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Aromatic Compounds

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Spices and herbs have long played a romantic role inthe course of history. Spices and herbs were among thefirst natural products studied by organic chemists.

Extract pure compounds with these desirablefragrances and flavors and determine their structures,then synthesize in large quantity, at low cost, andwithout danger

Aromatic substances have rather simple structurescontain a six-carbon unit C6H5that passes unscathedthrough various chemical reactions that alter only therest of the structure.

Benzaldehyde (isolated from the oil of bitter almonds),

benzyl alcohol (isolated from gum benzoin, a balsamresin obtained from certain Southeast Asian trees), andtoluene (a hydrocarbon isolated from tolu balsam)

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Benzene is the parent hydrocarbon of a classof substances that we now call aromaticcompounds, not because of their aroma, but

because of their special chemical properties,in particular, their stability.

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Some Facts About Benzene

Benzene, C6H6, a highly unsaturated structurebenzene but does not behave as unsaturated

No typical addition reactions of alkenes or alkynes.Instead, benzene reacts mainly by substitution

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The Kekul Structure of Benzene

Kekul : the single and double bonds exchange

positions around the ring so rapidly that thetypical reactions of alkenes cannot take place

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Resonance Model for Benzene

Kekuls model is nearly, but not entirely correct.Two structures differ only in the arrangement of theelectrons; all of the atoms occupy the samepositions in both structuresresonance

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Benzene is planar, carbon atom is at thecorner hexagon. C-C : 1.39 , intermediatebetween typical single (1.54 ) and double(1.34 )

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Orbital Model for Benzene

Each carbon is sp2-hybridized

Hexagonal shape, with H-C-C and C-C-C anglesof120

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Symbols for Benzene

The symbol with the inscribed circle emphasizesthe fact that the electrons are distributed evenlyaround the ring, and in this sense, it is perhapsthe more accurate of the two

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Nomenclature of Aromatic Compounds Common names are acceptedby IUPAC.

Monosubstituted benzenes do not have common names

accepted by IUPAC : derivatives of benzene

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ortho-, meta-, andpara- : o-, m-, andp-

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More than two substituents : numbering the ring.

The symbol Ar is used foran aryl group,

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The Resonance Energy of Benzene

Energy (heat) released is about 26 to 30 kcal/mol for each doublebond. Kekul structure (1,3,5-cyclohexatriene) should released 84

to 86 kcal/mol Stabilization energy, or resonance energy, is the difference between

the actual energy of the real molecule Benzene and other aromatic compounds usually react in such a way

as to preserve their aromatic structure and therefore retain theirresonance energy

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Electrophilic Aromatic Substitution

The most common reactions of aromatic compounds involvesubstitution of other atoms or groups for a ring hydrogen onthe aromatic unit

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The Mechanism of Electrophilic Aromatic Substitution

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Halogenation Chlorine or bromine is introduced into an aromatic ring by

using the halogen together with the corresponding iron halide

as a catalyst (that is, Cl2 + FeCl3 or Br2 + FeBr3).

Nitration

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SulfonationSO3 is a strong electrophile at sulfur

Alkylation and AcylationFriedelCrafts reaction

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Ring-Activating and Ring-Deactivating Substituents

benzene (left), toluene (middle), and nitrobenzene (right)

substituents that donate electrons to the ring will increase itselectron density and, hence, speed up the reaction;

substituents that withdraw electrons from the ring willdecrease electron density in the ring

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Ortho,Para-Directing and Meta Directing Groups

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Ortho,Para-Directing Groups

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Meta-Directing Groups

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Halogen are stronglyelectron withdrawing,they are ring deactivating;

Halogen have unsharedelectron pairs, they areortho,para directing.

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The Importance of Directing Effects in Synthesis

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Polycyclic Aromatic Hydrocarbons

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