Presentation on theme: "Aryl Halides Ar-X Organic compounds with a halogen atom attached to an aromatic carbon are very different from those compounds where the halogen is attached."— Presentation transcript:
Aryl Halides Ar-X Organic compounds with a halogen atom attached to an aromatic carbon are very different from those compounds where the halogen is attached to an aliphatic compound. While the aliphatic compounds readily undergo nucleophilic substitution and elimination reactions, the aromatic compounds resist nucleophilic substitution, only reacting under severe conditions or when strongly electron withdrawing groups are present ortho/para to the halogen.
Bond Lengths (Å) C—ClC—Br CH 3 —X1.771.91 C 2 H 5 —X1.771.91 sp 3 (CH 3 ) 3 C—X1.801.92 CH 2 =CH—X1.691.86 C 6 H 5 —X1.691.86 sp 2
In aryl halides, the carbon to which the halogen is attached is sp 2 hybrizided. The bond is stronger and shorter than the carbon-halogen bond in aliphatic compounds where the carbon is sp 3 hybridized. Hence it is more difficult to break this bond and aryl halides resist the typical nucleophilic substitution reactions of alkyl halides. The same is true of vinyl halides where the carbon is also sp 2 hybridized and not prone to nucleophilic substitution. In a manner analogous to the phenols & alcohols, we have the same functional group in the two families, aryl halides and alkyl halides, but very different chemistries.
Aryl halides, reactions: 1.Formation of Grignard reagent 2.EAS 3.Nucleophilic aromatic substitution (bimolecular displacement) (Ar must contain strongly electron withdrawing groups ortho and/or para to X) 4.Nucleophilic aromatic substitution (elimination-addition) (Ring not activated to bimolecular displacement)