Halogen compounds
A-Level Chemistry Topic 15 7:31 English narration · English + 中文 subtitles burned in
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Press the button on a can of freezer spray, and your skin goes numb with cold.
按下一罐冷冻喷雾的按钮，你的皮肤会被冻得发麻。
Inside is a halogenoalkane: an alkane with a halogen atom in place of one hydrogen.
罐子里装的是卤代烷： 一个烷烃分子，其中一个氢被卤素原子取代。
Molecules like these cooled our fridges and pushed our aerosols for fifty years.
这类分子曾为我们的冰箱制冷、 为气雾剂加压，用了整整五十年。
And every reaction in today's lesson comes from one bond — the bond between the carbon and the halogen.
而今天这节课里的每一个反应， 都来自同一根键——碳与卤素之间的那根键。
Welcome to halogen compounds.
欢迎来到卤素化合物。
We will make them, sort them into three classes, react them with nucleophiles, eliminate them to alkenes, and then take both substitution mechanisms apart.
我们会先制备它们，把它们分成三类， 让它们与亲核试剂反应，再消去成烯烃，最后把两种取代机理逐一拆解。
Let's begin.
让我们开始吧。
Start with the bond itself.
先从这根键本身讲起。
A halogen is more electronegative than carbon, so it pulls the shared pair of electrons towards itself.
卤素的电负性比碳大，所以它把共用电子对拉向自己。
That leaves the carbon slightly positive and the halogen slightly negative: a polar bond.
这就使碳带部分正电、卤素带部分负电：这是一根极性键。
And that carbon is the target.
而那个带部分正电的碳，就是被进攻的目标。
Anything carrying a lone pair of electrons is attracted to it, and attacks.
任何带有孤对电子的粒子都会被它吸引，并发起进攻。
How do you make one?
怎么制备它们呢？
Three routes, and the exam wants the conditions as well as the reagents.
有三条路线，而考试既要试剂，也要条件。
From an alkane: chlorine or bromine in ultraviolet light, by free radical substitution.
由烷烃制备：氯气或溴，在紫外光下，通过自由基取代。
From an alkene: a halogen, or a hydrogen halide, at room temperature, by electrophilic addition.
由烯烃制备：卤素单质，或卤化氢，在室温下，通过亲电加成。
From an alcohol: hydrogen halide gas, or phosphorus pentachloride, or phosphorus trichloride with heat.
由醇制备：卤化氢气体，或五氯化磷，或三氯化磷并加热。
Here is that middle route in motion.
我们来看中间那条路线的动画。
The double bond of an alkene is a loose, electron-rich pair sticking out of the molecule, so it reaches out and grabs the slightly positive hydrogen of a hydrogen bromide molecule.
烯烃的双键是一对松散的、 富电子的电子对，突出在分子外面，所以它伸出去抓住溴化氢分子里 那个带部分正电的氢。
Now sort them.
现在给它们分类。
Look only at the carbon that carries the halogen, and count how many other carbon atoms are joined to it.
只看那个连着卤素的碳，数一数有几个其他碳原子和它相连。
One carbon attached, and it is primary. Two carbons, and it is secondary. Three carbons, and it is tertiary.
连着一个碳，就是伯卤代烷；连着两个碳，就是仲卤代烷； 连着三个碳，就是叔卤代烷。
Ignore the hydrogens, and ignore the halogen itself.
不要数氢，也不要把卤素本身数进去。
This class decides the mechanism, so classify first, every time.
这个类别决定了分子走哪条机理，所以每一次都要先分类。
A nucleophile is a species with a lone pair of electrons that it can donate.
亲核试剂是指带有一对可以给出的孤对电子的粒子。
Hydroxide, cyanide and ammonia are the three you must know.
氢氧根、氰根和氨是你必须记住的三个。
It attacks the slightly positive carbon and forms a new bond to it, while the halogen leaves as a negative ion — the leaving group.
亲核试剂进攻那个带部分正电的碳， 与它形成一根新键，同时卤素以负离子的形式离去——它叫离去基团。
You draw this with two curly arrows: one from the lone pair to the carbon, one from the bond onto the halogen.
作答时要画两个弯箭头：一个从孤对电子指向碳，一个从旧键指向卤素。
Change the nucleophile, and you change the product.
换一个亲核试剂，产物就变了。
Warm it with aqueous sodium hydroxide, and you get an alcohol.
用氢氧化钠水溶液加热，得到醇。
Heat it with potassium cyanide dissolved in ethanol, and you get a nitrile — notice the chain has grown by one carbon, which makes this reaction precious in synthesis.
用溶在乙醇里的氰化钾加热，得到腈——注意碳链增加了一个碳， 这正是这个反应在有机合成中特别宝贵的原因。
Heat it with ammonia in ethanol, under pressure, and you get an amine.
用溶在乙醇里的氨，加压加热，得到胺。
There is a neat test to identify the halogen.
有一个巧妙的检验，用来鉴定卤素是哪一种。
Warm the halogenoalkane with silver nitrate in ethanol.
把卤代烷与溶在乙醇中的硝酸银一起温热。
Water slowly attacks the carbon, so the halide ion is released, and it meets a silver ion and drops out as a precipitate.
水会慢慢进攻那个碳， 于是卤离子被释放出来，遇到银离子就沉淀下来。
Now the colour tells you the halogen: white for chloride, cream for bromide, yellow for iodide.
沉淀的颜色就告诉你是哪种卤素：氯化银是白色，溴化银是奶油色，碘化银是黄色。
Same molecule, same reagent, different solvent, different product.
同一个分子，同一个试剂，溶剂不同，产物就完全不同。
With sodium hydroxide dissolved in water, and gentle warming, you get substitution and an alcohol.
用溶在水里的氢氧化钠，温和加热，发生取代，产物是醇。
With sodium hydroxide dissolved in ethanol, and strong heat, you get elimination and an alkene.
用溶在乙醇里的氢氧化钠，强热，发生消去，产物是烯烃。
In elimination the hydroxide never touches that carbon: it pulls a hydrogen off the next carbon along, and the electrons left behind swing in to form a double bond.
在消去反应中，氢氧根根本不碰那个碳。 它把相邻碳上的一个氢拉走，留下的电子对转过来形成一根双键。
Nucleophilic substitution can happen by two different routes, and here they are side by side.
亲核取代可以走两条不同的路线，这里把它们并排放在一起。
On top, the nucleophile comes in from behind and pushes the leaving group out in one movement, turning the molecule inside out.
上面一条：亲核试剂从背面进攻，一步就把离去基团顶出去， 整个分子像伞一样被翻了过来。
Below, the leaving group goes first, leaving a flat positive carbon.
下面一条：离去基团先走， 留下一个平面的、带正电的碳。
Let's take those routes apart, starting with S N two.
我们来把这两条路线仔细拆开，先看 S N 2。
It happens in one single step.
它只有一步。
The nucleophile comes in from the side opposite the halogen, and forms its bond while the old bond is still breaking.
亲核试剂从卤素的正对面进攻，在旧键还没断完时就开始形成新键。
Halfway through, the carbon is joined to five groups at once: that is the transition state.
进行到一半时，碳同时连着五个基团：这就是过渡态。
It is crowded, so bulky groups nearby slow it right down.
那里非常拥挤，所以碳周围体积大的基团会把它拖得很慢。
And since both molecules take part in that one step, the rate depends on both of them.
而由于两种分子都参与了这唯一的一步，速率同时取决于它们两个。
S N one works differently: two steps.
S N 1 则不同：它分两步。
In the slow first step the halogen leaves on its own, taking the bonding pair of electrons with it.
在慢的第一步中，卤素自己离去， 并把成键电子对一起带走。
What is left is a carbocation — a carbon with only three bonds and a positive charge, and it is flat.
剩下的是一个碳正离子—— 一个只有三根键、带正电荷的碳，而且它是平面的。
In the fast second step the nucleophile attacks that flat carbon.
在快的第二步中，亲核试剂进攻那个平面的碳。
Only the halogenoalkane appears in the slow step, so the rate depends only on the halogenoalkane.
慢步骤里只出现卤代烷，所以速率只取决于卤代烷。
So which molecule takes which route?
那么哪种分子走哪条路线呢？
Alkyl groups push electron density towards the carbon they are joined to: the inductive effect.
烷基会把电子密度推向与它相连的碳， 这叫做诱导效应。
The more alkyl groups around a positive carbon, the more its charge spreads out and the more stable the carbocation.
带正电的碳周围烷基越多，正电荷分散得越开， 碳正离子就越稳定。
So tertiary halogenoalkanes go by S N one: the carbocation is well stabilised, and three bulky groups block attack from behind.
所以叔卤代烷走 S N 1： 它的碳正离子很稳定，而三个大体积基团也挡住了背面的进攻。
Primary ones go by S N two, secondary ones use a mixture.
伯卤代烷走 S N 2，仲卤代烷则两者兼有。
Here is the classic exam question: predict the mechanism for hydrolysing these two bromides.
这是经典的考题：预测屏幕上这两种溴代烷水解时走什么机理。
Pause here and classify them yourself first.
先暂停，自己给它们分类。
The first carries its bromine on an end carbon with one carbon attached, so it is primary.
第一个的溴在链端的碳上， 那个碳只连着一个其他碳，所以它是伯卤代烷。
A nucleophile reaches the back of it easily, so it goes by S N two: one step, rate depending on both reactants.
亲核试剂很容易够到这个碳的背面，所以它走 S N 2： 一步完成，速率同时取决于两种反应物。
The second has its bromine on a carbon holding three methyl groups, so it is tertiary.
第二个的溴在一个连着三个甲基的碳上，所以它是叔卤代烷。
Those groups block the back attack but stabilise the carbocation, so it goes by S N one.
这些基团挡住了背面进攻，但也稳定了碳正离子，所以它走 S N 1。
And the twist: the crowded tertiary one hydrolyses faster.
而这里有个学生常忽略的反差：拥挤的那个叔卤代烷水解反而更快。
Finally: reactivity.
最后：反应活性。
Which halogenoalkane reacts fastest?
哪一种卤代烷反应最快？
One thing decides it — the strength of the bond between the carbon and the halogen.
只由一件事决定—— 碳与卤素之间那根键的强度。
Look at the bond energies, or bond enthalpy.
看屏幕上的键能数据，也就是键焓。
The carbon to iodine bond is by far the weakest, so it breaks most easily: iodoalkanes react fastest, and give their precipitate first.
碳碘键弱得多，所以最容易断，因此碘代烷反应最快， 也最先出现沉淀。
The carbon to chlorine bond is much stronger, so chloroalkanes are slowest.
碳氯键强得多，所以氯代烷最慢。
Careful: the answer is bond energy — bond enthalpy — not electronegativity.
这里要小心：答案是键能——键焓——不是电负性。
Three marks students throw away.
三个学生常丢的分。
First, curly arrows: each one starts at a lone pair or at a bond, never at an atom, and it points to where the electrons go.
第一，弯箭头：每一个都必须从孤对电子或一根键出发， 绝不能从一个原子出发，而且要指向电子去的地方。
Second, match the mechanism to the class: tertiary means two steps through a carbocation, primary means one step through a transition state.
第二，把机理和类别对应起来：叔卤代烷是经碳正离子的两步， 伯卤代烷是经过渡态的一步。
Third, when the question asks substitution or elimination, name the solvent.
第三，题目问取代还是消去时， 一定要写出溶剂。
Get those right, and this topic is yours.
把这些做对，这个专题就是你的了。