in this pic extracted from updates to cstoh chem books on
step-by-step website,
he cancelled out the slow and fast, for the SN2
mechanism
i think i know it is unnecessary(or is it Wrong) to write that
for SG-Cambridge A levels,
but is it wrong, chemically?
for my sch, the lect notes didn't put, but tutorial soln put the
'slow' and 'fast' ._. . anyway cstoh and C.K.S's books are 1000x
better, compact and organised and better info.
For reaction mechanisms, the "slow" and "fast" steps should only be
written if the Cambridge question asks for it, eg. particular if
it's a kinetics question.
Otherwise, you don't have to write it in. But it's ok if you
want to do so, and Cambridge will just ignore it (whether you got
it right or wrong) if the question didn't ask for it.
In CS Toh's update, the reason why it's incorrect to put either
"slow" or "fast" is because the structure shown is the transition
state, not the intermediate (there is no intermediate for SN2).
Note that "slow" and "fast" must refer to actual steps converting
reactant to intermediate, intermediate to intermediate, or
intermediate to product. Transition states are simply
human-imagined states in-between (ie. transitioning) actual steps,
and thus cannot be either "slow" or "fast".
