eeea2ta测oven h 5-6-d= engaaenea2aogaaene 8的ee6 c ddition 8d-05-d5= 227 An Ring: aa盟 语动8 Halogenation of phenols occurs readily and a catalyst is not required. Multiple halogenations are frequently observed but can be controlled by using a lower temperature and a less polar solvent. Para electrophilic attack frequently dominates because of steric effects. The ratio of para to ortho products is highly dependent upon reagents and reaction conditions. Under basic conditions, phenols can undergo electrophilic substitution through intermediate phenoxide ions, even with very mild electrophiles. The initial aldol products are unstable and dehydrate upon heating, giving reactive intermediates called quinomethanes. Quinomethanes are α,β-unsaturated carbonyl compounds, which can undergo Michael additions with excess phenoxide ion. Subsequent repeated hydroxymethylation and Michael addition eventually leads to the formation of a complex phenol￾formaldehyde copolymer called a phenolic resin. The precursor to asprin, o-hydroxybenzoic acid, can be prepared via the Kolbe reaction: An Electrocyclic Reaction of the Benzene Ring: The Claisen Rearrangement 22-7 The Claisen rearrangement is a concerted reaction with a transition state that accommodates the movement of six electrons