Carboxylic acids and derivatives (A2)
A-Level Chemistry Topic 33 8:08 English narration · English + 中文 subtitles burned in
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Open a medicine cabinet and pick up a packet of aspirin. That tablet is an ester.
打开药箱，拿出一板阿司匹林——这片药就是一种酯。
At AS you met one small group: the carboxyl group.
在 AS 阶段你学过一个小小的基团：羧基。
At A2 it becomes three new families — acyl chlorides, esters and amides.
到了 A2，它会变成三个新家族：酰氯、酯和酰胺。
And one reagent sits at the centre of them all, turning a quiet acid into a molecule that reacts the moment it meets water.
而有一种试剂位于它们的中心，它能把一个安静的酸， 变成一碰到水就立刻反应的分子。
Welcome to carboxylic acids and their derivatives.
欢迎来到羧酸及其衍生物。
We will make one from a benzene ring, rank three families by acid strength, build an acyl chloride, and draw the mechanism the exam asks for almost every year.
我们将从苯环出发制取一种羧酸， 把三类物质按酸性强弱排序，把羧酸变成酰氯， 并画出考试几乎年年都考的机理。
Where does a carboxylic acid come from?
羧酸是从哪里来的？
Start with an alkylbenzene: a benzene ring carrying an alkyl side chain, like methylbenzene.
从烷基苯开始：一个带有烷基侧链的苯环，例如甲苯。
Heat it with hot alkaline potassium manganate seven; that is your reagent. Then add dilute acid.
用热的碱性高锰酸钾加热它，这就是你要用的试剂；然后加入稀酸。
The product is benzoic acid.
产物就是苯甲酸。
And notice: the whole side chain, however long it was, is cut back to a single carboxyl group on the ring.
注意：不管侧链原来有多长， 它都会被切短，变成环上的一个羧基。
Two carboxylic acids can be oxidised further.
有两种羧酸还能被继续氧化。
Methanoic acid still carries a hydrogen atom on its carbonyl group, so it behaves like an aldehyde: Fehling's, Tollens' or acidified manganate all take it to carbon dioxide and water.
甲酸的羰基上还带着一个氢原子，所以它的表现有点像醛： 斐林试剂、多伦试剂或酸性高锰酸钾，都能把它氧化成二氧化碳和水。
Ethanedioic acid is simply two carboxyl groups joined together, and warm acidified manganate takes it to carbon dioxide too.
乙二酸就是两个羧基连在一起，温热的酸性高锰酸钾也能把它氧化成二氧化碳。
But every other carboxylic acid resists oxidation.
但其余的羧酸都不会再被氧化。
Now, acid strength.
接下来是酸性强弱。
Three families can lose a hydrogen ion, and you must be able to rank them.
有三类物质可以失去氢离子，你必须会给它们排序。
An alcohol is the weakest: its negative charge is stuck on one oxygen.
醇最弱：它的负电荷被困在一个氧原子上。
A phenol is stronger, because that charge spreads into the ring.
酚更强，因为这个负电荷可以分散到苯环里。
And a carboxylic acid is the strongest of the three: the charge is shared equally over two oxygen atoms.
而羧酸是三者中最强的：电荷平均分布在两个氧原子上。
Here is the rule — the more the charge spreads, the more stable the ion, and the stronger the acid.
规律就是：电荷分散得越开，离子越稳定，酸性也就越强。
Small changes matter too.
分子上的小改动同样重要。
Look at this series: ethanoic acid, then the same acid with one chlorine, two, then three.
看这一组：乙酸，然后是带一个氯、两个氯、三个氯的。
Each extra chlorine makes it stronger.
每多一个氯，酸性就更强。
Chlorine is very electronegative, so it pulls electron density along the bonds, away from the carboxyl group.
氯的电负性很大， 它会沿着化学键把电子密度拉走，远离羧基。
The charge on the ion spreads further, so the ion is more stable.
离子上的电荷分散得更开，于是离子更稳定。
That is the inductive effect — and the closer the chlorine sits, the bigger it is.
这就是诱导效应——而且氯离得越近，作用越大。
Now a real one, from the twenty twenty-five paper.
来做一道真题，取自二〇二五年的试卷。
Ethanol, ethanoic acid, bromoethanoic acid, chloroethanoic acid.
四种化合物：乙醇、乙酸、溴乙酸和氯乙酸。
Put them in order, most acidic first, and explain.
请按酸性由强到弱排序，并解释。
Pause and try it.
先暂停，自己试一试。
Here is the order: chloroethanoic acid, bromoethanoic acid, ethanoic acid, ethanol last.
顺序是：氯乙酸、溴乙酸、乙酸，乙醇最弱。
Now the marks.
现在是给分点。
One: electron-withdrawing groups stabilise the anion.
第一：吸电子基团能稳定负离子。
Two: chlorine is more electronegative than bromine, so it comes first.
第二：氯的电负性比溴大，所以它排在最前面。
Three: ethanol is an alcohol, and its ion cannot spread the charge at all.
第三：乙醇是醇，它的离子完全不能分散电荷。
Next, the acyl chloride.
接下来是酰氯。
You make one from a carboxylic acid, and there are three reagents to remember: phosphorus trichloride with heat, phosphorus pentachloride, or thionyl chloride.
它由羧酸制得，有三种试剂要记住： 三氯化磷并加热、五氯化磷，或者亚硫酰氯。
Here is the equation with thionyl chloride: ethanoic acid gives ethanoyl chloride, sulfur dioxide and hydrogen chloride.
这是用亚硫酰氯的方程式：乙酸生成乙酰氯、二氧化硫和氯化氢。
Thionyl chloride is neatest, because both by-products are gases and simply escape.
亚硫酰氯最省事，因为两种副产物都是气体，会自己跑掉。
And from a real paper: with two carboxyl groups, you need two thionyl chlorides.
来自真题的提醒：如果有两个羧基，就需要两个亚硫酰氯。
So what does an acyl chloride do?
那么酰氯会发生什么反应？
Five reactions, and they all follow the same pattern: room temperature, and hydrogen chloride every time. Add water, and you get the carboxylic acid back. Add an alcohol, and you get an ester.
五个反应，而且它们遵循同一个模式： 都在室温下进行，都放出氯化氢。
Add phenol, and you get an ester too. Add ammonia, and you get an amide. Add a primary or secondary amine, and you get an amide again.
加水，得到原来的羧酸；加醇，得到酯；加酚，同样得到酯； 加氨，得到酰胺；加伯胺或仲胺，还是得到酰胺。
That is why an acyl chloride is the chemist's connector.
这就是为什么酰氯是化学家的连接件。
The syllabus names two esters, so look at that route closely.
我们把制酯这条路线看得更仔细些，因为考纲点名了两个例子。
Take ethanol, add ethanoyl chloride at room temperature, and you get ethyl ethanoate.
取乙醇，在室温下加入乙酰氯，得到乙酸乙酯。
Now take phenol, with benzoyl chloride, and you get phenyl benzoate — phenol is a weak nucleophile, and this is exactly the job an acyl chloride is good at.
换成苯酚，加入苯甲酰氯，得到苯甲酸苯酯； 苯酚是很弱的亲核试剂，而这正是酰氯最擅长的场合。
And the naming: the alcohol part comes first, then the acid part.
至于命名：先写来自醇的那部分，再写来自酸的那部分。
Before the mechanism, one question: why is an acyl chloride so much more reactive than an ester?
在讲机理之前先问一句：为什么酰氯比酯活泼得多？
Look at the drawing.
看这张图。
The carbonyl carbon has an oxygen on one side and a chlorine on the other, and both pull electron density away.
羰基碳的一边是氧，另一边是氯，两者都在把电子密度拉走。
The carbon is left strongly positive, so it is wide open to a nucleophile.
这个碳带上很强的正电性，所以完全暴露在亲核试剂面前。
And chloride is a good leaving group: once it has gone, the ion is stable.
而且氯离子是好的离去基团：一旦离开，这个离子很稳定。
An ester is far less reactive: its own oxygen donates a lone pair into the carbonyl, reducing the positive charge, and its leaving group is poor.
酯要迟钝得多：它自己的氧会把孤对电子给回羰基，降低正电性，而且它的离去基团很差。
The mechanism's name is the plot: addition, then elimination.
现在讲机理，它的名字就是剧情：先加成，再消去。
First, addition.
第一步，加成。
A nucleophile attacks the carbonyl carbon, and the carbon oxygen double bond opens, pushing a pair of electrons onto the oxygen.
亲核试剂进攻羰基碳，碳氧双键打开，把一对电子推到氧上。
In the middle we get an intermediate: four groups on the carbon, and a negative oxygen.
中间得到一个中间体：碳上连着四个基团，氧带负电荷。
Second, elimination: hydrogen chloride is eliminated.
第二步，消去。
The double bond forms again, and chlorine is forced out as hydrogen chloride.
双键重新形成，氯被挤出去，以氯化氢的形式离开。
Overall it looks like a simple swap, but the route is addition first, elimination second.
整体看起来像一次简单的取代，但路线是先加成、后消去。
Now the exam question itself, worth five marks.
这就是那道真题，总共五分。
The paper used an acyl bromide, which behaves identically.
原题用的是酰溴，机理完全相同。
Ethanoyl chloride reacts with water.
乙酰氯与水反应。
Part one: name the mechanism.
第一问：写出机理的名称。
The answer is nucleophilic addition–elimination.
答案是亲核加成消去。
Part two: draw it.
第二问：把它画出来。
Start by marking the dipole: the carbonyl carbon slightly positive, the oxygen slightly negative.
先标出偶极：羰基碳带部分正电，氧带部分负电。
Now the first arrow: it starts on a lone pair of the water oxygen and points at the carbonyl carbon.
第一支箭：从水中氧的一对孤对电子出发，指向羰基碳。
The second arrow starts on the carbon oxygen double bond and points at the oxygen.
第二支箭：从碳氧双键出发，指向氧原子，双键随之打开。
That gives the intermediate.
这样就得到中间体。
Now the third arrow: it starts on a lone pair of the negative oxygen and points back at the carbon oxygen bond.
第三支箭：从带负电的氧的孤对电子出发，指回碳氧键，双键重新形成。
And the fourth arrow starts on the carbon chlorine bond and points at the chlorine, which leaves as a chloride ion.
第四支箭：从碳氯键出发，指向氯原子，氯以氯离子的形式离开。
Four arrows, four marks.
四支箭，四个分。
A favourite comparison: ease of hydrolysis — how easily do three different chlorides react with water?
一道很常考的比较题：三种不同的氯代物与水反应的难易如何？
The order is acyl chloride, then alkyl chloride, then aryl chloride.
顺序是酰氯、卤代烷、卤代芳烃。
An acyl chloride reacts violently with cold water — no heat, no catalyst.
酰氯与冷水就剧烈反应，不用加热，也不用催化剂。
An alkyl chloride is much slower; you warm it with aqueous alkali.
卤代烷慢得多，要用氢氧化钠水溶液加热。
And an aryl chloride does not react at all: the chlorine's lone pair joins the ring, which strengthens the carbon chlorine bond.
卤代芳烃完全不反应：氯的孤对电子与苯环相连，使碳氯键更强。
So always give both reasons — how positive the carbon is, and how good the leaving group is.
所以两个理由都要写：碳的正电性有多强，离去基团有多好。
Three marks students throw away.
三个学生常丢的分。
First, curly arrows: every arrow must start on a bond or a lone pair, and end where the electrons actually go.
第一，弯箭头：每一支箭都必须从一条键或一对孤对电子出发， 并指向电子真正去的地方。
Second, acidity: explain it through the ion, never the acid — the charge is spread out, so the ion is more stable.
第二，酸性：要通过离子来解释，而不是通过酸本身—— 电荷分散开了，所以离子更稳定。
Electron-withdrawing groups increase acid strength — explain via the carboxylate ion.
吸电子基团会增强酸性——用羧酸根离子来解释。
Third, comparisons: give both reasons, every time.
第三，比较题：每次都要给出两个理由。
Get those right, and this topic is yours.
把这三点做对，这个专题就是你的了。