http://s71.beta.photobucket.com/user/JogaBonito7/media/Screenshot2012-11-12atPM091256.png.html?sort=3&o=0
For Step 2, is the reagent concentrated or dilute HNO3?
I put concentrated HNO3 but apparently the answer given is dilute HNO3. Why is this so?
2 reasons :
1) As far as the benzene ring is concerned, concentrated HNO3 will also give the same product. But Cambridge wants to test if the student realizes that due to the 2 electron-donating by resonance and thus strong activator phenolic OH groups present, dilute HNO3 will suffice to substitute all 3 NO2 groups, and concentrated HNO3 would therefore be an overkill (which translates into a waste of resources and money, as well as an unncessary additional safety hazard since concentrated HNO3 is highly corrosive).
2) The more concentrated the acid, the higher the risk that the nearby C=C alkene group being reacted up, which is not desirable in this question. Any acid (eg. HNO3, H2SO4, H3PO4) is able to function as a catalyst to hydrate the alkene double bond, to generate an alcohol.
As far as A levels are concerned, reason #1 is more relevant and pertinent.