An introduction to AS Level organic chemistry
A-Level Chemistry Topic 13 8:56 English narration · English + 中文 subtitles burned in
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Chemists have found more than ten million hydrocarbons and carbon compounds.
化学家已经发现了一千多万种含碳化合物。
Ten million.
一千万。
So how can anyone learn organic chemistry?
那还怎么学有机化学呢？
Because almost all of them share one simple idea: a carbon chain, plus one small reactive part.
因为它们几乎都遵循同一个简单的思路：一条碳链，加上一个小小的活性部分。
That part decides how the whole molecule behaves.
正是这个部分决定了整个分子的性质。
Learn the parts, and you can predict molecules you have never seen.
学会这些部分，你就能预测从未见过的分子。
Welcome to organic chemistry.
欢迎来到有机化学。
We will learn to draw and name these molecules, the words that describe their reactions, their shapes, and the different ways the same atoms can be arranged.
我们将学习如何画出并命名这些分子、描述它们反应的术语、 它们的形状，以及同样的原子可以有哪些不同的排列方式。
There are five ways to write the same molecule, and the exam asks for one by name.
同一个分子有五种写法，而考试会点名要其中一种。
Take butane.
以丁烷为例。
The general formula is the pattern for a whole family.
通式是整个家族的通用模式。
The molecular formula counts every atom.
分子式数出每种原子的个数。
The structural formula lists the groups in order.
结构式按顺序列出各个基团。
The displayed formula draws every atom and every bond.
展开式把每个原子、每根键都画出来。
And the skeletal formula draws only the bonds, with a carbon at every corner and every free end.
骨架式只画键，每个折点和每个自由末端都是一个碳。
The empirical formula, the simplest ratio, follows from any of them.
实验式，也就是最简整数比，可以从其中任何一种读出来。
Here is a skeletal formula. Give its molecular formula.
这是一个骨架式，请写出它的分子式。
Pause if you want to try it.
想自己试的话可以先暂停。
First, count the carbons.
第一步，数碳。
Every corner and every free end of a line is a carbon — but not where an atom is written in, like this oxygen.
每一个折点、每一条线的自由末端都是一个碳， 但写出了原子的地方不算，比如这个氧。
So there are five.
所以一共五个碳。
Now the hydrogens.
接着数氢。
Every carbon must end up with four bonds, so add hydrogens until it does.
每个碳最后都必须有四根键，所以补氢补到四根为止。
Two, one, two, three and three make eleven, and the oxygen carries one more.
二、一、二、三、三，一共十一个，氧上还带着一个。
So the answer is five carbons, twelve hydrogens and one oxygen.
所以答案是五个碳、十二个氢和一个氧。
That molecule has a name: two-methyl-butan-one-ol. Read it with systematic nomenclature.
那个分子有一个名字：2-甲基丁-1-醇。
The stem tells you how many carbons are in the longest chain, and but means four.
把它拆开来读。 词干告诉你最长碳链上有几个碳，丁 就是四个碳。
The ending gives the functional group, and o l means an alcohol.
词尾给出官能团，醇 就是羟基。
Then the numbers say which carbon each part sits on.
数字则说明每一部分接在第几号碳上。
Three rules decide the name. Longest chain, lowest possible numbers, and branches in alphabetical order.
三条规则决定这个名字： 选最长的碳链、给出尽可能小的编号、支链按字母顺序排列。
Two words next.
接着是两个术语。
A homologous series is a family of compounds with the same functional group and the same general formula, where each member differs by one carbon and two hydrogens.
同系列是一族化合物，它们有相同的官能团和相同的通式， 相邻成员相差一个碳和两个氢。
That matters, because a family also shares similar chemical properties, and the mark scheme wants those words.
这一点很重要，因为同一族还有相似的化学性质， 评分标准要的就是这句话。
Saturated means only single carbon to carbon bonds. Unsaturated means there is a double bond.
饱和是指只有碳碳单键；不饱和是指含有双键。
A covalent bond can break in two ways.
共价键有两种断裂方式。
Homolytic fission splits the pair evenly, one electron to each atom, so you get two free radicals, and a radical is a species with a single unpaired electron.
均裂把这对电子平分，每个原子各得一个电子， 于是生成两个自由基，而自由基就是带有一个未成对电子的粒子。
Heterolytic fission is uneven. Both electrons go to one atom, so you get two ions, one positive and one negative.
异裂则是不均等的：两个电子都跑到同一个原子上， 于是生成两个离子，一正一负。
Now, who attacks what.
接下来，谁进攻谁。
A nucleophile has a lone pair of electrons and is attracted to a positive, electron-poor centre, like the carbon in a carbon-bromine bond.
亲核试剂带有一对孤对电子，会被带正电、缺电子的中心吸引， 比如碳溴键上的那个碳。
An electrophile is the opposite. It is pulled towards an electron-rich centre, such as a carbon-carbon double bond.
亲电试剂正好相反：它会被富电子的中心吸引， 比如碳碳双键。
Here is the way to remember it. Nucleus-loving goes to the positive part, and electron-loving goes to the electrons.
这样记就行：亲核就是爱核，奔着正电荷去； 亲电就是爱电子，奔着电子去。
Five reaction types. Classify each by type and reagent.
五种反应类型，你必须分类准确。
Addition joins two molecules into one.
加成：两个分子合成一个。
Substitution swaps one atom or group for another.
取代：一个原子或基团被另一个换掉。
Elimination removes a small molecule, and a double bond appears.
消去：脱去一个小分子，同时出现双键。
Hydrolysis splits a molecule apart with water.
水解：分子被水拆开。
Condensation joins two molecules and loses a small one, usually water.
缩合：两个分子结合，同时失去一个小分子，通常是水。
For organic redox, O in square brackets means one oxygen atom from an oxidising agent, and H in square brackets means one hydrogen atom from a reducing agent.
在有机氧化还原的方程式里，方括号里的 O 表示氧化剂提供的一个氧原子， 方括号里的 H 表示还原剂提供的一个氢原子。
In a mechanism, a curly arrow shows a pair of electrons moving. It must start at a bond or at a lone pair, and it must point to where the electrons go. Four mechanisms carry the whole AS course. Free-radical substitution happens in alkanes. Electrophilic addition happens in alkenes. Nucleophilic substitution happens in halogenoalkanes. And nucleophilic addition happens in carbonyl compounds.
在反应机理中，弯箭头表示一对电子的移动。 它必须从一根键或一对孤对电子出发， 并且指向电子去往的地方。 整个 AS 阶段有四种机理： 烷烃发生自由基取代；烯烃发生亲电加成；卤代烷发生亲核取代； 羰基化合物发生亲核加成。
Watch a real one.
来看一个真实的例子。
An alkene's double bond is an electron-rich pair sticking out, so it attacks.
烯烃的双键是一对向外突出的富电子电子对，所以是它主动进攻。
The curly arrow starts on the double bond, not on an atom, and points at the slightly positive hydrogen.
弯箭头从双键出发，而不是从某个原子出发，指向带部分正电的氢。
A new bond forms, and a positive carbon ion is left behind.
新键生成，留下一个带正电的碳正离子。
Which carbon takes the hydrogen?
哪个碳得到这个氢？
The one that gives the more stable ion.
能生成更稳定离子的那个碳。
A free-radical reaction runs as a chain, in three steps.
自由基反应是链式进行的，分三步。
Initiation makes the first radicals, when ultraviolet light splits a chlorine molecule evenly.
引发生成最初的自由基： 紫外光把一个氯分子均裂开。
Propagation is the chain itself. A chlorine radical takes a hydrogen from methane, and the methyl radical left behind attacks another chlorine molecule, making a new chlorine radical.
增长就是链本身： 一个氯自由基从甲烷上夺走一个氢，留下的甲基自由基再去进攻另一个氯分子， 又生成一个新的氯自由基。
Termination stops it, when two radicals meet and join.
终止让它停下来：两个自由基相遇并结合。
Organic molecules come as straight chains, branched chains, or cyclic rings.
有机分子有直链、支链和环状三种。
The shape at each carbon comes from hybridisation.
在每个碳上，形状由杂化决定。
An s p three carbon has four single bonds, so it is tetrahedral, at one hundred and nine point five degrees.
sp3 碳有四根单键，所以是四面体，键角一百零九点五度。
An s p two carbon has one double bond, so it is planar, meaning flat, at one hundred and twenty degrees. Ethene is planar for exactly that reason.
sp2 碳有一根双键，所以是平面的，也就是平的，键角一百二十度； 乙烯正是因为这样才是平面分子。
An s p carbon has a triple bond, so it is linear, at one hundred and eighty degrees.
sp 碳有一根三键，所以是直线形，键角一百八十度。
Now count the sigma and pi bonds. A single bond is one sigma; a double bond adds one pi, and a triple bond adds two.
再来数西格玛键和派键：单键是一根西格玛键；双键再加一根派键；三键再加两根派键。
Isomers have the same molecular formula but a different arrangement of atoms.
同分异构体的分子式相同，但原子的排列方式不同。
In structural isomerism the atoms are joined in a different order, and there are three kinds.
在结构异构中，原子的连接顺序不同，一共有三种。
Chain isomerism branches the carbon chain differently.
链异构是碳链的分支方式不同。
Positional isomerism moves the functional group to another carbon.
位置异构是官能团接在不同的碳上。
Functional group isomerism makes a completely different group from the same atoms, like an alcohol and an ether.
官能团异构是同样的原子组成了完全不同的官能团，比如醇和醚。
Stereoisomers are joined in the same order, but point in different directions in space.
立体异构体的原子连接顺序相同，但在空间中的指向不同。
So why does but-two-ene show geometrical isomerism — cis-trans — while but-one-ene does not?
那么为什么丁-2-烯有顺反异构，而丁-1-烯没有？
Two things are needed.
需要两个条件。
There must be restricted rotation about the double bond, and both alkenes have that.
第一，双键不能旋转，这一点两个烯烃都满足。
And each double-bond carbon must carry two different groups.
第二，双键上的每一个碳都必须连着两个不同的基团。
In but-two-ene each one carries a hydrogen and a methyl group, so the methyls sit on the same side, cis, or on opposite sides, trans.
在丁-2-烯中，每个双键碳都连着一个氢和一个甲基， 所以两个甲基可以在同一侧，即顺式，也可以在两侧，即反式。
In but-one-ene the end carbon carries two hydrogens, so swapping them changes nothing. Two identical groups on either carbon kills the isomerism.
而丁-1-烯的端位碳上连着两个氢，把它们对调什么也没有改变。
The other kind of stereoisomerism is optical isomerism.
另一种立体异构是旋光异构。
It happens at a chiral centre, a carbon with four different groups attached.
它发生在手性中心上， 也就是一个连着四个不同基团的碳。
The molecule and its mirror image look the same, but slide one onto the other and they never line up, just like your two hands.
分子和它的镜像看起来一样， 但把一个平移过去，它们永远无法重合，就像你的两只手。
The two forms are called enantiomers.
这两种形式叫做对映体。
How do you find a chiral centre?
那怎么找出手性中心呢？
Take the carbons one at a time, and ask whether that carbon carries four different groups.
一个一个碳地看， 问这个碳是不是连着四个不同的基团。
In lactic acid the middle carbon holds a hydrogen, an O H group, a methyl group and an acid group. All four differ, so mark it with a star.
在乳酸中， 中间那个碳连着一个氢、一个羟基、一个甲基和一个羧基，四个都不同， 所以给它打上星号。
In propan-two-ol the middle carbon holds two methyl groups. Two identical groups, so it is not chiral.
而丙-2-醇的中间碳连着两个甲基， 有两个相同的基团，所以它不是手性碳。
A molecule can have more than one chiral centre, so check every carbon.
一个分子可以有不止一个手性中心， 所以每个碳都要检查。
Three marks students throw away.
三个学生常丢的分。
First, define the isomerism properly. Structural isomers have the same molecular formula but a different structural formula. Stereoisomers have the same structural formula but a different arrangement in space.
第一，把异构的定义写准确： 结构异构体的分子式相同，但结构式不同；立体异构体的结构式相同， 但原子在空间中的排列不同。
Second, cis-trans isomerism needs both conditions. No rotation about the double bond, and two different groups on each carbon.
第二，顺反异构需要两个条件同时满足： 双键不能旋转，而且每个双键碳上连着两个不同的基团。
Third, a chiral centre needs four different groups on one carbon, not just a branch.
第三，手性中心需要同一个碳上连着四个不同的基团，仅仅有支链是不够的。
Get those right, and this topic is yours.
把这几点做对，这个专题就是你的了。