|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.
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.
- 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
- 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.
Animated reaction mechanisms