Carboxylic acids and derivatives
A-Level Chemistry Topic 18 8:02 English narration · English + 中文 subtitles burned in
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Open a bottle of vinegar and you know it at once.
打开一瓶醋，你立刻就能闻出来。
That sharp smell is ethanoic acid, and vinegar is only a weak solution of it in water.
那股刺鼻的味道来自乙酸，醋不过是它的稀水溶液。
Pour a little onto baking soda and watch it fizz.
把一点醋倒在小苏打上，它会剧烈冒泡，放出气体。
Now smell a pear drop, or a bottle of perfume. Sweet, fruity, nothing like vinegar at all.
再闻一闻水果糖或者一瓶香水： 甜甜的、带果香，和醋完全不一样。
Yet both smells come from the same small group of atoms, changed in one step.
但这两种气味，都来自同一小组原子， 只是被简单地改动了一下。
Welcome to carboxylic acids and their derivatives.
欢迎来到羧酸及其衍生物。
We will meet the carboxyl group, make these acids three ways, watch them react like any other acid, then turn them into esters and split those esters apart again.
我们会认识羧基，用三种方法制取羧酸， 看它们像普通的酸一样反应，然后把它们变成酯，再把酯拆开。 让我们开始吧。
Start with the group itself.
先看这个基团本身。
Every carboxylic acid ends in the carboxyl group.
每一种羧酸的末端都是羧基。
Look at it closely. One carbon carries two things: an oxygen joined by a double bond, and an oxygen hydrogen group beside it.
仔细看：一个碳原子上带着两样东西—— 一个用双键相连的氧，以及旁边的一个羟基。
Both on the same carbon.
两者连在同一个碳上，这一点最关键。
With the double bonded oxygen next door, the hydrogen on that oxygen is held loosely. It can leave as a hydrogen ion, so the molecule behaves as an acid.
因为旁边有双键氧，羟基上的那个氢结合得很松，可以以氢离子的形式离开， 所以这个分子表现为酸。
Exam papers write this group in two different ways, and they mean the same thing.
试卷上会用两种写法表示这个基团，含义完全相同。
The names always end in oic acid, and the carboxyl carbon is number one.
名称总是以“酸”结尾，而羧基上的碳永远是第一号碳。
How do you make one?
怎么制取羧酸？
There are three routes, each with its own conditions.
考纲给了三条路线，每一条都要连同条件一起记。
Route one: oxidation of a primary alcohol, or an aldehyde, with acidified potassium dichromate or acidified potassium manganate. Here the condition earns the mark.
路线一：用酸化的重铬酸钾或酸化的高锰酸钾，氧化伯醇或者醛。
You must heat it under reflux, so the alcohol goes all the way to the acid instead of stopping at the aldehyde.
这里条件就是得分点： 必须加热回流，醇才会一路氧化成酸，而不是停在醛。
Route two: take a nitrile and hydrolyse it with dilute acid, or dilute alkali, then acidify.
路线二：取一个腈，用稀酸水解，或者用稀碱水解后再酸化。
Route three: take an ester and hydrolyse that, with dilute acid or alkali and heat, then acidify.
路线三：取一个酯，同样用稀酸或稀碱加热水解，然后酸化。
Let's use route one on a real exam question.
我们用路线一来做一道真题。
This molecule is heated under reflux with excess acidified potassium dichromate. What is the final product?
这个分子与过量的酸化重铬酸钾一起加热回流， 直到不再反应为止，最终产物是什么？
Find the groups first. There are two of them, one at each end.
先找官能团：一共有两个，分子两端各一个。
Look at the left hand end: it is a primary alcohol, so with excess oxidising agent it is pushed past the aldehyde, all the way to a carboxylic acid.
看左边一端：它是伯醇，在过量氧化剂和回流条件下，会越过醛，一路氧化成羧酸。
Now the right hand end is already an aldehyde, and aldehydes oxidise to carboxylic acids too.
再看右边一端：它本来就是醛，而醛同样会被氧化成羧酸。
Both ends finish as carboxyl groups. The answer is ethanedioic acid.
两端最后都变成羧基，所以答案是乙二酸。
Why do we call it a weak acid?
为什么说它是弱酸？
In water, only a small fraction of the molecules give their hydrogen away. Most stay whole, so it is far less acidic than hydrochloric acid of the same concentration.
在水中，只有一小部分分子会把氢给出去，大多数保持完整， 所以同浓度下它的酸性远远弱于盐酸。
But the ion left behind is the delocalised carboxylate ion.
但剩下的那个离子却格外稳定：它是离域的羧酸根离子。
Its negative charge is not stuck on one oxygen. It is spread over both oxygen atoms equally, and a spread out charge is a stable charge.
它的负电荷不是固定在一个氧上，而是平均分布在两个氧原子上， 电荷分散开来就更稳定。
Anything that pulls electrons away steadies it further, so the acid gets stronger.
任何把电子拉走的基团，都会让这个离子更稳定，酸性也就更强。
Put chlorine atoms next to the carboxyl group, and the strength goes up.
在羧基旁边接上氯原子，酸性就会上升。
Carboxylic acids may be weak, but they do everything an acid should do.
羧酸虽然是弱酸，但酸该有的反应它都有。
With a reactive metal such as magnesium you get a salt and hydrogen gas, and that one is a redox reaction.
与镁这样的活泼金属反应，生成盐和氢气，这是一个氧化还原反应。
With an alkali such as sodium hydroxide you get a salt and water: plain neutralisation.
与氢氧化钠这样的碱反应，生成盐和水，这就是中和。
With a carbonate you get a salt, water and carbon dioxide, and the fizzing of that gas is the standard test for a carboxylic acid.
与碳酸盐反应，生成盐、水和二氧化碳，冒泡放出二氧化碳正是检验羧酸的标准方法。
Now the trap, straight from a past paper.
注意陷阱，真题考过：与金属反应放出的是氢气，不是水。
With a metal the gas is hydrogen, not water.
写反了，这一分就没了。
Two more reactions change the functional group itself.
还有两个反应会改变官能团本身。
Warm a carboxylic acid with an alcohol and a little concentrated sulfuric acid, and you make an ester. That fills the rest of this lesson.
把羧酸和醇以及少量浓硫酸一起加热， 就得到酯，这个反应占据了本节课余下的内容。
Or reduction of the acid with lithium aluminium hydride, and the carboxyl group goes back to a primary alcohol.
或者用四氢铝锂还原羧酸，羧基就会一路变回伯醇。
Notice what that is: route one, run backwards.
注意这是什么：正是路线一反过来走。
So what is an ester?
那么酯是什么？
Take the carboxyl group and swap the hydrogen of its oxygen hydrogen group for a carbon chain. What is left is the ester link: a carbon with a double bonded oxygen, joined through a single oxygen to another carbon.
把羧基上羟基的那个氢，换成一条碳链，剩下的就是酯基： 一个带双键氧的碳，通过一个单键氧连到另一个碳上。
Esters are the sweet smelling ones, in fruits and perfumes. Their names have two words.
酯就是那些有甜味香气的物质，存在于水果和香水里。
The alcohol gives the first word; the acid gives the second, ending in oate.
酯的名称有两个词，规则是：醇给出第一个词，酸给出第二个词。
Now make one. Warm a carboxylic acid together with an alcohol.
现在来制取一个酯。
Watch which atoms leave. The acid gives up the whole oxygen hydrogen group. The alcohol gives up only the hydrogen from its own.
把羧酸和醇一起加热，注意看哪些原子离开： 酸交出羧基上整个羟基，醇只交出自己羟基上的那个氢。
Those pieces leave as a molecule of water, and the two halves that remain bond straight to each other.
这些碎片一起以一分子水的形式离开，剩下的两半直接连在一起。
Because a small molecule is lost, this is a condensation reaction.
因为脱去了一个小分子，所以这是一个缩合反应。
The catalyst is concentrated sulfuric acid.
催化剂是浓硫酸。
But there is a catch, and it costs marks. Esterification never finishes.
但有一个要点，很容易失分：酯化反应永远不会进行到底。
As soon as ester and water have built up, they begin reacting back to the acid and the alcohol.
一旦生成了酯和水，它们就开始反过来生成酸和醇。
The forward rate falls, the reverse rate rises, and once the two are equal the amounts stop changing. That is dynamic equilibrium.
正反应速率下降，逆反应速率上升，当两者相等时，各物质的量不再变化， 这就是动态平衡。
So the yield is never complete, and the equation is written with a double arrow.
所以产率永远不可能是百分之百，方程式要用可逆符号。
Time for the classic naming question. Name the ester made from ethanol and propanoic acid.
来做经典的命名题：写出乙醇和丙酸生成的酯的名称。
Take the two words in order.
按顺序取两个词。
First, the alcohol. Ethanol gives an ethyl group, so the first word is ethyl.
第一，醇：乙醇提供乙基，所以第一个词是乙。
Second, the acid. Propanoic acid gives the propanoate part.
第二，酸：丙酸提供丙酸酯的部分。
Put them together, and the ester is ethyl propanoate.
合起来，这个酯就是丙酸乙酯。
Here is the equation, with water as the other product and a double arrow.
这里是方程式，另一个产物是水， 并且用可逆符号，因为反应是可逆的。
Written backwards it becomes propyl ethanoate: a different compound, from a different alcohol and acid.
如果把名称写反，就成了乙酸丙酯， 那是完全不同的化合物，来自不同的醇和不同的酸。 永远是醇在前，酸在后。
Esters can be split apart again. That is hydrolysis, and the conditions decide the products.
酯可以被重新拆开，这就是水解，而条件决定产物。
With dilute acid and heat, this is esterification running backwards, so it is reversible, and you get the carboxylic acid and the alcohol.
用稀酸加热，其实就是酯化反应反过来走，所以是可逆的，得到羧酸和醇。
With dilute alkali and heat, something better happens: the acid that forms is neutralised at once, so it can never react back.
用稀碱加热则更彻底：生成的酸立刻被中和，因此无法再反应回去。
The reaction goes to completion, and the products are the alcohol and the salt of the acid.
反应可以进行到底，产物是醇和这个酸的盐。
One last worked example, from a past paper.
最后一道例题，直接取自真题。
This ester is heated with aqueous sodium hydroxide.
这个酯与氢氧化钠水溶液共热，产物是哪一种化合物？ 分三步走。
Which compound is a product? Split the molecule at the single oxygen of the ester link. On the left is the acid half, the part with the double bonded oxygen; on the right is the alcohol half.
先在酯基的单键氧处把分子断开：左边是酸的一半，也就是带双键氧的部分； 右边是醇的一半。
Take the alcohol half first: three carbons with an oxygen hydrogen group, so that half is propan one ol.
先看醇的一半：三个碳加一个羟基，所以是丙一醇。 再看酸的一半。
Now the acid half. The conditions are alkaline, so do not stop at ethanoic acid: it is neutralised at once, so the product is sodium ethanoate, the salt.
条件是碱性的，所以不能停在乙酸：它会立刻被中和， 因此产物是乙酸钠这个盐。
Three marks students throw away.
三个学生常丢的分。
First, to tell a carboxylic acid from an alcohol or a phenol, add a carbonate. Only the carboxylic acid fizzes and gives carbon dioxide.
第一，要把羧酸与醇或酚区分开，就加碳酸盐： 只有羧酸会冒泡并放出二氧化碳。
Second, always write the conditions: reflux with the acidified oxidising agent, dilute acid or alkali for a hydrolysis, concentrated sulfuric acid for an ester.
第二，一定要写条件： 路线一是回流加酸化的氧化剂，水解是稀酸或稀碱，制酯是浓硫酸。
A reagent with no condition is half an answer.
只写试剂不写条件，只能算半个答案。
Third, name esters the right way round, alcohol first, and alkaline hydrolysis gives the salt, not the acid.
第三，酯的命名顺序不要反，醇在前； 还要记住碱性水解得到的是盐，不是酸。