Notes on Aryl Halides

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 AÂ ° 1. 86 AÂ ° 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. 1 3/22/2012 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 Â °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 2 3/22/2012 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 c annot form if both ortho positions are occupied by other groups 3 3/22/2012 D. Wurtz-Fittig Reaction 3. O-bromoanisole and m-bromoanisole give the same product under the reaction conditions. ANALYSIS: 1. Chemical tests: Test Reagent Br2/CCl4 KMnO4 AgNO3 Result (-) (-) (-)OTHER REACTIONS OF ARYL HALIDES 1. Metallation Reactions for aryl halides without reactive groups Grignard Reaction: 2. IR Spectroscopy C-X absorptions lie in the fingerprint region ? not useful for analysis 1000 – 1350 cm-1 C – F str. 750 – 850 cm-1 C – Cl str. 500 – 680 cm-1 C – Br str. 200 – 500 cm-1 C – I str. Important peak frequencies 1500, 1600 cm-1 3000 – 3100 cm-1 aromatic C – C str. aromatic C – H str. Reaction with Lithium Metal: Transmetallation – most successful with ArBr and ArI 4