Notes on Aryl Halides

3/22/2012 ARYL HALIDES 2. displacement reactions of diazonium salts ARYL HALIDES General formula ArX 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 unmistakably small. Bond C-Cl Pre paratroopertion 1. electrophilic aromatic rally useful only if unitary 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 radical 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 solutions 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 redolent Substitution (NAS or SNAr) ipso substitution ? an atom or group separate than H is replaced in the reaction reaction Mechanisms For NAS A. BIMOLECULAR DISPLACEMENT addition-elimination chemical appliance occurs beneath mild conditions ***the aromatic ring must contain strongly electron withdrawing or electron attracting groups 2 3/22/2012 Observations 1. fragment effect (Bunnett) Aryl halides do non 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 beneath forcing or vigorous conditions Evidence for the benzyne mechanism 1. scrambling of the label reaction mechanism 2. benzyne cannot form if both ortho positions are occupied by other groups 3 3/22/2012 D. Wurtz-F ittig 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