Most A Level organic reactions fit a small set of labels: electrophilic substitution, addition-elimination, condensation, hydrolysis, reduction and oxidation. Learn what each label means and the rest of the year becomes pattern recognition.
In electrophilic substitution an electrophile such as NO2+ attacks the delocalised electrons of a benzene ring and replaces an H atom, so the ring survives. In addition-elimination a nucleophile adds to the C=O carbon of an acyl chloride and then Cl− is eliminated, leaving HCl. A condensation reaction joins two molecules with the loss of a small molecule such as H2O or HCl. Hydrolysis breaks a bond by reaction with water, often catalysed by acid or alkali. Reduction adds hydrogen or removes oxygen (written [H]); oxidation adds oxygen or removes hydrogen (written [O]). Diazotisation turns phenylamine into a diazonium salt, which couples with phenol to make an azo dye.
Sort the reaction cards in the simulator, then follow an electrophile attacking a pi system in Fig. 1. Notice where each arrow starts: always from a lone pair or a bond.
| Reaction type | Attacking species | Typical substrate | What changes |
|---|---|---|---|
| electrophilic substitution | electrophile (NO2+, Br+ from Br2/FeBr3) | arene | H on the ring replaced; ring kept |
| electrophilic addition (AS) | electrophile | alkene | C=C becomes C–C |
| nucleophilic substitution (AS) | nucleophile | halogenoalkane | halogen replaced |
| addition-elimination | nucleophile (H2O, ROH, NH3, RNH2) | acyl chloride | Cl replaced; HCl lost |
These labels describe what happens to the molecule overall, whatever the mechanism.
| Type | Example | Conditions |
|---|---|---|
| condensation | CH3COCl + C2H5NH2 → CH3CONHC2H5 + HCl | room temperature |
| hydrolysis | CH3CONH2 + NaOH → CH3COONa + NH3 | heat under reflux |
| reduction | C6H5NO2 + 6[H] → C6H5NH2 + 2H2O | Sn and conc. HCl, heat, then NaOH(aq) |
| oxidation | C6H5CH3 + 3[O] → C6H5COOH + H2O | hot alkaline KMnO4, then acidify |
| diazotisation, coupling | phenylamine → benzenediazonium ion → azo dye with phenol | HNO2 below 10 °C; then alkaline phenol |
Benzene undergoes electrophilic substitution, not addition: an addition would destroy the stable delocalised ring. Addition-elimination happens at the C=O carbon of an acyl chloride, so it is not the same as nucleophilic substitution of a halogenoalkane. Hydrolysis and condensation are opposites: condensation releases a small molecule, hydrolysis uses water to split a bond.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What type of reaction is the nitration of benzene?
Which electrophile attacks benzene during nitration?
What is a condensation reaction?
CH3COCl reacts with water. What type of reaction is this?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
7.80 g of benzene (Mr 78.0) gives 9.84 g of nitrobenzene (Mr 123.0). What is the percentage yield?
How many [H] are needed to reduce nitrobenzene, C6H5NO2, to phenylamine, C6H5NH2?
Which reaction is a condensation?
What is the atom economy for making CH3CONHC2H5 (Mr 87.0) from CH3COCl (78.5) and C2H5NH2 (45.0)?
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