I saw acetic acid being used in two reactions: 

1) [![enter image description here][1]][1]

2)

[![enter image description here][2]][2]

 
I was asked to predict the outcome of the first reaction but I just couldn't becuase nitration requires generation of $\ce{NO2+}$ which in turn requires an acid stronger than nitric acid.

I would like to know how exactly is $\ce{AcOH}$ participating in these two reactions. *Is it supporting the generation of electrophile?* If yes, then how? I couldn't find any relevant information on the net. The second reaction is from Clayden's book and he doesn't highlight the role of $\ce{AcOH}$ there.  


  [1]: https://i.sstatic.net/1u7YS.png
  [2]: https://i.sstatic.net/t5n1r.png