IB Chemistry: Core Organic Chemistry Concepts | IB化学：有机化学核心考点梳理

📚 IB Chemistry: Core Organic Chemistry Concepts | IB化学：有机化学核心考点梳理

Organic chemistry is one of the most systematic and predictable areas of the IB Chemistry syllabus. Mastering a few key frameworks — functional groups, nomenclature, reaction mechanisms, and isomerism — can unlock the entire topic and dramatically improve your exam performance.

有机化学是IB化学课程中最具系统性和规律性的板块之一。掌握少数几个核心框架——官能团、命名、反应机理和异构现象——便能打通整个专题，显著提升你的考试成绩。

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1. Functional Groups: The Language of Organic Chemistry | 官能团：有机化学的语言

A functional group is an atom or group of atoms within a molecule that gives it characteristic chemical and physical properties. In IB Chemistry, you are expected to identify and name common functional groups quickly and confidently.

官能团是分子中对化学性质和物理性质起决定性作用的原子或原子团。IB化学要求你能够快速、准确地识别并命名常见官能团。

The table below summarises the key functional groups and their prefixes/suffixes used in IUPAC naming.

下表总结了主要官能团及其在IUPAC命名中的前缀和后缀。

Functional Group | 官能团	General Formula | 通式	Suffix / Prefix | 后缀 / 前缀	Example | 示例
Alkene | 烯烃	CₙH₂ₙ	-ene	Ethene | 乙烯
Alcohol | 醇	R-OH	-ol	Ethanol | 乙醇
Haloalkane | 卤代烃	R-X (X = F, Cl, Br, I)	fluoro-, chloro-, bromo-, iodo-	Chloroethane | 氯乙烷
Aldehyde | 醛	R-CHO	-al	Ethanal | 乙醛
Ketone | 酮	R-CO-R’	-one	Propanone | 丙酮
Carboxylic acid | 羧酸	R-COOH	-oic acid	Ethanoic acid | 乙酸
Ester | 酯	R-COO-R’	-yl -oate	Ethyl ethanoate | 乙酸乙酯
Amine | 胺	R-NH₂	-amine	Ethylamine | 乙胺
Amide | 酰胺	R-CONH₂	-amide	Ethanamide | 乙酰胺
Nitrile | 腈	R-C≡N	-nitrile	Ethanenitrile | 乙腈

Remember that the order of priority for the principal functional group in naming is: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > haloalkane.

请牢记命名时主官能团的优先顺序：羧酸 > 酯 > 酰胺 > 腈 > 醛 > 酮 > 醇 > 胺 > 烯烃 > 卤代烃。

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2. Nomenclature: From Alkane to Complex Molecules | 命名：从烷烃到复杂分子

IUPAC nomenclature follows a logical step-by-step procedure. The key is to always choose the longest continuous carbon chain containing the principal functional group, number it to give the lowest locants, and list substituents alphabetically.

IUPAC命名遵循一套逻辑清晰的步骤。关键在于：始终选择包含主官能团的最长碳链作为主链，从离主官能团最近的一端开始编号，并按字母顺序排列取代基。

• Step 1: Identify the longest chain containing the highest-priority functional group.

  步骤1：找出包含优先级最高官能团的最长碳链。

• Step 2: Number the chain so that the principal functional group gets the lowest possible locant.

  步骤2：给主链编号，使主官能团获得尽可能小的位次。

• Step 3: Name and locate substituents in alphabetical order; use di-, tri- for identical substituents (these prefixes are ignored when alphabetising).

  步骤3：按字母顺序命名并定位取代基；相同取代基使用二、三等前缀（字母排序时忽略这些前缀）。

• Step 4: Combine names, placing the principal functional group suffix at the end.

  步骤4：组合名称，将主官能团的后缀放在名称末尾。

For example, CH₃-CH₂-CH(CH₃)-CH₂-OH is 2-methylbutan-1-ol, not 3-methylbutan-4-ol. The OH group must have the lowest locant.

例如，CH₃-CH₂-CH(CH₃)-CH₂-OH 名为2-甲基丁-1-醇，而不是3-甲基丁-4-醇。羟基必须获得最小位次。

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3. Isomerism: Same Formula, Different Structure | 同分异构：相同分子式，不同结构

Isomerism is a favourite topic in IB exams because it tests conceptual understanding, not memorisation. There are two broad categories: structural isomerism and stereoisomerism.

同分异构是IB考试中的热门考点，因为它考查的是概念理解而非死记硬背。同分异构可分为两大类：构造异构和立体异构。

3.1 Structural Isomerism | 构造异构

Structural isomers have the same molecular formula but different connectivity of atoms. The three subtypes are chain, position, and functional group isomerism.

构造异构体具有相同的分子式，但原子连接方式不同。三种亚型分别是碳链异构、位置异构和官能团异构。

• Chain isomerism: different carbon skeleton; e.g. butane vs 2-methylpropane (C₄H₁₀).

  碳链异构：碳骨架不同；例如丁烷与2-甲基丙烷（C₄H₁₀）。

• Position isomerism: same skeleton, functional group at different positions; e.g. butan-1-ol vs butan-2-ol.

  位置异构：碳骨架相同，官能团位置不同；例如丁-1-醇与丁-2-醇。

• Functional group isomerism: same formula, different functional group; e.g. propanal vs propanone (C₃H₆O).

  官能团异构：分子式相同，官能团不同；例如丙醛与丙酮（C₃H₆O）。

3.2 Stereoisomerism | 立体异构

Stereoisomers have the same connectivity but different spatial arrangement. The two key types for IB are cis-trans isomerism and optical isomerism.

立体异构体的连接方式相同，但空间排布不同。IB涉及的两种关键类型是顺反异构和光学异构。

Cis-trans (geometric) isomerism occurs in alkenes when each carbon of the C=C has two different substituents. Cis means the same side; trans means opposite sides. This restricts rotation around the double bond.

顺反（几何）异构发生在烯烃中，当C=C双键的每个碳上各连有两个不同取代基时。顺式指同侧，反式指对侧。双键限制了旋转。

Optical isomerism occurs when four different groups are attached to a single carbon (chiral centre). The two non-superimposable mirror images are called enantiomers. They rotate plane-polarised light in opposite directions.

光学异构发生在一个碳原子连接四个不同基团（手性碳）时。两个不能重叠的镜像称为对映异构体。它们使平面偏振光向相反方向旋转。

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4. Alkanes and Free Radical Substitution | 烷烃与自由基取代反应

Alkanes are saturated hydrocarbons with only C-C and C-H sigma bonds. They are generally unreactive, but in the presence of ultraviolet light, they undergo free radical substitution with halogens.

烷烃是仅含C-C和C-H σ键的饱和烃。它们通常不活泼，但在紫外线照射下可与卤素发生自由基取代反应。

The chlorination of methane proceeds via a three-step free radical mechanism:

甲烷的氯化反应通过三步自由基机理进行：

• Initiation: Cl₂ → 2Cl• (UV light). Both chlorine atoms receive one electron from the homolytic bond fission.

  链引发：Cl₂ → 2Cl•（紫外光）。两个氯原子各获得一个电子，发生均裂。

• Propagation: Cl• + CH₄ → HCl + •CH₃; then •CH₃ + Cl₂ → CH₃Cl + Cl•. The radical is regenerated, perpetuating the chain.

  链增长：Cl• + CH₄ → HCl + •CH₃；然后•CH₃ + Cl₂ → CH₃Cl + Cl•。自由基再生，链反应持续进行。

• Termination: two radicals combine, e.g. Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆.

  链终止：两个自由基结合，例如 Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl，或•CH₃ + •CH₃ → C₂H₆。

A common exam question asks why a mixture of products is formed. The answer is that substitution can continue until all hydrogens are replaced, producing CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄.

一个常见考题是问为何会生成混合物。答案是取代反应可继续进行直到所有氢被替换，生成CH₃Cl、CH₂Cl₂、CHCl₃和CCl₄。

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5. Alkenes and Addition Reactions | 烯烃与加成反应

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond. The double bond consists of one sigma bond and one pi bond, making the molecule electron-rich and prone to electrophilic addition.

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键构成，使分子电子云密度高，容易发生亲电加成反应。

The pi bond is weaker than the sigma bond and acts as a source of electrons. In electrophilic addition, the electrophile is attracted to the electron-rich double bond.

π键弱于σ键，可作为电子源。在亲电加成中，亲电试剂被电子云密度高的双键吸引。

Key addition reactions of alkenes include:

烯烃的关键加成反应包括：

• Hydrogenation (with H₂ and Ni/Pd catalyst) → alkane. This is used in the food industry to convert unsaturated fats into saturated fats.

  加氢反应（H₂，Ni/Pd催化）→ 烷烃。食品工业中用于将不饱和脂肪转化为饱和脂肪。

• Halogenation (with Br₂ or Cl₂) → dihaloalkane. Bromine water decolourises, providing a test for unsaturation.

  卤素加成（Br₂或Cl₂）→ 二卤代烷。溴水褪色，可用于检验不饱和键。

• Hydrohalogenation (with HBr or HCl) → haloalkane. Follows Markovnikov’s rule.

  氢卤酸加成（HBr或HCl）→ 卤代烷。遵循马氏规则。

• Hydration (with steam and H₃PO₄ catalyst) → alcohol.

  水合反应（水蒸气，H₃PO₄催化）→ 醇。

Markovnikov’s rule states that when a protic acid HX adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached. The more stable carbocation intermediate forms preferentially.

马氏规则指出，当质子酸HX加合到不对称烯烃上时，氢原子加到原本连有更多氢原子的碳上。这是因为会优先生成更稳定的碳正离子中间体。

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6. Alcohols: Reactions and Oxidation | 醇：反应与氧化

Alcohols are classified as primary, secondary, or tertiary depending on how many carbon atoms are attached to the carbon bearing the OH group.

醇根据羟基所连碳原子连接的碳原子数分为伯醇、仲醇和叔醇。

Oxidation of alcohols is a core reaction. The oxidising agent commonly used is acidified potassium dichromate(VI), K₂Cr₂O₇ in H₂SO₄, which changes from orange to green as Cr(VI) is reduced to Cr(III).

醇的氧化是核心反应。常用氧化剂是酸化的重铬酸钾，即K₂Cr₂O₇与H₂SO₄的混合溶液，随着Cr(VI)被还原为Cr(III)，溶液由橙色变为绿色。

• Primary alcohols oxidise to aldehydes, and with further oxidation, to carboxylic acids.

  伯醇氧化生成醛，进一步氧化生成羧酸。

• Secondary alcohols oxidise to ketones only.

  仲醇只能氧化生成酮。

• Tertiary alcohols do not undergo oxidation under these conditions.

  叔醇在上述条件下不能被氧化。

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Elimination of water from alcohols (dehydration) produces alkenes. This requires concentrated H₂SO₄ or Al₂O₃ and heat. For example, ethanol produces ethene.

醇的脱水（消除反应）生成烯烃。该反应需要浓H₂SO₄或Al₂O₃并加热。例如，乙醇生成乙烯。

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7. Haloalkanes and Nucleophilic Substitution | 卤代烃与亲核取代

Haloalkanes contain a polar C-X bond because halogen atoms are more electronegative than carbon. The carbon atom becomes electron-deficient (δ+) and is attacked by nucleophiles.

卤代烃含有极性C-X键，因为卤素的电负性大于碳。碳原子因而电子云密度较低（δ+），容易受到亲核试剂的进攻。

The rate of nucleophilic substitution depends on the halogen: R-I > R-Br > R-Cl > R-F. The C-I bond is weakest, so iodoalkanes react fastest.

亲核取代的速率取决于卤素：R-I > R-Br > R-Cl > R-F。C-I键最弱，因此碘代烷反应最快。

Important nucleophilic substitution reactions:

重要的亲核取代反应：

• With aqueous NaOH → alcohol (OH⁻ substitutes X).

  与NaOH水溶液反应 → 醇（OH⁻取代X）。

• With alcoholic KOH → alkene (elimination, not substitution).

  与KOH醇溶液反应 → 烯烃（发生消除反应，而非取代）。

• With NH₃ → amine.

  与NH₃反应 → 胺。

• With KCN → nitrile. This extends the carbon chain by one carbon, a valuable synthetic tool.

  与KCN反应 → 腈。碳链增长一个碳，是重要的合成手段。

Two mechanisms exist: Sₙ1 (unimolecular, two-step, favoured by tertiary haloalkanes) and Sₙ2 (bimolecular, one-step, favoured by primary haloalkanes). Sₙ2 involves a backside attack and causes inversion of configuration.

亲核取代有两种机理：Sₙ1（单分子，两步反应，叔卤代烃占优势）和Sₙ2（双分子，一步反应，伯卤代烃占优势）。Sₙ2涉及背面进攻，导致构型翻转。

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8. Carboxylic Acids and Esters | 羧酸与酯

Carboxylic acids contain the -COOH group. They are weak acids that partially dissociate in water: RCOOH ⇌ RCOO⁻ + H⁺. Their acidity arises from the stability of the carboxylate anion due to resonance.

羧酸含有-COOH基团。它们是弱酸，在水中部分电离：RCOOH ⇌ RCOO⁻ + H⁺。其酸性源于羧酸根阴离子因共振而具有的稳定性。

Carboxylic acids react with bases, carbonates and metals to form salts. They also react with alcohols in the presence of concentrated H₂SO₄ to form esters and water, in a reversible reaction called esterification.

羧酸与碱、碳酸盐和活泼金属反应生成盐。在浓H₂SO₄催化下，羧酸与醇反应生成酯和水，这个可逆反应称为酯化反应。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Esters are characterised by a sweet, fruity smell and are used in flavourings and perfumes. The naming of esters uses the alkyl group from the alcohol first, then the -oate suffix derived from the carboxylic acid.

酯的特点是具有甜味、果香味，常用于食品香精和香水中。酯的命名先写来自醇的烷基，再写来自羧酸的-oatE后缀。

For example, methanol + butanoic acid → methyl butanoate.

例如，甲醇 + 丁酸 → 丁酸甲酯。

Esters can be hydrolysed back to carboxylic acid and alcohol by heating with dilute acid or base. Base hydrolysis (saponification) is irreversible and produces a carboxylate salt.

酯可通过与稀酸或稀碱加热水解回羧酸和醇。碱性水解（皂化反应）不可逆，生成羧酸盐。

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9. Amines and Amides | 胺与酰胺

Amines are organic bases. They contain nitrogen with a lone pair of electrons, which can accept a proton. Primary amines have the general formula RNH₂.

胺是有机碱。它们含有带孤对电子的氮原子，可以接受质子。伯胺的通式为RNH₂。

Amines are weaker bases than hydroxide ions but stronger bases than water. They react with acids to form ammonium salts, e.g. CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻.

胺的碱性弱于氢氧根离子，但强于水。它们与酸反应生成铵盐，例如 CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻。

Amides are derived from carboxylic acids by replacing the -OH with -NH₂. They are neutral and can be prepared by reacting acyl chlorides with ammonia or amines. The amide bond (-CO-NH-) is the key linkage in proteins.

酰胺由羧酸中的-OH被-NH₂取代而衍生。它们呈中性，可通过酰氯与氨或胺反应制备。酰胺键（-CO-NH-）是蛋白质中的关键连接。

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10. Reaction Pathways and Synthetic Design | 反应路线与合成设计

IB Chemistry requires you to design multi-step syntheses by working backwards from the target molecule. You should know which reagents convert one functional group into another.

IB化学要求你通过从目标分子逆推来设计多步合成路线。你需要知道哪些试剂能将一种官能团转化为另一种官能团。

Common interconversions include:

常见的官能团转化包括：

• Alkene → alkane (H₂/Ni), alcohol (H₂O/H₃PO₄), haloalkane (HX).

  烯烃 → 烷烃（H₂/Ni）、醇（H₂O/H₃PO₄）、卤代烃（HX）。

• Alcohol → alkene (conc. H₂SO₄), haloalkane (HX or PCl₃), aldehyde/carboxylic acid (K₂Cr₂O₇/H⁺).

  醇 → 烯烃（浓H₂SO₄）、卤代烃（HX或PCl₃）、醛/羧酸（K₂Cr₂O₇/H⁺）。

• Haloalkane → alcohol (NaOH/H₂O), nitrile (KCN, extends chain), amine (NH₃).

  卤代烃 → 醇（NaOH/H₂O）、腈（KCN，增长碳链）、胺（NH₃）。

• Nitrile → carboxylic acid (acid hydrolysis) or primary amine (reduction).

  腈 → 羧酸（酸性水解）或伯胺（还原）。

A typical exam question: starting from ethene, synthesise ethyl ethanoate. The route would be: ethene → ethanol (hydration) → ethanoic acid (oxidation) → ethyl ethanoate (esterification with ethanol).

一个典型的考题：以乙烯为原料合成乙酸乙酯。路线为：乙烯 → 乙醇（水合）→ 乙酸（氧化）→ 乙酸乙酯（与乙醇酯化）。

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11. Spectroscopic Identification | 波谱鉴定

Organic structure determination in IB Chemistry relies on mass spectrometry, infrared (IR) spectroscopy, and \(^1\)H NMR spectroscopy. Note: no LaTeX is used here — chemical shift values are written in plain text.

IB化学中的有机结构测定依赖于质谱、红外光谱和质子核磁共振氢谱。注意：此处不使用LaTeX，化学位移值以纯文本书写。

Mass spectrometry gives the molecular ion peak (M⁺) for molar mass, and fragmentation peaks for structural clues. IR spectroscopy identifies functional groups by characteristic absorption bands.

质谱提供分子离子峰（M⁺），用于确定摩尔质量；碎片峰则提供结构线索。红外光谱通过特征吸收带识别官能团。

Functional Group | 官能团	IR Absorption (cm⁻¹) | 红外吸收
O-H (alcohol) | 醇羟基	3200–3600 (broad) | 3200–3600（宽峰）
O-H (carboxylic acid) | 羧酸羟基	2500–3300 (very broad) | 2500–3300（很宽）
C=O (carbonyl) | 羰基	1630–1780 | 1630–1780
C-O (ester/alcohol) | C-O键	1000–1300 | 1000–1300
C=CH (alkene) | 烯烃碳氢	3100–3150 | 3100–3150

In ¹H NMR, the number of signals indicates the number of chemically distinct proton environments. The relative area under each peak gives the ratio of hydrogens. Splitting patterns (n+1 rule) reveal the number of neighbouring protons.

在¹H NMR中，信号数表示化学环境不同的质子种类数。峰面积比表示各类氢原子的数目比。裂分模式（n+1规则）揭示相邻质子的数目。

For example, ethanol (CH₃CH₂OH) shows three NMR signals: CH₃ (triplet), CH₂ (quartet), and OH (singlet), with area ratio 3:2:1.

例如，乙醇（CH₃CH₂OH）显示三组NMR信号：CH₃（三重峰）、CH₂（四重峰）和OH（单峰），面积比为3:2:1。

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12. Exam Tips and Common Pitfalls | 考试技巧与常见误区

Many students lose marks not because they do not understand the chemistry, but because they make avoidable errors. Here are the most frequent pitfalls and how to avoid them.

许多学生失分不是因为不理解化学，而是因为犯了可以避免的错误。以下是最常见的陷阱及应对方法。

• Always check the priority of functional groups when naming. Carboxylic acids always take priority over alcohols and alkenes.

  命名时务必检查官能团的优先顺序。羧酸的优先级永远高于醇和烯烃。

• Do not forget the locant for the principal functional group, even if it is on carbon 1.

  不要忘记标出主官能团的位次，即使它在1号碳上。

• When drawing mechanisms, show all curly arrows accurately. An arrow must start from a lone pair or a bond, never from an atom.

  画机理时，务必准确标出所有弯箭头。箭头必须从孤对电子或化学键出发，绝不是从原子出发。

• Remember that primary alcohols can oxidise twice (to aldehyde and then carboxylic acid), while secondary alcohols stop at ketone.

  记住伯醇可被氧化两次（先醛后羧酸），仲醇则止步于酮。

• In free radical substitution, write “UV light” over the arrow and show homolytic fission clearly.

  在自由基取代中，务必在箭头上写“紫外光”，并清楚画出均裂过程。

• Distinguish between aqueous NaOH (substitution) and alcoholic KOH (elimination) for haloalkanes.

  区分卤代烃与NaOH水溶液（取代）和KOH醇溶液（消除）的反应。

• For NMR questions, always state the number of environments, relative areas, and splitting pattern for full marks.

  对于NMR问题，要拿满分必须说明信号数、相对面积和裂分模式。

Finally, practise drawing full displayed formulae and skeletal formulae under timed conditions. Accuracy in structure drawing is essential for earning marks in mechanism and synthesis questions.

最后，请在计时条件下练习书写完整的结构式和键线式。结构绘制的准确性对于机理题和合成题得分至关重要。

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