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1 Why organic synthesis is the hardest exam topic
2 The master reaction map
3 Reagent-condition cheat sheet
Q: Is there a complete H2 Chemistry organic synthesis flowchart for the A-Level 9476 syllabus? A: Yes. This guide maps every functional group interconversion you need for Paper 2 and Paper 3 synthesis questions, with reagents, conditions, and worked retrosynthesis examples you can drill immediately.
TL;DR
Organic synthesis is not just reaction recall. It is route planning from a starting molecule to a target molecule.
Work backwards from the target functional group, check whether the carbon chain changes, then write each reagent and condition cleanly.
If the target has one more carbon, look for the nitrile route because the KCN step is the main H2 chain-extension move.
If you have...
Read this first
1 second
Organic synthesis is route planning, not isolated reaction recall.
Work backwards from the target, then choose reactions that change the right functional group and carbon count. The nitrile route is the common one-carbon extension.
Concrete example
To make an amide, work backwards to an acyl chloride, then find how to reach the required carboxylic acid.
Best next step
Pick the target functional group first, then write the previous molecule in the route.
Most JC students can recite individual organic reactions in isolation. Ask them to convert propan-1-ol into propanamide, though, and the route falls apart. The gap is not knowledge of single reactions but the ability to chain them into multi-step synthesis pathways, which is exactly what SEAB tests in the structured and free-response sections of Papers 2 and 3.
Work backwards to acyl chloride plus ammonia or amine
Target has one more carbon
Insert a halogenoalkane to nitrile step
Starting material is an alcohol
Decide whether it needs oxidation, substitution, or dehydration
The answer asks for conditions
Write reflux, distil, UV, room temperature, or sealed tube where relevant
1 Why organic synthesis is the hardest exam topic
Organic synthesis questions combine every reaction you have learnt across the entire organic section into a single multi-step problem. That makes them disproportionately difficult for three reasons:
Breadth of recall. You need instant access to 15-20 conversions, each with specific reagents and conditions.
Directionality. You must think backwards from the target molecule (retrosynthesis), not forwards from the starting material.
Carbon chain awareness. Some routes require extending or shortening the carbon chain, which means introducing a nitrile step that students often overlook.
A student who can answer every isolated reaction question may still score poorly on a synthesis question because the skill being tested is route planning, not individual reaction recall. The reaction map below is your tool for building that planning skill.
Multi-step synthesis planning is what separates A-grade from B-grade students. Students who can recall individual reactions but cannot chain them into a synthesis route from a given starting material to a target product lose significant marks in Paper 3 Section B. The skill is not knowing more reactions - it is working backwards from the target to identify the synthetic route.
2 The master reaction map
The table below organises the key functional group interconversions required by the SEAB 9476 syllabus. Read each row as a directed arrow: "From" can be converted to "To" using the stated reagent and conditions.
2.1 Alkanes, alkenes, and halogenoalkanes
From
To
Reagent
Conditions
Reaction type
Alkane
Halogenoalkane
ClX2 or BrX2
UV light
Free radical substitution
Alkene
Halogenoalkane
HBr or HCl
Room temperature
Electrophilic addition
Alkene
Alcohol
Steam (HX2O), HX3POX4
Alkene
Diol
Cold dilute KMnOX4
Alkaline, room temperature
Oxidation
Halogenoalkane
Alkene
Ethanolic NaOH or KOH
Heat under reflux
Elimination
2.2 Alcohols and carbonyls
From
To
Reagent
Conditions
Reaction type
Alcohol
Halogenoalkane
NaBr + conc HX2SOX4
Heat under reflux
Nucleophilic substitution
Alcohol
Chloroalkane
PClX3, PClX5
Primary alcohol
Aldehyde
KX2CrX2OX7
Secondary alcohol
Ketone
KX2CrX2OX7
Primary alcohol
Carboxylic acid
Excess KX2CrX2OX7
Aldehyde
Carboxylic acid
KX2CrX2OX7
Aldehyde
Primary alcohol
NaBHX4 in aqueous/methanol
Room temperature
Reduction (nucleophilic addition)
Ketone
Secondary alcohol
NaBHX4 in aqueous/methanol
Room temperature
Reduction (nucleophilic addition)
Aldehyde or ketone
Alcohol
LiAlHX4 in dry ether
Room temperature
Reduction
2.3 Carboxylic acids, acyl chlorides, and derivatives
From
To
Reagent
Conditions
Reaction type
Carboxylic acid
Ester
Alcohol + conc HX2SOX4 catalyst
Heat under reflux
Condensation (esterification)
Carboxylic acid
Acyl chloride
SOClX2 or PClX5
Acyl chloride
Ester
Alcohol
Room temperature
Nucleophilic acyl substitution
Acyl chloride
Amide
Concentrated NHX3 or amine
Room temperature
Nucleophilic acyl substitution
Acyl chloride
Carboxylic acid
HX2O
Room temperature
Hydrolysis
2.4 Nitriles, amines, and chain extension
From
To
Reagent
Conditions
Reaction type
Halogenoalkane
Nitrile
KCN in ethanol
Heat under reflux
Nucleophilic substitution (+1C)
Nitrile
Carboxylic acid
Dilute HX2SOX4 or dilute NaOH
Heat under reflux
Hydrolysis
Nitrile
Amine
LiAlHX4 in dry ether, or HX2
Halogenoalkane
Amine
Excess concentrated NHX3 in ethanol
Sealed tube, heat
Nucleophilic substitution
Key point on chain extension. The KCN step is the most important carbon chain extension pathway in the H2 syllabus. When a synthesis question requires a product with one more carbon than the starting material, you almost certainly need to go through a nitrile intermediate.
3 Reagent-condition cheat sheet
The table below consolidates every conversion into a single quick-reference. Print this, stick it on your wall, and drill until you can reproduce it from memory.
#
Conversion
Reagent
Conditions
Type
1
Alkane → halogenoalkane
ClX2 or BrX2
UV light
Free radical sub.
2
Alkene → halogenoalkane
HBr / HCl
r.t.
Electrophilic add.
3
Alkene → alcohol
Steam, HX3POX4
4
Alkene → diol
Cold dilute KMnOX4
Alkaline, r.t.
Oxidation
5
Halogenoalkane → alkene
Ethanolic NaOH
Reflux
Elimination
6
Alcohol → halogenoalkane
NaBr + conc HX2SOX4
7
Alcohol → chloroalkane
PClX5 / SOClX2
8
1∘ alcohol → aldehyde
KX2CrX2OX7
9
2∘ alcohol → ketone
KX2CrX2OX7
10
1∘ alcohol → carboxylic acid
Excess KX2CrX2OX7
11
Aldehyde → carboxylic acid
KX2CrX2OX7
12
Aldehyde → 1∘ alcohol
NaBHX4
13
Ketone → 2∘ alcohol
NaBHX4
14
Carboxylic acid → ester
Alcohol + conc HX2SOX4
15
Carboxylic acid → acyl chloride
SOClX2 / PClX5
16
Acyl chloride → amide
Conc NHX3 or amine
r.t.
Acyl sub.
17
Acyl chloride → ester
Alcohol
r.t.
Acyl sub.
18
Halogenoalkane → nitrile
KCN in ethanol
Reflux
Nucleophilic sub.
19
Nitrile → carboxylic acid
Dilute HX2SOX4
20
Nitrile → amine
LiAlHX4 / dry ether
Reflux
Reduction
21
Halogenoalkane → amine
Excess conc NHX3 / ethanol
Sealed tube, heat
Nucleophilic sub.
4 How to plan a synthesis route (retrosynthesis strategy)
In the exam, you are given a starting material and a target molecule. Here is the step-by-step strategy that converts a blank answer space into a full-mark response.
Step 1: Start from the target and work backwards
Identify the functional group in the target molecule. Ask: what could be the immediate precursor? Write that precursor down, then repeat the question until you reach the starting material. This backwards approach (retrosynthesis) is far more reliable than trying to work forwards, because it narrows your options at each step.
Step 2: Identify the functional group changes
List the functional group in the starting material and the functional group in the target. The difference tells you which rows of the reaction map you need.
Step 3: Check the carbon chain length
Count the carbons in the starting material and the target. If the target has more carbons, you need a chain extension step. In the H2 syllabus, this almost always means converting a halogenoalkane to a nitrile using KCN (row 18 in the cheat sheet), which adds one carbon. If the target has fewer carbons, look for an oxidative cleavage or consider whether the question is testing a different pathway.
Step 4: Write out each step with reagents and conditions
For every arrow in your route, state:
The reagent (exact formula, not just a name).
The conditions (reflux, distil, sealed tube, UV, room temperature, etc.).
The type of reaction, if the question asks for it.
Missing any one of these loses marks.
Why retrosynthesis beats trial-and-error
The most common mistake students make is trying to work forwards from the starting material, testing possible reactions until something leads to the target. This approach is slow, inconsistent under exam conditions, and breaks down completely when the route has three or more steps. Retrosynthesis - working backwards from the target - is faster and more systematic because it narrows your choices at each step: instead of asking "what can I make from this?", you ask "what must have come before this?" The latter question has far fewer valid answers at each stage.
5 Worked examples
Example 1: Ethanol → ethyl ethanoate (2 steps)
Target analysis. Ethyl ethanoate is an ester. Esters form from a carboxylic acid + an alcohol, or from an acyl chloride + an alcohol.
Route (using carboxylic acid pathway):
Step 1. Oxidise ethanol to ethanoic acid. Reagent: excess KX2CrX2OX7 / dilute HX2SOX4. Conditions: heat under reflux.
Step 2. React ethanoic acid with ethanol to form ethyl ethanoate. Reagent: ethanol + concentrated HX2SOX4 catalyst. Conditions: heat under reflux.
Note: you need a second portion of ethanol in step 2. This is a common detail students miss.
Example 2: 1-Bromopropane → propanoic acid (2 steps via nitrile)
Target analysis. Propanoic acid has 3 carbons; 1-bromopropane also has 3 carbons. However, the direct hydrolysis of a halogenoalkane gives an alcohol, not a carboxylic acid. The question specifies the nitrile route, which produces butanoic acid (4 carbons) if we are not careful. So let us restate: converting 1-bromopropane via KCN gives butanenitrile (4 C), which hydrolyses to butanoic acid.
If the target is genuinely propanoic acid from a 3-carbon halogenoalkane, the route uses bromoethane instead. Let us work the intended 1-bromopropane → butanoic acid route and then note the chain-extension point.
Step 1. Convert 1-bromopropane to butanenitrile. Reagent: KCN in ethanol. Conditions: heat under reflux. Carbon count: 3 C → 4 C (chain extended by 1).
Step 2. Hydrolyse butanenitrile to butanoic acid. Reagent: dilute HX2SOX4 (or dilute NaOH, followed by acidification). Conditions: heat under reflux.
Exam tip. Always count carbons before and after the KCN step. The nitrile adds one carbon; the hydrolysis does not change the chain length.
Example 3: Propan-1-ol → propanamide (3 steps)
Target analysis. Propanamide is an amide. Amides form from acyl chlorides + NHX3. Acyl chlorides form from carboxylic acids. Carboxylic acids form from primary alcohols.
Step 1. Oxidise propan-1-ol to propanoic acid. Reagent: excess KX2CrX2OX7 / dilute HX2SOX4. Conditions: heat under reflux.
Step 3. React propanoyl chloride with ammonia to form propanamide. Reagent: concentrated NHX3. Conditions: room temperature.
This three-step route (alcohol → carboxylic acid → acyl chloride → amide) is one of the most commonly tested pathways in the H2 exam.
6 Common mistakes that lose marks
6.1 Reflux vs distillation for alcohol oxidation
When oxidising a primary alcohol to an aldehyde, you must distil the product immediately to remove it from the oxidising mixture before it is further oxidised to a carboxylic acid. When oxidising all the way to a carboxylic acid, you heat under reflux with excess oxidising agent. Writing "heat" without specifying reflux or distillation is insufficient and will cost marks.
6.2 Not specifying "excess" or "limited" reagents
The oxidation of a primary alcohol gives different products depending on whether the KX2CrX2OX7 is limited (aldehyde) or in excess (carboxylic acid). Similarly, the reaction of a halogenoalkane with NHX3 requires excess concentrated ammonia to favour the primary amine and minimise further substitution. Always state the quantity qualifier where it matters.
6.3 Missing the carbon chain extension step
If the target molecule has more carbons than the starting material, you need a chain extension. The most common pathway is halogenoalkane → nitrile (via KCN), which adds exactly one carbon. Students who jump from a 2-carbon starting material to a 3-carbon product without a KCN step are proposing an impossible route.
6.4 Vague conditions
Writing "heat" is almost never sufficient. You must specify:
Heat under reflux for most substitution and oxidation reactions.
Distil immediately for partial oxidation of primary alcohols to aldehydes.
Sealed tube, heat for halogenoalkane + excess concentrated NHX3.
UV light for free radical substitution.
Room temperature where applicable (e.g. acyl chloride reactions, NaBHX4 reduction).
6.5 Confusing aqueous vs ethanolic conditions for halogenoalkanes
Aqueous NaOH with a halogenoalkane gives substitution (producing an alcohol). Ethanolic NaOH gives elimination (producing an alkene). Swapping the solvent changes the entire product, so always state the solvent explicitly.
7 Putting it all together: your revision plan
Memorise the cheat sheet. Cover the reagent and conditions columns and test yourself until you can reproduce every row.
Practise retrosynthesis. Take any two functional groups from the map and plan the shortest route between them.
Drill carbon counting. For every route you plan, verify that the carbon chain length is consistent at each step.
Write full answers. In practice, write out reagent, conditions, and reaction type for every step, even when the question only asks for reagents. This builds the habit of completeness.
The most common complaint from students is "I understood the reactions in class but couldn't apply them in the exam." This is because understanding a single reaction is passive knowledge. Chaining multiple reactions into a synthesis route under exam conditions requires active problem-solving - a skill that only develops through deliberate practice with multi-step synthesis questions, not by re-reading notes. Treat every synthesis question in past papers as a planning exercise, not a recall test.
