有机合成

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.

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).
• after warming with $\text{NaOH(aq)}$ and acidifying with dilute $\text{HNO}_3$, 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.

A map of the AS reactions

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

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, use a cyanide step. These are the only AS reactions that lengthen the chain: $\text{KCN}$ with a halogenoalkane gives a nitrile, and $\text{HCN}$ adding to an aldehyde or ketone gives a hydroxynitrile.

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.

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.

Common mistakes.

• Adding silver nitrate straight to a halogenoalkane. The halogen is bonded covalently, so there are no free halide ions: hydrolyse first with warm $\text{NaOH(aq)}$, acidify with dilute $\text{HNO}_3$, then add $\text{AgNO}_3$.
• Oxidising to a carboxylic acid by distilling. Distil to collect the aldehyde as it forms; heat under reflux to oxidise all the way to the carboxylic acid.
• Expecting a tertiary alcohol to turn dichromate green. Tertiary alcohols are not oxidised, so the orange colour stays.
• Getting the wrong alcohol from steam addition. By Markovnikov's rule propene gives mainly propan-2-ol, not propan-1-ol.
• Using $\text{KCN}$ in water. Use $\text{KCN}$ in ethanol, heated under reflux; in water the hydroxide ions give an alcohol instead.
• Forgetting the carbon count. Only the cyanide steps add a carbon; a route that needs one more carbon must include $\text{KCN}$ or $\text{HCN}$.

• 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.