第五章芳香族取代反应.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 5.5 萘环的定位规律 萘环的特性 （1）比苯环更易发生亲电取代反应。 α-位 β-位 （2）α位比β位的亲电取代反应活性高，E+优先进攻α位。 （3）某些反应在一定条件下可逆，使低温反应产物以α位为主，高温反应产物以β位为主。 ＜60℃磺化 ＞160℃磺化 160℃异构化 已有一个取代基的定位作用 （1）已有一个第一类取代基 大多使萘环活化，新取代基进入已有取代基的同环。 （2）已有一个第二类取代基 使苯环钝化，新取代基进入异环的α位。 5.6 蒽醌环的定位规律 蒽醌环的特点 （1）亲电取代反应活性低； （2）两个边环具有等同性； （3）α位比β位活泼。 5.7 Friedel–Crafts Alkylation傅克烷基化反应 Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3. Reactions of alkyl halide with Lewis acid produces a carbocation, which is the electrophile. Mechanism of the Friedel–Crafts Reaction傅-克反应的机理 Step 1 Step 2 Step 3 Protonation of Alkenes 烯烃的质子化 An alkene can be protonated by HF. This weak acid is preferred because the fluoride ion is a weak nucleophile and will not attack the carbocation. Alcohols and Lewis Acids 醇与Lewis酸作用 Alcohols can be treated with BF3 to form the carbocation. Limitations of Friedel–Crafts 傅克反应的局限 Reaction fails if benzene has a substituent that is more deactivating than halogens (基团的钝化). Rearrangements are possible （碳正离子重排）. The alkylbenzene product is more reactive than benzene, so polyalkylation occurs （多烷基化）. Rearrangements 重排 5.8 Friedel–Crafts Acylation傅克酰基化反应 Acyl chloride is used in place of alkyl chloride. The product is a phenyl ketone that is less reactive than benzene. Mechanism of Acylation酰基化反应的机理 Step 1: 产生酰基正离子. Step 2: 对苯环亲电进攻得到sigma络合物. Clemmensen Reduction 克莱门森还原 The Clemmensen reduction is a way to convert acylbenzenes to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc （盐酸+锌汞齐）. 5.9 Nucleophilic aromatic substitution 芳香亲核取代反应 Ar must contain strongly electron withdrawing groups ortho and/or para to the X. （芳基必须含强吸电子基团） (bimolecular displacement) Bimolecular Displacement (nucleophilic aromatic substitution) 机理: （强吸电子基团可稳定负离子中间体） Evidence for the bimolecular displacement mechanism: no element effect : Ar-I ? Ar-Br ? Ar-Cl ? Ar-F (the C—X