Chemistry A2 · An introduction to A Level organic chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Two new mechanism names are used at A Level.
Electrophilic substitution is the typical reaction of the benzene ring. An electrophile (an electron-pair acceptor, usually a positive ion such as NO2+) is attracted to the delocalised π electrons of the ring. It takes the place of a hydrogen atom, which leaves as H+. The ring keeps its delocalised system.
Addition–elimination is the typical reaction of acyl chlorides with water, alcohols, phenols, ammonia and amines. The nucleophile first adds to the δ+ carbon of the C=O group. Then the C=O bond forms again and a chloride ion is lost. Overall a small molecule, HCl, is eliminated, so the reaction is also called condensation.
Calling the reaction of benzene with bromine electrophilic addition because bromine adds to alkenes. In benzene a hydrogen atom is replaced and the ring stays intact, so it is substitution.
Answer: Addition–elimination; HCl is eliminated (with excess ammonia it is seen as NH4Cl).
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