Identifying groups and the reaction map
Identifying groups and the reaction map
- Organic synthesis doesn't add new reactions — it joins the ones you know.
- First, identify the functional groups present.
- Then use a map of reactions to get from one to another.
Functional group tests
- decolourises bromine water → a C=C (alkene).
- precipitate with silver nitrate → a halogenoalkane.
- orange dichromate → green → a primary/secondary alcohol.
- orange precipitate with 2,4-DNPH → an aldehyde or ketone.
- fizzes with a carbonate → a carboxylic acid.
Practice
A compound that decolourises bromine water contains:
Decolourising bromine water is the test for a C=C double bond.
Practice
An orange precipitate with 2,4-DNPH indicates:
2,4-DNPH detects the carbonyl group (aldehyde or ketone).
The reaction map
| Start | Reagent | Product |
|---|---|---|
| alkene | steam, $\text{H}_3\text{PO}_4$ | alcohol |
| halogenoalkane | NaOH(aq) | alcohol |
| halogenoalkane | KCN in ethanol | nitrile (+1 carbon) |
| alcohol | dichromate (distil/reflux) | aldehyde / carboxylic acid |
| aldehyde/ketone | $\text{NaBH}_4$ | alcohol |
| nitrile | dilute acid | carboxylic acid |
| acid + alcohol | conc. $\text{H}_2\text{SO}_4$ | ester |
- The KCN step is the only AS reaction that lengthens the carbon chain.
Practice
According to the reaction map, an alkene + steam (H₃PO₄) gives:
Electrophilic addition of water across the C=C gives an alcohol.
Practice
Which step lengthens the carbon chain by one?
KCN adds a CN group (a carbon) — the only AS reaction that lengthens the chain.
You've got it
Key idea
- synthesis joins known reactions to build a target
- tests: bromine water (alkene), AgNO₃ (halogenoalkane), dichromate (1°/2° alcohol), 2,4-DNPH (carbonyl), carbonate fizz (acid)
- learn the reaction map of group-to-group conversions; KCN is the only step that adds a carbon