Topic 20 · page 9 of 15
The reaction map
Every one-step conversion in the course, on two maps. Learn the arrows first and the reagents second, and mark every step that adds a carbon, because those are the steps a synthesis needs when the product has a longer chain.
How the reactions fit together 20.3(ii)
A synthesis question gives you a starting compound and a target. You build the answer from the one-step conversions you have already learnt. If you can see the whole set at once, the route is a matter of finding a path from the start to the target.
The two maps below show every conversion for aliphatic compounds. Each arrow has a number, and the tables after the maps give the reagent and the conditions for that number. The first map is built around the halogenoalkane and the second around the carboxylic acid. The alcohol is on both.
Map 1: alkane, alkene, halogenoalkane, alcohol 20.3(ii)
| No. | Conversion | Reagent | Conditions |
|---|---|---|---|
| 1 | alkane to halogenoalkane | Br₂ or Cl₂ | UV light |
| 2 | alkene to alkane | H₂ | nickel catalyst, heat |
| 3 | alkene to halogenoalkane | HBr (or HCl) | room temperature |
| 4 | alkene to alcohol | steam | H₃PO₄ catalyst, high temperature and pressure |
| 5 | alcohol to alkene | concentrated H₃PO₄ or H₂SO₄ | heat (dehydration) |
| 6 | halogenoalkane to alcohol | NaOH(aq) or KOH(aq) | heat under reflux |
| 7 | alcohol to halogenoalkane | PCl₅ (chloro); KBr with 50% H₂SO₄ (bromo); red phosphorus and I₂ (iodo) | room temperature for PCl₅; reflux for the others |
| 8 | halogenoalkane to alkene | KOH in ethanol | heat under reflux |
| 9 | halogenoalkane to nitrile (+1 C) | KCN in ethanol | heat under reflux |
| 10 | halogenoalkane to amine | excess concentrated NH₃ in ethanol | heat in a sealed tube |
| 11 | halogenoalkane to Grignard reagent | Mg | dry ether |
| 12 | Grignard reagent to alcohol (longer chain) | aldehyde or ketone | dry ether, then dilute acid |
| 13 | nitrile to amine | LiAlH₄ | dry ether |
Map 2: alcohol, carbonyl, acid and its derivatives 20.3(ii)
| No. | Conversion | Reagent | Conditions |
|---|---|---|---|
| 14 | primary alcohol to aldehyde | K₂Cr₂O₇ with dilute H₂SO₄ | heat, and distil the aldehyde off as it forms |
| 15 | secondary alcohol to ketone | K₂Cr₂O₇ with dilute H₂SO₄ | heat under reflux |
| 16 | aldehyde to carboxylic acid | K₂Cr₂O₇ with dilute H₂SO₄ | heat under reflux |
| 17 | aldehyde to primary alcohol | LiAlH₄ | dry ether |
| 18 | ketone to secondary alcohol | LiAlH₄ | dry ether |
| 19 | aldehyde to hydroxynitrile (+1 C) | HCN with a little KCN | room temperature |
| 20 | ketone to hydroxynitrile (+1 C) | HCN with a little KCN | room temperature |
| 21 | nitrile to carboxylic acid | dilute HCl | heat under reflux |
| 22 | hydroxynitrile to hydroxy acid | dilute HCl | heat under reflux |
| 23 | Grignard reagent to carboxylic acid (+1 C) | CO₂ | dry ether, then dilute acid |
| 24 | carboxylic acid to primary alcohol | LiAlH₄ | dry ether |
| 25 | carboxylic acid to acyl chloride | PCl₅ | room temperature, dry |
| 26 | acyl chloride to carboxylic acid | water | room temperature |
| 27 | carboxylic acid to ester | alcohol, with concentrated H₂SO₄ | heat under reflux |
| 28 | ester to carboxylic acid | dilute H₂SO₄ or HCl | heat under reflux (or NaOH(aq), then acid) |
| 29 | acyl chloride to ester | alcohol | room temperature |
| 30 | acyl chloride to amide | concentrated NH₃ (a primary amine gives an N-substituted amide) | room temperature |
| 31 | amine to N-substituted amide | acyl chloride | room temperature |
Aromatic compounds 20.3(ii)
Compounds with a benzene ring form their own small map. Benzene can be turned into other aromatic compounds, and into the amines and dyes of Topic 19.
| Conversion | Reagent | Conditions |
|---|---|---|
| benzene to nitrobenzene | concentrated HNO₃ with concentrated H₂SO₄ | about 50 °C |
| nitrobenzene to phenylamine | tin and concentrated HCl, then excess NaOH(aq) | heat under reflux |
| phenylamine to benzenediazonium ion | NaNO₂ and HCl | below 10 °C |
| diazonium ion to azo dye | phenol in NaOH(aq) | cold |
| phenylamine to N-phenylethanamide | ethanoyl chloride | room temperature |
| benzene to bromobenzene | Br₂ with an iron or FeBr₃ catalyst | room temperature |
| benzene to an alkylbenzene | halogenoalkane with AlCl₃ | dry, heat |
| benzene to a phenyl ketone | acyl chloride with AlCl₃ | dry, heat |
| benzene to benzenesulfonic acid | fuming sulfuric acid | heat |
Check yourself
- Give the reagent and conditions for each: (a) 1-bromopropane to propan-1-ol, (b) propan-1-ol to propanal, (c) propanoic acid to propanoyl chloride, (d) propene to 2-bromopropane, (e) butanenitrile to butylamine.
- State the numbers of the conversions on the maps that increase the length of the carbon chain, and give the reagent for each.
- Which conversions on the maps use LiAlH₄ in dry ether, and what class of compound is each product?
- Which conversions on the maps require dry conditions, and why?
- Explain why a halogenoalkane with ammonia gives an amine with the same number of carbons, but with KCN it gives a product with one more.
- State the difference in conditions between making an aldehyde and making a carboxylic acid from a primary alcohol.
Answers cover these until you have tried the questions
- (a) NaOH(aq), heat under reflux. (b) acidified K₂Cr₂O₇, heat, and distil off the propanal as it forms. (c) PCl₅, room temperature. (d) HBr at room temperature. (e) LiAlH₄ in dry ether.
- 9: KCN in ethanol, heat under reflux (+1 C). 19 and 20: HCN with a little KCN (+1 C). 23: CO₂ in dry ether, then dilute acid (+1 C). 12: a carbonyl compound with a Grignard reagent, which adds the whole R group.
- 13: nitrile to an amine. 17: aldehyde to a primary alcohol. 18: ketone to a secondary alcohol. 24: carboxylic acid to a primary alcohol.
- 11, 12, 23 (Grignard reagent): water destroys the reagent. 13, 17, 18, 24 (LiAlH₄): it reacts with water. 25 (PCl₅): it reacts with water to give HCl, so the sample must be dry.
- In the ammonia reaction the nucleophile is NH₃, which brings no carbon into the molecule. In the cyanide reaction the nucleophile is CN⁻, and its carbon becomes part of the new carbon chain.
- Aldehyde: warm with acidified dichromate and distil the aldehyde off as it forms, so it cannot be oxidised further. Carboxylic acid: use excess dichromate and heat under reflux, so the aldehyde stays in the flask and is oxidised to the acid.