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In my lab, acetanilide was added to bromine in aqueous acetic acid, and discolouration of the reaction mixture was obtained, indicating that bromine was consumed.

After some discussion, it was concluded that an electrophilic aromatic substitution might be taking place. This was backed up by a procedure we found online. What was left to do was to confirm the presence of the product, 4-bromoacetanilide.

The challenge is that the reaction mixture contains unreacted acetanilide as well as 4-bromoacetanilide, acetic acid, and possibly some polyhalogenated byproducts. Any conditions that could result in amide hydrolysis, e.g. heating with base, should also be avoided. No residue was observed in the reaction mixture.

So, what chemical property can be used to determine the presence of 4-bromoacetanilide in the reaction mixture?

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    $\begingroup$ Did you try to isolate and purify the product? $\endgroup$ – Klaus-Dieter Warzecha Oct 31 '16 at 17:31
  • $\begingroup$ @KlausWarzecha Actually, no. I'm quite sure that spectroscopy would quickly yield the answer, but as of now, both those techniques are out-of-bounds. This is why the question focuses on chemical properties. $\endgroup$ – Eashaan Godbole Oct 31 '16 at 17:36
  • $\begingroup$ @EashaanGodbole It's always easier when your material is purified. The most simple method to check whether bromination took place in the para position is to measure the melting point. The melting points of the bromoacetanilides strongly depend on the position of the bromo substituent, and they are all different from the parent acetanilide. $\endgroup$ – Klaus-Dieter Warzecha Oct 31 '16 at 17:45
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I advise against fishing in the mixture in this case. Try to isolate the product, purify it and determine its properties.

Have a look at the melting points of the starting material and the possible monobromination products:

  • acetanilide 114 °C
  • 2-bromoacetanilide 96.5–100.5 °C
  • 3-bromoacetanilide 81–85 °C
  • 4-bromoacetanilide 165–169 °C

This promising! You can of course use further spectroscopical methods (NMR), but I'd start with the melting point.

You mentioned possible complications for the workup, but I disagree.

The reaction is typically performed in glacial acetic acid with a slight excess of bromine to make sure that there's (almost) no starting material left.

For workup, you pour the whole mixture into a beaker with water that has been placed in a bowl with ice. If you want/need to destroy an excess of bromine, add some bisulfite solution until the aqueous phase is colourless.

There should be some crystals. Filter them off, and recrystallize the material from ethanol.

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