I want to get the molecular formula for this reaction:
$$\ce{{$trans$-stilbene} + {$m$-chloroperoxybenzoic acid} -> {$trans$-1,2-diphenyloxirane}}$$
and if you do you get this:
$$\ce{C14H12 + C7H5ClO3 -> C14H12O}$$
but what do I do about the chlorine not being present in the products, is it released as $\ce{Cl2}$?
The reason you cannot balance the reaction is because you did not consider everything that happens. In the Prilezhaev epoxidation, mCPBA is a stoichiometric reactant but only one single atom of mCPBA is transferred to the product. Therefore, these compounds are often written on the reaction arrow rather than as reactants and products:
$$\ce{C14H12 ->[C7H15ClO3] C14H12O}\tag{1}$$
However, that still does not tell you what exactly happened. If you take a close look at the mechanism, you will see that mCPBA remains essentially unchanged except that it loses its peroxy function; the second product is mCBA or meta-chlorobenzoic acid. Thus, a full reaction equation would be:
$$\ce{C14H12 + C7H15ClO3 -> C14H12O + C7H15ClO2}\tag{2}$$
Notice how this balances automatically.
