1. Designing Better Processes
Chemists try to make products in ways that are safer, cheaper and produce less waste. The 9620 specification asks you to explain four aims.
| Aim | Why it helps |
|---|---|
| No solvent | Solvents can be toxic and flammable. They must be removed and disposed of. They add cost and waste. |
| Non-hazardous starting materials | Safer for workers and the environment. The safety measures needed are cheaper. |
| Fewer steps | Each step loses some product. Fewer steps also mean less energy, time and waste. |
| High atom economy | More of the reactant atoms end up in the product, so there is less waste. |
Why fewer steps matter
The overall yield is the product of the yields of each step.
Overall yield = 0.8 × 0.8 × 0.8 = 0.512
Overall yield = 51%. Almost half of the possible product is lost, even though each step looks efficient.
Why atom economy matters
% atom economy = (Mr of the desired product ÷ total Mr of all reactants) × 100
- Addition reactions have 100% atom economy. All the reactant atoms end up in one product.
- Substitution and elimination reactions have a lower atom economy. They always make a second product, which is often waste.
When you explain one of these aims, link it to cost, safety or waste. For example: "fewer steps means a higher overall yield, so less starting material is wasted".
2. Reaction Map of 9620 Reactions
Use this table to plan a synthesis. Find your starting material in the first column. Then follow the reactions to your target.
| From | Reagent and conditions | To | Spec |
|---|---|---|---|
| alkane | Cl₂, UV light | chloroalkane | 3.3.2.4 |
| alkene | HBr | bromoalkane | 3.3.4.2 |
| alkene | conc. H₂SO₄, then water | alcohol | 3.3.4.2 |
| alkene | Br₂ | 1,2-dibromoalkane | 3.3.4.2 |
| halogenoalkane | NaOH(aq), warm | alcohol | 3.3.3.1 |
| halogenoalkane | KCN in ethanol/water, reflux | nitrile (one more C) | 3.3.3.1 |
| halogenoalkane | excess NH₃ in ethanol, heat in a sealed tube | primary amine | 3.3.3.1 |
| halogenoalkane | primary amine (excess halogenoalkane gives more) | secondary/tertiary amine, quaternary ammonium salt | 3.3.11.3 |
| halogenoalkane | KOH in ethanol, reflux | alkene | 3.3.3.2 |
| primary alcohol | acidified K₂Cr₂O₇, distil | aldehyde | 3.3.5.1 |
| primary alcohol | acidified K₂Cr₂O₇ (excess), reflux | carboxylic acid | 3.3.5.1 |
| secondary alcohol | acidified K₂Cr₂O₇, reflux | ketone | 3.3.5.1 |
| alcohol | conc. H₂SO₄ or H₃PO₄, heat | alkene | 3.3.5.2 |
| aldehyde or ketone | NaBH₄(aq) | alcohol | 3.3.8.1 |
| aldehyde or ketone | KCN then dilute acid | hydroxynitrile | 3.3.8.1 |
| carboxylic acid + alcohol | conc. H₂SO₄, warm | ester | 3.3.9.1 |
| ester | NaOH(aq) or dilute acid, reflux | carboxylate or acid + alcohol | 3.3.9.1 |
| acyl chloride or acid anhydride | water / alcohol / NH₃ / primary amine | acid / ester / amide / N-substituted amide | 3.3.9.2 |
| nitrile | H₂ with Ni catalyst (or LiAlH₄) | primary amine | 3.3.11.1 |
| benzene | conc. HNO₃ + conc. H₂SO₄, below 55 °C | nitrobenzene | 3.3.10.2 |
| nitrobenzene | Sn + conc. HCl, then NaOH | phenylamine | 3.3.11.1 |
| benzene | RCOCl + AlCl₃ | phenyl ketone | 3.3.10.2 |
| benzene | RCl + AlCl₃ | alkylbenzene | 3.3.10.2 |
| methylbenzene | Cl₂, UV light | (chloromethyl)benzene | 3.3.10.2 |
How to Plan a Synthesis
- Compare the start and the target. Which functional group changes? Does the number of carbon atoms change?
- Adding one carbon atom: use KCN. A halogenoalkane gives a nitrile. An aldehyde or ketone gives a hydroxynitrile.
- Adding carbons to a benzene ring: use an acyl chloride with AlCl₃ (Friedel-Crafts acylation).
- Work backwards from the target if you are stuck. Ask: "what could I make this from?"
- For each step, give the reagent, the conditions and the product.
Beyond the OxfordAQA specification: a nitrile can be hydrolysed to a carboxylic acid by heating with dilute acid. 9620 does not require this reaction.
3. Worked Syntheses
Each step names the reagent, the conditions and the product.
Step 1: KCN in ethanol/water, reflux → propanenitrile, CH₃CH₂CN.
CH₃CH₂Br + KCN → CH₃CH₂CN + KBr
Step 2: H₂/Ni (or LiAlH₄) → propylamine, CH₃CH₂CH₂NH₂.
CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂
Key idea: the target has one more carbon than the start, so the first step uses cyanide.
Step 1: Conc. H₂SO₄, then water → propan-2-ol, CH₃CH(OH)CH₃. This is the major product, because it forms via the more stable secondary carbocation.
Step 2: Acidified K₂Cr₂O₇, reflux → propanone.
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
Key idea: a secondary alcohol is oxidised to a ketone. The ketone is not oxidised further.
Step 1: Conc. HNO₃ and conc. H₂SO₄ below 55 °C → nitrobenzene.
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Step 2: Sn and conc. HCl, then NaOH → phenylamine.
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
Step 3: Ethanoyl chloride or ethanoic anhydride → N-phenylethanamide.
C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl
Step 1: Conc. H₂SO₄, then water → ethanol. Make enough for two portions.
CH₂=CH₂ + H₂O → CH₃CH₂OH (overall)
Step 2: Oxidise one portion with excess acidified K₂Cr₂O₇ under reflux → ethanoic acid.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Step 3: Warm the ethanoic acid with the second portion of ethanol and a few drops of conc. H₂SO₄ → ethyl ethanoate.
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
Key idea: the ester needs two pieces, an acid and an alcohol. Both come from the same ethanol.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
4. Practice Questions
Show Answer & Mark Scheme
Answer:
- Step 1: NaOH(aq), warm (1) → propan-1-ol (1).
- Step 2: acidified K₂Cr₂O₇ with the alcohol in excess, distilling off the aldehyde as it forms (1) → propanal (1).
Show Answer & Mark Scheme
Answer:
- Step 1: KCN in ethanol/water, reflux (1) → butanenitrile (1).
- Step 2: H₂/Ni or LiAlH₄ (1) → butylamine (1).
Show Answer & Mark Scheme
Answer: 0.90 × 0.70 × 0.85 = 0.5355, so the overall yield is 54% (53.6%) (1).
Show Answer & Mark Scheme
Answer: any two of (1 mark each):
- solvents can be toxic or flammable;
- they must be removed, which uses energy;
- they create waste that must be disposed of;
- they add cost.