📚 Core Content: A Comprehensive Review of Year 12 AQA Chemistry | Year 12 AQA 化学：核心知识点梳理
Year 12 AQA Chemistry lays the foundation for advanced chemical understanding, bridging GCSE concepts with the depth required at A-Level. Students often find the leap challenging, as the course demands not just factual recall but also the ability to apply principles quantitatively, interpret unfamiliar data, and visualise molecular-level processes. This article provides a structured walkthrough of the essential topics, from atomic structure and bonding through to organic chemistry and analytical techniques, highlighting the interconnected nature of the specification. Each section pairs clear English explanations with matched Chinese translations, making this a valuable resource for bilingual learners and those consolidating their knowledge ahead of end-of-year assessments.
AQA Year 12 化学课程为高级化学理解奠定基础，将GCSE概念与A-Level所需的知识深度衔接起来。学生们常常发现这一跨越具有挑战性，因为课程不仅要求记忆事实，还需要定量应用原理、解读陌生数据以及构想分子层面的过程。本文提供了一份结构化的知识要点梳理，涵盖从原子结构和化学键合到有机化学和分析技术的核心主题，突出课程规范中各部分的内在联系。每个部分都将清晰的英文解释与匹配的中文翻译配对呈现，对于双语学习者和在学年末评估前巩固知识的学生来说，这是一份宝贵的资料。
1. Atomic Structure & Electron Configuration | 原子结构与电子构型
At the heart of chemistry lies the atom. In Year 12 AQA, you must be confident describing the relative masses and charges of protons, neutrons and electrons. Protons and neutrons each have a relative mass of 1, while the electron's mass is approximately 1/1840. Protons carry a +1 charge, electrons carry a −1 charge, and neutrons are neutral. The atomic number (Z) defines the number of protons, uniquely identifying the element, while the mass number (A) is the sum of protons and neutrons. Isotopes are atoms of the same element with the same Z but different A, meaning they possess identical chemical properties but differ in mass and physical behaviour. Mass spectrometry provides evidence for isotopes and allows calculation of relative atomic mass (Aᵣ) from percentage abundances.
化学的核心在于原子。在Year 12 AQA课程中，你必须能够自信地描述质子、中子和电子的相对质量与电荷。质子和中子的相对质量均为1，而电子的质量约为1/1840。质子带+1电荷，电子带−1电荷，中子呈电中性。原子序数（Z）定义质子数，唯一地标识元素，而质量数（A）是质子与中子之和。同位素是同一元素中Z相同但A不同的原子，这意味着它们具有相同的化学性质，但质量和物理行为不同。质谱分析为同位素的存在提供了证据，并允许根据丰度百分比计算相对原子质量（Aᵣ）。
, 2, 3, 4, and within these shells reside sub-shells designated s, p, d and f. The order of filling is governed by the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Hund' per orbital with opposite spins. For AQA, you must be able to write both full and shorthand configurations for elements up to krypton (Z = 36), noting the key exception at chromium (Cr: [Ar] 4s¹3d⁵) and copper (Cu: [Ar] 4s¹3d¹⁰), where the stability of half-filled and fully filled d sub-shells overrides the expected filling order.
电子构型遵循一套基于量子理论的明确规则。电子占据由主量子数n = 1, 2, 3, 4标记的壳层，在这些壳层内存在着被标记为s、p、d和f的亚层。填充顺序由构造原理决定：1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p。洪特规则指出，电子在成对之前先单独填充简并轨道，泡利不相容原理则允许每个轨道最多容纳两个自旋相反的电子。对于AQA考试，你必须能够书写直至氪（Z = 36）的元素的全电子构型和简写构型，并注意关键例外情况：铬（Cr: [Ar] 4s¹3d⁵）和铜（Cu: [Ar] 4s¹3d¹⁰），其中半充满和全充满d亚层的稳定性压倒了预期的填充顺序。
2. Bonding, Structure & Intermolecular Forces | 化学键合、结构与分子间作用力
Three types of strong chemical bonding dominate the AQA Year 12 specification: ionic, covalent and metallic. Ionic bonding occurs between metals and non-metals, involving the electrostatic attraction between oppositely charged ions formed by electron transfer. The strength of an ionic bond is quantified by lattice enthalpy, and the structure adopted (e.g. NaCl-type, CsCl-type) depends on ionic radii and charge. Covalent bonding, found in non-metal compounds, involves the sharing of electron pairs. A dative covalent bond arises when both electrons in the shared pair originate from the same atom. Metallic bonding, present in metals and alloys, is described by a lattice of positive ions immersed in a 'sea' of delocalised electrons; this model explains electrical conductivity, malleability and ductility.
三种强化学键主导着AQA Year 12课程规范：离子键、共价键和金属键。离子键形成于金属与非金属之间，涉及由电子转移形成的带相反电荷的离子之间的静电吸引。离子键的强度通过晶格焓来量化，所采用的结构（例如NaCl型、CsCl型）取决于离子半径和电荷。共价键存在于非金属化合物中，涉及电子对的共享。当共享电子对中的两个电子均来自同一原子时，就产生了配位共价键。金属键存在于金属和合金中，通过浸没在离域电子'海'中的正离子晶格来描述；这一模型解释了导电性、可锻性和延展性。
Molecular shape is predicted using VSEPR (Valence Shell Electron Pair Repulsion) theory. Electron pairs around a central atom repel one another and arrange themselves to minimise repulsion. The shape is determined by the number of bonding pairs and lone pairs. Key geometries include linear (2 bonding pairs, 180°, e.g. BeCl₂), trigonal planar (3 bonding pairs, 120°, e.g. BF₃), tetrahedral (4 bonding pairs, 109.5°, e.g. CH₄), pyramidal (3 bonding pairs + 1 lone pair, 107°, e.g. NH₃) and bent (2 bonding pairs + 2 lone pairs, 104.5°, e.g. H₂O). Each lone pair reduces the bond angle by approximately 2.5° because lone pairs repel more strongly than bonding pairs.
分子形状通过VSEPR（价层电子对互斥）理论来预测。中心原子周围的电子对相互排斥，并排列成使排斥力最小化的构型。形状由键合电子对和孤对电子的数量决定。关键几何构型包括直线形（2对键合电子对，180°，如BeCl₂）、平面三角形（3对键合电子对，120°，如BF₃）、正四面体形（4对键合电子对，109.5°，如CH₄）、三角锥形（3对键合电子对+1对孤对电子，107°，如NH₃）和V形（2对键合电子对+2对孤对电子，104.5°，如H₂O）。每对孤对电子使键角减少约2.5°，因为孤对电子的排斥力比键合电子对更强。
Intermolecular forces determine bulk physical properties: melting point, boiling point and solubility. London dispersion forces arise from temporary fluctuations in electron distribution, creating instantaneous dipoles that induce dipoles in neighbouring molecules. These forces increase in strength with molecular size and surface contact. Permanent dipole-dipole interactions exist between polar molecules. Hydrogen bonding, the strongest intermolecular force at A-Level, occurs when hydrogen is covalently bonded to highly electronegative nitrogen, oxygen or fluorine and interacts with a lone pair on a neighbouring N, O or F atom. Water's unusually high boiling point, ice's lower density than liquid water, and the solubility of alcohols all stem from hydrogen bonding. Students must be able to explain trends in boiling points across homologous series using these intermolecular interactions.
分子间作用力决定了宏观物理性质：熔点、沸点和溶解度。伦敦色散力源于电子分布的瞬时波动，产生瞬时偶极，进而诱导邻近分子产生偶极。这些力的强度随着分子大小和表面接触的增加而增强。永久偶极-偶极相互作用存在于极性分子之间。氢键是A-Level阶段最强的分子间作用力，当氢原子与电负性很强的氮、氧或氟原子形成共价键，并与邻近的N、O或F原子上的孤对电子发生相互作用时，就形成氢键。水异常高的沸点、冰的密度低于液态水以及醇类的溶解度都源于氢键。学生必须能够利用这些分子间相互作用解释同系物中沸点的变化趋势。
3. The Mole, Equations & Stoichiometry | 摩尔、方程式与化学计量学
The mole is the chemist' d as the amount of substance containing exactly 6.022 × 10²³ elementary entities (Avogadro' grams (m), and molar mass in g mol⁻¹ (M). For gases at room temperature and pressure, the molar volume is approximately 24.0 dm³ mol⁻¹ (AQA uses 24 dm³ at 298 K and 100 kPa). For solutions, concentration c (mol dm⁻³) is related to amount n and volume V (dm³) by c = n / V. The ideal gas equation pV = nRT is also required, where p is pressure in Pa, V is volume in m³, n is moles, R is 8.31 J K⁻¹ mol⁻¹, and T is temperature in Kelvin.
摩尔是化学家的计数单位，定义为恰好包含6.022 × 10²³个基本实体（阿伏伽德罗常数）的物质的量。这一主题几乎是Year 12所有定量工作的基础。基本方程n = m / M将以摩尔为单位的物质的量（n）、以克为单位的质量（m）和以g mol⁻¹为单位的摩尔质量（M）联系起来。对于室温常压下的气体，摩尔体积约为24.0 dm³ mol⁻¹（AQA在298 K和100 kPa条件下使用24 dm³）。对于溶液，浓度c（mol dm⁻³）通过c = n / V与物质的量n和体积V（dm³）相关联。理想气体方程pV = nRT同样需要掌握，其中p是以Pa为单位的压强，V是以m³为单位的体积，n是摩尔数，R是8.31 J K⁻¹ mol⁻¹，T是以开尔文为单位的温度。
ermined experimentally from mass or percentage composition data. The molecular formula is a whole-number multiple of the empirical formula, requiring molar mass information from mass spectrometry. Balanced chemical equations provide mole ratios for reacting quantities. Calculations involving limiting reagents, percentage yield and atom economy are essential practical applications. Percentage yield = (actual yield / theoretical yield) × 100, while atom economy = (molar mass of desired product / total molar mass of all products) × 100. High atom economy is desirable for sustainable chemistry.
经验式和分子式是基础。经验式表示化合物中原子的最简整数比，通过实验从质量或百分比组成数据中确定。分子式是经验式的整数倍，需要来自质谱分析的摩尔质量信息。配平的化学方程式提供了反应物量的摩尔比。涉及限量试剂、产率和原子经济性的计算是必不可少的实际应用。产率= (实际产量/理论产量) × 100，而原子经济性= (目标产物的摩尔质量/所有产物总摩尔质量) × 100。高原子经济性对于可持续化学是可取的。
4. Energetics – Enthalpy Changes & Hess's Law | 能量学 – 焓变与盖斯定律
onditions defined as 100 kPa and a stated temperature, typically 298 K. An exothermic reaction releases heat (ΔH is negative), while an endothermic reaction absorbs heat (ΔH is positive). The standard enthalpy of combustion (ΔH°꜀ ns completely in oxygen under standard conditions. The standard enthalpy of formation (ΔH°բ ination uses calorimetry; the equation q = mcΔT relates heat energy transferred to the mass, specific heat capacity and temperature change of the surroundings, typically water.
热化学关注化学反应中的热能变化。焓变（ΔH）在恒压条件下测量，标准条件定义为100 kPa和指定温度，通常为298 K。放热反应释放热量（ΔH为负值），而吸热反应吸收热量（ΔH为正值）。标准燃烧焓（ΔH°꜀）是在标准条件下，一摩尔物质在氧气中完全燃烧时的焓变。标准生成焓（ΔH°բ）是在标准条件下，由其组成元素在标准状态下形成一摩尔化合物时的焓变。准确的实验测定使用量热法；方程q = mcΔT将传递的热能与周围物质（通常是水）的质量、比热容和温度变化联系起来。
Hess' f the route taken, provided the initial and final conditions are the same. This principle allows calculation of enthalpy changes that cannot be measured directly. Enthalpy cycles, typically drawn as triangular or multi-step diagrams, provide a visual method for applying Hess' n) − Σ (bonds formed). However, mean bond enthalpies are averaged values across many different compounds, so calculations using them yield approximate ΔH values rather than exact ones. Students must be proficient in constructing and manipulating enthalpy cycles for combustion, formation and bond enthalpy data.
盖斯定律指出，只要初始和最终条件相同，反应的总焓变与所采取的路径无关。这一原理允许计算无法直接测量的焓变。焓循环图通常以三角形或多步骤图的形式绘制，为应用盖斯定律提供了可视化方法。平均键焓提供了另一种途径：ΔH ≈ Σ（断裂的键）− Σ（形成的键）。然而，平均键焓是横跨许多不同化合物的平均值，因此使用它们进行的计算产生的是近似的ΔH值，而非精确值。学生必须熟练构建和操作基于燃烧、生成和键焓数据的焓循环图。
5. Kinetics – Collision Theory & Maxwell-Boltzmann | 动力学 – 碰撞理论与麦克斯韦-玻尔兹曼分布
For a reaction to occur, reactant particles must collide with sufficient energy to overcome the activation energy (Eₐ) and with the correct orientation. The activation energy is the minimum kinetic energy required for a collision to result in a reaction. The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. Factors affecting rate – concentration, pressure (for gases), temperature, surface area and catalysts – are all explained through collision theory. Increasing concentration or pressure raises the frequency of collisions. Increasing surface area exposes more reactive particles and also raises collision frequency.
要发生反应，反应物粒子必须以足够的能量克服活化能（Eₐ）并以正确的取向发生碰撞。活化能是碰撞导致反应所需的最低动能。化学反应速率定义为反应物或产物浓度每单位时间的变化。影响速率的因素——浓度、压强（对于气体）、温度、表面积和催化剂——都通过碰撞理论来解释。增加浓度或压强提高了碰撞频率。增加表面积暴露了更多反应粒子，同样提高了碰撞频率。
at a given temperature. The curve passes through the origin, rises to a peak at the most probable energy, and tails off at high energies. Crucially, only molecules in the shaded area beyond Eₐ have sufficient energy to react. Raising the temperature shifts the distribution to the right, flattening the curve: the peak lowers and moves to higher energy. The total area remains constant (representing the total number of molecules), but the proportion of molecules with energy ≥ Eₐ increases significantly. This explains why a modest temperature increase can produce a large rate enhancement. Catalysts provide an alternative reaction pathway with a lower activation energy, effectively shifting the Eₐ line to the left on the distribution; a far greater fraction of molecules now exceed this threshold, accelerating the reaction without being consumed.
温度的影响尤为显著。麦克斯韦-玻尔兹曼分布描绘了在给定温度下分子数量与其动能之间的关系。曲线经过原点，在最概然能量处达到峰值，并在高能量处逐渐趋于零。关键的是，只有Eₐ右侧阴影区域内的分子才具有足够的能量进行反应。升高温度使分布向右移动，曲线趋于平坦：峰值降低并向高能量方向移动。总面积保持不变（代表总分子数），但能量≥Eₐ的分子比例显著增加。这解释了为什么适度的温度升高可以产生很大的速率提升。催化剂提供了具有较低活化能的替代反应路径，实际上将分布图上的Eₐ线向左移动；现在有更大比例的分子超过这一阈值，从而在不被消耗的情况下加速反应。
6. Chemical Equilibria, Le Chatelier & Kc | 化学平衡、勒夏特列原理与Kc
rse reactions are equal, and the concentrations of reactants and products remain constant. Crucially, equilibrium is dynamic, not static: molecular-level processes continue, but with no net macroscopic change. The equilibrium constant Kc expresses the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient. For the general reaction aA + bB ⇌ cC + dD, Kc = [C]ᶜ Kc >> 1 implies a product-favoured position; Kc << 1 implies a reactant-favoured position.
许多化学反应是可逆的，同时向正向和反向进行。在平衡状态下，正反应和逆反应的速率相等，反应物和产物的浓度保持恒定。关键的是，平衡是动态的，而非静态的：分子层面的过程仍在持续，但没有宏观上的净变化。平衡常数Kc表示产物浓度与反应物浓度的比值，各自以其化学计量系数为幂次。对于一般反应aA + bB ⇌ cC + dD，Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ。对于给定温度下的给定反应，Kc值是恒定的。其大小指示平衡位置：Kc >> 1表示产物占优势的位置；Kc << 1表示反应物占优势的位置。
Le Chatelier's principle provides a qualitative tool: 'If a system at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose the change.' Increasing reactant concentration shifts equilibrium to the right, producing more products. Increasing pressure (for gaseous systems) favours the side with fewer moles of gas. For temperature changes, the direction of shift depends on whether the forward reaction is exothermic or endothermic. Increasing temperature favours the endothermic direction because the system absorbs heat to counteract the rise. Importantly, only temperature changes the value of Kc; concentration and pressure changes alter equilibrium position but not Kc itself. Catalysts speed up attainment of equilibrium but have no effect on position or Kc, as they accelerate forward and reverse reactions equally.
勒夏特列原理提供了一个定性工具：'如果处于平衡的系统受到浓度、压强或温度的变化，平衡位置会发生移动以抵消该变化。'增加反应物浓度使平衡向右移动，生成更多产物。增加压强（对于气体系统）有利于气体摩尔数较少的一侧。对于温度变化，移动方向取决于正向反应是放热还是吸热。升高温度有利于吸热方向，因为系统吸收热量以抵消温度的上升。重要的是，只有温度改变Kc的值；浓度和压强变化改变平衡位置，但不改变Kc本身。催化剂加快达到平衡的速度，但对平衡位置或Kc没有影响，因为它们同等地加速正反应和逆反应。
7. Redox Reactions & Oxidation States | 氧化还原反应与氧化态
Redox (reduction-oxidation) processes are unified by electron transfer. Oxidation Is Loss, Reduction Is Gain — the 'OIL RIG' mnemonic remains invaluable. More formally, oxidation involves an increase in oxidation state and reduction involves a decrease. The oxidation state (or oxidation number) is the hypothetical charge an atom would have if all bonds to atoms of different elements were fully ionic. Rules for assigning oxidation states include: elements in their standard states have oxidation state 0; oxygen is usually −2 (except in peroxides where it is −1, and in OF₂ where it is +2); hydrogen is +1 (except in metal hydrides where it is −1); Group 1 metals are +1, Group 2 are +2; the sum of oxidation states in a neutral compound is 0; the sum in a polyatomic ion equals the ion charge.
氧化还原过程通过电子转移来统一。氧化即失电子，还原即得电子——'OIL RIG'助记法仍然非常有用。更正式地说，氧化涉及氧化态的升高，还原涉及氧化态的降低。氧化态（或氧化数）是一个假设性的电荷，即如果某原子与不同元素原子之间的所有键都是完全离子性的，该原子将带有的电荷。分配氧化态的规则包括：标准状态下的元素氧化态为0；氧通常为−2（过氧化物中为−1，OF₂中为+2除外）；氢为+1（金属氢化物中为−1除外）；第1族金属为+1，第2族为+2；中性化合物中氧化态之和为0；多原子离子中的和等于离子电荷。
Half-equations separate the oxidation and reduction processes. In acidic solution, H⁺ and H₂O are used to balance oxygen and hydrogen atoms; electrons are added to balance charge. Combining half-equations yields the full redox equation, with electrons cancelling. Disproportionation is a special redox reaction where a single element undergoes both oxidation and reduction simultaneously; for example, in the reaction of chlorine with cold dilute sodium hydroxide, Cl₂ is both oxidised to ClO⁻ (oxidation state +1) and reduced to Cl⁻ (oxidation state −1).
半反应式将氧化和还原过程分开表示。在酸性溶液中，H⁺和H₂O用于平衡氧和氢原子；加入电子以平衡电荷。将两个半反应式合并得到完整的氧化还原方程式，电子相互抵消。歧化反应是一种特殊的氧化还原反应，其中单一元素同时发生氧化和还原；例如，在氯气与冷的稀氢氧化钠反应中，Cl₂既被氧化为ClO⁻（氧化态+1），又被还原为Cl⁻（氧化态−1）。
8. Organic Chemistry Fundamentals & Alkanes | 有机化学基础与烷烃
Organic chemistry is the study of carbon-based compounds. The ability of carbon to form four covalent bonds and catenate (bond to itself) produces an enormous diversity of structures. In AQA Year 12, nomenclature follows IUPAC rules: identify the longest continuous carbon chain to determine the root name (meth-, eth-, prop-, but-, pent-, hex-); identify the functional group to assign the suffix or prefix; number the chain to give substituents the lowest possible locants. Structural isomerism arises when molecules share the same molecular formula but differ in the arrangement of atoms: chain isomers have different carbon skeletons; position isomers have functional groups at different positions; functional group isomers have different functional groups altogether (e.g. ethanol and methoxymethane, both C₂H₆O).
有机化学是研究碳基化合物的学科。碳能够形成四个共价键并发生自联（与自身键合），产生了极其多样的结构。在AQA Year 12中，命名遵循IUPAC规则：识别最长的连续碳链以确定词根名称（甲-、乙-、丙-、丁-、戊-、己-）；识别官能团以分配后缀或前缀；给碳链编号，使取代基获得尽可能小的位次。当分子具有相同的分子式但原子排列不同时，就产生了结构异构现象：碳链异构体具有不同的碳骨架；位置异构体的官能团位于不同位置；官能团异构体则具有完全不同的官能团（例如乙醇和甲氧基甲烷，均为C₂H₆O）。
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Their primary reaction is combustion: complete combustion in excess oxygen yields CO₂ and H₂O, releasing large amounts of energy, making alkanes valuable fuels. Incomplete combustion produces CO (toxic) or C (soot). Alkanes also undergo radical substitution with halogens, initiated by ultraviolet light. The mechanism proceeds in three stages: initiation (homolytic fission of the halogen molecule by UV light produces two halogen radicals), propagation (a halogen radical abstracts a hydrogen from the alkane, forming HX and an alkyl radical; the alkyl radical then reacts with a halogen molecule, regenerating the halogen radical), and termination (two radicals combine to form a stable molecule). This is a chain reaction, and the product is typically a mixture of mono- and poly-substituted haloalkanes. Students must be able to write equations for each step using displayed formulae and curly arrows for single-electron movement.
烷烃是通式为CₙH₂ₙ₊₂的饱和烃。它们的主要反应是燃烧：在过量氧气中完全燃烧生成CO₂和H₂O，释放大量能量，使烷烃成为有价值的燃料。不完全燃烧产生CO（有毒）或C（碳烟）。烷烃还会在紫外光引发下与卤素发生自由基取代反应。该机理分三个阶段进行：引发（卤素分子在紫外光下发生均裂，产生两个卤素自由基）、增长（卤素自由基从烷烃中夺取一个氢原子，形成HX和一个烷基自由基；烷基自由基随后与卤素分子反应，再生卤素自由基）和终止（两个自由基结合形成稳定分子）。这是一个链式反应，产物通常是单取代和多取代卤代烷的混合物。学生必须能够使用结构式和单电子移动的弯箭头书写每一步的方程式。
9. Halogenoalkanes, Alkenes & Alcohols | 卤代烷、烯烃与醇
Halogenoalkanes contain a polar carbon-halogen bond, making the carbon atom electron-deficient (δ+) and susceptible to nucleophilic attack. Nucleophilic substitution is their characteristic reaction. The hydroxide ion (OH⁻), cyanide ion (CN⁻) and ammonia (NH₃) are key nucleophiles in the AQA specification. With aqueous NaOH, halogenoalkanes hydrolyse to form alcohols; this reaction proceeds faster with iodoalkanes than bromoalkanes, and faster with bromoalkanes than chloroalkanes, because the C−I bond is weaker and breaks more readily. With ethanolic KCN, nitriles are formed, extending the carbon chain by one atom — a synthetically useful transformation. With excess ethanolic ammonia, primary amines are produced. The mechanism involves the nucleophile attacking the δ+ carbon from the opposite side to the departing halide ion, inverting the configuration.
卤代烷含有极性的碳-卤键，使碳原子缺电子（δ+）并容易受到亲核攻击。亲核取代是其特征反应。在AQA课程规范中，氢氧根离子（OH⁻）、氰根离子（CN⁻）和氨（NH₃）是关键的亲核试剂。在水溶液中与NaOH反应时，卤代烷水解生成醇；该反应在碘代烷中比在溴代烷中进行得更快，在溴代烷中比在氯代烷中进行得更快，因为C−I键更弱，更容易断裂。在乙醇溶液中与KCN反应时，生成腈类，使碳链延长一个原子——这是一种在合成上有用的转化。与过量的乙醇氨溶液反应时，生成伯胺。机理涉及亲核试剂从离去卤离子对面一侧攻击δ+碳原子，导致构型反转。
bond. The double bond consists of a σ bond and a π bond; the π bond is formed by sideways overlap of p orbitals and is weaker than the σ bond, making alkenes far more reactive than alkanes. The characteristic reaction is electrophilic addition. With HBr, the alkene adds HBr across the double bond; with unsymmetrical alkenes, Markovnikov' tion intermediate (tertiary > secondary > primary). With Br₂, the bromine molecule is polarised as it approaches the electron-rich double bond, and addition yields a dibromoalkane; the decolorisation of bromine water serves as a test for unsaturation. With concentrated H₂SO₄, alkyl hydrogensulfates form, which can be hydrolysed to produce alcohols. Alkenes also undergo addition polymerisation, forming long-chain polymers such as poly(ethene) and poly(propene).
烯烃是含有至少一个C=C双键的不饱和烃。双键由一个σ键和一个π键组成；π键由p轨道的侧向
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