when asked to 'give equation for the reaction'
e.g. pre u paper 2014 p3 qn 5b ii
can we draw the molecules, instead of giving
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr? will it be marked
correct?
Depends on context. Even in contexts where drawing out the
structures and/or mechanisms is acceptable by Cambridge as an
alternative to the written equation, you'll need to be careful in
the stoichiometry and balancing the reaction correctly. If unsure
of acceptability of context, always play safe and give both (ie.
draw out structures and/or mechanism if it helps you, *and* write
out the balanced equation), as long as you know your chemistry well
and have good time management.
In the context of your particular question, if all you mean is
drawing out the benzene ring, yes it's acceptable to do so (instead
of just writing the formula), particularly since in this case
phenol is tribrominated (on all 3 of the ortho and para positions)
and hence in this case there are no positional isomer ambiguity
problems (eg. if it was only monobrominated or dibrominated, then
you would have a mixture of different positional isomers, in which
case it would be more correct to write the condensed structural
formula C6H4BrOH instead of drawing it out).