Organic synthesis

This topic does not add new reactions. Instead it asks you to join up the reactions you already know, so you can build a target molecule in several steps.

Medicines are built up from simple starting materials by multi-step organic synthesis

Identifying functional groups

A molecule may have more than one functional group 官能团. Use the test reactions from the syllabus to identify each one, and then predict how the molecule will behave. For example:

• decolourises bromine water → a C=C double bond (an alkene 烯烃).
• gives a precipitate with silver nitrate → a halogenoalkane 卤代烷.
• orange $\text{K}_2\text{Cr}_2\text{O}_7$ turns green → a primary or secondary alcohol 醇.
• orange precipitate with 2,4-DNPH → an aldehyde 醛 or ketone 酮.
• fizzes with a carbonate → a carboxylic acid 羧酸.

The standard identification tests: each reagent gives a characteristic observation that points to one functional group

A map of the AS reactions

Each row turns one functional group into another. Learn it as a map you can travel around:

A map of the AS reactions: each arrow turns one functional group into another. Work backwards from your target to plan a route

Planning a multi-step route

To devise a synthetic route 合成路线:

1. compare the target with the starting material — what has changed (the functional group, the number of carbons)?
2. work backwards from the target: which single reaction could make it, and from what?
3. repeat until you reach the starting material.
4. write each step with its reagent 试剂 and conditions.

If you need to add a carbon, the $\text{KCN}$ step is the key — it is the only AS reaction that lengthens the chain.

Plan a route by working backwards from the target, one reaction at a time, until you reach the starting material

Analysing a route

When you are given a route, for each step state the type of reaction (such as oxidation 氧化, reduction 还原, substitution, addition or elimination) and the reagent used. Also think about possible by-products 副产物 — for example, making an amine from a halogenoalkane also gives a mixture of further-substituted amines, so the yield of the simple amine is low.

For every step in a given route, ask its reaction type, reagent and by-products

Name the reaction type at each step: oxidation, reduction, substitution, addition or elimination

Worked example. Devise a route from propene to propanone, $\text{CH}_3\text{COCH}_3$. Work backwards from the target. A ketone comes from oxidising a secondary alcohol, so the step before propanone is propan-2-ol with acidified $\text{K}_2\text{Cr}_2\text{O}_7$ under reflux. Propan-2-ol comes from propene by adding steam over an $\text{H}_3\text{PO}_4$ catalyst - and Markovnikov's rule conveniently puts the $\text{OH}$ on the middle carbon, which is exactly the secondary alcohol needed. So the route is: propene, then steam with $\text{H}_3\text{PO}_4$, giving propan-2-ol; then acidified $\text{K}_2\text{Cr}_2\text{O}_7$ under reflux, giving propanone. Give a reagent and its conditions on every arrow: a route with the right intermediates but no reagents scores very little.

• Learn the reagents and conditions for each conversion — that is exactly what synthesis questions test.
• Plan multi-step routes by functional group and watch the carbon count (KCN adds one carbon).
• Choose the shortest valid route and state every reagent and condition.