Description Of Aromatic Compounds

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CHEM 220B: Organic Chemisty

Aromatic Compounds
 Formerly used to describe fragrant substances
 Now used to refer to the class of compounds that contain six-membered benzene-like rings with three double bonds
14.2. Nomenclature of Benzene Derivatives
 Common names are as important as IUPAC names
OH

CH3

phenol hydroxybenzene toluene methylbenzene O

H
C CH2

styrene

C

acetophenone

aniline aminobenzene O

CH3

OCH3

NH2

C

benzaldehyde

anisole methoxybenzene O

H

C

benzoic acid

OH

 When two substituents are present, the prefixes ortho, meta, and para (abbreviated o-, m-, and p-) or the numbers are used to indicate the relative positions

 When three or more substituents are present, the lowest possible numbers are used to indicate the positions; The substituents are listed alphabetically when writing the name  Alkyl substituent smaller than the ring (6 or fewer carbons), named as an alkyl substituted benzene; Alkyl substituent larger than the ring (7 or more carbons), named as a phenyl-substituted alkane

14.3. -14.6. Reactions, Structure, and Stability of Benzene
 Benzene is much less reactive than typical alkene and fails to undergo the usual alkene reactions
 Benzene reacts only slowly with Br2 and gives the substitution product C6H5Br

 All six carbon atoms and all six p orbitals in benzene are equivalent; Benzene is a hybrid of two equivalent forms

Kekulé structure

 Benzene is more stable than the hypothetical 1,3,5cyclohexatriene by 152 kJmol-1; the difference is the resonance energy
 Molecular orbitals

14.7. Hückel’s rule: The 4n + 2 p electron rule
Aromatics
 Structure must be cyclic with conjugated pi bonds.
 Each atom in the ring must have an unhybridized p orbital.
 The p orbitals must overlap continuously around the ring.
(usually planar structure)
 Compound is more stable than its open-chain counterpart.
 If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic.

Antiaromatics
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