[viet]

I need a refresher on how to figure out the percentage yield for an experiment.
in lab i use 4g of tryptophan (1equiv), it was added to acetyl chloride (3equiv) and MeOH to make tryptophan methyl ester.
The actual amount was 3.3g.

i converted 4g tryptophan to moles and got 0.01959 mol of tryptophan.
what else do i need?
 

[BetaAmyloid]

You need the balanced chemical equation of the reaction and would need to figure out the limiting reactant.

This being in the lab, I would assume that the acetyl chloride and methanol would be in excess (but you need to calculate this for the moles of each you actually used just to be sure). I'm pretty sure the reaction would yield a 1:1 ratio of tryptophan + acetyl chloride giving the tryptophan methyl ester. You now need to use this information to calculate the theoretical yield of the reaction, so start with 4 grams of tryptophan, go to moles, molar ratio, and back to grams using the molar mass of tryptophan methyl ester.

Then use the given actual yield of 3.3 grams to use (actual yield/theoretical yield) * 100 = percentage yield.

[viet]

i'm having a hard time writing the chemical equation for this reaction.
I think its
C11H12N2O2 + C3H6O2  C12H14N2O2 + OH

would that be right? its not balanced tho

[Dan]

In this reaction, the acetyl chloride first reacts with methanol to produce HCl - this HCl catalyses the condensation of tryptophan with methanol.

See: http://en.wikipedia.org/wiki/Fischer_esterification

1. In your reaction equation, what is C₃H₆O₂ supposed to be? This formula does not correspond to any of the reagents.
2. OH radicals are not produced - something else is.

So, the reaction equation is:

Tryptophan + methanol Tryptophan methyl ester + ?

1. If you have 1 mol of tryptophan, how many moles of tryptophan methyl ester could be produced?
2. If you have X mol of tryptophan, how many moles of tryptophan methyl ester could be produced?
3. How many moles of tryptophan (X) did you start with? So how many moles of the methyl ester could you theoretically produce?
4. What is the mass of that theoretical amount of the methyl ester?
5. How does that compare to the mass you actually got? Express the actual mass as a percentage of the theoretical mass (from 4.).   

[viet]

thanks that made more sense. Tryptophan + methanol  Tryptophan methyl ester + water

i started with 0.0196 mol of tryptophan = 0.0196 mol trp methyl ester.
the theoretical mass is 4.9784 g trp methyl ester.

% yeild = actual / theoretical = 3.3042g / 4.9784 = 69%

[Dan]

i started with 0.0196 mol of tryptophan = 0.0196 mol trp methyl ester.
the theoretical mass is 4.9784 g trp methyl ester.

Your theoretical mass is for tryptophan methyl ester hydrochloride. So, if you isolated the product as the hydrochloride salt, then that's fine (you did not mention this before). If you isolated tryptophan methyl ester, you have used the wrong molecular weight.

Otherwise, method is correct.