Example Synthesis of 1,4-dibromobenzene

The procedure below has been used as a laboratory exercise at the author s university during the first semester undergraduate course in organic chemistry. 

Assume that you wish to run the synthesis of 1,4-dibromebenzene on a larger scale and that this particular procedure is a candidate for scale-up. To allow for an optimization of the yield it will be necessary to identify the important experimental variables. 

Bromination To 5 ml of benzene in a 100 ml Erlenmeyer flask is added one small piece of iron filings. Cool the flask in ice-water and add cautiously 6.0 ml of bromine. Swirl the flask and allow it to stand at room temperature until the next day. 

Weigh the product to determine the yield, and determine tbe melting point. Compare the observed melting point with literature data. 

The procedure is divided into two steps, bromination and work-up. The critical step is the bromination. A maximum conversion in this step is necessary for a maximum final yield. Hence, variables involved in the first step will be the most important. A first screening can therefore be limited to these. An overall optimization can possibly be achieved through a step-wise approach First optimize the yield of the bromination step, then adjust the work-up procedure to fit an optimum bromination. This is not always true, sometimes by-products interfere with the work-up procedure. 

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