GCSE有机化学 反应路径 烷烃 烯烃 醇 酯
有机化学在GCSE化学考试中占分稳定，而反应路径图（reaction pathway）是把零散知识点串成一条线的最佳工具。考试常要求判断某物质如何转化为另一种物质、写出反应条件与催化剂、或补全反应流程图。本文用一张有机反应地图帮你理清烷烃、烯烃、醇、羧酸、酯五大类物质的转化关系，从石油分馏讲到聚合反应，一步一个考点。
Organic chemistry carries a stable share of marks in GCSE Chemistry, and the reaction pathway is the best tool for linking scattered facts into one clear picture. Exams frequently ask you to identify how one compound converts into another, to state the conditions and catalysts, or to complete a reaction flow chart. This article maps the five main families (alkanes, alkenes, alcohols, carboxylic acids, esters) from crude oil distillation to polymerisation, one exam point at a time.
1. 原油分馏与烷烃：反应路径的起点
反应路径的起点是原油。原油通过分馏（fractional distillation）按沸点不同分离成不同馏分：石油气、汽油、煤油、柴油、润滑油和沥青，沸点从上到下逐渐升高。馏分主要由烷烃（alkanes）组成。烷烃是饱和烃，通式为CnH2n+2，只含碳碳单键和碳氢单键，化学性质稳定，主要发生取代反应和燃烧反应。完全燃烧生成二氧化碳和水：CH4 + 2O2 -> CO2 + 2H2O；不完全燃烧则生成一氧化碳或碳粒，一氧化碳有毒且无色无味，这是考试必考的安全问题。长链烷烃通过裂化（cracking）生成短链烷烃和烯烃，例如C10H22 -> C5H12 + C5H10，裂化产物中的烯烃正是后续反应路径的关键原料。
The pathway starts with crude oil. Fractional distillation separates crude oil into fractions by boiling point: refinery gas, petrol, kerosene, diesel, lubricating oil and bitumen, with boiling points rising from top to bottom. The fractions are mainly alkanes. Alkanes are saturated hydrocarbons with the general formula CnH2n+2, containing only single carbon-carbon and carbon-hydrogen bonds, so they are fairly unreactive and mainly undergo substitution and combustion. Complete combustion gives carbon dioxide and water: CH4 + 2O2 -> CO2 + 2H2O, while incomplete combustion produces toxic carbon monoxide or soot. Carbon monoxide is colourless and odourless, a compulsory safety point in exams. Long-chain alkanes are cracked into shorter alkanes and alkenes, for example C10H22 -> C5H12 + C5H10, and the alkene products are the key feedstock for the rest of the pathway.
2. 烯烃：不饱和烃的加成反应
烯烃（alkenes）含有碳碳双键C=C，是不饱和烃，通式为CnH2n。双键使烯烃比烷烃活泼得多，主要发生加成反应（addition reactions）：与氢气加成生成烷烃，需要镍催化剂并加热；与溴水加成使橙色溴水褪色，这是鉴别烯烃与烷烃的经典方法，答案必须写"橙色溴水褪色"而非"溴水变无色"；与水蒸气加成生成醇，需要磷酸催化剂、高温高压；与卤化氢加成生成卤代烷。考试中加成反应是最高频考点，务必同时写对反应物、条件和产物。
Alkenes contain the carbon-carbon double bond C=C, making them unsaturated hydrocarbons with the general formula CnH2n. The double bond makes alkenes far more reactive than alkanes, and they mainly undergo addition reactions: hydrogenation with hydrogen over a nickel catalyst with heat forms alkanes; addition of bromine water decolourises the orange solution, the classic test for unsaturation (write "orange bromine water turns colourless", a precise phrase examiners reward); hydration with steam over a phosphoric acid catalyst at high temperature and pressure forms alcohols; addition of hydrogen halides forms haloalkanes. Addition reactions are the most tested topic, so always state the reactants, conditions and products together.
3. 醇：发酵与催化水合两条路线
乙醇是GCSE有机化学的代表醇，工业上有两条生产路线。发酵法（fermentation）让葡萄糖在酵母和无氧条件下分解：C6H12O6 -> 2C2H5OH + 2CO2，条件温和、原料可再生，但速度慢、产物浓度低，还需要蒸馏提纯。乙烯催化水合法让乙烯与水蒸气在磷酸催化、高温高压下直接生成乙醇，连续生产、产率高、纯度高，但依赖石油资源且能耗大。两条路线各有优缺点，是"比较题"的高频素材，答题时要从原料、条件、速度、纯度、环保五个维度对比。此外，醇可被氧化为羧酸：乙醇在氧化剂作用下生成乙酸，这是从醇通往羧酸的关键一步。
Ethanol is the representative alcohol in GCSE organic chemistry and has two production routes. Fermentation breaks down glucose with yeast in anaerobic conditions: C6H12O6 -> 2C2H5OH + 2CO2. It is gentle, uses renewable feedstock, but is slow and gives dilute ethanol that needs distillation. Catalytic hydration of ethene reacts ethene with steam over a phosphoric acid catalyst at high temperature and pressure, producing pure ethanol continuously with high yield, but it depends on oil and consumes much energy. Comparing these routes is a classic question; contrast feedstock, conditions, speed, purity and environmental impact. Alcohols can also be oxidised to carboxylic acids: ethanol forms ethanoic acid with an oxidising agent, the key step from alcohols to acids.
4. 羧酸与酯：酯化反应三得分点
羧酸（carboxylic acids）含有羧基-COOH，是弱酸，能与碱发生中和反应，也能与活泼金属反应放出氢气，酸性比盐酸、硫酸弱得多。乙酸与乙醇在浓硫酸催化下发生酯化反应（esterification）生成乙酸乙酯和水：CH3COOH + C2H5OH -> CH3COOC2H5 + H2O。酯类有水果香味，常用作食品香精和溶剂。酯化反应是可逆反应，浓硫酸既作催化剂又作吸水剂，吸水可推动平衡向右移动、提高产率。反应条件、催化剂的双重作用、产物的水果气味是三个最常考的得分点，一个都不能漏。
Carboxylic acids contain the -COOH group and are weak acids: they neutralise bases and release hydrogen with reactive metals, but they are far weaker than hydrochloric or sulfuric acid. Ethanoic acid and ethanol undergo esterification with concentrated sulfuric acid to form ethyl ethanoate and water: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O. Esters smell fruity and are used as flavourings and solvents. Esterification is reversible; the concentrated acid acts as both catalyst and dehydrating agent, and removing water shifts the equilibrium to the right to raise yield. Conditions, the double role of the catalyst, and the fruity smell are the three most rewarded marks, so never miss any of them.
5. 聚合反应：从烯烃到高分子
烯烃还能发生加成聚合反应（addition polymerisation）：大量乙烯分子在高温高压和催化剂作用下首尾相连，生成聚乙烯poly(ethene)，n C2H4 -> poly(ethene)。书写聚合物链节时，把双键改成单键，并在链节两端各加一条短线表示延伸。考试常见两种问法：给出单体写链节，或给出链节判断单体。注意区分：加成聚合的链节与单体原子组成相同，没有小分子生成；缩聚反应是A-Level内容，GCSE只需掌握加成聚合。聚合的另一面是环境问题：塑料难降解、焚烧产生有毒气体，回收、减少使用与可降解塑料是高频讨论题。
Alkenes also undergo addition polymerisation: many ethene molecules join end to end under high temperature, pressure and a catalyst to form poly(ethene), n C2H4 -> poly(ethene). When drawing the repeat unit, change the double bond into a single bond and add a short bond at each end to show continuation. Two question types appear: write the repeat unit from a monomer, or identify the monomer from a repeat unit. Remember that in addition polymerisation the repeat unit has the same atoms as the monomer and no small molecule is released; condensation polymerisation belongs to A-Level. The environmental side is also tested: plastics resist biodegradation, burning releases toxic gases, and recycling, reducing use and biodegradable plastics are frequent discussion topics.
6. 反应路径图总览：一张图记住全流程
把以上转化连成一张图，就是考试最爱的反应路径图：烷烃经裂化得到烯烃，烯烃加水生成醇、加溴水可鉴别、加氢回烷烃，醇氧化成羧酸，醇与羧酸酯化生成酯，烯烃聚合得到高分子。记忆时抓住三条主线：一是"烷烃到烯烃靠裂化"，二是"烯烃到醇靠水合或发酵"，三是"醇到羧酸靠氧化、醇加羧酸成酯"。作答有机流程题时，先写出反应物与产物的结构式，再补充反应条件（催化剂、温度、压力），最后检查方程式配平，按这个固定顺序答题最不容易丢分。
Joining all the conversions together gives the reaction pathway map examiners love: cracking turns alkanes into alkenes; hydration turns alkenes into alcohols, bromine water tests for them, and hydrogenation returns them to alkanes; oxidation turns alcohols into carboxylic acids; esterification joins an alcohol and an acid into an ester; polymerisation builds polymers from alkenes. Remember three main lines: cracking makes alkenes from alkanes, hydration or fermentation makes alcohols from alkenes, and oxidation makes acids from alcohols while esterification joins an alcohol and an acid. When answering flow-chart questions, first write the structural formulas of reactants and products, then add conditions such as catalyst, temperature and pressure, then check that the equation balances. This fixed order loses the fewest marks.
7. 常见错误与考试技巧
四个高频失分点要特别警惕。第一，混淆饱和与不饱和：烷烃通式CnH2n+2、烯烃通式CnH2n，千万不要记反。第二，漏写反应条件：裂化要高温，催化加氢要镍和加热，水合要磷酸和高温高压，酯化要浓硫酸，条件不写必扣分。第三，酯化反应忘记写可逆符号。第四，燃烧题不看氧气量，无法判断完全燃烧还是不完全燃烧。复习建议：把反应路径图至少画三遍，第一遍对照课本画，第二遍合上书默写，第三遍只根据题目关键词补全流程。考试时先通读全题再动笔，有机题的关键信息往往藏在题目给出的结构式里。
Watch out for four frequent mark-losers. First, confusing saturated with unsaturated: alkanes are CnH2n+2, alkenes are CnH2n, never swap them. Second, forgetting conditions: cracking needs high temperature, hydrogenation needs nickel and heat, hydration needs phosphoric acid with high temperature and pressure, esterification needs concentrated sulfuric acid. Third, missing the reversible arrow in esterification. Fourth, ignoring whether oxygen is in excess, so you cannot judge complete or incomplete combustion. Revision advice: redraw the pathway map at least three times, once from the textbook, once from memory with the book closed, and once by completing the flow from question keywords alone. In the exam, read the whole question before writing; organic questions usually hide key clues inside the given structural formulas.
8. 考点自查：四道自测题
下面四道题覆盖本文全部主线，先自己做再对答案。第一题：如何用简单方法鉴别甲烷和乙烯？答案：分别通入溴水，乙烯能使橙色溴水褪色，甲烷不能，现象不同即可区分。第二题：写出乙醇完全燃烧的化学方程式。答案：C2H5OH + 3O2 -> 2CO2 + 3H2O，注意配平后碳、氢、氧原子数两边相等。第三题：工业制乙醇的两种方法分别是什么，各举一个优缺点。答案：发酵法（可再生原料、条件温和，但慢且产物稀）与乙烯催化水合法（连续、纯度高，但依赖石油）。第四题：聚乙烯的链节是C2H4吗？答案：不是，链节是-CH2-CH2-，书写时双键必须变成单键并保留两端短线。
These four questions cover every main line of this article; try them before reading the answers. Q1: How do you distinguish methane from ethene with a simple test? A: Bubble each gas into bromine water; ethene decolourises the orange solution while methane does not, so the different observations separate them. Q2: Write the equation for complete combustion of ethanol. A: C2H5OH + 3O2 -> 2CO2 + 3H2O, balanced so that carbon, hydrogen and oxygen atoms match on both sides. Q3: Name the two industrial routes to ethanol with one advantage and one disadvantage each. A: Fermentation (renewable feedstock, mild conditions, but slow and dilute) and catalytic hydration of ethene (continuous, pure, but oil-dependent). Q4: Is the repeat unit of poly(ethene) C2H4? A: No, it is -CH2-CH2-; the double bond must become a single bond and the end bonds stay.
如果四道题全部答对，说明反应路径图已经基本掌握；答错也不要紧，回到对应章节重看一遍，重点盯住条件与方程式配平。有机化学的复习不需要死记硬背全部反应，抓住"裂化、水合、氧化、酯化、聚合"五个关键词，把每条转化关系画一遍，考试时就能快速调动知识。
If you answered all four correctly, you have basically mastered the pathway map. If you made mistakes, return to the matching section and re-read it, focusing on conditions and balanced equations. You do not need to memorise every reaction blindly; grasp the five keywords of cracking, hydration, oxidation, esterification and polymerisation, draw each conversion once, and you can recall the knowledge quickly in the exam.
海外直订AQA GCSE Chemistry Student Book AQA GCSE化学学生手册¥588.0📱 去京东购买京东精选需要一对一辅导？Need One-on-One Tutoring?
A-Level / GCSE / IB / KS2-KS3 全科辅导
或截图后用「扫一扫 → 相册」选择本图