📚 Electron Sharing Reactions | 电子共享反应
In IB Chemistry, Reactivity 3.3 explores how atoms achieve stability by sharing electrons, forming covalent bonds. This process underpins the structure and reactivity of molecular substances, from simple diatomic molecules to complex organic compounds. Understanding electron sharing mechanisms—including coordinate bonding, homolytic and heterolytic fission, and radical chain reactions—is crucial for predicting reaction outcomes.
在IB化学中，反应性3.3探讨了原子如何通过共享电子形成共价键而达到稳定。这一过程奠定了分子物质从简单双原子分子到复杂有机化合物的结构和反应性基础。理解电子共享机制（包括配位键、均裂和异裂以及自由基链式反应）对于预测反应结果至关重要。
1. The Nature of Electron Sharing | 电子共享的本质
Atoms share electrons to achieve a full outer shell, typically an octet, by forming covalent bonds. Each shared pair of electrons constitutes a single covalent bond, and the shared electrons are attracted to the nuclei of both bonding atoms, lowering the overall potential energy of the system.
原子通过形成共价键共享电子以达到满外层（通常为八隅体）。每一对共享电子构成一个单共价键，共享的电子同时受到两个成键原子核的吸引，从而降低体系的总势能。
The Lewis model represents valence electrons as dots and uses shared pairs to illustrate bonding. For example, in a chlorine molecule, Cl₂, each chlorine atom contributes one electron to form a shared pair: Cl–Cl. Non‑bonding electron pairs (lone pairs) are also shown.
路易斯模型用点表示价电子，用共享电子对表示成键。例如，在氯分子 Cl₂ 中，每个氯原子提供一个电子形成一对共享电子：Cl–Cl。非键电子对（孤对电子）也会被标示出来。
2. Lewis Structures: Mapping Shared Pairs | 路易斯结构：描绘共享电子对
r triple) appear when atoms share more than one pair of electrons, as in O₂ (O=O) or N₂ (N≡N).
绘制准确的路易斯结构需要计算总价电子数、确定中心原子，并在可能的情况下排列原子以满足八隅规则。当原子共享超过一对电子时，就会形成多重键（双键或三键），如 O₂ (O=O) 或 N₂ (N≡N)。
Example: For the sulfate ion, SO₄²⁻, the Lewis structure shows six bonding pairs (four S– s in resonance) and lone pairs on oxygen. Formal charges help determine the most plausible distribution.
示例：对于硫酸根离子 SO₄²⁻，路易斯结构显示了六个成键对（四个 S–O 单键和两个 S=O 双键的共振）以及氧上的孤对电子。形式电荷有助于判断最合理的分布。
| Molecule / Ion | Valence electrons | Shared pairs | Lone pairs on central atom |
|---|---|---|---|
| CH₄ | 8 | 4 | 0 |
| NH₃ | 8 | 3 | 1 |
| H₂O | 8 | 2 | 2 |
3. Coordinate Covalent Bonds | 配位共价键
A coordinate (dative) covalent bond forms when both electrons in the shared pair originate from the same atom. This is common in reactions between Lewis bases (electron‑pair donors) and Lewis acids (electron‑pair acceptors).
配位共价键（又称配价键）形成时，共享电子对的两个电子均来自同一个原子。这类键常见于路易斯碱（电子对供体）与路易斯酸（电子对受体）之间的反应。
For instance, when ammonia reacts with a hydrogen ion, the nitrogen atom donates its lone pair to form the ammonium ion, NH₄⁺. Similarly, boron trifluoride, BF₃, accepts a lone pair from NH₃ to form the adduct F₃B–NH₃. In both cases, a new covalent bond is created exclusively from one partner's electrons.
例如，当氨与氢离子反应时，氮原子提供孤对电子形成铵离子 NH₄⁺。同样，三氟化硼 BF₃ 接受来自 NH₃ 的孤对电子生成加合物 F₃B–NH₃。在这两种情形下，新的共价键完全由一个反应伙伴的电子形成。
NH₃ + H⁺ → NH₄⁺ | BF₃ + :NH₃ → F₃B–NH₃
4. Unequal Sharing: Bond Polarity | 不均等共享：键的极性
When two atoms with different electronegativities share electrons, the electron density is unequally distributed. The more electronegative atom pulls the bonding pair closer, creating a polar covalent bond with a partial negative charge (δ⁻) and a partial positive charge (δ⁺) on the other atom.
当两个电负性不同的原子共享电子时，电子密度分布不均。电负性更强的原子将成键电子对拉得更近，形成极性共价键，一个原子带部分负电荷 (δ⁻)，另一个带部分正电荷 (δ⁺)。
Electron sharing in polar bonds still involves a shared pair, but the bond has a dipole moment. For example, in hydrogen chloride, HCl, the chlorine atom carries a δ⁻ because of its higher electronegativity. The extent of unequal sharing can be judged by the difference in electronegativity values; a difference greater than ~1.8 often indicates ionic character, while smaller differences represent polar covalent sharing.
极性键中的电子共享仍然涉及一对共享电子，但键具有偶极矩。例如，在氯化氢 HCl 中，氯原子因较高的电负性而带 δ⁻。不均匀共享的程度可通过电负性差值判断；差值大于约 1.8 通常意味着离子性，而较小的差值代表极性共价共享。
5. Homolytic Bond Fission | 均裂
Bonds can break either homolytically or heterolytically. Homolytic fission occurs when a covalent bond breaks symmetrically, with each atom retaining one electron from the shared pair. This process generates two neutral free radicals, each with an unpaired electron.
化学键可以通过均裂或异裂发生断裂。均裂是指共价键对称断裂，每个原子保留共享电子对中的一个电子。该过程产生两个中性自由基，各带一个未成对电子。
Homolytic fission is favoured for non‑polar bonds or under high‑energy conditions such as ultraviolet light or high temperatures. The resulting radicals are highly reactive due to their unpaired electrons and tend to seek out another electron to complete an octet, initiating chain reactions.
非极性键或在紫外光、高温等高能量条件下容易发生均裂。生成的自由基由于存在未成对电子而高度活泼，倾向于寻找另一个电子以完成八隅体，从而引发链式反应。
Cl–Cl → 2 Cl· (each chlorine atom takes one electron)
6. Heterolytic Bond Fission | 异裂
Heterolytic fission involves the unsymmetrical breaking of a covalent bond, where both electrons of the shared pair remain with one atom. This produces a cation and an anion. The atom that receives both electrons becomes a negatively charged nucleophile, while the other forms a positive electrophile.
异裂指共价键的不对称断裂，共享电子对的两个电子都留在一个原子上。这会产生一个阳离子和一个阴离子。得到两个电子的原子带负电成为亲核体，另一个原子则成为正电性的亲电体。
Polar bonds are more susceptible to heterolytic cleavage because the electron density is already shifted towards the more electronegative atom. For example, the H–Cl bond can break heterolytically to give H⁺ and Cl⁻. Understanding this concept is essential for predicting electrophiles and nucleophiles in organic reaction mechanisms.
极性键更容易发生异裂，因为电子密度已经偏向电负性较大的原子。例如，H–Cl 键可异裂生成 H⁺ 和 Cl⁻。理解这一概念对于预测有机反应机理中的亲电体和亲核体至关重要。
H–Cl → H⁺ + Cl⁻ (chlorine keeps both electrons)
7. Radical Substitution Reactions | 自由基取代反应
Cl. The mechanism proceeds through three stages: initiation, propagation, and termination.
自由基取代反应（如甲烷的氯化）是通过自由基中间体展现电子共享反应性的经典实例。总反应为：CH₄ + Cl₂ → CH₃Cl + HCl。其机理分为三个阶段：引发、增长和终止。
Initiation: Ultraviolet light provides energy to break a Cl–Cl bond homolytically, forming two chlorine radicals.
引发：紫外光提供能量使 Cl–Cl 键均裂，生成两个氯自由基。
Cl₂ → 2 Cl·
Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical. The methyl radical then reacts with a Cl₂ molecule, producing chloromethane and a new chlorine radical, which continues the chain.
增长：氯自由基从甲烷中夺取一个氢原子，生成 HCl 和甲基自由基。随后甲基自由基与 Cl₂ 分子反应，生成氯甲烷和一个新的氯自由基，从而继续链式反应。
Cl· + CH₄ → HCl + CH₃·
CH₃· + Cl₂ → CH₃Cl + Cl·
Termination: ble termination steps include Cl· + Cl· → Cl₂, CH₃· + CH₃· → C₂H₆, and CH₃· + Cl· → CH₃Cl.
终止：两个自由基结合形成稳定分子，中断循环。可能的终止步骤有 Cl· + Cl· → Cl₂、CH₃· + CH₃· → C₂H₆ 以及 CH₃· + Cl· → CH₃Cl。
8. Electrophilic Addition: A Sharing Story | 亲电加成：共享的故事
Electrophilic addition to alkenes illustrates how electron sharing is reorganised during a reaction. In the addition of HBr to ethene, the π‑bond electrons are shared between the two carbon atoms, but the double bond is electron‑rich and acts as a nucleophile. The mechanism involves heterolytic fission of H–Br and formation of a carbocation intermediate.
烯烃的亲电加成反应展示了反应中电子共享如何重新组织。在 HBr 与乙烯的加成反应中，π 键电子被两个碳原子共享，但双键是富电子的，起到亲核体的作用。其机理涉及 H–Br 的异裂和碳正离子中间体的形成。
Step 1: The π‑electrons attack the partially positive hydrogen, forming a C–H bond while the H–Br bond breaks heterolytically, yielding a carbocation and a bromide ion.
步骤1：π 电子进攻略带正电的氢，形成 C–H 键，同时 H–Br 键异裂，生成碳正离子和溴离子。
H₂C=CH₂ + H–Br → CH₃–CH₂⁺ + Br⁻
Step 2: The bromide ion donates a lone pair to the carbocation, forming a new C–Br bond. This final sharing step completes the addition product, bromoethane.
步骤2：溴离子将孤对电子提供给碳正离子，形成新的 C–Br 键。这个最终的共享步骤生成了加成产物溴乙烷。
9. Lewis Acids and Bases: Electron Pair Donors and Acceptors | 路易斯酸与碱：电子对供体与受体
A broader view of electron sharing includes reactions where a Lewis base (electron‑pair donor) forms a coordinate bond with a Lewis acid (electron‑pair acceptor). No protons need be exchanged; the reaction is purely about sharing a pair of electrons.
更广义的电子共享包括路易斯碱（电子对供体）与路易斯酸（电子对受体）之间形成配位键的反应。无需转移质子；该反应完全是关于共享一对电子。
Consider the reaction between ammonia and boron trifluoride. NH₃ has a lone pair; BF₃ has an incomplete octet and can accept a pair. The product, F₃B–NH₃, features a new covalent bond formed by electron sharing. Similarly, the formation of complex ions like [Cu(NH₃)₄]²⁺ involves multiple coordinate bonds from NH₃ ligands to the Cu²⁺ centre.
以氨与三氟化硼的反应为例：NH₃ 有孤对电子，BF₃ 八隅体不完整，可接受一对电子。产物 F₃B–NH₃ 中有一个通过电子共享形成的新共价键。类似地，络离子如 [Cu(NH₃)₄]²⁺ 的形成涉及多个来自 NH₃ 配体向 Cu²⁺ 中心提供的配位键。
This donor‑acceptor sharing is reversible, and the strength of the coordinate bond depends on the ability of the donor to share and the acceptor's affinity for electrons.
这种供受体共享是可逆的，配位键的强度取决于供体共享电子的能力以及受体对电子的亲和力。
10. Summary and Exam Tips | 总结与备考要点
Electron sharing reactions underpin covalent bonding, molecular shape, and a wide range of reaction mechanisms. Key distinctions include homolytic versus heterolytic fission, the role of radicals, and the concept of coordinate bonds as a special class of electron sharing.
电子共享反应是共价键、分子形状以及众多反应机理的基础。关键区分包括均裂与异裂、自由基的作用，以及配位键作为一类特殊电子共享的概念。
- Lewis structures: Always count valence electrons and check formal charges; use resonance where needed.
- 路易斯结构：始终计算价电子数并检查形式电荷；必要时使用共振结构。
- Fission: Homolytic fission gives two radicals (one‑electron movement); heterolytic fission gives ions (two‑electron movement).
- 键断裂：均裂产生两个自由基（单电子移动）；异裂产生离子（双电子移动）。
- Radical substitution: Remember initiation → propagation → termination, and use half‑curved arrows to show single‑electron transfer.
- 自由基取代：记住引发 → 增长 → 终止，并用半弯箭头表示单电子转移。
- Electrophilic addition: The π‑bond is the electron‑sharing region that attacks the electrophile.
- 亲电加成：π 键是电子共享区域，负责进攻亲电体。
- Coordination: Look for lone‑pair donors combining with electron‑deficient acceptors.
- 配位作用：寻找孤对电子供体与缺电子受体的结合。
Master these electron‑sharing patterns, and you will be well prepared to tackle structure and mechanism questions on the IB Chemistry examination.
掌握这些电子共享模式，你将能从容应对IB化学考试中关于结构和机理的试题。
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