[greenmoon]

Hi!
I can't make ends meet with this problem.
What quantity of tribromophenol can be obtained from 1000 g Br,the yield of the reaction is 80%?

And I wrote the equation:

C6H6O+ 3 Br2->C6H3Br3O + 3 HBr

Then I calculated the number of moles of Br which is 1000/80=12.5 moles.
Then I calculated the number of moles of C6H3Br3O from the equation like this:12.5/3=4.16.
After that I calculated how many grams of C6H3Br3O there are:4.16*331=1376,96 grams.
Considering the yield I have:(80*1376.96)/100=1101.568 grams.
Though I'm supposed to obtain 551.66.What did I do wrong?

[fledarmus]

What is the mechanism of your reaction?

Where did you get the 3 Br^2-?

[sjb]

Br atoms or Br₂ molecules?

It's unclear whether your 80% yield is taking into account the excess phenol, so let's assume for a minute that it doesn't. Effectively, you have 1000g x 80% Br₂ molecules reacting. What is the mass, and hence the number of moles of bromine here? From your balanced equation, how many moles of TBP can you make from this. What is the mass of that amount of TBP?

[greenmoon]

Where did you get the 3 Br^2-?

Well,don't you have to add Br₂ to phenol for the bromination? and the 3 in front is from balancing the equation.

Br atoms or Br₂ molecules?

Effectively, you have 1000g x 80% Br₂ molecules reacting.

It's Br₂ molecules.
But aren't I supposed to apply the yield to the TPB not to the Br₂?

[fledarmus]

I'm sorry, I misread your equation - without the subscripts and superscripts, I thought you were proposing a Br atom dianion.

Yes, you use molecular bromine added to phenol for the bromination, but what is the mechanism? How does the bromine molecule react with the phenol? What is added to the phenol and what is left behind?

[sjb]

It's Br₂ molecules.
But aren't I supposed to apply the yield to the TPB not to the Br₂?

OK, let's try it this way. How many moles of Br₂ are there then in 1000g? What number of moles of TBP would you get from this amount of bromine? If the yield is only 80% how many moles are there, which has what mass?

[greenmoon]

There are 12.5 moles of bromine in 1000g.Cause 1000/80=12.5,and after applying the 80 % yield there are 10 moles of bromine left.So you get 10/3=3.33 moles of TBP.Then the molecular mass of TBT=331.So that would be 3.33*331=1102 grams of TBP which is exactly double compared to what I'm supposed to get.

Yes, you use molecular bromine added to phenol for the bromination, but what is the mechanism? How does the bromine molecule react with the phenol? What is added to the phenol and what is left behind?

Sorry I made a pig's ear of the equation by not using subscripts.
Well 3 bromines substitute 3 H from the aromatic nucleus of the phenol and a H and a Br gets left behind.They form the HBr and after you balance the equation there is 1 mole of TBP and 3 of HBr.

[sjb]

12.5 mol of bromine atoms, yes, but not of bromine molecules, which is how your equation is written...

[fledarmus]

12.5 mol of bromine atoms, yes, but not of bromine molecules, which is how your equation is written...

Exactly. The bromine molecules separate heterolytically, leaving a Br⁺ to react with the phenol, and a Br^- that is left behind as HBr. Each molecule of bromine can only contribute 1 atom to bromination, not 2.

[greenmoon]

Sorry for not replying.I've found the problem the moles of bromine had to be 1000/160=6.25.
Thanks for help.