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orgchem lec
unit 7
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Aromatic compounds
Class of compounds that contain six-membered benzene-like rings with three double bonds
Early aromatic compounds
benzaldehyde (from cherries, peaches, and almonds)
toluene (from Tolu balsam)
benzene (from coal distillate)
Benzene (C6H6)
Has eight fewer hydrogens than alkane (C6H14) and is clearly unsaturated
A six-membered ring with alternating double and single bonds
All C-C-C bond angles are 120°
All six carbon atoms are sp2-hybridized
Each carbon has a p orbital perpendicular to the plane
Benzene causes leukopenia on prolonged exposure and should not be used as laboratory solvent
Major source of aromatic compounds
Coal and Petroleum
Pharmaceutical preparations
Steroids (e.g. estrone, Lipitor)
Benzene
Aromatic compounds are generally nonpolar and immiscible with water
Aromatic compounds are often unreactive, making them useful as solvents for other nonpolar compounds
Due to their high ratio of carbon to hydrogen, aromatic compounds are characterized by a sooty yellow flame
benzene
Less reactive than alkenes because of their resonance
Cyclohexene reacts rapidly with Br2
Gives the addition product 1,2-dibromocyclohexane
Benzene reacts slowly with Br2
Gives the substitution product C6H5Br
Benzene
The "circle" representation must be used carefully since it doesn't indicate the number of π electrons in the ring
Hückel's rule for aromaticity
Molecule must have a planar, monocyclic system of conjugation
It must contain a total of 4n + 2π molecules where n is an integer n = 0,1,2,3...
Molecules with 2, 6, 10, 14, 18 .... π electrons can be aromatic
Antiaromatic - planar conjugate molecule with 4nπ electrons (4,8,12,16)
Electrophilic aromatic substitution
A process in which an electrophile (E+) reacts with an aromatic ring and substitutes for one of the hydrogens
Mechanism of electrophilic bromination of benzene
Reaction occurs in two steps and involves a resonance-stabilized carbocation intermediate
Other electrophilic aromatic substitution reactions
Aromatic halogenation (fluorination, chlorination, iodination)
Aromatic nitration
Aromatic sulfonation
Substituents affect the reactivity and orientation of a reaction
Substituents can be classified as meta-directing deactivators, ortho-para directing deactivators, and ortho-para directing activators
Ortho
Substituents at the 1 and 2 positions on an aromatic compound, adjacent to the primary carbon
Meta
Substituents at the 1 and 3 positions on an aromatic compound
Para
Substituents at the 1 and 4 positions on an aromatic compound, directly opposite the primary carbon
Activating groups
Donate electrons to the ring, making it more electron-rich, stabilizing the carbocation intermediate, and lowering the activation energy
Deactivating groups
Withdraw electrons from the ring, making it more electron-poor, destabilizing the carbocation intermediate, and raising the activation energy
Iodination
biosynthesis of thyroxine (growth regulating hormone)
Sulfonation
mixture of SO3 and H2SO4
Friedel-crafts alkylation reaction
treating aromatic with alkyl chloride