[denisepr]

You have 4 test tubes with:

#1 water and pure acetanilide
#2 water and pure salicylic acid
#3 water and a mixture of acetanilide and salicylic acid
#4 water (control)

Then you add a drop of a solution of iron chloride 1%

My question is: Why the tube with the presence of salicylic acid (#2 & #3) turns purple? I know its because it has a phenol group but I want to know why? What reacts with what, why the tube #1 didn't turn purple and stayed clear, things like that.

Thank you so very much!

[organicadam94]

I'm not sure if this is as much detail as you wanted, but the -OH of the phenol forms a violet complex with the iron chloride. If I remember correctly, it goes something like:

FeCl3 + 6 Ph-OH >> H3[Fe(PhO)6] + 3 HCl

Since the test only gives positive results for phenols (and other aromatics with hydroxyl groups), then I assume the purple colour is related to the presence of the aromatic group (aromatic groups have lots of conjugation and complexes/charged species of aromatic groups are often coloured).     Obviously, the test does not go positive for simple alcohols such as methanol or ethanol

The positive result for phenols, and not for simple alcohols could also have some relation to the ease at which the hydroxyl group can be "deprotonated", since in the purple complex, you essentially have PhO- coordinating to the iron.

Hope that slightly helps...
A