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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 $\ce{Cl2}$?

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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.

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  • $\begingroup$ THANK YOU! @Jan! Can you help me further to reach the answer I was looking for, which was what the theoretical yield of the C14H12O would be when I used 1,8g of trans-Stilbene and 3,45g of mCPBA with an obtained product of 0,247g. I get the theoretical yield to be 33g but that must be wrong! $\endgroup$ – O.kth Mar 1 '17 at 19:26
  • $\begingroup$ @O.kth The typical steps are: 1) Calculate the amount of substance of each reactant 2) determine the limiting reagent 3) using that and the equation to determine the expected amount of product 4) calculating the expected mass of product. Using your numbers, the theoretical yield should be $\pu{1.96g}$. $\endgroup$ – Jan Mar 1 '17 at 21:33

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