Session XIV-1: Synthesize Esters

[Jeff Buterbaugh]

We had a disappointing experience with this lab today and I cannot for the life of me figure out where we went wrong. I'm a Ph.D. chemist by training (but it's been awhile) and I know that sometimes labs just don't "work" but this one has me baffled.

I watched my sons do the lab step by step and I don't think they missed a beat. We used all of the chemicals from the kit and had 95% Ethanol and 99% isopropanol. We have a nice hotplate with temperature control at 65 degrees C.

As far as I can tell, no reaction occurred with the possible exception of test tube E. That was the only one that had a visible separation of layers. Furthermore, the salicylic acid never seemed to go into solution.

If none of them had reacted I could wonder whether there was something wrong with the HCl solution or the hotplate, but since test tube E seemed to be successful that doesn't seem likely.

How critical are the volumes / concentrations of the reagents? I wouldn't expect much dependence.

The only thought I had was that maybe the excess acid should be neutralized with NaOH solution prior to checking for the ester aroma. But I'm not sure this would explain the complete lack of an organic layer?

Interested in any ideas others might have?

Jeff B

[The Home Scientist]

That is odd. As you know, Fischer esterification is generally reliable and has been used on an industrial scale for decades.

Salicylic acid is only very slightly soluble in water, so it's normal for solid salicylic acid to remain after the esterification. The quantities of reactants are very small in this lab and the refluxing method is crude, so it's normal for the yield of the ester to be small. Also, there are evaporative losses of both reactant and product. When I last ran this lab (back in 2007, when I was writing the Illustrated Guide book), I got small amounts of each of the six esters, but sufficient that each was detectable by the sniff test. In each case, the organic layer was a very thin layer on top of the aqueous layer, but enough to draw off a few drops for sniff-testing.

If I had to guess, I think it's likely that you didn't allow refluxing to proceed  long enough to produce enough of the esters in all but tube E. Also, because each of the esters is soluble in water to a greater or lesser extent, it's possible that the esters you did produce were in small enough quantities to be soluble in the aqueous (only) layer. If that's the case, as you say neutralizing the HCl with NaOH should allow the esters in solution to be detected by smell.

[Jeff Buterbaugh]

Thanks Robert. As you read back through the procedure, are there any steps where it would be important to swirl (or not swirl) the test tubes in order to obtain the desired result? For example, is there any time after adding the cotton balls where the student should do anything to agitate the test tube? Or any opinions on whether heating to higher temperature and/or longer reaction time would be more fruitful? I might try to run the lab again (myself) to give the boys a chance to smell the esters but I'd like to get as many thoughts as possible before doing so. Thanks again.

Jeff

[The Home Scientist]

Certainly you should swirl the tube to mix the reactants initially, but additional agitation shouldn't be necessary. Convection within the liquid should mix the reactants thoroughly as you heat them. You don't want to heat them to much or they'll boil vigorously. The ideal temperature is that at which the solutions are near or at a gentle boil. Within reason, longer reaction time should of course result in a higher yield.

[Jeff Buterbaugh]

So now I am wondering if the hot water bath was actually hot enough. I don't recall seeing any boiling at all. I know we were supposed to watch for boiling that was too vigorous, but it never occurred to me that maybe we weren't warm enough? As mentioned previously, our hotplate has a temperature probe and thermostat control and was set to 63 C. I never observed any reflux and the cotton balls were dry.

When I run it myself I will probably give it 20 minutes or so and I will heat until I see some reflux.

Jeff