Notes on Aryl Halides

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3/22/2012

ARYL HALIDES
2. displacement reactions of diazonium salts

ARYL HALIDES General formula: Ar–X where X = F, Cl, Br or I

Examples

Structure:
 C-X bonds are shorter and stronger Bond C-Cl C-Br Ar-X 1.69 A° 1.86 A° R-X 1.77 1.91

 Dipole moments are unusually small. Bond C-Cl Preparation: 1. electrophilic aromatic substitution – useful only if one product is obtained C-Br Ar-X 1.7 D 1.7 D R-X 2.10 D 2.15 D

 Dipole moments may cancel out depending on the geometry of the molecule.

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PHYSICAL PROPERTIES:  Boiling points – similar to those of alkyl halides; same trends  Melting point – Among disubstituted aryl dihalides, para isomer has unusually high MP; ~70 – 100 °C higher than ortho and meta isomers  Solubility behavior – insoluble in water; soluble in nonpolar organic solvents – para isomer is less soluble than ortho or meta in any given solvent  can be readily purified by recrystallization

Consider:

REACTIONS: 1. Electrophilic aromatic substitution (EAS) – recall directing power (o, p director) but ring is deactivated toward EAS.

2. Nucleophilic Aromatic Substitution (NAS or SNAr) ipso substitution  an atom or group other than H is replaced in the reaction

Reaction Mechanisms For NAS:
A. BIMOLECULAR DISPLACEMENT: addition-elimination mechanism; occurs under mild conditions ***the aromatic ring must contain strongly electron – withdrawing or electron – attracting groups

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Observations: 1. Element effect (Bunnett): Aryl halides do not show much difference in reactivities toward NAS via bimolecular displacement. 2. Aryl fluorides are most reactive.

II. Addition - fast

 removal of the halogen as X- is not the rate
determining step .

B. The Benzyne Mechanism: Elimination-addition mechanism - occurs under forcing or vigorous conditions

Evidence for the benzyne mechanism: 1. scrambling of the label

- reaction mechanism:

2. benzyne cannot form if both ortho positions...
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