compounds Many common names: Memorize them to master naming

of aromatic compounds

5Naming Aromatic Compounds Functional group suffix = benzene Functional group prefix = phenyl

6The Phenyl Group

When a benzene ring is a substituent, the term phenyl Ph is used (for C6H5)

Benzyl refers to C6H5CH2

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)

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)

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

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

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

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