📚 GCSE Chemistry: Alkenes Revision Guide | GCSE 化学：烯烃 考点精讲
Alkenes are a fascinating family of hydrocarbons that contain a carbon-carbon double bond, making them unsaturated and highly reactive. They are the starting materials for countless products, including plastics, solvents and alcohols, and they feature prominently in GCSE Chemistry. This revision guide breaks down every key concept, reaction and exam technique you need to master alkenes, with clear bilingual explanations to reinforce your understanding.
烯烃是一类含有碳碳双键的烃，这种不饱和结构使它们具有很高的反应活性，是塑料、溶剂和醇类等无数产品的基础原料，也是GCSE化学的重要考点。本文以中英双语逐一拆解烯烃的核心概念、重要反应和应试技巧，帮助你彻底掌握这一主题。
1. What are Alkenes? | 什么是烯烃？
ond is their functional group and the site of most chemical reactions. Because they are unsaturated, they have fewer hydrogen atoms per carbon than the corresponding alkanes.
烯烃是含有至少一个碳碳双键(C=C)的不饱和烃。双键就是它们的官能团，也是大多数化学反应的位点。由于不饱和，它们每个碳原子所带的氢原子数比相应的烷烃少。
2. General Formula and Homologous Series | 通式与同系物
Alkenes with one double bond follow the general formula CₙH₂ₙ. This allows them to form a homologous series, where each successive member differs by a CH₂ unit. Physical properties such as boiling point increase gradually with chain length, while chemical properties remain similar due to the same functional group.
含一个双键的烯烃通式为CₙH₂ₙ。它们形成同系物，相邻成员相差一个CH₂单元。随着碳链增长，沸点等物理性质逐渐升高，但由于官能团相同，化学性质保持相似。
- Ethene: C₂H₄
- Propene: C₃H₆
- Butene: C₄H₈
- Pentene: C₅H₁₀
3. Naming Alkenes | 烯烃的命名
IUPAC names of alkenes end in '-ene'. The position of the double bond is indicated by the lowest possible number placed before the suffix. The carbon chain is numbered from the end nearest to the double bond. For example, but-1-ene has the double bond between carbon 1 and carbon 2, while but-2-ene has it between carbon 2 and carbon 3.
烯烃的IUPAC命名以“-烯”结尾。双键的位置用最小的编号标注在词尾之前。碳链从离双键最近的一端开始编号。例如，丁-1-烯的双键在C1和C2之间，丁-2-烯的双键在C2和C3之间。
- CH₂=CH₂: ethene (乙烯)
- CH₂=CHCH₃: propene (丙烯)
- CH₂=CHCH₂CH₃: but-1-ene (丁-1-烯)
- CH₃CH=CHCH₃: but-2-ene (丁-2-烯)
4. Structure and Bonding | 结构与键合
d uses three of its four outer electrons in sigma bonds: two to hydrogen or carbon atoms, and one sigma bond to the other carbon. The remaining p orbital electron on each carbon overlaps sideways to form a pi (π) bond. The combination of one sigma and one pi bond makes a double bond.
碳碳双键中每个碳原子用三个最外层电子形成σ键：两个与氢或其他碳原子成键，一个与另一个碳原子形成σ键。剩余的一个p轨道电子从侧面重叠，形成π键。一个σ键和一个π键共同构成双键。
The pi bond restricts rotation, so the two carbon atoms and the four atoms attached to them lie in the same plane, with bond angles of approximately 120° around each double-bonded carbon.
π键限制了旋转，因此双键的两个碳原子以及直接相连的四个原子处于同一平面，每个双键碳周围的键角约为120°。
5. Isomerism in Alkenes | 烯烃的同分异构
Alkenes exhibit both structural isomerism and geometric (cis-trans) isomerism. Structural isomers occur when the carbon skeleton or the position of the double bond differs. For example, but-1-ene and but-2-ene are position isomers, while but-1-ene and 2-methylpropene are chain isomers.
烯烃存在结构异构和几何异构(顺反异构)。当碳骨架或双键位置不同时，产生结构异构，如丁-1-烯和丁-2-烯是位置异构，丁-1-烯和2-甲基丙烯是碳链异构。
Geometric isomerism arises because the double bond cannot rotate. If each carbon of the double bond is attached to two different groups, cis and trans isomers exist. In cis-but-2-ene the two methyl groups are on the same side; in trans-but-2-ene they are on opposite sides.
由于双键不能旋转，如果双键的每个碳上都连有两个不同的基团，就会产生几何异构。顺-丁-2-烯中两个甲基在双键同侧，反-丁-2-烯中两个甲基在异侧。
cis-but-2-ene: CH₃ \ / CH₃
sp; C=C
/ \
H H
trans-but-2-ene: CH₃ \ / H
sp; C=C
/ \
H CH₃
6. Addition Reactions: Overview | 加成反应概述
Alkenes are far more reactive than alkanes because the pi bond is relatively weak and can be broken easily. In an addition reaction, the double bond opens up and two atoms or groups attach to the carbon atoms, forming a saturated compound. No atoms are lost; the reaction involves only addition.
烯烃远比烷烃活泼，因为π键相对较弱，容易断裂。在加成反应中，双键打开，两个原子或基团加到双键碳原子上，形成饱和化合物。反应中没有任何原子丢失，只有加成。
The typical reagents include hydrogen, halogens, hydrogen halides and steam. Each follows the same pattern: one part of the reagent attaches to each carbon of the original double bond.
典型的试剂包括氢气、卤素、卤化氢和水蒸气。它们都遵循相同的模式：试剂的每个部分分别加在原来双键的两个碳上。
7. Reaction with Halogens: The Bromine Test | 与卤素反应：溴水试验
Alkenes react rapidly with bromine at room temperature without UV light. The orange-brown colour of bromine water disappears immediately as the bromine adds across the double bond to form a colourless dibromoalkane. This is the standard chemical test for unsaturation.
烯烃在室温下无需紫外光就能与溴迅速反应。溴水的橙棕色会立即消失，因为溴加成到双键上，生成无色的二溴代烷。这是检验不饱和度的标准化学试验。
C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
For propene: CH₂=CHCH₃ + Br₂ → CH₂BrCHBrCH₃
If an organic compound decolorises bromine water, it indicates the presence of a carbon-carbon double bond. This distinguishes alkenes from alkanes, which do not react with bromine in the dark.
如果有机物能使溴水褪色，就说明存在碳碳双键。这一现象可区分烯烃与烷烃，因为烷烃在避光条件下不与溴反应。
8. Hydrogenation | 加氢反应
Alkenes react with hydrogen gas in the presence of a nickel catalyst at around 150 °C. The hydrogen adds across the double bond, converting the alkene into the corresponding alkane. This reaction is called catalytic hydrogenation.
烯烃在约150 °C和镍催化剂存在下与氢气反应。氢气加成到双键上，烯烃转化为相应的烷烃。该反应称为催化加氢。
C₂H₄ + H₂ → C₂H₆
ially hydrogenated to raise their melting points, producing solid spreads such as margarine.
工业上利用加氢反应硬化植物油。含C=C双键的不饱和脂肪通过部分加氢提高熔点，制成固体涂抹脂，如人造黄油。
9. Hydration: Reaction with Steam | 水蒸气反应：水合
Alkenes can be converted into alcohols by direct hydration with steam. Ethene reacts with steam at high temperature (around 300 °C) and high pressure (60-70 atm) in the presence of a phosphoric acid catalyst. Water adds across the double bond to produce ethanol.
烯烃可通过与水蒸气直接水合转化为醇。乙烯在高温(约300 °C)、高压(60-70 atm)和磷酸催化剂存在下与水蒸气反应，水加成到双键上生成乙醇。
C₂H₄ + H₂O ⇌ C₂H₅OH
This is a reversible reaction, so conditions are optimised to shift the equilibrium towards the alcohol. Hydration of alkenes is an important industrial route to alcohols, competing with fermentation.
这是一个可逆反应，因此需要通过优化条件使平衡向生成醇的方向移动。烯烃水合是工业制醇的重要路线，与发酵法形成竞争。
10. Addition Polymerisation | 加成聚合
Under high pressure and with an appropriate catalyst, alkene molecules can undergo addition polymerisation. The double bond opens up, and monomers join together in a long chain without the loss of any small molecules. The resulting polymer is a saturated, long-chain alkane backbone with repeating units.
在高压和合适催化剂作用下，烯烃分子可发生加成聚合。双键打开，单体连接成长链，没有小分子脱去。所得到的聚合物具有饱和的长链烷烃骨架和重复单元。
n CH₂=CH₂ → [ CH₂ CH₂ ]ₙ poly(ethene)
n CH₂=CH(CH₃) → [ CH₂ CH(CH₃) ]ₙ poly(propene)
The repeating unit is written by removing the double bond and adding brackets with a subscript n. It is essential to show the continuation bonds through the brackets.
书写重复单元时，去掉双键，加上方括号并标注下标n。必须通过方括号表示键的延续。
11. Poly(ethene) and Poly(propene) | 聚乙烯与聚丙烯
Poly(ethene) is the most common plastic, used in plastic bags, bottles and films. Poly(propene) is tougher and used for containers, car bumpers and ropes. Both are thermoplastics, meaning they can be melted and remoulded.
聚乙烯是最常见的塑料，用于塑料袋、瓶子和薄膜。聚丙烯更坚韧，用于容器、汽车保险杠和绳索。两者都是热塑性塑料，可熔化和重塑。
Despite their utility, these addition polymers are non-biodegradable and persist in the environment. Recycling, reuse and reduction of single-use plastics are essential to minimise pollution.
尽管用途广泛，这些加成聚合物不可生物降解，会长期存在于环境中。回收、重复使用和减少一次性塑料制品对于减少污染至关重要。
12. Key Points and Exam Tips | 关键考点与考试技巧
Reactions to remember: Test for unsaturation with bromine water (orange to colourless); hydrogenation (Ni catalyst, 150 °C); hydration (H₃PO₄ catalyst, 300 °C, 60 atm); polymerisation (high pressure, catalyst). Always state the colour change for the bromine test.
重点反应：溴水试验检验不饱和度(橙色褪为无色)；加氢反应(镍催化剂，150 °C)；水合反应(磷酸催化剂，300 °C，60 atm)；聚合反应(高压，催化剂)。溴水试验务必写出颜色变化。
Naming: The double bond position takes the lowest number. Check for cis/trans when both carbons carry two different groups. Use prefixes cis- or trans- (or E/Z if required by your syllabus).
命名：双键位次取最低编号。当双键两个碳都连有两个不同基团时，需考虑顺反异构，用cis-或trans-前缀(或根据考纲使用E/Z)。
Polymer drawing: Show the repeating unit with the side groups correctly oriented, and extend bonds through the brackets. Never leave double bonds inside the polymer backbone.
聚合物画法：正确标出侧基方向，延伸键穿越括号。主链中不得残留双键。
Equations: Practice balancing addition equations. For hydration, show the reversible arrow (⇌) and conditions. For polymerisation, write 'n' as the coefficient.
方程式：练习配平加成反应方程式。水合反应应标明可逆符号(⇌)及条件。聚合反应须用'n'作为系数。
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