[gjbaum]

I would greatly appreciate anyone who could explain this to me.  In the following reaction:

4-bromoacetanilide was mixed with HCl and acetic acid.  The mixture was heated on a steam bath and then cooled to zero degrees.  Then sodium chlorate was added with water.  At this point a precipitate formed and the mixture was to sit for one hour.  

I need to know why it is important that the mixture not be allowed to sit for more than one hour.  Will it lead to multiple substitutions?  The expected product is 4-bromo-2-chloroacetamide. Could the chlorine add at both of the ortho positions.?  Thanks.

[Organic_lover*]

Heym gjbaum!

The first step it's analise what you are using on the reaction. Well, the "main" reagent is the 4-bromoacetanilide and the others reagents used are HCl and CH₃-COOH. So you have to question yourself: "What will happen if I do this mixture?" and "Is my expected product formed exactly when the mixture of this reagents occur?".

Ok, thinking on the concepts involved in Eletrophilic Substitutions, we can see that the "main" step for this kind of reaction is the formation of a eletrophilic specie on the reactional mixture. So the difficult involved on this reaction is obtaining a Cl⁺ specie. Looking for the reagents that we got, the source of chloride is the HCl. So how can we turn this chloride atom in a Cl⁺?

Continuing with our thinking, we clearly assume that this reaction occurs on a aqueous solution. So the HCl added in the beginning of the procedure will be in the form of ions H⁺ and Cl^-. The acetic acid, on the other hand, will be in the form CH₃-COO^- and H⁺ but as acetic acid is a weak acid, he will tend to stay in the CH₃-COOH form. This is a key idea to solve this problem, because we can ask yourselves: "So why do I use sodium chlorate and water in this procedure?"

http://books.google.com.br/books?id=UbPpoeV-v1sC&pg=PA619&lpg=PA619&dq=4-bromoacetanilide+%2B+HCl+%2B+acetic+acid&source=bl&ots=xgTox-9Pxe&sig=bpJ3doOBLgAGJkn3v902G9_pJ18&hl=pt-BR&sa=X&ei=QCYaT9GcKs69gAfLuLCEDA&ved=0CCsQ6AEwAQ#v=onepage&q=4-bromoacetanilide%20%2B%20HCl%20%2B%20acetic%20acid&f=false

According to this huge link, some importants aspects were omitted:
-The heat used on the mixture was for a best solubilization of the 4-bromoacetanilide;
-After the addition of the sodium chlorate and water, occur the release of chlorine gas.

This indicates that the sodium chlorate added (NaClO₃) "combines" with the Cl^- ions to form Cl₂! This is the more important idea involved in this reaction!

So, we reach until this point. The mechanism for this reaction will depends as well of the orientation in the ortho, meta or para positions (which varies for each substituent on the benzene unit). The bromine atom of the 4-bromoacetanilide will favor the ortho and para positions and the amide group of the molecule will favor the "same" positions (ortho and para in relation of the amide group), but this last effect on the reactivity of the benzene ring is weakened by the ressonance effect of the carbonyl group on the unbounding electrons of the nitrogen atom. (a little confused this part, don't you think? sorry for this.) So we can conclude that the ortho positions of the  4-bromoacetanilide (in relation of the bromine atom) are the most possible positions to be occupied by the chlorine formed in the reactional mixture.

Finally answering your question, according to others halogens reactions of this type, may occur a second substitution on the benzene ring, resulting in a compound with two chloro atoms.

I hope my explanation has helped you ^^'''''

Bye bye