[plu]

Hello there,

I recently began a course that's based heavily on organic synthesis experiments and I have been finding difficulty with improving my product yields.  Many of the experiments that I have been conducting have involved simple mixing of reactants, refluxing, and filtrating products.  However, for some reason, I always end up with relatively poor products yields, no matter how careful I am with my laboratory technique.  Any suggestions?

[Yggdrasil]

It's hard to suggest methods of improving product yield without more detailed descriptions of the procedures and reactions involved, but here are some general tips.  One common mistake of undergraduates is to add too much drying agent.  Now, drying agent is good at absorbing water which also mean that drying agents will also absorb other polar compounds such as compounds containing OH groups.  Washing your filtrate with too much solvent or solvent which is not ice cold (use ice cold solvent for washing because this lowers the solubility of your product in the solvent) is another commmon mistake.  Finally, you may want to check the yield of your reaction (e.g. using TLC) after reflux since your reaction may have not proceeded to completion.

[plu]

Some specifics of an experiment I recently conducted where my yield was <50% :
- mixing of two reactants (one in slight excess)
- refluxing for 2 hours (as suggested)
- crystallization of product in ice bath
- vacuum filtration

The reaction was quite certainly complete after the 2 hours of refluxing and my filtration technique was nearly spotless (I was very very careful with it!).  What could have gone wrong?

[Yggdrasil]

Do you have a literature value for the yield?  Some reactions just give bad yields.

[plu]

The yield was expected to be around 90%   

[Dan]

- crystallization of product in ice bath

Was the solution hot (boiling) and saturated before you cooled it?

[plu]

Was the solution hot (boiling) and saturated before you cooled it?

No, I first allowed the reaction flask to cool to room temperature after the reflux period before subjecting it to the ice bath.  I do not understand what you mean by "saturated" though 

[Borek]

You don't know what is saturated solution?

What Dan aims at is that your yield could be much higher, you have just not separated all your product by crystallization.

[Dan]

No, I first allowed the reaction flask to cool to room temperature after the reflux period before subjecting it to the ice bath.

In that case, you probably retained a signifiacnt amount of product in solution which was lost. By saturating your solution at higher T, more of it will crystallise at low T (beause anything is more soluble in a hotter solvent - this is why you recrystallise with boiling solvent).

[plu]

Ah, I see.  So are you suggesting that I simply use less solvent?

[Dan]

Well, you need to saturate your solution before crystallisation.
Standard procedure in my undergrad lab was to take off all the solvent on a rotary evaporator, then dissolve the product (if solid) in a minimum amount of boiling solvent - then cool, and your product crystallises - this is recrystallisation.

If you don't have a rotary evaporator at your disposal, you can boil your solution to drive off excess solvent, and achieve a hot saturated solution that way. That's what we did back in my school days.

[Dude]

I noticed a lot of that in organic chemistry.  The comment about making sure you use a minimum of solvent during crystallization is very good.  The question I have would be where are you getting your procedure from?  If you are using something from "Organic Syntheses" or Vogel's, the yields are peer-reviewed and generally represent what you should get with attention to detail based upon a laboratory round robin from several labs.  On the other hand, if you are adapting a journal article to a lab, good luck trying to get that yield.  Most authors do NOT report isolated yields or they report the optimal yield (ie perhaps they ran the reaction 25 times and they reported the highest yield instead of an average).  When I was in school, I remember harassing professors about this and the answer I received was that although there may be guidelines, there was no universal protocol for yield reporting.  I have seen and heard all kinds of marketing including using crude yields (ie reporting a weight before distillation as the yield even though the experimental procedure clearly indicates a distillation process, yield before column chromatography, yield before recrystallization, etc).  Any purification step will cut your yield.  You should only report ISOLATED yields with confirmation of the approximate purity (even if it is a simple statement by TLC - include Rf value, solvent, and stationary phase (ie silica). 

[plu]

If you don't have a rotary evaporator at your disposal, you can boil your solution to drive off excess solvent, and achieve a hot saturated solution that way. That's what we did back in my school days.

Does this mean that I should simply heat the reaction mixture without a condenser and allow the solvent to boil off before cooling and crystallizing?  Also, when recrystallizing and filtrating, is it alright to swirl the solution of crystals constantly in order to mobilize them before pouring them out onto the filter paper or is it better to just use more solvent to flush the crystals out of the reaction flask?

[Borek]

or is it better to just use more solvent to flush the crystals out of the reaction flask?

More solvent means lower yield - as crystals get dissolved.

[technologist]

Just to estimate your conversion, after crystal separation if possible, analyze the product concentration in the solvent.

Leave the separation technique to be followed, it can be designed based on the number u obtain after analysis. It also depends binary system of your product & solvent both.

Many times, its not easier to follow conventional techniques. Once u know the no U can think of different options for separation.

Mind it, it will give u the no for confirming your total conversion, which probably u r assuming to be 100%.

Reducing solvent in refluxing environment sometimes reduces overall conversion if it is already not too much.