Carbonyl compounds
A-Level Chemistry Topic 17 8:03 English narration · English + 中文 subtitles burned in
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Open a bottle of nail polish remover, and that sharp smell is propanone.
打开一瓶洗甲水，那股刺鼻的气味就是丙酮。
Your own body makes a little of it every day.
你自己的身体每天也会产生一点点。
All of that comes from one small group of atoms: a carbon joined to an oxygen by a double bond.
这一切都来自一个很小的原子团： 一个碳原子用双键连着一个氧原子。
We call it the carbonyl group.
我们叫它羰基。
Learn how that one group behaves, and a whole family of reactions and tests opens up.
学会这一个基团的行为，一整个家族的反应和检验就都打开了。
Welcome to carbonyl compounds.
欢迎来到羰基化合物。
We will meet aldehydes and ketones, make them, react them, follow one mechanism carefully, and finish with the tests that tell them apart.
我们会认识醛和酮，学会制备它们、让它们反应、 仔细走一遍机理，最后用几个检验把它们区分开。
Let's begin.
让我们开始吧。
Every carbonyl compound contains that same carbon-oxygen double bond.
每一种羰基化合物都含有同样的碳氧双键。
What changes is where it sits.
不同的是它长在哪里。
In an aldehyde, the carbonyl carbon is at the end of the chain, so it also carries a hydrogen atom.
在醛中，羰基碳在链的末端，所以它还带着一个氢原子。
In a ketone, the carbonyl carbon is in the middle, with a carbon on each side.
在酮中，羰基碳在中间，两边各连着一个碳。
That one hydrogen is the whole story of this topic: it is what lets an aldehyde be oxidised, and it is why the tests later on work.
这一个氢就是整章的关键：正是它让醛容易被氧化， 也正是它让本课最后那些检验能够成立。
So how do we make them?
那么怎么制备它们呢？ 氧化醇。
By oxidation of an alcohol. Warm a primary alcohol with acidified potassium dichromate, or acidified potassium manganate, and use distillation to take the product off as it forms: you get an aldehyde.
把伯醇（primary alcohol）与酸化的重铬酸钾、 或酸化的高锰酸钾一起加热，并把产物边生成边蒸馏出来，得到的就是醛。
With the dichromate, watch the colour change from orange to green.
用重铬酸钾时，注意颜色由橙色变成绿色。
If you leave the aldehyde in the flask it is oxidised again, all the way to a carboxylic acid, so take it out quickly.
如果把醛留在烧瓶里，它会被再次氧化，一路变成羧酸，所以要尽快把它蒸出来。
A secondary alcohol gives a ketone, and there it stops. A ketone will not oxidise further.
仲醇（secondary alcohol）则给出酮，到此为止——酮不会再被氧化。
Now run it backwards.
现在把它倒过来做。
Reduction adds hydrogen across the double bond, and turns a carbonyl compound into an alcohol again.
还原是把氢加到双键上，让羰基化合物重新变回醇。
The reagent is sodium borohydride, or lithium aluminium hydride.
试剂是硼氢化钠，或者四氢铝锂。
And the product tells you where you started: an aldehyde gives a primary alcohol, a ketone gives a secondary alcohol.
而产物会告诉你起点是什么： 醛给出伯醇，酮给出仲醇。
Now the reaction the exam loves.
接下来是考试最爱的反应。
Warm a carbonyl compound with hydrogen cyanide, using a little potassium cyanide as the catalyst.
把羰基化合物与氰化氢一起加热， 用少量氰化钾作催化剂。
A hydrogen atom and a cyanide group add across the double bond, and the product is a hydroxynitrile: an alcohol group and a nitrile group on the very same carbon.
一个氢原子和一个氰基加到双键两端， 产物就是羟基腈：同一个碳上既有羟基又有腈基。
Now count the carbons.
现在数一数碳原子。
The cyanide brought one with it, so the chain is one carbon longer.
氰基自己带来了一个碳，所以碳链长了一个碳。
Examiners test that carbon count again and again, so always check it before you answer.
考官一次又一次地考这个碳数，所以选答案之前一定要先数一遍。
Remember where you have met a nucleophile before.
在讲机理之前，回忆一下你在哪里见过亲核试剂。
In a halogenoalkane, a nucleophile attacked a slightly positive carbon and pushed the halogen out. That is nucleophilic substitution.
在卤代烷中， 亲核试剂进攻一个略带正电的碳，把卤素挤了出去，那是亲核取代。
A carbonyl carbon is slightly positive too, so a nucleophile attacks it in the same way.
羰基上的碳也略带正电，所以亲核试剂会用同样的方式进攻它。
But here there is nothing to push out: both new parts stay on.
但这里没有东西可以被挤出去，两个新加上的部分都留在分子上。
This one is nucleophilic addition.
所以这一个叫亲核加成。
Here is why it happens.
来看它为什么会发生。
Oxygen is much more electronegative than carbon, so it pulls the shared electrons towards itself.
氧的电负性比碳大得多，所以它把共用电子拉向自己。
The oxygen end becomes slightly negative, and the carbon becomes slightly positive.
氧的一端略带负电，碳则略带正电。
That is exactly what a nucleophile is hunting for.
而略带正电的碳，正是亲核试剂在寻找的目标。
The cyanide ion carries a lone pair, and that lone pair attacks the carbon.
氰离子带有一对孤对电子，这对孤对电子进攻那个碳。
As the new bond forms, the double bond has to break, and those two electrons move up onto the oxygen.
新键形成的同时，双键必须断开，那两个电子移到氧上去。
That leaves the intermediate: a negatively charged oxygen, on a carbon that now holds the cyanide group.
这样就得到了中间体：一个带负电的氧，连在如今带着氰基的碳上。
It does not last long.
它存在不了多久。
The negative oxygen takes a hydrogen ion from a hydrogen cyanide molecule, and you have your hydroxynitrile.
带负电的氧从一个氰化氢分子上夺走一个氢离子， 羟基腈就做成了。
Look at what is released: a cyanide ion, ready to attack the next molecule.
注意被放出来的东西：一个氰离子，随时可以去进攻下一个分子。
That is why only a little potassium cyanide is needed.
这就是为什么只需要少量氰化钾。
And in the exam, every curly arrow must start on a lone pair or on a bond, and point to where those electrons end up.
另外在考试中， 每一支弯箭头都必须从一对孤对电子或者一根键出发，并指向电子最终去的地方。
Now the tests, and each one answers a different question.
现在来看检验，每一个检验回答一个不同的问题。
First question: is there a carbonyl group at all?
第一个问题： 到底有没有羰基？
Add the D N P H reagent.
加入 2，4-DNPH 试剂。
An orange precipitate says yes, this compound is an aldehyde or a ketone.
出现橙色沉淀就说明有， 这个化合物是醛或者酮。
But that is all it tells you.
但它能告诉你的就只有这些。
It cannot tell an aldehyde from a ketone, and students lose marks every year claiming that it can.
它不能区分醛和酮，每年都有学生因为说它能区分而丢分。
Second question: which of the two is it?
第二个问题：它是醛还是酮？
Think back to that hydrogen on the aldehyde carbon.
回想一下醛的碳上那个氢。
It makes an aldehyde easy to oxidise into a carboxylic acid; a ketone has none, so it resists.
它让醛很容易被氧化成羧酸，而酮没有这个氢，所以不容易被氧化。
Two tests use exactly that difference.
有两个检验正是利用这个差别。
Warm the compound with Tollens' reagent, and an aldehyde coats the tube with a silver mirror.
把化合物与托伦试剂一起温热， 醛会在试管壁上镀出一层银镜。
Warm it with Fehling's solution, which is blue, and an aldehyde gives a brick red precipitate.
把它与蓝色的斐林试剂一起温热， 醛会生成砖红色沉淀。
A ketone does nothing in either test, and here no change is itself the answer.
酮在这两个检验中都没有变化—— 在这两个检验里，"没有变化"本身就是答案。
Third question: is there a methyl group joined straight to the carbonyl carbon? That is the iodoform test.
第三个问题：羰基碳上有没有直接连着一个甲基？
Warm the compound with alkaline aqueous iodine.
把化合物与碱性碘水一起温热。
A pale yellow precipitate of iodoform — tri-iodomethane — says yes, and the rest of the molecule leaves as a carboxylate ion.
出现淡黄色的三碘甲烷沉淀就说明有， 分子的其余部分则以羧酸根离子的形式离开。
So propanone, ethanal and butanone all pass this test.
所以丙酮、乙醛和丁酮都能通过这个检验。
An alcohol with the same pattern passes too, because the iodine oxidises it to a carbonyl compound first.
具有同样结构特征的醇也能通过，因为碘会先把它氧化成羰基化合物。
Let's use all three.
我们把三个检验一起用。
A liquid gives an orange precipitate with D N P H, no silver mirror, and a yellow precipitate with alkaline iodine.
某液体与 2，4-DNPH 反应生成橙色沉淀， 与托伦试剂没有银镜，与碱性碘水生成黄色沉淀。
Identify it.
请鉴定它。
Take the tests one at a time.
一个一个来看。
The orange precipitate proves there is a carbonyl group.
橙色沉淀证明有羰基。
No silver mirror rules out an aldehyde, so it must be a ketone.
没有银镜排除了醛， 所以它一定是酮。
The yellow precipitate proves a methyl group beside that carbonyl.
黄色沉淀证明羰基旁边有一个甲基。
The simplest compound that fits all three is propanone.
同时满足这三点的最简单化合物就是丙酮。
Each test did one job; only together did they pin the molecule down.
注意每个检验只做一件事， 只有把它们合起来，才能把这个分子定下来。
Now one for you.
现在轮到你了。
Seven aldehydes and ketones each contain one oxygen atom and four carbons or fewer.
七种醛和酮，每一种都只含一个氧原子，碳原子不超过四个。
How many of them give a yellow precipitate with alkaline iodine?
其中有几种能与碱性碘水生成黄色沉淀？
Pause the video and count.
先暂停，数一数。
Ready?
好了吗？
You need a methyl group joined to the carbonyl carbon.
你需要的是直接连在羰基碳上的甲基。
Methanal has no methyl group at all, and in propanal, butanal and methylpropanal the methyl group sits too far away.
甲醛根本没有甲基； 在丙醛、丁醛和甲基丙醛中，甲基离羰基太远。
That leaves ethanal, propanone and butanone.
剩下的就是乙醛、丙酮和丁酮。
The answer is three.
答案是三种。
If you counted four, a methyl group one carbon too far away probably slipped in.
如果你数出了四种，多半是把一个隔了一个碳的甲基也算进去了。
Let's put the whole topic on one map.
我们把整章放在一张图上。
Oxidise a primary alcohol and distil, and you get an aldehyde. Oxidise a secondary alcohol and you get a ketone.
氧化伯醇并蒸馏，得到醛；氧化仲醇，得到酮。
Reduce either one, and you go straight back to the alcohol.
把它们中的任何一个还原，就直接回到醇。
Add hydrogen cyanide, and you move forward to a hydroxynitrile, one carbon longer.
加入氰化氢，就向前得到羟基腈， 碳链长了一个碳。
And underneath sit the three tests: any carbonyl, aldehyde only, methyl group.
而下面是那三个检验：任何羰基、只有醛、以及甲基。
Keep that map in your head, and every question in this topic has a place on it.
把这张图记在脑子里，这一章的每一道题都能在上面找到自己的位置。
Three marks students give away.
三个学生常送掉的分。
First, the D N P H test proves a carbonyl group and nothing more; only Tollens' or Fehling's separates an aldehyde from a ketone.
第一，2，4-DNPH 只能证明有羰基，仅此而已； 只有托伦试剂或斐林试剂才能区分醛和酮。
Second, when you draw the mechanism, start every curly arrow on a lone pair or a bond, and never forget the negative oxygen in the middle step.
第二，画机理时， 每一支弯箭头都要从一对孤对电子或一根键出发， 中间那一步不要漏掉带负电的氧。
Third, when hydrogen cyanide adds, count the carbons again: the chain has grown by one.
第三，加成氰化氢之后， 再数一遍碳原子：碳链多了一个碳。
Get those three right, and this topic is yours.
把这三点做对，这个专题就是你的了。