Been working on the chemistry science fair project the past few weeks with
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bensing,
yongzi, choon boon and liang en. Basically our project involves playing around with chitosan (a polymer of glucosamine - the thing that GNC seems to sell for dunno what health reasons) and modifying it to adsorb metal ions.
This is chitosan:
We're trying to cross-link the chitosan monomers / molecules with all sorts of other chemicals, such as esters. Cross-linking is to join two chitosan molecules to each other with another molecule as a bridge. It's somewhat like how cellulose molecules are cross-linked with H-bonds. In the process, we introduce new functional groups into the compound (e.g. amides, carbonyl), which supposedly help to adsorb the metal ions. So we'll experiment with different methods of modifying chitosan, and see which creates the best product for removing the metals.
What we've been working on for now has been synthesising a diester (diethyl adipate), to cross-link chitosan. On diethyl adipate, the two ester groups react with the amine groups on 2 separate chitosan molecules to give 2 amide groups. This joins the 2 chitosan molecules together.
Diethyl adipate:
But first we need diethyl adipate, which we can synthesise from ethanol and adipic acid (hexanedioic acid). It's a textbook example of ester synthesis, but us being noobs at practical organic chem, we've taken 3 lab sessions so far to actually make it.
Stripped of detail, the protocol looks like this:
1) Measure out and dump ethanol, adipic acid, toluene (solvent) and conc sulphuric acid (catalyst) in a flask.
2) Heat.
3) Distill out the water and some of the excess alcohol and toluene.
4) Remove the water from the distillate and dump it back in the flask.
5) Heat.
6) Distill off the excess alcohol and solvent and throw it away.
7) Distill off the ester (our product!) at low pressure.
Round 1Theory is always much harder in practice. The first session had us figuring out what the equipment was supposed to be used for, and how to connect it all together. The lame thing was that we had to walk all the way from the college lab to the high school's new research centre just for a stupid weighing balance because the labs were moving over there. =.= We weren't exactly 100% sure of our setup, and when all the reagents are just colourless liquids it's quite difficult to say whether everything's working or not :p We didn't have any funnels (!) to pour our chemicals, so it got spilt outside the flask... and at the end I was quite worried about contamination. The distillation didn't really work out, because all the liquid was just condensing in the flask and flowing back in, instead of condensing in the condenser (lol) and flowing to the collecting flask. So we stoned for over 2 hours waiting for nothing to distill, playing Risk on my laptop in the meantime. And at the end at 7pm when we were packing up we realised the stuff in the reaction flask had turned green o_O wth?
Lessons for the next time:
1) Use a funnel (=p)
2) Use an insulating cloth so that the vapour doesn't just condense in the reaction flask.
3) Use a better holder for the thermometer since the one we used didn't seem to be sealed.
4) Use a water pump in a basin with water instead of running tap water (waste water!)
5) Cool the running water with ice so that the condenser works better.
Round 2Here's the distillation setup on our 2nd try. Paiseh for the poor photo quality because I was just using my handphone camera.
There's an oil bath on the hot plate on the right. We need an oil bath because the reaction needs lots of heat and the ester only distills above 100°C (i.e. cannot use water bath). There's a round-bottomed flask with 2 necks in the oil bath. This is connected on its left to a water-cooled condenser, which leads to a collecting flask filled with potassium carbonate.
The thermometer on the extreme right is measuring the temperature of the solution in the flask, while the thermometer on the extreme left is measuring the temperature of the vapour that's about to into the condenser. The thermometer in the middle is supposed to measure the oil bath temperaure but since it's so pok and can measure up to 110°C we couldn't really use it.
The cloth tied around the neck of the flask is supposed to insulate it, so that any liquid that's boiled won't condense so easily. The vapour is then supposed to go to the left, into the water-cooled condenser, where it contacts the cool surface and condenses and flows further to the extreme left into our collecting flask. The black tubes are connected to a water pump in the green basin on the left. The green basin contains water with ice.
This shows the part of the condenser nearest the reaction flask. It's blurry but you can see liquid condensing on the sides of the inner tube. Seeing the first drops of condensate appearing and flowing down was very relieving and almost exhilarating. The last time we spent 2 hours waiting for nothing to come out, when the textbook says you should expect 2-3 drops of condensate per *second*.
Finally, here's the collection flask. It's filled with potassium carbonate, which is supposed to dry the distillate by removing any water. This is important because the reaction is driven to completion by removing water. After filtering away the carbonate, we dump the filtrate (which contains ethanol and solvent, but without water) back in to continue the reaction.
Then we heat the reaction mixture again, before doing the final distillation. We do this at reduced pressure (20 mmHg, which is really low) by connecting a vacuum pump to our setup to lower the pressure, before heating the mixture to distill it. Reducing the pressure lowers the boiling point, which is important since very high temperatures will supposedly either degrade the product or make the experiment just plain inconvenient. The excess ethanol and solvent came out pretty quickly, so all that was left was our ester and excess adipic acid. At first it was unnerving because the stuff inside was starting to turn brown again :s But then we raised the temperature some more and the vapour temperature quickly started rising to the expected 134°C, and our ester product finally came out :D Plus, it was sweet-smelling, as esters are supposed to be. We got 12.1g of the stuff, which is about 60% yield. Poor compared to the textbook's 96% expected yield, but we consider that good for the first time :)
amused
