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Few topics generate as much excitement at teacher feedback meetings as that of reaction mechanisms. This is partly because teachers have come to believe that there are certain ways of showing mechanisms that are 'required', a belief reinforced by the publication of mark schemes which are then treated as model answers.

Please note that these pages represent my views, and do not constitute a policy document from Edexcel or any other examining body.

  • Markschemes are not model answers. The published markscheme is an attenuated version of the one used by examiners to mark the papers. It does not address all the possible routes via which credit can be obtained, the principle being that correct chemistry scores. The art of marking examinations (a process unlike that of marking homework or classwork) requires an appreciation of all the nuances of meaning that candidates can produce, an understanmding which comes from the examiners meeting and the consideration given there to the way in which the paper has been answered.
  • The representation of mechanisms does not require certain hoops to be jumped. The problem with a mechanism is that you have to represent as static drawings something that is a dynamic process. An examiner can get a feel for whether the candidate understands the mechanism, and therefore deserves the credit for this, without getting a ruler out to see if the arrows are of exactly the right length. However, the candidate has a duty to make intentions clear by drawing as carefully as possible. Some pointers will be found in each of the mechanism pages.
  • General principles.
  • Curly arrows show the movement of electron pairs. The tail of the arrow is where the electrons come from, and the head can either point at the attacked atom, or can finish at the midpoint of where the new bond will be.
  • Draw the mechanisms so that as far as is possible electrons move, but atoms do not do so very much.
  • In mechanisms involving nucleophiles show the lone pairs. This is not required, but is very helpful.
  • Don't draw arrows to or from charges. Yes, I know that several highly respected and fine textbooks do it, but it can cause problems of interpretation and this is one thing that examiners do not like.
Electrophilic addition
Addition of bromine to a carbon-carbon double bond.
Electrophilic substitution
Nitration of benzene; the Friedel-Crafts reaction.
Nucleophilic substitution SN1
Hydroxide ions with 2-bromo-2-methylpropane
Nucleophilic substitution SN2
Hydroxide ions with bromomethane
Nucleophilic addition
Cyanide ions and carbon-oxygen double bond.

Animated reaction mechanisms
Animations, but no curly arrows


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