[Violagirl]

About a week ago, we had to prepare semicarbazone derivatives of aldehydes and ketones with semicarbazide hydrochloride in the presense of a buffer. When determining the limiting reactant, it looked like the semicarbazide was the limiting reactant but wanted to verify if this was correct or not. Can the semicarbazide be a limiting reactant since it had been mixed with a buffer? We had to mix it with potassium hydgrogen phosphate (the buffer) before adding it with the cyclohexanone and 2-furaldehyde. Or can it not be the limiting reactant since it was the mixed with the buffer? (I know that catalysts are not considered to be limiting reactants and wondered if it would apply in this case since it was mixed with the buffer)

[Violagirl]

Also had one more question, when a test was performed at room temperature, the product was found to have had a melting range of 181.8-187.5 degrees C. The literature melting point of cyclohexanone semicarbazone is 166 degrees C and for 2-furaldehyde semicarbazone is 202 degrees C. It seems like the melting range is closer to the 2-furaldehyde product but impurities also seemed to be the present. I know that from other tests performed, that 2-furaldehyde was formed under high temperatures and is the thermodynamic product as cyclohexanone semicarbazone formed under low temperatures and is the kinetic product. Would my thinking be correct in that 2-furaldehyde was the product formed at room temperature since thermodynamic products are more stable?

[Violagirl]

Nevermind, looking at the physical descriptions of both products, the room temperature reaction product matched the physical description to that of cyclonhexanone semicarbazone so it would be a kinetic reaction. Still confused on which would be the limiting reactant from my first post, if anyone could clarify that for me, I'd appreciate it. 

[Doc Oc]

You need to calculate the moles of each reagent.  The one in lowest quantity is your limiting reagent.  It's not going to be the buffer or anything you use as a solvent, those are in huge excess.

[Violagirl]

Ok thanks a lot! So it sounds like the semicarbazide would be factored in as a potential limiting reagent then and the potassium hydrogen phosphate would be the buffer then. Thanks a lot!