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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: chrisso80b on January 13, 2012, 06:57:18 AM

Title: Balancing equation to find theoretical yield!
Post by: chrisso80b on January 13, 2012, 06:57:18 AM

I need to calculate the theoretical yield of a reaction, am I right in thinking I need to find the balanced chemical equation?
The reaction is reacting 2.0g Glucose with 20ml Acetic anhydride (with NaOAc catalyst). I have calculated my actual yield using the mass of product produced (penta-acetate glucose) which was 7.334g, and dividing this by the Mr of the product.
However this is where I get stuck, once I have found the actual yield, how do I find the theoretical yield???
Thanks!!

Title: Re: Balancing equation to find theoretical yield!
Post by: fledarmus on January 13, 2012, 07:28:37 AM

Yes, you need to start with a balanced equation.

Title: Re: Balancing equation to find theoretical yield!
Post by: Honclbrif on January 13, 2012, 07:35:03 AM

The theoretical yield is the maximum yield you could get if all of your starting material was converted to product with no loss and can be calculated before you even start the reaction. You can calculate it in either moles or mass, and as fledarmus says, the first step is starting with a balanced equation.

A simplified method of determining theoretical yield would be: What's your limiting reagent? How many moles of limiting reagent do you have? How many moles of product would that produce? What would be the mass of those moles of product?

Title: Re: Balancing equation to find theoretical yield!
Post by: chrisso80b on January 13, 2012, 07:40:53 AM

I thought it was the balanced equation I needed, but Im having serious problems balancing it.
I have this so far:
C₆H₁₂O₆ + C₄H₆O₃  :rarrow: C₁₆H₂₂O₁₁ + C₂H₄O₂
For glucose + acetic anhydride  :rarrow: penta-acetate glucose + acetic acid
And I cant seems to balance it, unless I have the equation wrong? The catalyst used was NaOAc.
How would I go about finding the limiting reagent?
Thanks!

Title: Re: Balancing equation to find theoretical yield!
Post by: sjb on January 13, 2012, 07:45:55 AM

Is it worth creating a pseudoelement of Ac (not actinium) for this, replacing the acetyl group.

So for e.g. the acylation of ethanol with acetic anhydride you get

C₂H₅OH + Ac₂O  :rarrow: C₂H₅OAc + AcOH; then replace each Ac with C₂H₃O once you're finished balancing?

Title: Re: Balancing equation to find theoretical yield!
Post by: fledarmus on January 13, 2012, 07:56:28 AM

As sjb indicated, this is one case where knowing the mechanism of the reaction and the structure of the reactants will make it much easier to balance! Look at how acetic anhydride reacts with one alcohol group, then apply that to all of the hydroxy groups in your glucose.