Organic Synthesis

Organic synthesis is the art and science of building target molecules from simpler starting materials through a planned sequence of reactions. At A-Level, you are expected to plan multi-step synthesis routes, choose appropriate reagents and conditions for each step, and understand the logic of retrosynthetic analysis.

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The Goal of Organic Synthesis

In organic synthesis, you begin with a starting material (a simple, readily available compound) and transform it through a series of chemical reactions into a target molecule (the desired product). Each step in the sequence changes the functional group, the carbon skeleton, or both.

The challenge lies in choosing the right reactions in the right order, ensuring that each step is chemically feasible and that side reactions are minimised.

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Functional Group Interconversions

The core of A-Level synthesis planning is knowing which functional group can be converted into which other functional group, and what reagents and conditions are required for each conversion.

Key Conversions from Alkenes

Starting Group	Target Group	Reagent/Conditions
Alkene	Alcohol	Steam (H2O), H3PO4 catalyst, high T and P
Alkene	Halogenoalkane	HBr or HCl (gas) -- electrophilic addition
Alkene	Diol	Cold, dilute KMnO4 (alkaline)
Alkene	Dihalide	Br2 (in organic solvent) -- decolourises
Alkene	Polymer	High pressure, catalyst (addition polymerisation)

Key Conversions from Alcohols

Starting Group	Target Group	Reagent/Conditions
Primary alcohol	Aldehyde	K2Cr2O7/H2SO4, distil immediately
Primary alcohol	Carboxylic acid	K2Cr2O7/H2SO4, heat under reflux
Secondary alcohol	Ketone	K2Cr2O7/H2SO4, heat under reflux
Alcohol	Halogenoalkane	NaBr + H2SO4, or SOCl2, or PCl5
Alcohol	Alkene	Conc. H2SO4 or conc. H3PO4, heat (elimination)
Alcohol	Ester	Carboxylic acid + conc. H2SO4 catalyst, reflux

Critical distinction: Distillation vs reflux for primary alcohol oxidation. This is one of the most commonly tested points. Distillation removes the aldehyde product as it forms, preventing further oxidation. Reflux keeps everything in the flask, allowing the aldehyde to be oxidised further to the carboxylic acid.

Key Conversions from Halogenoalkanes

Starting Group	Target Group	Reagent/Conditions
Halogenoalkane	Alcohol	NaOH(aq), heat under reflux
Halogenoalkane	Amine	Excess NH3, sealed tube, heat
Halogenoalkane	Nitrile	KCN in ethanol/water, heat under reflux
Halogenoalkane	Alkene	NaOH in ethanol, heat under reflux

Key Conversions from Carbonyls and Acids

Starting Group	Target Group	Reagent/Conditions
Aldehyde	Primary alcohol	NaBH4/H2O (reduction)
Ketone	Secondary alcohol	NaBH4/H2O (reduction)
Aldehyde	Carboxylic acid	K2Cr2O7/H2SO4, reflux
Aldehyde/Ketone	Hydroxynitrile	HCN + KCN (nucleophilic addition)
Nitrile	Carboxylic acid	Dilute acid or dilute NaOH, reflux (hydrolysis)
Nitrile	Primary amine	LiAlH4 in dry ether, then dilute acid
Carboxylic acid	Acyl chloride	SOCl2 or PCl5
Carboxylic acid	Primary alcohol	LiAlH4 in dry ether
Carboxylic acid	Ester	ROH + conc. H2SO4, reflux
Acyl chloride	Ester	ROH, room temperature
Acyl chloride	Amide	RNH2 or NH3, room temperature

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Master Synthesis Route Map

flowchart TD
    ALK[Alkene] -->|"Steam, H3PO4<br>high T and P"| ALC[Alcohol]
    ALK -->|HBr| HAL[Halogenoalkane]
    ALC -->|"K2Cr2O7/H2SO4<br>distil"| ALD[Aldehyde]
    ALC -->|"K2Cr2O7/H2SO4<br>reflux"| CA[Carboxylic Acid]
    ALC -->|"NaBr/H2SO4<br>or SOCl2"| HAL
    ALC -->|"Conc. H2SO4<br>heat"| ALK
    ALD -->|"K2Cr2O7/H2SO4<br>reflux"| CA
    ALD -->|NaBH4, water| ALC
    ALD -->|"HCN + KCN"| HN[Hydroxynitrile]
    HAL -->|"NaOH(aq), reflux"| ALC
    HAL -->|"KCN, reflux"| NIT[Nitrile +1C]
    HAL -->|"Excess NH3<br>sealed tube"| AM[Amine]
    HAL -->|"NaOH in ethanol<br>heat"| ALK
    NIT -->|"Dilute acid, reflux"| CA2[Carboxylic Acid +1C]
    NIT -->|"LiAlH4, dry ether"| AM2[Amine +1C]
    CA -->|SOCl2| ACL[Acyl Chloride]
    CA -->|"LiAlH4, dry ether"| ALC
    ACL -->|ROH| EST[Ester]
    ACL -->|"RNH2"| AMD[Amide]

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Retrosynthetic Analysis

Retrosynthetic analysis is a problem-solving strategy where you work backwards from the target molecule to the starting material. Instead of asking "what can I make from this starting material?", you ask "what could I make this target molecule from?"

The Approach

1. Identify the target molecule and its functional group(s)
2. Ask: what reaction could produce this functional group? What would the immediate precursor look like?
3. Repeat: work backwards from the precursor to an even simpler molecule
4. Continue until you reach a simple, readily available starting material
5. Write the forward synthesis by reversing the retrosynthetic steps and adding reagents and conditions

In retrosynthesis notation, a special arrow (=>) is used pointing from target to precursor, indicating "can be made from."

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Worked Synthesis Route Examples

Example 1: Ethanol to Ethylamine (2 steps)

Target: CH3CH2NH2 (ethylamine -- an amine) Starting material: CH3CH2OH (ethanol)

Retrosynthetic analysis:

• Ethylamine <= Bromoethane + excess NH3
• Bromoethane <= Ethanol + NaBr/H2SO4

Forward synthesis:

1. CH3CH2OH --> CH3CH2Br (using NaBr + conc. H2SO4)
2. CH3CH2Br --> CH3CH2NH2 (using excess NH3, sealed tube, heat)

Example 2: Propan-1-ol to Propanoyl Chloride (2 steps)

Target: CH3CH2COCl Starting material: CH3CH2CH2OH

Forward synthesis:

1. Propan-1-ol --> Propanoic acid (K2Cr2O7/H2SO4, heat under reflux)
2. Propanoic acid --> Propanoyl chloride (SOCl2; by-products SO2 and HCl are gases)

Example 3: Bromoethane to Propanoic Acid (2 steps, chain extension)

Target: CH3CH2COOH (propanoic acid, 3C) Starting material: CH3CH2Br (bromoethane, 2C)

The chain must grow by 1 carbon. The nitrile route achieves this:

1. CH3CH2Br + KCN --> CH3CH2CN (propanenitrile) -- KCN in ethanol/water, reflux. Chain extended by 1C.
2. CH3CH2CN + H2O --> CH3CH2COOH -- Dilute acid (or dilute NaOH), heat under reflux. Hydrolysis of nitrile to carboxylic acid.

Example 4: Ethanol to Ethyl Ethanoate (Starting from Ethanol Only) (3 steps)

Target: CH3COOC2H5 (ethyl ethanoate) Starting material: Ethanol (the only organic reagent)

You need both the acid component (ethanoic acid) and the alcohol component (ethanol):

1. Oxidise some ethanol to ethanal: K2Cr2O7/H2SO4, distil
2. Oxidise ethanal to ethanoic acid: K2Cr2O7/H2SO4, reflux
3. React ethanoic acid with ethanol: conc. H2SO4 catalyst, reflux --> ethyl ethanoate + H2O

Key insight: You must reserve some unreacted ethanol for step 3. If you oxidise all of it, you have no alcohol left to make the ester.

Example 5: But-1-ene to Butanoic Acid (2 steps)

1. But-1-ene --> Butan-1-ol (steam, H3PO4 catalyst, high T and P)
2. Butan-1-ol --> Butanoic acid (K2Cr2O7/H2SO4, heat under reflux)

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Common Multi-Step Route Patterns

Pattern: Alcohol --> Halogenoalkane --> Amine

Standard route for introducing an amine group:

1. Convert alcohol to halogenoalkane (NaBr/H2SO4 or SOCl2)
2. React with excess ammonia in sealed tube

Pattern: Alcohol --> Aldehyde --> Carboxylic Acid (Oxidation Ladder)

Controlled by distillation vs reflux:

1. Oxidise primary alcohol to aldehyde (K2Cr2O7/H2SO4, distil)
2. Further oxidise aldehyde to carboxylic acid (K2Cr2O7/H2SO4, reflux)

Pattern: Halogenoalkane --> Nitrile --> Carboxylic Acid/Amine (Chain Extension)

Adds one carbon to the chain:

1. React halogenoalkane with KCN (nitrile, +1C)
2. Hydrolyse to carboxylic acid OR reduce to amine

Pattern: Carboxylic Acid --> Acyl Chloride --> Ester/Amide (Activation)

For efficient ester/amide formation:

1. Convert acid to acyl chloride (SOCl2)
2. React acyl chloride with alcohol (ester) or amine (amide) -- fast, irreversible

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Protecting Groups

Sometimes a molecule has two functional groups but you only want to react one of them. A protecting group is a temporary modification applied to one functional group to prevent it from reacting while you perform a transformation on the other group. After the desired reaction, the protecting group is removed to regenerate the original functional group.

Example

If you have a molecule with both an -OH group and a C=C double bond, and you want to react only the C=C:

1. Protect the -OH group by converting it to an ester (e.g., react with ethanoyl chloride)
2. Perform the desired reaction on the C=C double bond
3. Deprotect by hydrolysing the ester back to the -OH group

Protecting groups are essential in complex synthesis because many reagents are not selective enough to react with only one functional group in a polyfunctional molecule.

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Planning Tips for Exam Questions

1. Identify the starting material and target -- compare their functional groups and carbon chain lengths
2. Count carbons. If the chain has grown, a nitrile step (KCN) is needed. If the chain is the same length, only functional group interconversions are needed.
3. If the chain has been extended by one carbon, look for the nitrile route (halogenoalkane + KCN)
4. Write out each step with: starting material --> product, reagent, conditions
5. Check each step is chemically reasonable -- does the mechanism make sense?
6. Consider the order -- some transformations must be done before others (e.g., you cannot oxidise an alcohol that has already been converted to a halogenoalkane)
7. State reagents precisely -- "acidified dichromate" is not enough; write "K2Cr2O7/H2SO4" and specify "distil" or "reflux"

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Common Exam Mistakes

1. Confusing distillation and reflux conditions. Distillation = collect the aldehyde as it forms. Reflux = keep everything in the flask and get the carboxylic acid. This single word choice changes the product.

2. Forgetting to specify excess ammonia. Without "excess," you get a mixture of amines. Always write "excess ammonia, sealed tube, heat."

3. Using NaBH4 to reduce a carboxylic acid. NaBH4 is too mild. Use LiAlH4 in dry ether for carboxylic acids.

4. Mixing up aqueous and ethanolic NaOH. Aqueous NaOH + halogenoalkane = substitution (alcohol product). Ethanolic NaOH + halogenoalkane = elimination (alkene product). State the solvent.

5. Not accounting for all reagents needed. In the "ethanol to ethyl ethanoate" synthesis, students often forget that ethanol is needed as both the oxidation substrate AND the esterification alcohol. Reserve some.

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Summary

Organic synthesis at A-Level requires you to memorise a toolkit of functional group interconversions and to apply retrosynthetic thinking. The key is to work backwards from the target, identify the functional group transformations needed, and then write the forward synthesis with specific reagents and conditions for each step. Mastery of synthesis routes is one of the most demanding but rewarding aspects of organic chemistry.

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A-Level Deep Dive: Organic Synthesis

Spec mapping

Edexcel 9CH0 specification, Topic 18 — Organic synthesis (synoptic across Topics 6–18) covers the design of multi-step synthetic routes from a given starting material to a target molecule, the use of functional group interconversions (FGI) with appropriate reagents and conditions, the consideration of selectivity (chemoselectivity, regioselectivity and stereoselectivity), the use of protecting groups at an introductory level, the recognition of chirality in synthesis (racemic vs enantiopure products) and the interpretation of synthesis problems involving aliphatic, aromatic and bifunctional substrates (refer to the official specification document for exact wording). Examined directly on Paper 2 with high-tariff multi-step questions and on Paper 3 (synoptic problem-solving). Connects to CP6 (distillation), CP7 (preparation of organic solid), CP14 (aspirin), CP15 (ester preparation) and CP16 (chromatographic identification).

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Worked example with full mark scheme

Question (8 marks):

(a) Outline a 3-step synthesis of ethyl ethanoate (CH3COOCH2CH3) starting from ethanol (CH3CH2OH). Give reagents and conditions for each step. (4)

(b) Outline a 3-step synthesis of 4-aminobenzoic acid (NH2C6H4COOH) starting from methylbenzene (C6H5CH3). State the regioselectivity issues at each step. (4)

Solution with mark scheme:

(a) Step 1 — oxidise ethanol to ethanoic acid. Reflux with excess acidified potassium dichromate(VI) (K2Cr2O7 + dilute H2SO4). The strong oxidising conditions take the primary alcohol all the way to the carboxylic acid (not stopping at the aldehyde). Solution turns from orange to green.

M1 — correct reagents/conditions for oxidation to carboxylic acid (note: distillation of an aldehyde would require gentle heating; reflux ensures full oxidation to -COOH).

Step 2 — esterify ethanoic acid with a second portion of ethanol. Reflux with concentrated H2SO4 catalyst.

M1 — esterification step correctly identified.

Step 3 — purification. Distil off the ester (b.p. 77 °C); wash with aqueous Na2CO3 to remove residual acid; dry with anhydrous Na2SO4; redistil.

M1 A1 — purification step / overall product correctly identified as ethyl ethanoate.

(b) Step 1 — nitration of methylbenzene. Reagents: conc. HNO3 + conc. H2SO4 at 50–55 °C. The methyl group is 2,4-directing (alkyl groups donate electron density into the ring by hyperconjugation/+I), so the major product is 4-nitromethylbenzene (with some 2-isomer as minor product, separated by recrystallisation).

M1 — nitration with correct regioselectivity (4-position favoured by steric arguments over 2-position).

Step 2 — oxidation of methyl to carboxylic acid. Reagents: hot acidified KMnO4 (or hot K2Cr2O7), prolonged reflux. The alkyl side chain is oxidised to -COOH. Product: 4-nitrobenzoic acid.

M1 — oxidation step correctly identified.

Step 3 — reduction of -NO2 to -NH2. Reagents: Sn + concentrated HCl, reflux; then add NaOH to liberate the free amine. Product: 4-aminobenzoic acid (PABA).

M1 A1 — reduction step + final product correctly identified.

Total: 8 marks (M7 A1).

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Specimen question modelled on the Edexcel 9CH0 Paper 2 format

Question (8 marks): Suggest a synthesis of 2-aminopropanoic acid (alanine, CH3CH(NH2)COOH) from propanal (CH3CH2CHO).