- for an organic molecule containing several functional groups: (a) identify organic functional groups using the reactions in the syllabus (b) predict properties and reactions
- devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus
- analyse a given synthetic route in terms of type of reaction and reagents used for each step of it, and possible by-products
Organic synthesis
A-Level Chemistry · Topic 36
7:52
Organic synthesis (A2)
Three very different things: a bright orange dye, a painkiller, and a tough plastic. All three begin as the same colourless liquid — benzene. But no single…
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36.1
Organic synthesis
Syllabus
Source: Cambridge International syllabus
Like the AS synthesis topic, this asks you to join up known reactions to build a target molecule. Now you also have the A Level reactions of arenes, phenol, amines, amides and acyl chlorides.
Identifying functional groups
A molecule may carry several functional group 官能团 types. Use the test reactions to spot each one, then predict its behaviour. For example:
- decolourises bromine water with no catalyst, giving a white precipitate → a phenol or a phenylamine (the ring is activated).
- reacts violently with cold water, giving fumes of $\text{HCl}$ → an acyl chloride 酰氯.
- gives a purple colour with neutral $\text{FeCl}_3$ → a phenol 苯酚.
A map of the A Level reactions
| Start | Reagent and conditions | Product |
|---|---|---|
| benzene (an arene 芳烃) | $\text{HNO}_3$ / $\text{H}_2\text{SO}_4$ (electrophilic substitution 亲电取代) | nitrobenzene |
| nitrobenzene | $\text{Sn}$ / conc $\text{HCl}$, then $\text{NaOH}$ (reduction 还原) | phenylamine |
| phenylamine | $\text{HNO}_2$, below $10\,°\text{C}$ | a diazonium salt |
| diazonium salt | warm water | phenol; or couple with phenol → azo dye |
| methylbenzene | hot $\text{KMnO}_4$ (oxidation 氧化) | benzoic acid |
| carboxylic acid 羧酸 | $\text{SOCl}_2$ or $\text{PCl}_5$ | acyl chloride |
| acyl chloride | alcohol / phenol | an ester 酯 |
| acyl chloride | ammonia / amine 胺 | an amide 酰胺 |
| amide or nitrile 腈 | $\text{LiAlH}_4$ | an amine |
| halogenoalkane | $\text{KCN}$ | a nitrile (adds one carbon) |
Planning and analysing a route
To devise a synthetic route 合成路线, work backwards from the target: which single reaction makes it, and from what? Repeat until you reach the starting material, then write each step with its reagent 试剂 and conditions.
When you analyse a route, state the type of reaction for each step (for example electrophilic substitution, addition–elimination 加成消去, oxidation or reduction) and watch for likely by-products 副产物 — for example, making an amine from a halogenoalkane also gives over-substituted amines, lowering the yield.
Worked example. Devise a route from benzene to phenylamine, $\text{C}_6\text{H}_5\text{NH}_2$. Work backwards: phenylamine comes from reducing nitrobenzene, and nitrobenzene comes from nitrating benzene - so the route is two steps. Step 1: benzene with concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$ at $55\ °\text{C}$; electrophilic substitution gives nitrobenzene (keep below $55\ °\text{C}$, or further substitution follows). Step 2: reduce the nitrobenzene with tin and concentrated $\text{HCl}$, then add $\text{NaOH}$ to free the amine from its salt. There is no direct route - you cannot put an $\text{–NH}_2$ straight onto a ring, which is exactly why this nitrate-then-reduce pair is worth knowing by heart.
Explore
A-Level synthesis planner
Build a route by matching functional-group changes to reagents.
Vocabulary
Train
| English |
|---|
| functional group/ˈfʌŋkʃənl ɡruːp/ |
| acyl chloride/əˈkɪl ˈklɔːraɪd/ |
| phenol/ˈfenɒl/ |
| arene/ˈæren/ |
| electrophilic substitution/ɪˌlektrəʊˈfɪlɪk ˌsʌbstɪˈtjuːʃn/ |
| reduction/rɪˈdʌkʃn/ |
| oxidation/ˌɒksɪˈdeɪʃn/ |
| carboxylic acid/ˌkɑːbəkˈsɪlɪk ˈæsɪd/ |
| ester/ˈestə/ |
| amine/ˈæmaɪn/ |
| amide/əˈmaɪd/ |
| nitrile/ˈnaɪtraɪl/ |
| synthetic route/sɪnˈθetɪk ruːt/ |
| reagent/rɪˈeɪdʒənt/ |
| addition–elimination/əˈdɪʃn ɪˌlɪmɪˈneɪʃn/ |
| by-product/baɪ ˈprɒdʌkt/ |
36.1
Exam tips
- Combine AS and A-level steps and watch for benzene-ring reactions and carbon-count changes.
- State every reagent and condition, and note when a step produces a racemic mixture.
- Choose the shortest route that reaches the target functional group.
Interactive lessons on this topic
Work through it step by step, with instant-check exercises.