Organic synthesis

Chemistry A2 · Organic synthesis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A molecule with several functional groups behaves as the sum of its groups. To predict its reactions, find each group and ask what the reagent does to that group. One reagent may react with more than one group, so check them all.

To plan a synthetic route, compare the starting molecule with the target. Ask whether the carbon chain has changed and which groups are different. Then work forwards from the start and backwards from the target until the two meet.

For each step, know the reagent, the conditions and the type of reaction, such as substitution, addition, oxidation, reduction, hydrolysis or condensation. Think about by-products too, for example further substitution.

Rules to remember

2,4-dinitrophenylhydrazine: orange precipitate with aldehydes and ketones
Tollens' reagent (silver mirror) or Fehling's solution (red precipitate): aldehydes only
Alkaline aqueous iodine: yellow precipitate of CHI₃ with CH₃CO– or CH₃CH(OH)– groups
Na₂CO₃ or NaHCO₃: CO₂ with carboxylic acids only, not with phenols or alcohols
NaOH(aq) reacts with phenols and carboxylic acids; sodium metal reacts with every –OH group (alcohol, phenol, acid) to give H₂
Br₂(aq): decolourised by C=C; white precipitate with phenol or phenylamine
One more carbon: halogenoalkane + KCN in ethanol, heat → nitrile; then hydrolyse to a carboxylic acid or reduce to an amine

Common mistake

Students deal with only the most obvious functional group. In a molecule with two groups, a reagent such as sodium, NaOH or an acyl chloride may react with both, so count the moles used by every group.

Worked example

Compound Z is CH₃COCH₂CH₂COOH. What is seen when separate samples of Z are tested with 2,4-dinitrophenylhydrazine, Tollens' reagent, alkaline aqueous iodine and Na₂CO₃(aq)?

Z has a ketone group and a carboxylic acid group.
The ketone gives an orange precipitate with 2,4-dinitrophenylhydrazine but no silver mirror with Tollens' reagent, because it is not an aldehyde.
The ketone is a CH₃CO– group, so alkaline aqueous iodine gives a yellow precipitate.
The –COOH group gives bubbles of CO₂ with Na₂CO₃(aq).

Answer: Orange precipitate; no silver mirror; yellow precipitate; bubbles of carbon dioxide

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