Characteristic Organic Reactions

· Organic mechanisms show the step-by-step movement of electrons during a reaction.
· For CIE A-Level, the key A Level mechanism types here are electrophilic substitution and addition–elimination.
· Always use curly arrows to show movement of an electron pair.
· A curly arrow must start from a bond or lone pair, not from an atom label or charge.
· Mechanism answers should clearly show attacking species, intermediates, charges, and products.

This image helps students recognise species that donate electron pairs in mechanisms. It is useful for reinforcing where curly arrows often begin. Source

Electrophilic Substitution

· Electrophilic substitution = an electrophile replaces an atom or group in an organic molecule.
· In aromatic chemistry, benzene/arenes undergo substitution rather than addition because substitution restores the delocalised π system.
· General idea: arene + E⁺ → substituted arene + H⁺.
· Step 1: the π electrons of the aromatic ring attack the electrophile, E⁺, forming a positively charged intermediate.
· Step 2: loss of H⁺ restores aromatic stability and forms the substituted product.
· Exam focus: show the first curly arrow from the π system to E⁺, then show removal of H⁺ to regenerate the ring.

Electrophilic Substitution: Exam Pattern

· Electrophile generation may be required before the main mechanism.
· Example pattern: strong acids/catalysts can generate a more powerful E⁺.
· The aromatic ring behaves as an electron-rich π system.
· The intermediate is often drawn as a carbocation with disrupted aromaticity.
· The final step is deprotonation, restoring the delocalised π system.
· Common exam errors: drawing addition products, missing positive charges, or failing to regenerate the aromatic ring.

This image sequence shows how benzene reacts with an electrophile and then loses H⁺. It is a clean example of the standard electrophilic substitution mechanism. Source

Addition–Elimination

· Addition–elimination = a two-stage mechanism: addition first, then elimination.
· It is typical of reactions at a carbonyl carbon in compounds such as acyl chlorides.
· The carbonyl carbon is δ⁺ because oxygen is more electronegative, so it is attacked by a nucleophile.
· Step 1: nucleophilic addition to the carbonyl carbon forms a tetrahedral intermediate.
· Step 2: the C=O bond reforms and a leaving group, commonly Cl⁻, is eliminated.
· Overall, the leaving group is replaced, but the mechanism must be described as addition–elimination, not simple substitution.

This diagram clearly separates the addition and elimination stages. It is ideal for practising curly-arrow mechanisms involving acyl chlorides. Source

Addition–Elimination: Exam Pattern

· Identify the nucleophile: usually a species with a lone pair such as H₂O, NH₃, an alcohol, phenol, or amine.
· Identify the electrophilic carbonyl carbon in RCOCl.
· Draw the curly arrow from the lone pair on the nucleophile to the carbonyl carbon.
· Move the C=O π electrons onto oxygen to form the tetrahedral intermediate.
· Reform the C=O bond and eliminate Cl⁻.
· Finish by showing any required proton transfer to form the neutral organic product and HCl.

Mechanism Keywords

· Electrophile = electron-pair acceptor; attracted to electron-rich regions.
· Nucleophile = electron-pair donor; usually has a lone pair or negative charge.
· Curly arrow = movement of an electron pair, not movement of atoms.
· Intermediate = species formed during a mechanism but not present in the overall equation.
· Leaving group = atom/group that departs with an electron pair, e.g. Cl⁻ in acyl chloride reactions.
· Aromatic stabilisation = stability from the delocalised π system in benzene/arenes.

Common Exam Traps

· Do not draw curly arrows starting from positive charges; start from a bond or lone pair.
· Do not call electrophilic substitution in benzene electrophilic addition.
· Do not forget that electrophilic substitution must restore the delocalised π system.
· Do not skip the tetrahedral intermediate in addition–elimination.
· Do not forget charges on intermediates, nucleophiles, electrophiles, and leaving groups.
· Do not describe acyl chloride reactions as just “substitution” when the required mechanism is addition–elimination.