International A Level (IAL)ChemistryEdexcelRevision Notes5. Transition Metals & Organic Nitrogen ChemistryOrganic SynthesisPlanning Reaction Schemes

Planning Reaction Schemes (Edexcel International A Level (IAL) Chemistry): Revision Note

Exam code: YCH11

Richard Boole

Written by: Richard Boole

Reviewed by: Caroline Carroll

Updated on

Reaction Schemes

  • A large number of organic products are made from a few starting compounds using appropriate reagents and conditions

  • Knowing how organic functional groups are related to each other is key to the synthesis of a given molecule

  • The main functional groups you need to know are

    • Alkanes

    • Alkenes

    • Haloalkanes

    • Nitriles

    • Amines

    • Alcohols

    • Carbonyls (aldehydes & ketones)

    • Hydroxynitriles

    • Carboxylic acids

    • Esters

    • Acyl chlorides

    • Primary and secondary amides

Aliphatic Reaction Pathways

  • The key interconversions between functional groups are summarised here:

Aliphatic Reactions Table

Reactant

Product

Reagents

Reaction

Alkene

Haloalkane

X2 / HX

Electrophilic addition

Alkene

Alcohol

Steam + H2SO4 / heat

Hydration

Alkene

Alkane

Hydrogen + Ni catalyst / 150 oC

Addition (hydrogenation)

Alcohol

Alkene

Al2O3 or conc. H2SO4 / heat

Elimination / dehydration

Alcohol

Haloalkane

NaX + H2SO4 / reflux (X = Cl or Br; red P + I2 for iodoalkanes)

Nucleophilic substitution

Haloalkane

Alcohol

NaOH (aq) / reflux

Nucleophilic substitution

Alkane

Haloalkane

Halogen / UV light

Free radical substitution

Primary alcohol

Aldehyde

K2Cr2O7 / H2SO4 / distil

Oxidation

Secondary alcohol

Ketone

K2Cr2O7 / H2SO4 / Heat

Oxidation

Primary alcohol

Carboxylic acid

K2Cr2O7 / H2SO4 / reflux

Oxidation

Aldehyde

Primary alcohol

NaBH4 / H2O

Reduction / nucleophilic addition

Ketone

Secondary alcohol

NaBH4 / H2O

Reduction / nucleophilic addition

Haloalkane

Nitrile

Aqueous ethanolic KCN / Heat

Nucleophilic substitution

Halogenoalkane

Amine

NH3 / ethanol

Nucleophilic substitution

Nitrile

Carboxylic acid

H2O / HCl

Hydrolysis

Aldehyde

Hydroxynitrile

NaCN / H+

Nucleophilic addition

Alcohol

Ester

Carboxylic acid / H2SO4

Esterification

Carboxylic acid

Ester

Alcohol / H2SO4

Esterification

Ester

Carboxylate salt and alcohol

NaOH (aq)

Alkaline hydrolysis

Ester

Carboxylic acid

Dilute acid

Acid hydrolysis

Carboxylic acid

Acyl chloride

PCl5 (or SOCl2)

Chlorination

Acyl chloride

Carboxylic acid

H2O

Hydrolysis

Acyl chloride

Primary amide

NH3

Nucleophilic addition elimination

Acyl chloride

Secondary amide

Primary amine

Nucleophilic addition elimination

Aromatic Reaction Pathways

  • The key aromatic reactions are summarised here:

Aromatic Reactions Table

Reactant

Product

Reagents

Reaction

Benzene

Methylbenzene / toluene

CH3Cl / AlCl3

Alkylation / Electrophilic substitution

Benzene

Bromobenzene

Br2 / FeBr3

Bromination / Electrophilic substitution

Benzene

Chlorobenzene

Cl2 / AlCl3

Chlorination / Electrophilic substitution

Benzene

Nitrobenzene

HNO3 / H2SO4

Nitration / Electrophilic substitution

Nitrobenzene

Aminobenzene / phenylamine / aniline

Sn / HCl

Reduction

Aminobenzene 

2,4,6-tribromoaminobenzene / 2,4,6-tribromoaniline

Bromine

Electrophilic substitution

Benzene

Phenylethanone

CH3COCl / AlCl3

Acylation / Electrophilic substitution

Phenylethanone

1-phenylethanol

NaBH4

Reduction

Designing a Reaction Pathway

  • The given molecule is usually called the target molecule and chemists try to design a synthesis as efficiently as possible

  • Designing a reaction pathway starts by drawing the structures of the target molecule and the starting molecule

  • Determine if they have the same number of carbon atoms

    • If you need to lengthen the carbon chain you can put on a nitrile group by nucleophilic substitution, or use a Grignard reagent with carbon dioxide or a carbonyl compound

  • Work out all the compounds that can be made from the starting molecule and all the molecules that can be made into the target molecule

    • Match the groups they have in common and work out the reagents and conditions needed

Worked Example

Suggest how the following syntheses could be carried out:

a) Chloroethane to ethanoic acid

b) Ethene to 1-aminopropane

Answer 1

Reaction pathway from chloroethane to ethanoic acid: NaOH(aq) under reflux gives ethanol, then acidified K2Cr2O7 under reflux oxidises it to ethanoic acid

Chloroethane to ethanoic acid (two steps):

  1. Chloroethane to ethanol: react with NaOH (aq) and heat under reflux (nucleophilic substitution)

  2. Ethanol to ethanoic acid: oxidise with K2Cr2O7 and H2SO4, heating under reflux

 

Answer 2

Reaction pathway from ethene to 1-aminopropane: HCl at room temperature, KCN in ethanol under reflux, then reduction with LiAlH4 in dry ether

Ethene to 1-aminopropane (three steps):

  1. Ethene to chloroethane: react with HCl at room temperature

  2. Chloroethane to propanenitrile, CH3CH2CN: react with KCN in ethanol, heating under reflux (this adds the extra carbon)

  3. Propanenitrile to 1-aminopropane, CH3CH2CH2NH2: reduce with LiAlH4 in dry ether and heat

Examiner Tips and Tricks

You could be required to design a synthesis with up to four steps

Part of this process can include identifying appropriate control measures to reduce risk, based on data about hazards

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Author
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Richard Boole

Author: Richard Boole

Expertise: Curriculum Expert

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.