A LevelChemistryOCRRevision Notes6. Organic Chemistry & AnalysisOrganic SynthesisSynthetic Routes

Synthetic Routes (OCR A Level Chemistry A): Revision Note

Exam code: H432

Richard Boole

Written by: Richard Boole

Reviewed by: Caroline Carroll

Updated on

Multi-Stage Organic Synthetic Routes

  • 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

Examiner Tips and Tricks

You also need to be able to identify the functional groups of these chemicals in structures that are given to you

Aliphatic Reaction Pathways

  • The key interconversions between functional groups are summarised here:

Reactant

Product

Reagents and conditions

Reaction type

Alkene

Haloalkane

Hydrogen halide

Electrophilic addition

Alkene

Alcohol

Steam + H3PO4 catalyst, 300 °C, 60 atm

Hydration

Alkene

Alkane

H2 + Ni catalyst, 200 °C

Electrophilic addition / hydrogenation

Alcohol

Alkene

Al2O3 or conc. acid, heat

Elimination / dehydration

Alcohol

Haloalkane

NaX + H2SO4, heat under reflux

Nucleophilic substitution

Haloalkane

Alcohol

NaOH(aq), heat under reflux

Nucleophilic substitution

Alkane

Haloalkane

Halogen, UV light

Free radical substitution

Primary alcohol

Aldehyde

K2Cr2O7 / H2SO4, distillation

Oxidation

Secondary alcohol

Ketone

K2Cr2O7 / H2SO4, heat

Oxidation

Primary alcohol

Carboxylic acid

K2Cr2O7 / H2SO4, heat under reflux

Oxidation

Aldehyde

Primary alcohol

NaBH4 / H2O

Reduction

Ketone

Secondary alcohol

NaBH4 / H2O

Reduction

Haloalkane

Nitrile

NaCN

Nucleophilic substitution

Haloalkane

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

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:

Reactant

Product

Reagents

Reaction type

Benzene

Methylbenzene

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

Phenylamine

Sn / HCl

Reduction

Phenylamine

2,4,6-tribromophenylamine

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 will need to put on a nitrile group by nucleophilic substitution

  • 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 synthesis could be carried out:

Ethene to 1-aminopropane

Answer 

Reaction pathway from ethene to 1-aminopropane: step 1 ethene with HCl at room temperature gives chloroethane; step 2 KCN in ethanol with heat under reflux gives propanenitrile; step 3 reduction with LiAlH4 in dry ether gives 1-aminopropane

Answer, ethene to 1-aminopropane (three steps):

  1. Ethene + HCl at room temperature gives chloroethane, CH3CH2Cl

  2. Chloroethane + KCN in ethanol, heat under reflux gives propanenitrile, CH3CH2CN (the chain is lengthened by one carbon)

  3. Reduce propanenitrile with LiAlH4 in dry ether to give 1-aminopropane, CH3CH2CH2NH2

Examiner Tips and Tricks

Sound knowledge of all of the different reactions is beneficial as the A-level course simply states that you should be able to design a multistage synthesis

Past papers generally go to four steps in a multistep reaction although there is no clear limit stated

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Author
Reviewer
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.