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5.6.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Planning Reaction Schemes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 39 of 43 Unit quiz

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Part 1 of 5Key Conversions to Know

5.6.2 Planning Reaction Schemes

A route is built by joining conversions one after another.

Alkene to halogenoalkane
  • Addition of a hydrogen halide such as HBr at room temperature gives a bromoalkane.
Halogenoalkane to alcohol
  • Heating under reflux with aqueous sodium hydroxide substitutes the halogen for –OH.
Alcohol to carbonyl or acid
  • Acidified potassium dichromate(VI) gives an aldehyde on distillation, a carboxylic acid under reflux,
    • or a ketone from a secondary alcohol.
Alcohol to alkene
  • Heating with concentrated phosphoric or sulfuric acid removes water and forms an alkene.
ConversionReagentCondition
Halogenoalkane to nitrileKCN in ethanolReflux
Nitrile to amineLiAlH₄ in dry etherRoom temperature
Nitrobenzene to phenylamineTin and concentrated HCl, then NaOHReflux
Carboxylic acid to esterAlcohol with concentrated H₂SO₄Reflux

Part 2 of 5How to Plan a Route

1
Write down the starting material and the target molecule and note which functional groups change.
2
Work backwards from the target, asking which compound could be turned into it in one step.
3
Check whether the carbon chain length changes, since only a few reactions add carbon atoms.
4
Choose reagents and conditions for each step and confirm that no step would affect another functional group.
5
Write the full scheme with balanced reagents above the arrows and name each intermediate.

Part 3 of 5Practical Considerations

Practical Considerations
Number of steps
  • Each extra step lowers the overall yield, so shorter routes are preferred.
Overall yield
  • Multiply the percentage yields of each stage; three 80% steps give 0.80 × 0.80 × 0.80 = 51%.
Selectivity
  • Reagents such as LiAlH₄ reduce many groups at once, so consider which groups are present first.
Safety and green choices
  • Avoid toxic reagents such as cyanide when a safer route reaches the same product.

Part 4 of 5Worked Example

Question

  • A three-step synthesis has step yields of 90%, 75% and 80%.
  • Calculate the overall yield.

  1. Convert each to a fraction0.90, 0.75 and 0.80.
  2. Multiply0.90 × 0.75 = 0.675.
  3. Then 0.675 × 0.80 = 0.540.

Answer Overall yield = 54%

Part 5 of 5Exam Checklist & Tips

5.6.2 – Key Points for the Exam

  • Functional group interconversion Changing one functional group into another.
  • Reagent A substance added to cause a chemical change.
  • Reflux Heating a mixture with a condenser fitted vertically to stop volatile loss.
  • Overall yield The product of the fractional yields of every step in a synthesis.

Common Mistakes

  • Writing the product without stating the reagent and conditions on the arrow.
  • Using dichromate and distilling when a carboxylic acid is needed; reflux is required.
  • Forgetting that aqueous NaOH gives substitution but ethanolic NaOH gives elimination.
  • Adding up percentage yields rather than multiplying them.

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