BIOL 3305 Lecture Notes - Lecture 18: Aryne, Nucleophilic Aromatic Substitution, Chlorobenzene
Document Summary
There are two different possible reaction mechanisms for nas: addition elimination mechanism, elimination addition mechanism (the benzyne mechanism) Consider the reaction of hydroxide ion with 2,4-dinitrochlorobenzene. When the nucleophile attacks the carbon bearing the chlorine, a negatively charged sigma complex is generated. The negative charge is delocalized over the ortho and para positions, and further delocalized into the electron withdrawing groups (conveniently located at these positions). Loss of chloride from the sigma complex generates 2,4-dinitrophenol. (this is like the mechanism for eas, but with the benzene reacting with a nucleophile instead of an electrophile). The previous addition elimination reaction mechanism required powerfully electron withdrawing groups on the benzene ring. However, under forcing conditions, unactivated halobenzenes can react with strong bases. For example, phenol is produced commercially via the reaction of sodium hydroxide with chlorobenzene. Analogously, aniline is produced via reaction of chlorobenzene with sodium amide.
