2. displacement reactions of diazonium salts
ARYL HALIDES General formula: Ar–X where X = F, Cl, Br or I
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.
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
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
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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