A-Level — chemistryHard4 min read

Organic Synthesis and Reaction Pathways

Master multi-step synthesis, retrosynthetic analysis, and functional group interconversions for A-Level Chemistry.

Tutor AI Team
Tutor AI Team

# Organic Synthesis and Reaction Pathways

Organic synthesis involves planning a route from starting material to target molecule using the reactions you've learned. This is where all of A-Level organic chemistry comes together. You need to select appropriate reagents, conditions, and reaction sequences.

1. Key Functional Group Interconversions

From Alkanes

  • Alkane → halogenoalkane: UV light + halogen (free radical substitution)

From Alkenes

  • Alkene → alcohol: steam + H₃PO₄ catalyst (hydration)
  • Alkene → halogenoalkane: add HBr or HCl (electrophilic addition)
  • Alkene → dihalide: add Br₂ or Cl₂
  • Alkene → polymer: addition polymerisation

From Halogenoalkanes

  • Halogenoalkane → alcohol: aqueous NaOH, reflux (nucleophilic substitution)
  • Halogenoalkane → nitrile: KCN in ethanol/water, reflux (extends C chain by 1)
  • Halogenoalkane → amine: excess NH₃ in ethanol, heat in sealed tube
  • Halogenoalkane → alkene: ethanolic NaOH, reflux (elimination)

From Alcohols

  • Primary alcohol → aldehyde: K₂Cr₂O₇/H₂SO₄, distil
  • Primary alcohol → carboxylic acid: K₂Cr₂O₇/H₂SO₄, reflux
  • Secondary alcohol → ketone: K₂Cr₂O₇/H₂SO₄, reflux
  • Alcohol → alkene: conc. H₂SO₄ or H₃PO₄, heat (dehydration)
  • Alcohol → halogenoalkane: HBr or NaBr/H₂SO₄
  • Alcohol → ester: carboxylic acid + acid catalyst, reflux

From Nitriles

  • Nitrile → carboxylic acid: dilute acid, reflux (hydrolysis)
  • Nitrile → amine: LiAlH₄ in dry ether (reduction)

From Carbonyls

  • Aldehyde/ketone + HCN → hydroxynitrile (nucleophilic addition)
  • Aldehyde → carboxylic acid: K₂Cr₂O₇/H₂SO₄ (oxidation)

From Carboxylic Acids

  • Carboxylic acid + alcohol → ester (condensation)
  • Carboxylic acid → acyl chloride → more reactive derivatives
  • Carboxylic acid → primary alcohol: LiAlH₄ (reduction)

2. Strategies for Synthesis

Retrosynthetic Analysis

Work backwards from the target molecule:

  1. What is the last reaction to make the target?
  2. What intermediate is needed?
  3. What reaction makes that intermediate?
  4. Continue until you reach a simple starting material

Key Strategies

  • Extending the carbon chain: Use KCN + halogenoalkane → nitrile (adds 1 C)
  • Shortening the chain: Not commonly required at A-Level
  • Changing functional group: Follow the interconversion routes above
  • Introducing a functional group: Consider what's available and work backwards

Worked Example: Ethanol → Ethanoic Acid

Problem

CH3CH2OH→K2Cr2O7/H2SO4,refluxCH3COOH\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4, \text{reflux}} \text{CH}_3\text{COOH}

One step: oxidation with acidified dichromate under reflux.

Solution

Worked Example: 1-Bromopropane → Butanoic Acid

Problem

Route:

  1. CH3CH2CH2Br+KCN→CH3CH2CH2CN\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{KCN} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CN} (nucleophilic substitution — extends chain)
  2. CH3CH2CH2CN+2H2O→dilute HCl, refluxCH3CH2CH2COOH+NH4Cl\text{CH}_3\text{CH}_2\text{CH}_2\text{CN} + 2\text{H}_2\text{O} \xrightarrow{\text{dilute HCl, reflux}} \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{NH}_4\text{Cl} (hydrolysis)
Solution

Worked Example: Propene → Propylamine

Problem
  1. CH3CH=CH2+HBr→CH3CHBrCH3\text{CH}_3\text{CH=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CHBrCH}_3 (electrophilic addition)
  2. CH3CHBrCH3+2NH3→CH3CH(NH2)CH3+NH4Br\text{CH}_3\text{CHBrCH}_3 + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CH(NH}_2\text{)CH}_3 + \text{NH}_4\text{Br} (nucleophilic substitution)
Solution

4. Practical Techniques

Technique Purpose
Reflux Heating for extended time without loss of volatile reactants/products
Distillation Collecting a product as it forms (prevents further reaction)
Separating funnel Separating immiscible liquids
Drying Removing water using anhydrous drying agent (e.g. MgSO₄, CaCl₂)
Recrystallisation Purifying solid products
Filtration Separating solid from liquid

Reflux vs Distillation

  • Reflux: vapour condenses and drips back into flask → maximises yield, prevents loss
  • Distillation: product is collected as it evaporates → prevents further reaction (used for aldehydes to prevent oxidation to carboxylic acids)

5. Practice Questions

    1. Outline how you would convert ethanol to ethylamine. Give reagents and conditions for each step.
    1. Suggest a two-step synthesis of propan-1-ol from propene.
    1. How would you convert 1-bromoethane to propanoic acid? (Hint: extend the carbon chain first.)
    1. Describe the practical technique of reflux and explain when it is used.
    1. A student needs to make ethyl ethanoate. Starting from ethanol, outline all the steps.

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6. Exam Tips

  • Draw a flow chart of all conversions and keep it updated as revision
  • Always state reagents, conditions, and type of reaction for each step
  • Retrosynthesis questions: work backwards from the target
  • The KCN route for extending the carbon chain is extremely popular in exams
  • Be clear about reflux vs distillation and when to use each

Summary

  • Organic synthesis requires knowledge of all functional group interconversions
  • Retrosynthesis: work backwards from target to starting material
  • Key chain-extending step: halogenoalkane + KCN → nitrile (adds 1 C)
  • Always specify reagents, conditions, and reaction type
  • Practical techniques: reflux (long reaction), distillation (collect volatile product), separating funnel, drying
Tags:
#a-level#chemistry#organic#synthesis

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