Edexcel化学 有机化学 烷烃 烯烃 同系物
Edexcel IGCSE 化学的有机化学部分词多、结构密，看上去像要背下几十个名字和方程式。把线索理一遍就会发现，考纲只围绕三件事转：碳原子怎样连成链、官能团决定什么性质、每一类物质有哪些典型反应。剩下的命名、结构式、反应类型题，都是从这三件事推出来的。
Organic chemistry in Edexcel IGCSE Chemistry looks crowded at first, with long lists of names, formulas and reactions. The syllabus actually tests a small set of ideas. Carbon forms four bonds, the functional group decides how a molecule behaves, and each family of compounds reacts in a predictable way. Naming questions, displayed formulas and reaction-type questions all follow from those three ideas.
同系物与官能团：把上万种有机物归成几类
同系物是一组化学性质相似、分子式相差整数个 CH2 的有机物，它们共用同一个通式。烷烃是 CnH2n+2，烯烃是 CnH2n，醇是 CnH2n+1OH，羧酸是 CnH2COOH。判断两种物质算不算同系物，先看官能团是否相同，再看碳原子数是不是只差整数个 CH2。很多学生以为分子式相差 CH2 就成立，其实乙醇和乙烷碳数相邻，一个是醇一个是烷烃，官能团不同就不算同系物。丙烷 C3H8 与丁烷 C4H10 相差一个 CH2，属于同一系列；乙烯 C2H4 与丙烷 C3H8 碳数也只差一个，却不是同系物，差别就在官能团。官能团是决定化学性质的那一个原子团，碳链长短只影响沸点和密度这类物理量，所以认出官能团，反应类型基本就定了：有双键就走加成，有羟基就能酯化，有羧基就显酸性。Edexcel IGCSE Chemistry 4CH1 specification 要求写出前四个成员的分子式、结构式和显示式，命名用前缀表示碳数（meth、eth、prop、but），用后缀表示官能团（ane、ene、ol、oic acid）。
Homologous series are families of compounds that share one functional group and one general formula, each member differing from the next by a single CH2 unit; alkanes follow CnH2n+2, alkenes CnH2n, alcohols CnH2n+1OH and carboxylic acids CnH2COOH. To decide whether two compounds belong together, check the functional group first and the carbon count second, because ethanol and ethane sit next to each other in carbon number yet belong to different series. Physical properties drift along a series as well, with boiling point rising and density increasing as the chain grows, which is why methane and propane are handled differently in the laboratory. Propane C3H8 and butane C4H10 differ by a single CH2 and belong to the same series, while ethene C2H4 and propane C3H8 do not, because their functional groups differ. For the first four members you need the molecular formula, the displayed formula and a name built from a prefix (meth, eth, prop, but) plus a suffix (ane, ene, ol, oic acid).
烷烃：取代反应需要的条件和产物
烷烃来自原油的分馏，碳链越长沸点越高，所以分馏塔顶部出来的是小分子，底部出来的是大分子。汽油、煤油、柴油和沥青的差别就在于链长，链太长而用处又不大的部分要经过裂解变成小分子燃料。烷烃的化学性质不活泼，考纲只要求两个反应。充分燃烧生成二氧化碳和水，氧气不足时生成一氧化碳和碳黑，这两种产物经常出现在同一道题里。取代反应要把烷烃和氯气放在紫外光下，方程式写成 CH4 + Cl2 -> CH3Cl + HCl，产物是氯甲烷、二氯甲烷、三氯甲烷和四氯化碳的混合物。常见的错误是把它写成加成反应，或者漏掉光照条件，阅卷时这一类失分最多。甲烷完全燃烧的方程式是 CH4 + 2O2 -> CO2 + 2H2O，配平的关键在于两边的氧原子数要相等，写完之后数一遍氧原子能省下一分。一氧化碳有毒，所以用燃气热水器时必须通风，这类应用问题在资料分析题里出现过多次。
Alkanes come from the fractional distillation of crude oil, where boiling point rises with chain length, so short molecules leave at the top of the column and long ones leave at the bottom; petrol, kerosene, diesel and bitumen differ only in chain length, and very long chains are broken down by cracking to make useful fuels. Chemically alkanes are unreactive, so the specification asks for two reactions only: complete combustion giving carbon dioxide and water, and incomplete combustion giving carbon monoxide and soot. Substitution needs ultraviolet light, CH4 + Cl2 -> CH3Cl + HCl, and the product is a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane. Many students lose marks by calling this addition, or by leaving the light condition out of the equation. The balanced equation for complete combustion is CH4 + 2O2 -> CO2 + 2H2O, and counting oxygen atoms on both sides is the quickest check; carbon monoxide is toxic, so a gas heater always needs ventilation.
烯烃：加成反应与溴水的检验作用
烯烃含一个碳碳双键，通式 CnH2n，所以它们的反应几乎都是加成。检验方法只有一步：把气体通入溴水，橙色褪去说明有烯烃，烷烃通进去颜色不变。这一步在实验题里几乎年年出现，因为一次实验就把两个族分开了。加成反应中双键打开，两个碳上各接一个原子或基团，加氢得到烷烃，加水得到醇，加溴得到二溴乙烷。乙烯还能发生加聚，n 个乙烯分子连成聚乙烯，写成 n CH2=CH2 -> (CH2CH2)n。画重复单元时要保留两个碳原子，并且把双键写成打开的状态，把重复单元画成单体的答案很常见。显示式画的是每个原子的连接位置，结构式写成 CH2=CH2，分子式 C2H4 只说明组成，题目给出的多是显示式，判断依据就是有没有双键。举个数值例子：0.1 mol 乙烯完全加成需要 0.1 mol 溴，若溴水浓度为 0.02 mol/dm3，恰好反应完需要 5 dm3，这类换算在 Paper 2 的计算部分出现过。
Alkenes contain a carbon to carbon double bond and follow CnH2n, so nearly every reaction they take part in is addition. The test is a single step: bubble the gas through bromine water and the orange colour disappears, while an alkane leaves it untouched; this one experiment separates the two families and turns up in most exam papers. In an addition reaction the double bond opens and one atom or group attaches to each carbon, so hydrogen gives an alkane, water gives an alcohol and bromine gives a dibromo compound. Ethene also undergoes addition polymerisation, written as n CH2=CH2 -> (CH2CH2)n, and the repeating unit keeps two carbon atoms with the double bond shown as opened. A worked example: 0.1 mol of ethene needs 0.1 mol of bromine for complete addition, so a 0.02 mol/dm3 solution has to supply 5 dm3.
乙醇：发酵与乙烯加水两条路线
乙醇有两种制法。发酵用酵母，温度控制在 25 到 35 摄氏度之间，产物是乙醇和二氧化碳；温度过高酵母失去活性，反应停止，解释题常问这个原因。工业路线是乙烯加水，用磷酸做催化剂，温度约 300 摄氏度并加压。制得的乙醇可以燃烧，也可以被酸性高锰酸钾氧化成乙酸，氧化过程中溶液由紫色变成无色，这个颜色变化本身就是得分点。乙醇与钠反应还会放出氢气，说明羟基上的氢比较活泼。燃料用途上，乙醇燃烧更充分，但每克释放的能量比汽油低，这条比较常出现在资料分析题里。
Ethanol has two routes. Fermentation uses yeast at roughly 25 to 35 degrees Celsius and produces ethanol with carbon dioxide, and if the mixture overheats the yeast is killed and the reaction stops, which is a standard explain question; the industrial route adds water to ethene with a phosphoric acid catalyst at about 300 degrees Celsius and raised pressure. Ethanol burns in air, is oxidised to ethanoic acid by acidified potassium manganate(VII), and reacts with sodium to release hydrogen. Ethanol releases less energy per gram than petrol, and its oxidation shows a colour change from purple to colourless that examiners often ask for.
羧酸与酯：酯化反应的写法与命名
羧酸的通式是 CnH2COOH，乙酸是学生最早接触的一个。它们溶于水显酸性，与碳酸盐反应放出二氧化碳，用这一步可以把它和醇区分开，石蕊试纸则更适合选择题里的快速判断。酯化反应是羧酸与醇在浓硫酸催化下加热，生成酯和水，乙酸与乙醇生成乙酸乙酯，产物有水果气味，常用作溶剂和香精。写这个可逆反应时不能漏掉水，也不能把浓硫酸写成反应物。酯的命名是醇在前、酸在后，并把酸的词尾改成 -ate。很多学生会写成 ethanoic ethyl，顺序一颠倒这一分就没了。酯化是可逆反应，想让平衡向右移动，可以多加一种反应物，也可以把生成的酯及时蒸出，两种做法都能提高产率。
Carboxylic acids follow CnH2COOH, and ethanoic acid is the first one students meet. They dissolve in water to give an acidic solution and release carbon dioxide from carbonates, a single test that separates them from alcohols; litmus paper gives the same information more quickly in multiple choice questions. Esterification joins an acid and an alcohol with concentrated sulfuric acid as catalyst and heat, giving an ester and water, so ethanoic acid with ethanol gives ethyl ethanoate, a sweet smelling liquid used as a solvent and in flavourings. Water must appear in the equation, the sulfuric acid stays a catalyst, and the ester is named from the alcohol first with the acid ending changed to -ate. The reaction is reversible, so yield improves when one reactant is in excess or the ester is distilled off as it forms.
聚合物：加聚反应与塑料处置
加聚只发生在含碳碳双键的单体上，产物是长链聚合物，整个过程没有其他小分子生成，这一点把它和缩聚区分开。聚乙烯、聚丙烯、聚氯乙烯都是常见的加聚产物，各自的重复单元不同，画的时候要把单体里的双键打开。重复单元和单体可以互相推：给出单体能写出重复单元，给出重复单元也能倒推单体名称，这类转换题在 4CH1 里出现频率很高。塑料的处置是这一节的第二个考点：它们不易被微生物分解，焚烧又可能放出有毒气体，所以回收、再利用和改用可降解塑料是三个常被问到的方向。看清楚题目问的是反应类型还是环境影响，两种情况要写的答案完全不同。
Addition polymerisation happens only with monomers that contain a carbon to carbon double bond. The product is a long chain and no small molecule is released, which is what separates it from condensation polymerisation; poly(ethene), poly(propene) and PVC are the standard examples, and each repeating unit is drawn with the double bond opened. Disposal is the second half of the topic, because these polymers are not broken down by microorganisms and burning them can release toxic gases, so recycling, reuse and biodegradable alternatives are the answers examiners look for. Converting a monomer into a repeating unit and back again appears often, so practise both directions.
沸点、溶解性与分离方法
有机物的沸点主要取决于分子大小和分子间作用力，碳链越长，分子之间接触面积越大，沸点就越高。短链烷烃在常温下是气体，中等长度的多为液体，长链则变成蜡状固体。小分子有机物大多不溶于水而易溶于有机溶剂，乙醇和乙酸是明显的例外，因为它们能与水形成氢键，这也是乙醇能与水以任意比例混合的原因。分离液体混合物用分馏，温度计的水银球要放在支管口，按沸点范围收集不同馏分；分离互不相溶的两种液体用分液漏斗，下层从下口放出。实验题里这两个装置图出场频率很高，装置细节常常就是那一分。
Boiling point in organic compounds depends on molecular size and the forces between molecules, and longer chains have more surface contact and boil higher. Short alkanes are gases at room temperature, medium ones are liquids, and long chains are waxy solids. Most small organic molecules do not dissolve in water, ethanol and ethanoic acid being the exceptions because they form hydrogen bonds with water, which is why ethanol mixes with water in any proportion. Fractional distillation separates liquid mixtures, with the thermometer bulb at the side arm and each fraction collected over a narrow boiling range, while two immiscible liquids are separated in a funnel with the lower layer run off first.
结构式书写与 4CH1 的三类问法
Edexcel IGCSE Chemistry 4CH1 的有机化学题集中在三件事上：认结构、写方程、解释现象。认结构要求从显示式里数出碳原子并认出官能团，写方程要求配平并带上紫外光、催化剂或温度这些条件，解释题通常问沸点为什么升高、液体为什么不导电、反应为什么会停止，答案要落到分子间作用力、离子和酶失活这些具体概念上。不要把条件写成一大串罗列，题目问现象就只写现象，问原因再补解释。把历年真题里的有机题按这三类分开练，比从头再读一遍课本省时间。
Most organic questions in Edexcel IGCSE Chemistry 4CH1 ask for one of three things: recognise a structure, write an equation, or explain an observation. Recognition means counting carbons and spotting the functional group in a displayed formula. Equations must be balanced and carry the right condition, whether that is ultraviolet light, a catalyst or a temperature. Explanation questions usually ask why a boiling point rises, why a liquid does not conduct, or why a reaction stops, and the marks sit in intermolecular forces, ions and enzyme damage.
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