Nitrogen compounds
A-Level Chemistry Topic 34 7:49 English narration · English + 中文 subtitles burned in
Chapters
Transcript
Look at three very different things. A sweet, dyed bright orange. A painkiller tablet.
看三样完全不同的东西：一颗染成亮橙色的糖、一片止痛药，还有你肌肉里的蛋白质。
And the protein in your muscles. They share almost nothing — except one atom.
它们几乎没有共同点——除了一种原子。
All three are built around nitrogen.
这三者都以氮为核心。
And in all three, the same small pair of electrons on that nitrogen decides how the molecule behaves.
而且在这三者里，氮上同样的那一小对电子，决定了分子的行为。
Welcome to nitrogen compounds.
欢迎来到含氮化合物。
We will make amines, explain why some are far stronger bases than others, build a bright dye from benzene, and finish with amides and amino acids.
我们会制备胺，解释为什么有些胺的碱性强得多， 从苯出发做出一种鲜艳的染料，最后学习酰胺和氨基酸。
An amine is ammonia with the hydrogens swapped for carbon groups.
胺就是把氨分子里的氢换成碳基团。
Swap one hydrogen, and you have a primary amine. Swap two, and you have a secondary amine.
换掉一个氢，就得到伯胺；换掉两个氢， 就得到仲胺。
Count the carbon groups on the nitrogen — that is all those words mean.
数一数氮上连了几个碳基团——这两个词的意思仅此而已。
And notice what never changes: the nitrogen still keeps one pair of electrons to itself.
再注意一件始终不变的事：氮上仍然保留着一对属于自己的电子。
That pair runs this whole lesson.
整节课都围绕这对电子展开。
Four ways to make an amine, and the exam wants the reagent and the conditions every time.
制备胺有四条路，考试每次都要求写出试剂和条件。
One: heat a halogenoalkane with ammonia in ethanol, under pressure.
第一：把卤代烷与溶在乙醇中的氨 加压加热。
Two: use a primary amine in place of the ammonia — that gives a secondary amine.
第二：用伯胺代替氨——得到仲胺。
Three: reduce an amide with lithium aluminium hydride.
第三：用氢化铝锂还原酰胺。
Four: reduce a nitrile, with the same reagent or with hydrogen over nickel.
第四：用同样的试剂，或者用氢气和镍催化剂，还原腈。
So what does that lone pair do?
那么这对孤对电子有什么用？
Job one: it makes the amine a base.
第一件事：它让胺成为碱。
Basicity comes from that lone pair: a base accepts a hydrogen ion, and a lone pair is exactly what you need to hold one.
碱性就来自那对孤对电子： 碱就是能接受氢离子的物质，而孤对电子正是抓住氢离子所需要的。
Put an amine in water, and the nitrogen takes a hydrogen ion from a water molecule, leaving a hydroxide ion behind.
把胺放进水里， 氮就从水分子上夺走一个氢离子，留下一个氢氧根离子。
So the solution turns alkaline.
所以溶液呈碱性。
Which is the strongest base?
接下来是考试最爱考的问题：谁的碱性最强？
Ammonia is the reference, in the middle.
氨在中间，作为参照。
Ethylamine is stronger, because its alkyl group pushes electron density onto the nitrogen, so the lone pair is more available.
乙胺更强， 因为它的烷基把电子密度推向氮，使孤对电子更容易被利用。
Phenylamine is much weaker, because its lone pair is pulled into the benzene ring and spread out.
苯胺弱得多， 因为它的孤对电子被拉进苯环并分散开来。
So the order is phenylamine, ammonia, ethylamine.
所以顺序是：苯胺、氨、乙胺。
Phenylamine is a benzene ring with an amine group. Build it from benzene in two steps.
苯胺就是带有胺基的苯环，可以从苯经两步制得。
Step one, nitration: concentrated nitric acid with concentrated sulfuric acid, at about fifty five degrees.
第一步，硝化：用浓硝酸和浓硫酸， 约五十五摄氏度。
Step two, reduction: tin with concentrated hydrochloric acid, heated.
第二步，还原：用锡和浓盐酸，加热。
That leaves a salt, so you add sodium hydroxide solution to set the amine free.
这一步得到的是盐， 所以还要加氢氧化钠溶液，把游离的胺释放出来。
Nitrate first, then reduce.
先硝化，再还原。
Phenylamine has two reactions to learn.
苯胺有两个反应要记。
First, bromine water, at room temperature.
第一，室温下的溴水。
No catalyst and no heat: the amine group switches the ring on, three bromines go straight on, and a white precipitate drops out.
不用催化剂，也不加热： 胺基活化了苯环，三个溴直接取代上去，生成白色沉淀。
Second, diazotisation: nitrous acid — from sodium nitrite and dilute hydrochloric acid — below ten degrees, giving the diazonium salt.
第二，重氮化：亚硝酸—— 由亚硝酸钠和稀盐酸现制——温度在十摄氏度以下，得到重氮盐。
Let it warm up with water instead, and you get phenol.
如果让它和水一起升温，得到的就是苯酚。
Now for the colour.
现在讲颜色。
Take that cold diazonium salt and add phenol dissolved in sodium hydroxide solution.
取那份冷的重氮盐，加入溶于氢氧化钠溶液的苯酚。
The two rings join through a pair of nitrogen atoms.
两个苯环通过一对氮原子连接起来。
Look at the link between them: a nitrogen to nitrogen double bond.
看看它们之间的这个连接：一个氮氮双键。
That is the azo group, and you must be able to point at it.
这就是偶氮基，你必须能把它指出来。
Azo compounds are strongly coloured — which is why so many dyes are made this way.
偶氮化合物颜色很深， 所以很多染料都是这样制成的。
Starting from benzene, give the reagents and conditions for every step that leads to an azo dye.
我们把它串起来。 从苯出发，写出制得偶氮染料每一步的试剂和条件。
Pause here and try it.
先暂停，自己试一试。
Step one: concentrated nitric acid and concentrated sulfuric acid, fifty five degrees.
准备好了吗？ 第一步：浓硝酸和浓硫酸，五十五摄氏度。
Step two: tin and concentrated hydrochloric acid, then sodium hydroxide solution.
第二步：锡和浓盐酸，然后加氢氧化钠溶液。
Step three: sodium nitrite and dilute hydrochloric acid, below ten degrees.
第三步：亚硝酸钠和稀盐酸， 十摄氏度以下。
Step four: phenol in sodium hydroxide solution.
第四步：溶于氢氧化钠溶液的苯酚。
Four steps, and every mark is a reagent with its condition.
四步，每一分都是一个试剂加它的条件。
Put the nitrogen next to a carbonyl group and you have an amide.
把氮放在羰基旁边，就得到酰胺。
To make one, add ammonia to an acyl chloride at room temperature; hydrogen chloride is lost, so it is a condensation.
制备方法是：室温下把氨加到酰氯上； 过程中失去氯化氢，所以是缩合反应。
Use a primary amine instead, and the nitrogen carries a carbon group.
如果改用伯胺，氮上就带一个碳基团。
Hydrolysis: aqueous acid gives the carboxylic acid, aqueous alkali gives the carboxylate plus ammonia.
水解：与稀酸共热得到羧酸，或与碱溶液共热得到羧酸盐和氨。
And reduction: lithium aluminium hydride gives an amine.
还原：氢化铝锂把酰胺变成胺。
Here is the surprise.
这里有个意外。
An amide has a nitrogen with a lone pair, just like an amine — but an amide is a far weaker base. Almost neutral. Why?
酰胺的氮上也有一对孤对电子，和胺一样——但酰胺的碱性弱得多， 几乎是中性的。
It is delocalised into the neighbouring carbon to oxygen double bond, and a pair that is spread out cannot accept a hydrogen ion.
为什么？ 因为这对孤对电子并没有留在氮上， 而是离域到旁边的碳氧双键上；分散开来的电子对无法接受氢离子。
Same story as phenylamine.
和苯胺是同一个道理。
Last part: amino acids.
最后一部分：氨基酸。
An amino acid carries two groups at once — a basic amine group and an acidic carboxylic acid group.
氨基酸同时带有两个基团——一端是碱性的胺基， 另一端是酸性的羧基。
So it reacts with itself. The acid end hands its hydrogen ion to the amine end, giving an ion with a positive end and a negative end, but no overall charge.
所以它会跟自己反应：酸的一端把氢离子交给胺的一端， 形成一个一端带正电、一端带负电、但整体不带电荷的离子。
That is a zwitterion.
这就是两性离子。
The one pH where almost every molecule is a zwitterion is the isoelectric point. Learn that wording exactly.
而几乎所有分子都以两性离子形式存在的那个 pH，就叫等电点。
Two amino acids can also join together.
两个氨基酸之间也能连接起来。
The acid group of one meets the amine group of the other.
一个分子的羧基遇上另一个分子的胺基。
An oxygen and a hydrogen leave one side, a hydrogen leaves the other, and together they go as water.
一边失去一个氧和一个氢，另一边失去一个氢，它们合起来以水的形式离开。
The new link is the peptide bond — and look closely, because it is simply an amide link.
新形成的连接就是肽键——仔细看，它其实就是一个酰胺键。
Two amino acids make a dipeptide, three make a tripeptide, and thousands of them make a protein.
两个氨基酸生成二肽，三个生成三肽，几千个就生成一条蛋白质。
Now we can separate them.
现在我们可以把它们分开了。
In electrophoresis the mixture is placed on a gel at a chosen pH, and a voltage is switched on.
在电泳中，把混合物点在选定 pH 的凝胶上，然后通电。
An amino acid that is positive is pulled to the negative electrode.
带正电的氨基酸被拉向负极；带负电的被拉向正极。
One that is negative is pulled to the positive electrode. And one at its own isoelectric point has no net charge, so it does not move at all.
而正好处在自己等电点上的氨基酸没有净电荷，所以完全不移动。
That is how a mixture separates.
混合物就是这样被分开的。
An amino acid has an isoelectric point of pH six.
某氨基酸的等电点是 pH 六。
Give its charge, and the electrode it moves to, at pH two, six and eleven.
写出它在 pH 二、六和十一时的电荷，以及它移向哪个电极。
Pause and try all three.
先暂停，三种情况都试一试。
At pH two, well below, the amine group takes a hydrogen ion, so the molecule is positive and moves to the cathode — the negative electrode.
准备好了吗？ 在 pH 二，远低于等电点，胺基获得一个氢离子，分子带正电，所以移向阴极——负极。
At pH six, exactly at the isoelectric point, it is the zwitterion and stays put.
在 pH 六，正好是等电点，它是两性离子，所以不动。
At pH eleven, well above, the acid group loses a hydrogen ion, so it is negative and moves to the anode — the positive electrode.
在 pH 十一，远高于等电点，羧基失去氢离子，分子带负电，所以移向阳极——正极。
Always compare with the isoelectric point, never with seven.
永远和等电点比较，不要和七比较。
Three marks students throw away.
三个学生常丢的分。
First, phenylamine: asked why it is a weaker base, say the lone pair is delocalised into the ring.
第一，苯胺：被问到它为什么碱性更弱时， 要写孤对电子离域进了苯环。
Second, the diazonium salt: give the reagent and the temperature together — miss the temperature and you have made phenol.
第二，重氮盐：试剂和温度必须一起写， 漏了温度，你做出来的就是苯酚。
Third, amino acids: compare the pH with the isoelectric point, never with seven.
第三，氨基酸：把 pH 和等电点比较，不要和七比较。
Get those right, and this topic is yours.
这三点做对，这个专题就是你的了。