quenching? organic lab

hi, wondering if anyone could give me a clue to my organic lab write up;
in the bromination 3-nitroaniline using glacial acetic acid and 25% bromine soln, a sodium sulfite/water solution is added to ''quench'' the reaction.
the question asks 'why is the reaction quenched with sodium sulfite'
I'm kinda stuck because i dont' really understand what 'quench' means, i'm thinking it means maybe to stop the reaction but why would i do that so i'm guessing that's wrong.
Any help would be much appreciated.
also what does 'the dominant directing effect' mean, does it just mean why that product?
thanks

Reply 1

charco

Study Forum Helper

quench does indeed mean to stop the reaction by destroying one of the reagents.

In this case it's my guess that you are quenching a possible further reaction...

Sodium sulfite is a reducing agent so perhaps it mops up excess bromine and stops the ring disubstituting bromine.

In the reaction of bromine with aniline the product is tribromoaniline as the bromine activates the ring causing further substitution.

I imagine that the sodium sulfite allows only monosubstitution by preventing excess bromine reacting.

Reply 2

charco

Study Forum Helper

ah yes the dominant directing product means that the substitution occurs at a specific carbon as directed by the substtuents already on the ring.
For example the NH2 group is 2,4,6 directing.

Basically, the reason for this is down to the resonances that can occur on th ering leading to regions of greater electron density ot a specific carbon or extra stability of an intermediate.
It's rather complex to explain here but suffice to say that some substituents activate the ring by providing extra electrons conjugating with the ring electrons (the case of NH2 which has a lone pair) These activate the ring and direct substitution to the 2,4,and 6 positions (it's called a positive mesomeric effect or +M)
There are also electron withdrawing groups such as the nitro group NO2 (-M) and then there are groups that just withdraw electrons by pulling due to electronegativity (-I, or negative inductive) eg COOH, and others that seem to push electrons into the ring (+I, or positive inductive effect) eg alkyl chains.

Each of these has a different substitution directing effect.
Roughly speaking
-M and -I cause 3,5, substitution
+M and +I cause 2,4,6 substitution

the halogens are a little peculiar as they activate the ring but cause 3,5 substitution - it seems that both +M and -I effects occur together.

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