[lividtea]

We reacted biphenyl and t-butyl chloride (used in excess) with FeCl2 and dichloromethane. After the reaction, we used HCl to wash the organic layer. Why do we use HCl?

I'm having a hard time understanding washing step. I know it's used to remove unwanted impurities. Since we're using HCl, I'm assuming we're removing a basic impurity. I know we're trying to remove tertbutyl chloride and FeCl2 from the organic layer.... But how does HCl achieve that?

[critzz]

Personally, when doing an extraction I like to analyze where all the reactants/product like to be, in de organic layer or in the (acidic) water layer. HCl doesnt just magically destroy or remove an impurity.

So in your reaction mixture you have: your biphenyl product, excess t-butylchloride, formed HCl from the reaction, and FeCl₂ (are you sure you didn't use FeCl₃?).
Definately, the product and the excess tbutylchloride stay in the organic layer. HCl and the Iron salt will go to the water layer. (Anorganic salt generally go to the water layer.

So you are left with your product and excess tbutylchloride in the dichloromethane layer after washing. How do you seperate these 2 from eachother? What physical property do they really differ in when you look at them?

[lividtea]

Personally, when doing an extraction I like to analyze where all the reactants/product like to be, in de organic layer or in the (acidic) water layer. HCl doesnt just magically destroy or remove an impurity.

So in your reaction mixture you have: your biphenyl product, excess t-butylchloride, formed HCl from the reaction, and FeCl₂ (are you sure you didn't use FeCl₃?).
Definately, the product and the excess tbutylchloride stay in the organic layer. HCl and the Iron salt will go to the water layer. (Anorganic salt generally go to the water layer.

So you are left with your product and excess tbutylchloride in the dichloromethane layer after washing. How do you seperate these 2 from eachother? What physical property do they really differ in when you look at them?

Thank you so much for your response critzz!!! I really need to understand how workups work before my lab exam next week and unfortunately, my lab coordinator is leaving me in the dust.

Yes, I meant to say FeCl₃.
Okay so the difference between the product and excess t-butyl chloride is that they contain different halides and the product contains two aromatic ring structure. (Not too sure where this is going xD)

[critzz]

Okay so the difference between the product and excess t-butyl chloride is that they contain different halides and the product contains two aromatic ring structure. (Not too sure where this is going xD)

Yes, and the biphenyl product is much larger than the t-butyl chloride, and what do you know about boiling points of molecules?

[lividtea]

OH YEAH!!!! Tertbutyl chloride has a low bp (around 50°C)!!! We did a simple distillation but I thought it was to remove DCM only. Thank you that makes so much sense!

But what was the point of washing with HCl? There was a dark colour change when HCl was added.

[kriggy]

YOu neeed acid/water to decompose the intermediate that leads to form the ketone
http://www.organic-chemistry.org/namedreactions/friedel-crafts-acylation.shtm

[critzz]

YOu neeed acid/water to decompose the intermediate that leads to form the ketone
http://www.organic-chemistry.org/namedreactions/friedel-crafts-acylation.shtm

He did a friedelcrafts alkylation though. But I guess you use HCl to quench the reaction and keep all salts desolved? If you'd use NaOH solution for example you would get nasty insoluble Fe(OH)₃ salts. I'm not sure though in this case.

The dark (brown?) colour probably comes from the hydrolized FeCl₃ salts (Fe(H₂O)₆).

[kriggy]

Right, My mistake :-)