chapter 15 -organic- 2

8/7/2019 chapter 15 -organic- 2

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2011 PearsonEducation, Inc.

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Chapter 15: Aromaticity & Reactions of Benzene

Aromatic compounds such as benzene are unusually

stable because of electron delocalization (resonance).

The p orbitals overlap around the ring (b) to form a bonding

molecular orbital with electron density above and below the plane

of the ring (c).


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resonance

energy

Resonance energy of benzene

{151 KJ/mole (36 Kcal/mole)}


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The Two Criteria for Aromaticity

(Huckels Rule: The 4n+2p Electron Rule)

1. A compound must have an uninterrupted cyclic cloud

ofT electrons above and below the plane of the

molecule: {cyclic, planar, conjugated (a continuous

system ofp orbitals )}

2. The Tcloud must contain an odd number of pairs

ofTelectrons, or 4n + 2 (n = 0, 1, 2 ) total electrons.Benzene is aromatic: it is planar, cyclic, has a p

orbital at every carbon, and 6 p electrons (n=1). There

is a polygon-and-circle methodfor deriving the

relative energies of orbitals of a system with a cyclic

continuous array ofp orbitals: Aromatic compounds are

stable because they have filled bonding p molecular

orbitals


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Monocyclic hydrocarbons with alternating single and

double bonds are called annulenes:

not aromatic aromatic Even # ofp odd # ofp Even # ofp

electron pairs electron pairs electron pairs

Not stable highly stable stable: tub shape


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not aromatic not aromatic aromatic

The resonance hybrid shows that all the carbons in

the cyclopentadienyl anion are equivalent. Eachcarbon has exactly one-fifth of the negative charge

associated with the anion.


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Aromatic Heterocyclic Compounds

Heteroatom donates two electronsHeteroatom

donates one

electron


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The Effect of Aromaticity on the pKaValues of Some Compounds

Why is the pKa of cyclopentadiene so much lower than that of ethane?

The conjugate base is aromatic:

The cycloheptatrienyl cation is aromatic:

Aromaticity influences chemical reactivity:


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A compound is antiaromatic if it is a planar, cyclic with continuous

loop ofp orbitals and an even number of pairs ofTelectrons:

Antiaromatic compounds are highly unstable, but the nonplanarversions are stable.

Antiaromaticity

benzene cyclopentadienyl cation

cyclobutadienecyclopentadienyl cation


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Nomenclature of Monosubstituted Benzenes


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Reactions of benzene:

Aromatic compounds such as benzene undergo

electrophilic aromatic substitution reactions:

Benzene is a nucleophile that reacts with an

electrophile

Mechanism for Electrophilic Aromatic Substitution of Benzene


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Halogenation of Benzene

The Mechanism:


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Nitration of Benzene


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Sulfonation of Benzene


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FriedelCrafts Acylation Reactions

The electrophile is an acylium ion:


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FriedelCrafts Alkylation of Benzene


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Methodologies Used for the Reduction Step

One needs to consider an alternative if another


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One needs to consider an alternative if another

functional group is present in the compound:

Ch i l M difi ti f S b tit t f B


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Chemical Modification of Substituents of Benzene

Reactions of alkyl substituents:

Remember that halo-substituted alkyl groups can also


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Remember that halo substituted alkyl groups can also

undergo E2 and E1 reactions (Section 9.8)

Substitutions with double and triple bonds can undergo

catalytic hydrogenation (Sections 4.11 and 6.9)

Oxidation of an alkyl group bonded to a benzene ring


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Oxidation of an alkyl group bonded to a benzene ring

Provided that a hydrogen is bonded to the benzylic carbon,

The same reagent that oxidizes alkyl substituents will


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The same reagent that oxidizes alkyl substituents will

oxidize benzylic alcohols:

However, aldehydes or ketones can be generated if a

milder oxidizing agent is used:


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chapter 15 -organic- 2

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