Lewis Acid–Base Theory: Key Concepts for IB Chemistry HL | 路易斯酸碱理论要点：IB化学HL精讲

📚 Lewis Acid–Base Theory: Key Concepts for IB Chemistry HL | 路易斯酸碱理论要点：IB化学HL精讲

The Lewis theory of acids and bases, proposed by Gilbert N. Lewis in 1923, offers a broader and more general framework than the Arrhenius and Brønsted–Lowry models. Instead of focusing on protons or hydroxide ions, this theory centres on the transfer of electron pairs, making it essential for understanding a vast range of chemical reactions, including those in non-aqueous solvents and coordination chemistry.

路易斯酸碱理论由吉尔伯特·N·路易斯于1923年提出，比阿伦尼乌斯理论和布朗斯特-洛瑞理论更为广泛和普适。该理论不再聚焦于质子或氢氧根离子，而是以电子对的转移为核心，使其成为理解非水溶剂中反应和配位化学等广泛领域的关键工具。

1. The Lewis Definition | 路易斯定义

A Lewis acid is defined as any species that can accept an electron pair. A Lewis base is any species that can donate an electron pair. The product formed when a Lewis acid and a Lewis base react is called a Lewis adduct or Lewis acid–base complex.

路易斯酸定义为任何能够接受电子对的物种；路易斯碱定义为任何能够给出电子对的物种。路易斯酸与路易斯碱反应生成的产物称为路易斯加合物或路易斯酸碱配合物。

In the reaction between BF₃ and NH₃, we can identify the electron-pair acceptor and donor clearly:

在BF₃与NH₃的反应中，我们可以清晰地识别电子对的受体与给体：

BF₃ + NH₃ → F₃B←NH₃

Here, the boron atom in BF₃ has only six valence electrons and therefore lacks a full octet. It accepts the lone pair from nitrogen. BF₃ is the Lewis acid; NH₃ is the Lewis base.

在这里，BF₃中的硼原子只有六个价电子，未达到八隅体结构，因此它接受来自氮的孤对电子。BF₃是路易斯酸，NH₃是路易斯碱。

Key point: the arrow in the adduct F₃B←NH₃ points from the donor (N) towards the acceptor (B), from the Lewis base to the Lewis acid.

要点：加合物F₃B←NH₃中的箭头从给体（N）指向受体（B），即从路易斯碱指向路易斯酸。

2. Comparing Lewis with Brønsted–Lowry | 路易斯理论与布朗斯特-洛瑞理论的比较

All Brønsted–Lowry acids are Lewis acids, because a proton (H⁺) is an electron-pair acceptor. However, many Lewis acids are not Brønsted–Lowry acids — the latter model is a subset of the former. Similarly, all Brønsted–Lowry bases are Lewis bases, because any species that accepts a proton must donate an electron pair to form the H–B bond.

所有布朗斯特-洛瑞酸都是路易斯酸，因为质子（H⁺）是电子对受体。然而，许多路易斯酸并不是布朗斯特-洛瑞酸——后者只是前者的一个子集。同样地，所有布朗斯特-洛瑞碱都是路易斯碱，因为任何能够接受质子的物种必定要给出电子对来形成H–B键。

Arrhenius model: limited to aqueous solutions; acid produces H⁺, base produces OH⁻.

阿伦尼乌斯模型：仅限于水溶液；酸产生H⁺，碱产生OH⁻。
Brønsted–Lowry model: applies to proton transfer in any solvent.

布朗斯特-洛瑞模型：适用于任何溶剂中的质子转移。
Lewis model: applies to any electron-pair transfer, including reactions with no proton involvement at all.

路易斯模型：适用于任何电子对转移，包括完全不涉及质子的反应。

Consider the reaction between CaO and SO₃ to form calcium sulfate:

考虑CaO与SO₃反应生成硫酸钙：

CaO + SO₃ → CaSO₄

Here, O²⁻ donates an electron pair to S⁶⁺. No proton transfer occurs. This reaction would be difficult to classify under Brønsted–Lowry theory but is straightforward under Lewis theory.

此反应中O²⁻向S⁶⁺给出电子对，没有质子转移发生。用布朗斯特-洛瑞理论难以分类，但用路易斯理论则一目了然。

3. Identifying Lewis Acids and Bases | 识别路易斯酸与碱

To identify Lewis acids and bases efficiently, students should look for common structural indicators.

为了高效识别路易斯酸与碱，学生应寻找常见的结构特征。

Lewis acids include: species with incomplete octets (BF₃, AlCl₃, FeCl₃), positively charged ions or electron-deficient cations (H⁺, Ag⁺, Cu²⁺, Fe³⁺), and molecules with polar multiple bonds where the central atom is electron-deficient (CO₂, SO₂, SO₃).

路易斯酸包括：具有不完整八隅体的物种（BF₃、AlCl₃、FeCl₃）、带正电荷的离子或缺电子阳离子（H⁺、Ag⁺、Cu²⁺、Fe³⁺），以及中心原子缺电子的极性多重键分子（CO₂、SO₂、SO₃）。

Lewis bases include: all anions (Cl⁻, OH⁻, CN⁻, F⁻), molecules with lone pairs (NH₃, H₂O, CO, R–OH), and species with pi-electron systems that can donate electron density (C₆H₆, alkenes).

路易斯碱包括：所有阴离子（Cl⁻、OH⁻、CN⁻、F⁻）、含孤对电子的分子（NH₃、H₂O、CO、R–OH），以及能够提供电子密度的π电子体系物种（C₆H₆、烯烃）。

For CO₂, the carbon atom is bonded to two highly electronegative oxygen atoms. The electron density around carbon is heavily withdrawn, exposing a positive region that can accept electron pairs from Lewis bases such as OH⁻.

对于CO₂，碳原子与两个高电负性的氧原子成键，碳周围的电子密度被严重拉走，暴露出一个正电区域，可以接受来自OH⁻等路易斯碱的电子对。

4. The Lewis Adduct and Dative Covalent Bonds | 路易斯加合物与配位共价键

The bond formed between a Lewis acid and a Lewis base is called a coordinate covalent bond or dative covalent bond. In such a bond, both shared electrons originate from the same atom — the Lewis base. Once formed, there is no distinction between a dative bond and an ordinary covalent bond in terms of bond strength or length.

路易斯酸与路易斯碱之间形成的键称为配位共价键。在这类键中，两个共享电子均来自同一原子——即路易斯碱。一旦形成，配位键与普通共价键在键能和键长上并无区别。

A typical example is the reaction between ammonia and boron trifluoride:

典型的例子是氨与三氟化硼的反应：

NH₃ + BF₃ → H₃N→BF₃

In H₃N→BF₃, the N–B bond is formed using the lone pair on nitrogen. The arrow in the structural formula points towards the acceptor atom.

在H₃N→BF₃中，N–B键由氮上的孤对电子形成。结构式中的箭头指向受体原子。

Another familiar example is the formation of the hydroxonium ion (H₃O⁺). Water donates one lone pair to a proton, forming a dative bond. Similarly, when ammonia accepts a proton, the ammonium ion (NH₄⁺) is formed with one dative bond.

另一个熟悉的例子是水合氢离子（H₃O⁺）的形成。水向质子给出一个孤对电子，形成配位键。类似地，氨接受质子时，铵离子（NH₄⁺）形成并含有一个配位键。

In coordination complexes such as [Cu(H₂O)₆]²⁺, each water ligand donates an electron pair to the Cu²⁺ ion. The Cu²⁺ ion acts as a Lewis acid and each H₂O acts as a Lewis base. This framework is central to transition metal chemistry.

在配位配合物如[Cu(H₂O)₆]²⁺中，每个水配体向Cu²⁺离子给出一个电子对。Cu²⁺离子作为路易斯酸，每个H₂O作为路易斯碱。这一框架是过渡金属化学的核心。

5. HSAB Principle: Hard and Soft Acids and Bases | 硬软酸碱（HSAB）原理

The HSAB principle, introduced by Ralph Pearson, classifies Lewis acids and bases as ‘hard’ or ‘soft’ based on their size, charge density and polarisability. Hard species are small, highly charged and not very polarisable. Soft species are large, low-charge and highly polarisable.

HSAB原理由拉尔夫·皮尔逊提出，根据体积大小、电荷密度和极化性将路易斯酸与碱分为”硬”和”软”两类。硬物种体积小、电荷高、不易极化；软物种体积大、电荷低、高度可极化。

The key rule is: hard acids prefer to bind with hard bases, and soft acids prefer to bind with soft bases. This preference is governed primarily by electrostatic interactions for hard species and by covalent interactions for soft species.

核心规则是：硬酸偏好与硬碱结合，软酸偏好与软碱结合。这种偏好对于硬物种主要由静电作用主导，对于软物种主要由共价作用主导。

Hard Acids \| 硬酸	H⁺, Li⁺, Na⁺, Mg²⁺, Ca²⁺, Al³⁺, Fe³⁺
Soft Acids \| 软酸	Ag⁺, Cu⁺, Hg²⁺, Pt²⁺, Au⁺
Hard Bases \| 硬碱	F⁻, OH⁻, H₂O, NH₃, CO₃²⁻, NO₃⁻
Soft Bases \| 软碱	I⁻, CN⁻, S²⁻, CO, P(CH₃)₃, SCN⁻

This principle explains many observations in chemistry. For example, Ag⁺ (a soft acid) forms a stable complex with I⁻ (a soft base) rather than with F⁻. In medicine, the toxicity of heavy-metal ions such as Hg²⁺ is related to their affinity for soft sulphur-containing sites in proteins.

该原理解释了化学中的许多现象。例如，Ag⁺（软酸）与I⁻（软碱）形成稳定配合物而非与F⁻结合。在医学中，Hg²⁺等重金属离子的毒性与其对蛋白质中含硫软位点的亲和力有关。

6. Strengths of Lewis Acids and Bases | 路易斯酸碱的强度

The strength of a Lewis acid is determined by its ability to attract an electron pair. For example, the Lewis acidity of boron trihalides follows the order: BF₃ < BCl₃ < BBr₃. This may seem counterintuitive on the basis of electronegativity. The more electronegative fluorine should withdraw more electron density from boron, making BF₃ the most electron-deficient and thus the most acidic. However, in practice BF₃ is the weakest Lewis acid among the boron trihalides. This is because the halogen lone pairs can donate electron density into the empty p-orbital of boron, forming p–π back-bonding. Fluorine's 2p orbitals match well with boron's 2p orbital, providing the most effective stabilisation.

路易斯酸的强度由其吸引电子对的能力决定。例如，三卤化硼的路易斯酸性顺序为：BF₃ < BCl₃ < BBr₃。这看起来似乎与电负性直觉相悖。电负性更强的氟应该从硼上拉走更多电子密度，使BF₃最缺电子、酸性最强。然而实践中BF₃是三卤化硼中最弱的路易斯酸。原因是卤素的孤对电子可以向硼的空p轨道提供电子密度，形成p–π反馈键。氟的2p轨道与硼的2p轨道匹配良好，提供了最有效的稳定化作用。

For Lewis bases, gas-phase basicity indicates that NH₃ is a stronger Lewis base than PH₃ because nitrogen is more electronegative and holds its lone pair more tightly. However, in aqueous solution the trend can reverse due to solvation effects.

对于路易斯碱，气相碱性表明NH₃比PH₃是更强的路易斯碱，因为氮的电负性更高，能更紧密地持有孤对电子。然而在水溶液中，由于溶剂化效应，该趋势可能发生反转。

7. Applications of Lewis Acid–Base Theory | 路易斯酸碱理论的应用

Lewis acid–base theory is indispensable for understanding organic reaction mechanisms. In electrophilic addition to alkenes, the electrophile (E⁺) acts as a Lewis acid and the alkene pi-bond acts as a Lewis base. In nucleophilic substitution, the nucleophile is the Lewis base and the substrate carbon acts as the Lewis acid.

路易斯酸碱理论是理解有机反应机理不可或缺的工具。在烯烃的亲电加成中，亲电体（E⁺）作为路易斯酸，烯烃的π键作为路易斯碱。在亲核取代反应中，亲核体是路易斯碱，底物的碳是路易斯酸。

A common illustrative example is the reaction of aluminium chloride with an acyl chloride in Friedel–Crafts acylation:

一个常见的示例是Friedel-Crafts酰基化中氯化铝与酰氯的反应：

CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻

Here, AlCl₃ accepts a chloride ion (a Lewis base), generating the acylium ion CH₃CO⁺, which is a strong electrophile in aromatic substitution.

此反应中AlCl₃接受氯离子（路易斯碱），生成酰基正离子CH₃CO⁺，后者是芳香取代反应中的强亲电体。

Lewis acid–base theory also explains the catalytic role of transition metals in industrial processes. In heterogeneous catalysis, reactant molecules donate electron density to the metal surface, weakening their internal bonds and lowering the activation energy. In enzyme catalysis, metal ions in active sites act as Lewis acids to stabilise negatively charged transition states.

路易斯酸碱理论还解释了过渡金属在工业催化的作用。在多相催化中，反应物分子向金属表面提供电子密度，削弱其内部键合并降低活化能。在酶催化中，活性位点中的金属离子作为路易斯酸来稳定带负电荷的过渡态。

8. Common Pitfalls and Exam Tips | 常见误区与考试提示

Students often confuse the Lewis definition with the oxidation-state concept. Remember: Lewis acidity depends on electron-pair acceptance, not on oxidation state. For example, CO₂ has carbon in the +4 oxidation state, but it is a Lewis acid because the carbon can accept an electron pair from a base.

学生常将路易斯定义与氧化态概念混淆。记住：路易斯酸性取决于接受电子对的能力，而非氧化态。例如CO₂中碳为+4氧化态，但它是路易斯酸，因为碳可以接受来自碱的电子对。

Another common error is claiming that all metal cations are strong Lewis acids without considering charge density. A small, highly charged cation such as Al³⁺ is a stronger Lewis acid than a large, singly charged cation such as Na⁺.

另一个常见错误是认为所有金属阳离子都是强路易斯酸，而未考虑电荷密度。小体积高电荷的阳离子如Al³⁺比大体积单价阳离子Na⁺是更强的路易斯酸。

In exam questions, students may be asked to identify the Lewis acid and base in a given reaction. The safest strategy is to follow the electrons: locate the species that provides the lone pair and the species that accepts it.

在考题中，学生常被要求识别给定反应中的路易斯酸和碱。最稳妥的策略是追踪电子：找到提供孤对电子的物种和接受孤对电子的物种。

When drawing reaction mechanisms, always use a curved arrow. The arrow must originate from the lone pair of the Lewis base and point towards the atom of the Lewis acid that forms the new bond.

绘制反应机理时，务必使用弯箭头。箭头必须从路易斯碱的孤对电子出发，指向路易斯酸中形成新键的原子。

Finally, do not claim that H⁺ exists as a bare proton in aqueous solution. It is always hydrated to form H₃O⁺ or more complex species such as H₅O₂⁺.

最后，不要声称H⁺在水溶液中以裸质子形式存在。它总是水合形成H₃O⁺或更复杂的物种如H₅O₂⁺。

9. Worked Example | 例题精解

Consider the reaction: Ag⁺ + 2NH₃ → [Ag(NH₃)₂]⁺

考虑反应：Ag⁺ + 2NH₃ → [Ag(NH₃)₂]⁺

Identify the Lewis acid, the Lewis base and the dative bond.

请识别路易斯酸、路易斯碱和配位键。

Solution: Ag⁺ is the Lewis acid because it accepts electron pairs from two ammonia molecules. Each NH₃ is a Lewis base because nitrogen donates its lone pair to Ag⁺. Each Ag–N bond is a dative covalent bond.

解答：Ag⁺是路易斯酸，因为它接受来自两个氨分子的电子对。每个NH₃是路易斯碱，因为氮向Ag⁺给出孤对电子。每个Ag–N键都是配位共价键。

In this complex, silver has a coordination number of 2 with a linear geometry. The dative bonds are equivalent in strength and length to any ordinary covalent bond of the same type.

在该配合物中，银的配位数为2，呈直线型几何构型。两个配位键在强度和键长上与同类型的普通共价键完全等同。

10. Summary | 总结

The Lewis acid–base theory extends far beyond proton transfer, providing a unified framework for understanding acids and bases in both aqueous and non-aqueous systems, organic and inorganic chemistry, and catalysis. Mastering the ability to identify electron-pair donors and acceptors is essential for success in IB Chemistry HL.

路易斯酸碱理论远远超越了质子转移的范畴，为理解水溶液和非水体系、有机化学和无机化学以及催化反应中的酸碱行为提供了统一框架。掌握识别电子对给体和受体的能力是IB化学HL取得成功的关键。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Lewis Acid–Base Theory: Key Concepts for IB Chemistry HL | 路易斯酸碱理论要点：IB化学HL精讲