last updated Monday, February 02, 2015

Q1: The reaction does not seem to initiate on its own. What can I do to initiate the reaction?

A: There are several possible answers to this question.

1. Did you dry the glassware properly as described in the lab manual? If not, it is very unlikely that you will get the reaction started. The best course of action here is to start over.

2. Did you get the correct compounds? It is not uncommon that students use the wrong chemicals in the "heat of the battle".

3. Did you activate the Mg-turnings by crushing them up? How do the Mg-turnings look like? Shiny and silver or black? If they are black, they are old and do usually not react very well. Get new ones from the lab support.

4. Apply some heat using a warm water bath to initiate the reaction and see if this gets the reaction started. Do not use the heat gun for this step!

5. Add an iodine crystal to the mixture and see if this gets the reaction started.

6. Did you notice that the reagent bottles were open?

7. Sometimes, patience pays off as well because there are little details that are not perfect and they add up in slowing the initiation. If you encounter problems, try to solve them yourself first before you ask your TA for help.

Q2: I ran the reaction and do not seem to get much product.

A: Again, there are several possible answers to this question.

1. Are you sure that you isolated the correct layer? The benzoic acid is initially dissolved in ether (top layer) and then is extracted into the aqueous layer using the sodium hydroxide (bottom layer, usually a 'dirty' brown).

2. How fast did you add the bromobenzene solution? If you added it too fast, you obtained a lot of biphenyl and the yield of benzoic acid will be very low. In addition, you probably observe that a lot of the Mg-turnings did not dissolve either.

3. Did you check the pH-value to see if the solution is acidic (pH~2-3). If not, the better part of the product will stay in solution. Don't forget to mix the solution before you check the pH.