[Puresimple]

HI,

I was wondering if anyone has ever done synthesis of diphenylacetylene by eliminating  stilbene dibromo with KOH, and if so what was your yield?...

we started with 1.5 g of stilbene and retrieved and actual of 0.32g of the product (21%). I was wondering if this is acceptable and wondering why there is such a low yield. The only thing that i can think of is possibly side reactions contributing to the low yield?

[OrgXemProf]

1. A procedure for preparing diphenylaceylene from stilbene dibromide appears online c/o the following URL: http://www.slideshare.net/chem3221/diphenylacetylene-from-stilbene.

1. KOH-promoted E2 elimination of the first equivalent of HBr from stilbene dibromide should proceed easily. However, the corresponding elimination of the second equivalent of HBr from the resulting monobromostilbene requires more stringent environmental conditions; see Part B in the procedure that appears in the slideshare.net reference.

Hope this helps.

[orgopete]

Re: yield

Check your stoichiometry!

It depends. Is this the crude weight or recrystallized? It's a pretty big molecule to just disappear. How pure was the starting material? Etc, etc,.

[Puresimple]

I had taken a look beforehand on the slide share you posted. My procedure was similar with slight differences:

Place about 0.75 g of KOH in a 100ml round bottom flask and add 6ml of diethylene glycol. Swirl the mixture and warm until most of the KOH dissolves. Add 1.5 g of Stilbene dibromide along with two boiling chips. Attach a reflux condenser to the flask. Note: Do not run water through the condenser; diethylene glycol has such a high boiling point, 160-170 oC, that water cooling is unnecessary. Heat the mixture until boiling and maintain a gentle reflux for 20 minutes.

At the end of the reflux period decant the hot solution into a 150ml Erlenmeyer flask and allow it to cool to room temperature. Slowly add 36ml of water while swirling the flask. The product should separate as a yellow, semi-crystalline mass. Allow the mixture to stand for 15 minutes, cool it in an ice bath and wait until most of the precipitate has settled towards the bottom. Carefully decant the solution leaving the precipitate in the flask. Suspend the solid in cold distilled water and then filter it by suction using a Buchner funnel. Rinse the flask in cold distilled water and pour it over the solid in the funnel. Note: the aqueous filtrate must be discarded in organic waste.

Weigh the solid and calculate the overall yield of diphenylacetylene from stilbene. Determine the melting point.

The stilbene dibromo was given to us ready so I would think that this was pretty pure. I followed all these instructions exactly as it is written, but still retrieved a low yield, the purity was ok though. What would be a possible expected yield for such an experiment? I'm pretty sure 21% is low. still not sure where i went wrong...

[orgopete]

What is the MW of the dibromide and the diphenylacetylene?

[Puresimple]

340.07 and 178.22 respectively.

[orgopete]

Okay, since the HBr is nearly half the weight, what was the theoretical yield?

[Puresimple]

I did the calculations and it came out to 0.786g...looks like the outcome (theoretically) should be about 52%?

[Babcock_Hall]

Your calculation of theoretical yield is questionable.  Might be best to lay out the calculations, so that we can see and discuss them.

[Puresimple]

ok...Here goes nothing....

since the equation is: stilbene dibromo(SD) + 2KOH :rarrow:diphenylacetylene (DPA) + 2H₂O + 2KBr

The limiting reagent is stilbene dibromo since it gives the least amount of moles of product.

The grams of limiting reactants is 1.500g

moles of limiting reactants is 1.500g x (1 mol/340.07g)= 4.411x10^-3moles

Theoretical yield in grams calculation: 1.500g stilbene dibromo (SD) x (1 mole/ 340.07g) x (1 mole DPA/ 1 mole SD) x (178.22g/ 1 mole DPA) = 0.786 g as the theoretical yield.

Actual yield: 0.32g

% yield: 0.32g /0.786g = 40%

Not too sure why i put 52% on the last post. Probably because I was doing it in my head. is this correct? it's still a pretty low yield...

[Babcock_Hall]

It is a somewhat lower yield than one would like to see.  I have not yet looked at the slides, but it could be just a matter of letting the reaction proceed a little bit longer, within reason.  Generally one wants to be able to monitor a reaction to completion, and often TLC is used to do this.  However, there are two particulars in your system that work against one's ability to do so.  First is the fact that you are actually doing two consecutive reactions, which will complicate the analysis.  Second is the presence of a high boiling solvent that would be difficult to remove.  Its presence in a TLC experiment might make all of the R_f values high.
EDT
Your temperature was a bit lower than what was used in the procedure linked above, and your time of reaction was only five minutes longer.  The solvent was also slightly different.

[Puresimple]

Thanks for the response.

I suppose I could list variables that could have caused the low yield. I was actually looking at how to monitor a reaction through TLC (just out of curiosity), but not too sure of the mechanics behind it. Do you have a link that maybe explains this better?

[Babcock_Hall]

http://www.chemicalforums.com/index.php?topic=28041.0

This old thread may help.