📚 Addition | 加成
Addition reactions sit at the heart of organic chemistry, especially in the study of alkenes. An addition reaction transforms an unsaturated compound, typically containing a carbon–carbon double bond, into a saturated product by adding atoms or groups across the multiple bond. These reactions are not just laboratory curiosities; they are the industrial pathways to polymers, alcohols, and halogenated compounds that shape our everyday life. Understanding addition means mastering electron‑rich and electron‑poor interactions, reaction mechanisms, and regioselectivity rules such as Markovnikov's principle.
加成反应处于有机化学的核心，尤其是烯烃化学的研究中。加成反应将不饱和化合物（通常含有碳碳双键）通过在多键上加上原子或基团，转化为饱和产物。这些反应不仅仅是实验室的奇观，更是通向聚合物、醇和卤代化合物等日常生活物质的工业途径。理解加成意味着要掌握富电子与缺电子相互作用、反应机理以及马尔科夫尼科夫规则等区域选择性原则。
1. What is an Addition Reaction? | 什么是加成反应？
specifically occurs at unsaturated centres, most commonly at alkenes (C=C) and alkynes (C≡ bond into stronger sigma bonds, releasing energy and making addition reactions exothermic.
在有机化学中，加成反应被定义为两个或多个反应物结合形成单一产物，且没有原子丢失的过程。它特指在不饱和中心进行，最常见的是烯烃（C=C）和炔烃（C≡C）。π键断裂并形成两个新的σ键，生成一个饱和分子。通用方程式可以写作：A−B + >C=C< → A−C−C−B。其驱动力是将较弱的π键转化为更强的σ键，同时释放能量，使得加成反应通常是放热的。
The unsaturation provides a region of high electron density. This electron‑rich cloud is susceptible to attack by electrophiles – species that are electron‑deficient or carry a positive charge. Therefore, most addition reactions of alkenes follow an electrophilic addition mechanism. Nucleophilic additions also exist, but they are typical of carbonyl compounds rather than simple alkenes in the IB syllabus.
不饱和度提供了一片高电子密度区域。这个富电子云容易受到亲电试剂的进攻——亲电试剂是缺电子或带有正电荷的物种。因此，烯烃的大多数加成反应遵循亲电加成机理。亲核加成也存在，但在IB大纲中它们更典型地发生在羰基化合物上，而不是简单烯烃。
2. The Mechanism of Electrophilic Addition | 亲电加成机理
Electrophilic addition proceeds in two main steps. In the first, rate‑determining step, the pi electrons of the double bond attack an electrophile (E⁺), forming a carbocation intermediate and a negatively charged species. For example, when hydrogen bromide adds to ethene, the H⁺ part of the polar H−Br molecule acts as the electrophile. The double bond opens, and a C−H bond forms, leaving a carbocation on the more substituted carbon if the alkene is unsymmetrical.
亲电加成分两步进行。第一步是速率决定步骤，双键的π电子进攻亲电试剂（E⁺），形成一个碳正离子中间体和一个带负电荷的物种。例如，当溴化氢与乙烯加成时，极性H−Br分子中的H⁺部分充当亲电试剂。双键打开，形成一个C−H键，如果烯烃不对称则碳正离子留在取代程度较高的碳上。
In the second, fast step, the negatively charged species (nucleophile, Nu⁻) donates its electron pair to the carbocation, forming a new covalent bond. Continuing with the HBr example, the bromide ion Br⁻ attacks the carbocation, yielding bromoethane. The overall stereochemistry is not controlled in simple cases because the planar carbocation can be attacked from either side, often leading to racemic mixtures if a chiral centre is created.
在第二步快速步骤中，带负电荷的物种（亲核试剂，Nu⁻）将它的电子对提供给碳正离子，形成一个新的共价键。继续以HBr为例，溴离子Br⁻进攻碳正离子，生成溴乙烷。在简单情形下，整体立体化学不受控制，因为平面碳正离子可以从任一面受到进攻；如果产生了手性中心，通常会得到外消旋混合物。
It is essential to show the movement of electron pairs using curly arrows. From the double bond to the electrophile, and from the nucleophile to the carbocation. This mechanistic understanding is a core requirement in IB Chemistry, underpinning prediction of products and explanation of regioselectivity.
用弯箭头表示电子对的移动至关重要：从双键到亲电试剂，再从亲核试剂到碳正离子。这种机理的理解是IB化学的核心要求，是预测产物和解释区域选择性的基础。
3. Addition of Halogens (Br₂, Cl₂) | 卤素（Br₂, Cl₂）的加成
Alkenes react rapidly with bromine or chlorine at room temperature in the absence of light. This reaction is often used as a test for unsaturation: the reddish‑brown colour of bromine water is decolourised. The overall equation for ethene and bromine is displayed below. The mechanism involves a polarisable halogen molecule that behaves as an electrophile due to an induced dipole when it approaches the electron‑rich double bond.
烯烃在室温下、没有光照时与溴或氯迅速反应。该反应常被用作不饱和度的检验：溴水的红棕色会褪去。乙烯与溴的总方程式如下。机理涉及一个可极化的卤素分子，当它靠近富电子的双键时产生瞬时偶极，从而作为亲电试剂。
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
In the first step, the pi electrons induce a dipole in Br₂, making the nearer bromine slightly positive. This Brδ⁺ accepts electrons, forming a cyclic bromonium ion and a bromide ion. The bromonium ion is a three‑membered ring with a positive charge on bromine. In the second step, the bromide ion attacks the bromonium ion from the opposite face (backside attack) to open the ring, giving a vicinal dibromide with anti stereochemistry.
第一步中，π电子在Br₂中诱导出偶极，使得较近的溴略带正电。这个Brδ⁺接受电子，形成一个环状溴鎓离子和一个溴离子。溴鎓离子是一个三元环，溴上带正电荷。第二步中，溴离子从反面进攻溴鎓离子，开环得到邻二溴化物，具有反式立体化学特征。
Thus, addition of bromine to cyclopentene gives trans‑1,2‑dibromocyclopentane exclusively. This stereospecificity is a powerful piece of evidence for the bromonium ion mechanism over a simple carbocation pathway. Chlorine behaves similarly but is less frequently used in the IB lab due to its gaseous state and toxicity.
因此，溴与环戊烯加成只得到反式‑1,2‑二溴环戊烷。这种立体专一性是溴鎓离子机理的有力证据，否定了简单的碳正离子路径。氯的行为类似，但由于其气体状态和毒性，在IB实验室中使用较少。
4. Addition of Hydrogen Halides (HX) | 卤化氢（HX）的加成
Hydrogen halides such as HCl, HBr, and HI add across alkenes to produce haloalkanes. Gaseous HX is bubbled through the alkene, or the reaction can be carried out in a non‑aqueous solvent. The reactivity order follows the acid strength: HI > HBr > HCl, correlating with bond polarity and the stability of the halide leaving group as a nucleophile after heterolytic cleavage.
卤化氢如HCl、HBr和HI与烯烃加成得到卤代烷。将气态HX通入烯烃，或在非水溶剂中进行反应。反应活性顺序遵循酸性强度：HI > HBr > HCl，这与键的极性和卤离子作为离去基团在异裂后的稳定性相关。
The electrophile is the proton H⁺, generated by heterolytic fission of the H−X bond, although this fission is often assisted by the alkene's pi electrons. The mechanism proceeds via the most stable carbocation possible (Markovnikov addition), which will be discussed in detail later. For symmetrical alkenes like ethene, a single product is formed: adding HBr gives bromoethane.
亲电试剂是质子H⁺，由H−X键异裂产生，尽管这种断裂常常由烯烃的π电子协助。机理经过尽可能最稳定的碳正离子（马尔科夫尼科夫加成），稍后将详细讨论。对于像乙烯这样的对称烯烃，只生成一种产物：加HBr得溴乙烷。
CH₂=CH₂ + HBr → CH₃CH₂Br
In the case of unsymmetrical alkenes two regioisomers are theoretically possible, but one usually predominates. The reaction is useful for synthesising secondary and tertiary haloalkanes since primary carbocations are rarely formed under electrophilic conditions.
对于不对称烯烃，理论上可能生成两种区域异构体，但通常其中一种占主导。该反应对于合成仲和叔卤代烷十分有用，因为在亲电条件下很少形成伯碳正离子。
5. Addition of Water (Hydration) | 水合加成反应
Alkenes can be hydrated to alcohols in the presence of an acid catalyst, typically concentrated sulfuric acid or phosphoric acid. Industrially, ethanol is manufactured from ethene by direct hydration using a phosphoric acid catalyst on a solid support at around 300 °C and 60–70 atm. The equation is straightforward but the mechanism is a classic example of electrophilic addition followed by hydrolysis.
烯烃可以在酸催化剂存在下水合生成醇，常用的催化剂是浓硫酸或磷酸。工业上，乙醇是由乙烯在固态磷酸催化剂上、约300 °C和60–70 atm下直接水合生产的。方程式简单，但机理是亲电加成后进行水解的经典范例。
CH₂=CH₂ + H₂O → CH₃CH₂OH
The mechanism involves protonation of the alkene to form a carbocation. Water then acts as a nucleophile, attacking the carbocation to yield a protonated alcohol, which loses a proton to give the final alcohol. For unsymmetrical alkenes, the initial protonation occurs so that the more stable carbocation is formed, following Markovnikov's rule: the OH group ends up on the more substituted carbon.
机理包括烯烃被质子化以形成碳正离子。水作为亲核试剂进攻碳正离子，生成质子化的醇，后者失去一个质子得到最终的醇。对于不对称烯烃，初始质子化方式使得更稳定的碳正离子形成，遵循马尔科夫尼科夫规则：OH基团最终连在取代更多的碳上。
Indirect hydration via alkyl hydrogen sulfate formation is an alternative laboratory method: alkene is absorbed in cold concentrated H₂SO₄ to form alkyl hydrogen sulfate; subsequent dilution with water and heating hydrolyses the ester to the alcohol and regenerates sulfuric acid. This two‑step process avoids the high temperatures of direct hydration.
通过烷基硫酸氢盐形成的间接水合是一种实验室替代方法：烯烃在冷的浓硫酸中被吸收，形成烷基硫酸氢盐；随后加水稀释并加热，水解酯得到醇并再生硫酸。这种两步过程避免了直接水合所需的高温。
6. Markovnikov's Rule | 马尔科夫尼科夫规则
When an unsymmetrical reagent (H−X, H₂O) adds to an unsymmetrical alkene, the major product can be predicted by Markovnikov's rule. The rule states that the hydrogen atom (or more generally the electrophilic part) adds to the carbon of the double bond that has the greater number of hydrogen atoms already attached, while the nucleophilic part adds to the more substituted carbon. In modern terms, the addition proceeds via the most stable carbocation intermediate.
当不对称试剂（H−X, H₂O）与不对称烯烃加成时，主要产物可由马尔科夫尼科夫规则预测。该规则指出，氢原子（或更一般地，亲电部分）加到双键上原先带有更多氢原子的碳上，而亲核部分则加到取代较多的碳上。用现代语言表述，加成经过最稳定的碳正离子中间体进行。
mary (1°) > methyl. Thus, when propene (CH₃CH=CH₂) reacts with HBr, the major product is 2‑bromopropane, where Br attaches to the central carbon (secondary), not 1‑ minal CH₂ group because that path generates a 2° carbocation rather than a 1° carbocation.
碳正离子的稳定性随烷基取代增加而增大，原因是超共轭效应和诱导效应：叔（3°）> 仲（2°）> 伯（1°）> 甲基。因此，当丙烯（CH₃CH=CH₂）与HBr反应时，主要产物是2‑溴丙烷，溴连在中间的碳上（仲），而不是1‑溴丙烷（伯）。质子加在末端的CH₂基团上，因为那条路径生成2°碳正离子而非1°碳正离子。
Markovnikov's rule is not a law; there are exceptions, most notably in the presence of peroxides with HBr. The peroxide effect leads to anti‑Markovnikov addition via a free‑radical mechanism. However, under standard electrophilic conditions without peroxides, the rule holds reliably for predicting major products.
马尔科夫尼科夫规则并非定律，存在例外，最显著的是在过氧化物存在下与HBr的反应。过氧化物效应通过自由基机理导致反马氏加成。但在标准亲电条件下、无过氧化物时，该规则可可靠地预测主要产物。
7. Addition to Unsymmetrical Alkenes | 不对称烯烃的加成
Unsymmetrical alkenes are those where the two carbon atoms of the double bond carry different numbers or types of substituents. Propene, but‑1‑ene, and 2‑methylpropene are common examples. The regioselectivity of addition depends on the competing stabilities of possible carbocation intermediates. Attack at the less substituted end generally gives the more substituted carbocation, which is more stable and forms faster.
不对称烯烃是指双键两个碳原子上带有不同数量或类型的取代基。丙烯、丁‑1‑烯和2‑甲基丙烯是常见例子。加成的区域选择性取决于可能的碳正离子中间体的稳定性竞争。在取代较少的一端进攻通常得到取代更多的碳正离子，这更稳定且形成更快。
Consider the addition of HCl to 2‑methylpropene. Protonation at the CH₂ end gives a tertiary carbocation (CH₃)₃C⁺, whereas protonation at the central carbon would give a primary carbocation (CH₃)₂CHCH₂⁺. The tertiary carbocation pathway dominates, leading to 2‑chloro‑2‑methylpropane as the major product.
考虑HCl与2‑甲基丙烯的加成。在CH₂端质子化给出叔碳正离子(CH₃)₃C⁺，而在中心碳上质子化则会给出伯碳正离子(CH₃)₂CHCH₂⁺。叔碳正离子路径占主导，生成2‑氯‑2‑甲基丙烷作为主要产物。
When water is added under acidic conditions, the same principle applies. Hydration of propene yields propan‑2‑ol (isopropyl alcohol) rather than propan‑1‑ol. This regioselectivity is exploited industrially to produce secondary and tertiary alcohols from readily available alkenes.
在酸性条件下加水时，同样原则适用。丙烯的水合生成丙‑2‑醇（异丙醇）而非丙‑1‑醇。工业上利用这种区域选择性从易得的烯烃生产仲醇和叔醇。
8. Addition of Hydrogen (Hydrogenation) | 加氢反应（氢化）
Hydrogenation is the addition of dihydrogen (H₂) to an alkene in the presence of a metal catalyst, typically nickel, palladium, or platinum. The reaction converts unsaturated fats to saturated fats and is a key step in margarine production. The process is heterogeneous catalysis: H₂ and alkene adsorb onto the metal surface, the H−H bond breaks, and hydrogen atoms add stepwise to the double bond.
加氢反应是在金属催化剂（通常是镍、钯或铂）存在下，将氢气（H₂）加到烯烃上的反应。该反应将不饱和脂肪转化为饱和脂肪，是人造黄油生产的关键步骤。该过程是多相催化：H₂和烯烃吸附在金属表面，H−H键断裂，氢原子逐步加到双键上。
CH₂=CH₂ + H₂ → CH₃CH₃
The mechanism is not electrophilic in the classic sense; it is a surface reaction where both hydrogen atoms add to the same face of the alkene, resulting in syn addition. For example, hydrogenation of an alkene using a palladium catalyst yields the corresponding alkane with cis addition stereochemistry, although this is rarely an issue in simple open‑chain molecules because of free rotation in the saturated product.
该机理并非经典意义上的亲电过程，而是一个表面反应，两个氢原子都加在烯烃的同一面，导致顺式加成。例如，使用钯催化剂氢化烯烃生成相应的烷烃，具有顺式加成立体化学，不过在简单的开链分子中由于饱和产物可自由旋转，这通常不是问题。
Hydrogenation is highly exothermic but requires a catalyst to lower the activation energy. The degree of unsaturation in fats and oils is often measured by the iodine number, while hydrogenation reduces the number of double bonds. This topic links to the IB option on biochemistry and food chemistry.
加氢高度放热，但需要催化剂以降低活化能。脂肪和油的不饱和度常用碘值来衡量，而氢化会降低双键数目。该主题与IB生物化学与食品化学选修内容相关联。
9. Addition Polymerisation | 加成聚合
Addition polymerisation is a chain reaction in which alkene monomers join together without the loss of any atoms to form a long‑chain polymer. Poly(ethene), poly(propene), poly(chloroethene) (PVC), and poly(tetrafluoroethene) (PTFE) are all produced in this way. The reaction is initiated by free radicals, cations, or coordination catalysts, depending on the type of polymerisation.
加成聚合是一种链式反应，烯烃单体连接在一起而不丢失任何原子，形成长链聚合物。聚（乙烯）、聚（丙烯）、聚（氯乙烯）（PVC）和聚（四氟乙烯）（PTFE）都是通过这种方式生产的。根据聚合类型的不同，反应可由自由基、阳离子或配位催化剂引发。
In the IB curriculum, free‑radical addition polymerisation is emphasised. It features three stages: initiation (radical generation and addition to monomer), propagation (chain growth by repeated addition), and termination (coupling or disproportionation of radicals). The repeating unit of the polymer retains the basic carbon skeleton of the monomer, and the polymer name is simply 'poly' followed by the monomer name in parentheses if substituted.
在IB课程中，着重强调的是自由基加成聚合。它包含三个阶段：引发（自由基生成并加成到单体上）、增长（通过反复加成实现链增长）和终止（自由基偶合或歧化）。聚合物的重复单元保留了单体基本的碳骨架，聚合物的命名在取代情况下为“聚”后跟括号中的单体名称。
The physical properties of addition polymers depend on the nature of the side groups and the degree of chain branching. High‑pressure poly(ethene) has extensive branching and is low density (LDPE), while a Ziegler–Natta catalyst produces linear, high‑density poly(ethene) (HDPE). These examples beautifully illustrate the relevance of addition reactions to materials science.
加成聚合物的物理性质取决于侧基的性质和链支化程度。高压聚乙烯支化程度高，为低密度聚乙烯（LDPE），而用齐格勒–纳塔催化剂可生产线性高密度聚乙烯（HDPE）。这些例子极好地说明了加成反应与材料科学的相关性。
10. Summary and Importance | 总结与重要性
Addition reactions are a unifying theme across organic chemistry, linking laboratory synthesis, industrial manufacturing, and biological processes. From the decolourisation test of bromine to the production of millions of tonnes of polymers, the addition pathway proves its utility. The key concepts – electrophilic attack, carbocation stability, Markovnikov's rule, and stereochemical outcomes – form a conceptual framework that IB students must confidently apply to novel scenarios.
加成反应是有机化学的一个统一主题，连接着实验室合成、工业生产和生物过程。从溴的褪色检验到数以百万吨计的聚合物生产，加成路径证明了它的实用性。亲电进攻、碳正离子稳定性、马尔科夫尼科夫规则和立体化学结果这些关键概念构成了一个概念框架，IB学生必须能够自信地应用于新情境。
The ability to draw full mechanisms with curly arrows, predict major products, and explain the underlying reasons for regioselectivity is a fundamental skill assessed in examinations. Moreover, appreciating the limitations of Markovnikov's rule, such as the peroxide effect, demonstrates a deeper, analytical understanding that distinguishes top‑level answers.
能够用弯箭头画出完整机理、预测主要产物并解释区域选择性的根本原因，是考试中评估的基本技能。此外，认识到马尔科夫尼科夫规则的局限性，例如过氧化物效应，展示出更深层次的分析性理解，这正是高水平答案的特征。
In the broader context, addition reactions highlight the interplay between thermodynamics (exothermic nature) and kinetics (activation energy, catalysts). They show how a single mechanistic motif can be adapted to diverse reagents – halogens, hydrogen halides, water, hydrogen – each with subtle variations that enrich our grasp of molecular reactivity.
在更广泛的背景下，加成反应突显了热力学（放热性质）与动力学（活化能、催化剂）之间的相互作用。它们展示了同一个机理主题如何适应于各种试剂——卤素、卤化氢、水、氢气——每一个都带有微妙的变化，丰富了我们对于分子反应性的理解。
Mastering addition places students in a strong position to tackle more complex organic reaction pathways, including electrophilic aromatic substitution and nucleophilic addition to carbonyls, which build on the same fundamental principles of electron density and charge stabilisation.
掌握加成为学生应对更复杂的有机反应路径奠定了坚实基础，包括亲电芳香取代和羰基的亲核加成，这些均建立在电子密度与电荷稳定这一相同的基本原理之上。
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