苯环取代基的定位效应Directing effects of substituents on the benzene ring
苯环上已有的取代基决定两件事：新的亲电取代比苯快还是慢（活化 / 钝化），以及新基团进到哪个位置（邻对位 / 间位）。原因是取代基通过诱导效应（I，沿 σ 键传递）和共轭效应（M，通过 π 体系传递）改变环上电子密度，并稳定或削弱反应中间体芳基正离子（σ 络合物）。
A substituent already on a benzene ring decides two things: whether further electrophilic substitution is faster or slower than for benzene (activating / deactivating), and where the new group goes (ortho/para or meta). It does so through its inductive effect (I, through σ bonds) and mesomeric effect (M, through the π system), which change the ring's electron density and stabilise or destabilise the arenium-ion (σ-complex) intermediate.
规律The rules
有孤对电子直接连在环上的基团（–NH₂、–OH、–OR、–NHCOR）共轭给电子（+M），把电子推向邻、对位：活化、邻对位定位。
Groups with a lone pair on the atom joined to the ring (–NH₂, –OH, –OR, –NHCOR) donate by resonance (+M), pushing electron density to the ortho and para positions: activating, ortho/para-directing.
烷基通过诱导效应和超共轭给电子（+I）：弱活化、邻对位定位。
Alkyl groups donate through induction and hyperconjugation (+I): weakly activating, ortho/para-directing.
与环相连的原子带正电或有极性重键（–NO₂、–CN、–COR、–COOH、–SO₃H、–CF₃）同时吸电子（−I、−M），邻、对位缺电子最多：钝化、间位定位。
When the atom joined to the ring is positive or carries a polar multiple bond (–NO₂, –CN, –COR, –COOH, –SO₃H, –CF₃) the group withdraws electrons (−I, −M), most strongly from ortho and para: deactivating, meta-directing.
卤素是例外：电负性大，诱导吸电子（−I）占上风，所以钝化；但孤对电子的共轭给电子（+M）仍能稳定邻、对位进攻的中间体，所以邻对位定位。
Halogens are the exception: strong −I wins, so they deactivate; but resonance donation (+M) from their lone pairs still stabilises attack at ortho and para, so they direct ortho/para.
苯酚、苯胺活化很强，与溴水在室温下就生成 2,4,6-三溴代物，不需要催化剂；苯则需要 FeBr₃ 或 AlBr₃。
Phenol and phenylamine are so strongly activated that bromine water gives the 2,4,6-tribromo product at room temperature without a catalyst; benzene needs FeBr₃ or AlBr₃.
结构讲解What the structures show
给电子基团：苯酚 Electron-donating group: phenol
氧原子的孤对电子与苯环 π 体系共轭，把电子推入环内。画出共振式可以看到，负电荷只出现在羟基的两个邻位和对位，所以亲电试剂优先进攻这三个位置，而且整个环比苯更富电子，反应更快。
The lone pair on oxygen is conjugated with the ring's π system and pushes electron density into the ring. The resonance structures put the negative charge only on the two ortho positions and the para position, so electrophiles attack there, and the whole ring is more electron-rich, and faster to react, than benzene.
氧的孤对电子推入环内
The oxygen lone pair is pushed into the ring
负电荷在邻位
Negative charge at ortho
负电荷在对位
Negative charge at para
负电荷在另一个邻位
Negative charge at the other ortho position
吸电子基团：硝基苯 Electron-withdrawing group: nitrobenzene
硝基通过共轭把环上的 π 电子拉向自己。共振式中正电荷落在硝基的两个邻位和对位，这三个位置最缺电子；间位没有正电荷，相对富电子，所以亲电试剂进攻间位，而且整个环比苯更难反应。
The nitro group pulls π electrons out of the ring by resonance. The resonance structures put positive charge on the two ortho positions and the para position, which become the most electron-poor; the meta positions carry no positive charge and are relatively electron-rich, so electrophiles attack meta, and the ring as a whole reacts more slowly than benzene.
环上的 π 电子被拉向硝基
Ring π electrons are pulled toward the nitro group
正电荷在邻位
Positive charge at ortho
正电荷在对位
Positive charge at para
正电荷在另一个邻位；间位始终不带正电
Positive charge at the other ortho position; meta never carries it
为什么邻对位进攻更有利：中间体的稳定性 Why ortho/para attack wins: the intermediate
亲电试剂进攻后生成芳基正离子（σ 络合物），正电荷分布在三个碳上。苯酚在对位（或邻位）被进攻时，其中一个共振式的正电荷正好落在连接羟基的碳上，氧的孤对电子可以再给出一个共振式：氧鎓离子，此时每个原子都满足八隅体，非常稳定。间位进攻的中间体正电荷始终不在这个碳上，得不到这种额外的稳定。
Attack by the electrophile gives an arenium ion (σ-complex) with the positive charge spread over three carbons. When phenol is attacked para (or ortho), one resonance structure puts the positive charge on the carbon bearing OH, and the oxygen lone pair then gives one more structure, an oxonium ion in which every atom has an octet, which is especially stable. In the meta-attack intermediate the charge never reaches that carbon, so this extra stabilisation is missing.
对位进攻：正电荷在连羟基的碳上
Para attack: positive charge on the carbon bearing OH
氧鎓离子：每个原子都满足八隅体
Oxonium ion: every atom has an octet
活化 / 钝化与定位方向Activation and directing effects
按活化能力从强到弱排列。结构图中绿圈标出新取代基进入的位置。I = 诱导效应，M = 共轭效应；“+”给电子，“−”吸电子。Ordered from most activating to most deactivating. Green circles mark where a new substituent goes. I = inductive, M = mesomeric; “+” donates, “−” withdraws.
| 基团 Group | 结构 Structure | 名称 Name | 对反应速率 Rate | 定位 Directs to | I | M |
|---|---|---|---|---|---|---|
| –O⁻ | 氧负离子（酚盐）oxido (phenoxide) | 强活化strongly activating | 邻、对位ortho / para | − | + | |
| –N(CH₃)₂ | 二甲氨基dimethylamino | 强活化strongly activating | 邻、对位ortho / para | − | + | |
| –NH₂ | 氨基amino | 强活化strongly activating | 邻、对位ortho / para | − | + | |
| –OH | 羟基hydroxy | 强活化strongly activating | 邻、对位ortho / para | − | + | |
| –OCH₃ | 甲氧基methoxy | 中等活化moderately activating | 邻、对位ortho / para | − | + | |
| –NHCOCH₃ | 乙酰氨基acetamido | 中等活化moderately activating | 邻、对位ortho / para | − | + | |
| –CH₃ | 甲基methyl | 弱活化weakly activating | 邻、对位ortho / para | + | — | |
| –C₂H₅ | 乙基ethyl | 弱活化weakly activating | 邻、对位ortho / para | + | — | |
| –C₆H₅ | 苯基phenyl | 弱活化weakly activating | 邻、对位ortho / para | — | ± | |
| –H | 氢（参照）hydrogen (reference) | 参照reference | —— | — | — | |
| –F | 氟fluoro | 弱钝化weakly deactivating | 邻、对位ortho / para | − | + | |
| –Cl | 氯chloro | 弱钝化weakly deactivating | 邻、对位ortho / para | − | + | |
| –Br | 溴bromo | 弱钝化weakly deactivating | 邻、对位ortho / para | − | + | |
| –I | 碘iodo | 弱钝化weakly deactivating | 邻、对位ortho / para | − | + | |
| –CHO | 甲酰基（醛基）formyl | 中等钝化moderately deactivating | 间位meta | − | − | |
| –COCH₃ | 乙酰基acetyl | 中等钝化moderately deactivating | 间位meta | − | − | |
| –COOH | 羧基carboxy | 中等钝化moderately deactivating | 间位meta | − | − | |
| –COOCH₃ | 甲氧羰基methoxycarbonyl | 中等钝化moderately deactivating | 间位meta | − | − | |
| –SO₃H | 磺基sulfo | 强钝化strongly deactivating | 间位meta | − | − | |
| –CF₃ | 三氟甲基trifluoromethyl | 强钝化strongly deactivating | 间位meta | − | — | |
| –CN | 氰基cyano | 强钝化strongly deactivating | 间位meta | − | − | |
| –NO₂ | 硝基nitro | 强钝化strongly deactivating | 间位meta | − | − | |
| –N(CH₃)₃⁺ | 三甲铵基trimethylammonio | 强钝化strongly deactivating | 间位meta | − | — |
Hammett 取代基常数（进阶）Hammett substituent constants (advanced)
Hammett 方程 lg(k/k₀) = ρσ 定量描述间位、对位取代基对反应速率或平衡的影响。σ 是取代基常数：σ > 0 吸电子，σ < 0 给电子；σm 主要反映诱导效应，σp 同时包含共轭效应，所以 –OH、–OCH₃ 的 σm 为正而 σp 为负。ρ 是反应常数，衡量反应对电子效应的敏感度；以苯甲酸在 25 °C 水中的电离为标准，ρ = 1。
The Hammett equation log(k/k₀) = ρσ quantifies how meta and para substituents change rates or equilibria. σ is the substituent constant: σ > 0 withdraws electrons, σ < 0 donates; σm mostly reflects induction, while σp also includes resonance, which is why –OH and –OCH₃ have positive σm but negative σp. ρ, the reaction constant, measures how sensitive the reaction is; ionisation of benzoic acid in water at 25 °C defines ρ = 1.
| 基团 Group | 名称 Name | σm | σp |
|---|---|---|---|
| –N(CH₃)₂ | 二甲氨基dimethylamino | −0.16 | −0.83 |
| –O⁻ | 氧负离子（酚盐）oxido (phenoxide) | −0.47 | −0.81 |
| –NH₂ | 氨基amino | −0.16 | −0.66 |
| –OH | 羟基hydroxy | +0.12 | −0.37 |
| –OCH₃ | 甲氧基methoxy | +0.12 | −0.27 |
| –CH₃ | 甲基methyl | −0.07 | −0.17 |
| –C₂H₅ | 乙基ethyl | −0.07 | −0.15 |
| –C₆H₅ | 苯基phenyl | +0.06 | −0.01 |
| –NHCOCH₃ | 乙酰氨基acetamido | +0.21 | 0.00 |
| –H | 氢（参照）hydrogen (reference) | 0.00 | 0.00 |
| –F | 氟fluoro | +0.34 | +0.06 |
| –I | 碘iodo | +0.35 | +0.18 |
| –Cl | 氯chloro | +0.37 | +0.23 |
| –Br | 溴bromo | +0.39 | +0.23 |
| –CHO | 甲酰基（醛基）formyl | +0.35 | +0.42 |
| –COOH | 羧基carboxy | +0.37 | +0.45 |
| –COOCH₃ | 甲氧羰基methoxycarbonyl | +0.37 | +0.45 |
| –COCH₃ | 乙酰基acetyl | +0.38 | +0.50 |
| –CF₃ | 三氟甲基trifluoromethyl | +0.43 | +0.54 |
| –CN | 氰基cyano | +0.56 | +0.66 |
| –NO₂ | 硝基nitro | +0.71 | +0.78 |
| –N(CH₃)₃⁺ | 三甲铵基trimethylammonio | +0.88 | +0.82 |
| –SO₃H | 磺基sulfo | — | — |
σ 值引自 Hansch, Leo, Taft, Chem. Rev. 1991, 91, 165。σ values from Hansch, Leo and Taft, Chem. Rev. 1991, 91, 165.