Halogen compounds (A2)
A-Level Chemistry Topic 31 7:56 English narration · English + 中文 subtitles burned in
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A farmer sprays a field.
农民在田里喷药。
Many of the pesticides and weedkillers in that tank are halogenoarenes: a chlorine atom bonded straight onto a benzene ring.
药罐里的许多农药和除草剂都是卤代芳烃：一个氯原子直接连在苯环上。
Now look at two molecules. Each one contains a single carbon to chlorine bond.
现在看这两个分子，它们都只含有一个碳氯键。
Warm the first with sodium hydroxide solution and it becomes an alcohol in minutes.
把第一个和氢氧化钠溶液加热， 几分钟内它就变成了醇。
Warm the second the same way, and nothing happens at all.
用同样的方法加热第二个，却什么都不会发生。
This is the A2 side of halogen chemistry: halogen compounds built on a benzene ring.
这是卤素化学的 A2 部分：建立在苯环上的卤代化合物。
We will make one, follow the mechanism, choose between the ring and the side chain, and then explain why a halogen on a ring hardly reacts at all.
我们会制备一个，跟着机理走一遍， 在环上和侧链之间做出选择，最后解释为什么环上的卤原子几乎不反应。
Start with three molecules and three names, because the exam marks the words.
先记住三个分子和三个名称，因为考试就是按用词给分的。
Chloroethane is a halogenoalkane: its chlorine sits on a carbon chain.
氯乙烷是卤代烷： 氯原子连在碳链上。
Chlorobenzene is a halogenoarene, also called an aryl halide: its chlorine is joined straight to a carbon of the ring.
氯苯是卤代芳烃，也叫芳基卤：氯原子直接连在环上的碳原子上。
The third molecule is the trap. It has a ring, but the chlorine is not on the ring — it hangs off the side chain.
第三个分子是陷阱：它有环，但氯不在环上——它挂在侧链上。
So how do we make one?
那么怎样制备它呢？
Take benzene, add chlorine or bromine, and add a catalyst: aluminium chloride for chlorine, aluminium bromide for bromine.
取苯，加入氯气或溴，再加一种催化剂：用氯气时是氯化铝， 用溴时是溴化铝。
Room temperature is enough.
室温就够了。
One hydrogen on the ring is replaced by the halogen, and it leaves as hydrogen chloride gas.
环上的一个氢被卤原子取代，这个氢以氯化氢气体离开。
Benzene gives chlorobenzene.
苯就生成氯苯。
Nothing adds across the ring, because the ring keeps its delocalised electrons.
没有任何东西加成到环上，因为环保留着它的离域电子。
Look at what the catalyst is really doing.
看看催化剂真正在做什么。
Aluminium chloride is a halogen carrier: it pulls a chloride ion off a chlorine molecule, and what is left is a very strong electrophile.
氯化铝是卤素载体：它从一个氯分子上夺走一个氯离子， 剩下的就是一个非常强的亲电试剂。
Step one: a pair of electrons from the delocalised ring attacks that electrophile and forms a bond to it.
第一步：离域环上的一对电子进攻这个亲电试剂， 并与它成键。
The delocalised ring is broken for a moment, and the intermediate carries a positive charge.
此时离域环被暂时破坏，中间体带正电荷。
Step two: the pair of electrons in the carbon to hydrogen bond falls back into the ring.
第二步：碳氢键上的那对电子 重新落回环中。
A hydrogen ion leaves, and the stable ring is restored.
一个氢离子离开，稳定的环就恢复了。
Now start from methylbenzene instead.
现在改从甲苯出发。
The methyl group pushes electrons into the ring, and it also decides where the chlorine lands: next door at position two, or straight across at position four.
甲基把电子推入环中，同时也决定了氯落在哪里： 相邻的 2 号位，或者正对面的 4 号位。
So methylbenzene and chlorine give two products, two-chloromethylbenzene and four-chloromethylbenzene.
所以甲苯与氯气会生成两种产物： 2-氯甲苯和 4-氯甲苯。
Learn that pair.
把这一对记住。
An alkyl group, a hydroxyl group and an amino group all direct to positions two and four.
烷基、羟基和氨基都定位到 2 号位和 4 号位。
Now a warning. The same two chemicals can give a completely different product, because the conditions decide.
现在提醒一句：同样的两种药品可以给出完全不同的产物，因为条件说了算。
With the aluminium chloride catalyst and no ultraviolet light, the chlorine goes onto the ring, and you get a halogenoarene.
有氯化铝催化剂、没有紫外光时，氯进入环上，得到卤代芳烃。
With ultraviolet light and no catalyst, the chlorine attacks the side chain instead, by free radical substitution, and that product behaves like a halogenoalkane.
有紫外光、没有催化剂时，氯转而进攻侧链，走自由基取代，产物的表现就像卤代烷。
Read the conditions before you write the product.
写产物之前，先看清条件。
Before we explain the difference, remember what a halogenoalkane does.
在解释差别之前，先回忆卤代烷会做什么。
It reacts by nucleophilic substitution.
它通过亲核取代反应。
Its carbon to halogen bond is polar, so that carbon is slightly positive.
它的碳卤键是极性的， 所以那个碳带部分正电荷。
A nucleophile is pulled towards it, attacks it, and pushes the halogen out as a halide ion.
亲核试剂被它吸引，进攻它，并把卤原子以卤离子的形式挤出去。
Primary molecules go by the one step route, tertiary ones lose the halide first — but either way, the halogen leaves easily.
伯卤代烷走一步的路线，叔卤代烷先失去卤离子——但无论哪一种，卤原子都很容易离开。
Here is the comparison the syllabus asks for by name: chloroethane against chlorobenzene.
这就是考纲点名要求的比较：氯乙烷对氯苯。
Warm chloroethane with aqueous sodium hydroxide, and it is hydrolysed to ethanol.
把氯乙烷与氢氧化钠水溶液加热， 它会水解成乙醇。
Warm chlorobenzene with the same aqueous sodium hydroxide, under the same ordinary conditions, and there is no reaction at all.
把氯苯与同样的氢氧化钠水溶液、在同样的普通条件下加热， 却完全不反应。
A halogenoarene is very unreactive towards nucleophiles.
卤代芳烃对亲核试剂很不活泼。
Two reasons explain it, and a full answer needs both.
原因有两个，完整的答案两个都要写。
Reason one, and this is the sentence that earns the mark.
原因一，这也是能拿分的那句话。
The chlorine atom carries lone pairs.
氯原子带有孤对电子。
One lone pair sits in a p orbital, and that orbital overlaps sideways with the delocalised ring, so the lone pair spreads into the ring.
其中一对孤对电子位于一个 p 轨道上， 这个轨道与离域环侧向重叠，于是孤对电子扩展进入环中。
The carbon to chlorine bond now has partial double bond character.
碳氯键因此具有部分双键的性质。
It is shorter and stronger than the carbon to chlorine bond in chloroethane, and a stronger bond is far harder to break.
它比氯乙烷中的碳氯键更短、更强，而更强的键要断开就困难得多。
Reason two.
原因二。
The delocalised ring is a cloud of electrons above and below the carbons, so the ring is electron rich.
离域环是碳原子上方和下方的一团电子云，所以环是富电子的。
A nucleophile is electron rich as well: it carries a lone pair, and often a negative charge.
亲核试剂同样富电子：它带有孤对电子，而且常常带负电荷。
Like charges repel, so the nucleophile is pushed away before it ever reaches the carbon.
同种电荷相斥， 所以亲核试剂在靠近那个碳之前就被推开了。
Put the two reasons together: the bond is too strong to break, and the attacker cannot get close.
把两个原因合起来： 键太强断不开，进攻者又靠不近。
Now a real exam question.
来看一道真题。
Compound A is bromobenzene. Compound B is bromomethylbenzene.
化合物 A 是溴苯，化合物 B 是（溴甲基）苯，也叫苄基溴。
Each one is warmed with aqueous silver nitrate in ethanol. State what you would see, and explain. Start with B.
把它们分别与乙醇中的硝酸银水溶液加热，说出你会看到什么，并解释。
Its bromine is on the side chain, so it behaves like a halogenoalkane: the carbon to bromine bond breaks, bromide ions go into the solution, and a cream precipitate of silver bromide appears.
先看 B： 它的溴在侧链上，所以表现得像卤代烷：碳溴键断裂，溴离子进入溶液， 出现浅黄色（cream）的溴化银沉淀。
Now A. Its bromine is on the ring, so there is no change: no precipitate.
再看 A：它的溴在环上，所以没有变化，没有沉淀。
For the explanation, use the mark scheme words. The lone pair on the bromine is delocalised into the ring, so the carbon to bromine bond is stronger.
解释时要用评分标准的原话：溴上的孤对电子离域进入环中，因此碳溴键更强。
One more real question: why is chlorobenzene so much less reactive than ethanoyl chloride?
再看一道真题：为什么氯苯比乙酰氯的活性低那么多？ 把它想成一个梯子。
Think of a ladder. Ethanoyl chloride is at the top. Its carbon is bonded to two electronegative atoms, an oxygen and a chlorine, so that carbon is very electron deficient, and water attacks it at once.
乙酰氯在最上面：它的碳原子连着两个电负性大的原子——一个氧和一个氯， 所以那个碳非常缺电子，水一碰它就反应。
A halogenoalkane sits in the middle: one polar bond, and it needs warming with alkali.
卤代烷在中间：只有一个极性键， 需要与碱共热。
Chlorobenzene is at the bottom, because its lone pair is delocalised into the ring.
氯苯在最下面，因为它的孤对电子离域进入了环中。
So the whole topic comes down to one decision. Find the halogen, and ask what it is joined to.
所以整个专题归结为一个判断：找到卤原子，看它连在什么上面。
Joined to a ring carbon, it is a halogenoarene: unreactive, no precipitate with silver nitrate, and a catalyst was needed to put it there.
连在环上的碳原子上，它就是卤代芳烃：不活泼，与硝酸银不产生沉淀， 而且当初要靠催化剂才能接上去。
Joined to a side chain carbon, it is really a halogenoalkane: it hydrolyses, it gives a precipitate, and ultraviolet light put it there.
连在侧链的碳原子上，它其实就是卤代烷： 会水解，会产生沉淀，而且当初是靠紫外光接上去的。
Three marks students throw away.
三个学生常丢的分。
First, when you explain the low reactivity, write both halves: the lone pair on the halogen overlaps with the delocalised ring, and the carbon to halogen bond gains partial double bond character.
第一，解释活性低时，两半都要写：卤原子上的孤对电子与离域环重叠， 碳卤键因此具有部分双键的性质。
Saying only that the ring is stable earns nothing.
只说“环很稳定”是拿不到分的。
Second, read the conditions before you name the product: a halogen carrier catalyst means the ring, ultraviolet light means the side chain.
第二，写产物名称之前先看条件： 有卤素载体催化剂就是环上，有紫外光就是侧链。
Third, in the silver nitrate test, know which one gives nothing: on the ring, no precipitate; on the side chain, a precipitate.
第三，硝酸银实验中要清楚哪一个没有现象： 在环上，没有沉淀；在侧链上，有沉淀。
Get those right, and this topic is yours.
把这三点做对，这个专题就是你的了。