Organic Mechanisms (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the organic mechanisms covered in the AQA A-Level course.

There are eight of them, and the notes give the general form of each one, so it can be drawn for whichever specific reaction an exam question asks about.

Throughout the A-Level course you have met several organic mechanisms. There are eight in total: free radical substitution, electrophilic addition, nucleophilic substitution, elimination, nucleophilic addition, nucleophilic addition-elimination, electrophilic substitution, and the formation of amides. What the notes set out is the general mechanism for each. In the exam, you have to draw them specific to the reaction in the question.

We'll take free radical substitution first, then the electrophilic mechanisms, then the nucleophilic ones, and elimination last.

Free radical substitution proceeds through three stages: initiation, propagation and termination. Initiation uses ultraviolet light to break a bond homolytically, propagation is a chain reaction, and termination is two radicals combining. It is the mechanism where curly arrows are not used: half-headed arrows are not required, but double-headed arrows will be penalised, and radical dots must be shown. You meet it in the alkanes topic.

Two of the eight mechanisms are electrophilic. In electrophilic addition, the electron-rich pi bond in an alkene attacks the electrophile to form a carbocation intermediate, and you meet that one in alkenes. In electrophilic substitution, the aromatic ring is preserved, because the delocalised system is restored after the substitution has happened. That one comes up in aromatic chemistry.

Four of the eight mechanisms involve a nucleophile. In nucleophilic substitution, a nucleophile attacks a delta-positive carbon as the halide ion leaves. With ammonia as the nucleophile, a further deprotonation step removes a hydrogen to form the amine product. You meet these in halogenoalkanes.

In nucleophilic addition, the nucleophile attacks the planar delta-positive carbon of a carbonyl group, which is the aldehydes and ketones topic. Nucleophilic addition-elimination adds to the carbonyl carbon first and then loses the leaving group, and it appears in carboxylic acids and their derivatives. Amide formation is an example of that same mechanism, between an amine and an acyl chloride.

Elimination produces an alkene by removing a hydrogen atom and a halide from adjacent carbons. Like nucleophilic substitution, you meet it in the halogenoalkanes topic.

You must be able to provide these mechanisms in full if you are asked for them, and to apply what happens in one reaction mechanism to other reactions. Curly arrows must originate from a lone pair or from a bond, because an arrow starting at an atom scores zero. When you show a bond forming, the arrow must go directly to the relevant atom, not to a formal charge. Drawing extraneous curly arrows also loses marks.

Eight mechanisms cover the whole of the organic course: free radical substitution, two electrophilic mechanisms, four nucleophilic ones, and elimination. Free radical substitution runs in three stages and uses no curly arrows. The electrophilic mechanisms start with an electron-rich system attacking an electrophile. The nucleophilic ones start with a nucleophile attacking a delta-positive carbon. Elimination takes a hydrogen and a halide from adjacent carbons. What the notes give you is the general form of each, and the exam asks you to draw it for the specific reaction in front of you.