Benzene Chromium Tricarbonyl and Challenge

On Tuesday 10-4-11 we checked via IR to see if we had made our desired compound. First, we got an IR of the pure Cr(CO)6 in methylene chloride to compare to our experiment. There was only one carbonyl peak at 2000 1/cm. We pipetted out a small sample of our solution from the flask. There was both a solid yellow-green precipitate and a similarly colored liquid in the reaction flask. We used the liquid for the IR and got a spectrum that had two carbonyl peaks that were slightly lower than the peak on chromium hexacarbonyl. The values were very close to tabulated values for the complex, so we are confident that we got the desired complex. We then suction-filtered the contents of the flask to get a light green precipitate but there were also some black specks in the solution. We recrystallized our product by putting the precipitate in a mininum of methylene chloride and letting it sit overnight to evaporate the methylene chloride. The next day when we went to go check the product, we had green crystals in the beaker:



We also wanted to take a C-13 NMR and an H-NMR of our product. We did this by scraping out some of the crystals and placing them in an NMR tube with some deuterated DMSO. Our product dissolved in this and we got both NMR spectra. They both correspond very well to expected values for our complex.

We had to come up with a demonstration for National Chemistry Week that had to do with health, hygiene, or medicine. Purple Chrome decided to do a demonstration about acid erosion of teeth. Pepsi and other soft drinks contain phosphoric acid, so we wanted to test and see what prolonged exposure to your teeth actually does. We made a 2 M and a 7 M solution of phosphoric acid, a 2 M solution of NaF (which should theoretically make the tooth stronger, not weaken it), and we also had some Pepsi. We massed 4 teeth and placed one in each of the solutions. Immediately, the teeth in the acid and in the Pepsi started fizzing:

The tooth in the NaF didn't really do anything once it was put in the solution. We left the teeth sit in the solution for 4 and a half days, checking in on them periodically. We could see the erosion of the teeth in the acid happening very quickly. Even the tooth in the Pepsi wasn't safe from some acid erosion! We took them out on Tuesday 10-11-11 to dry, and then we massed them. The teeth that had been in the acid were very soft and almost disintegrated when we tried to get them out onto the watchglasses to mass them. Our hypothesis was correct that the teeth in the acid would lose mass due to erosion ("demineralization") and the tooth in NaF would gain some mass due to remineralization. Here is what each of them looked like after we took them out of the solutions:

Look at how the coloring in the Pepsi stained the tooth! It was just white before it sat in solution.

The tooth in NaF wasn't eroded like the teeth that were in the acid.

This tooth, before it sat in the acid, actually looked a lot like the tooth that was in NaF. See how the top of the tooth (facing right) is almost completely eaten away, and there are just deep craters remaining!

This one USED to be a whole tooth, but it was so soft and degraded that it fell apart when we tried to get it out of the vial of acid. In places, it was so thin it was translucent!

This experiment definitely made us think twice about drinking Pepsi!!



Not Quite Perfect

The next day, we went up to the lab after the solution had 20 hours to reflux to find that during the night, we had an air leak and all of our solvent had boiled away. Nothing was left except a black residue on the inside of the flask and all over the stir bar. We got this cleaned up and decided to start the reaction again on Thursday.

On Thursday 9-29-11 we set the reaction up in the same way except for two differences: we used a smaller flask so that the contents would have more volume in the bottom of the flask (hopefully this would prevent the solvent from boiling away too fast) and we used a two neck flask instead of a three neck flask.

This would have one less way for the solvent to escape, so the reaction has one less way to fail. We didn't want to leave the reaction to run overnight again in case it would fail, so we set it up at the end of lab, and one of us could come in on Friday morning to turn the heat on.

On Friday 9-30-11 we turned the heat on at about 10:30 AM and let it go all day until 4 PM. The heating mantle was turned to 3 this time instead of 4. We still got the black residue on the inside of the flask, but we could tell that we still had solvent in the flask because nothing had bubbled up through the joints and there was still liquid dripping from the condenser back into the flask.

On Monday 10-3-11 we started a second round of heating at 11 AM and didn't turn it off again until 5. The total cook time at this point was 11.5 hours, which is less than the 20 hours we originally decided to do but we decided to do an IR spectrum of the solution the next day during lab to see if the reaction had proceeded at all, or whether it had failed again.

Finishing Ferrocene and Beginning Benzene

FINALLY...we got our acetylation of ferrocene to work. We ran the GC on Tuesday 9-27-11 and the highest peak on the spectrum was monoacetylated ferrocene. That is the most compelling evidence we have, and the only evidence we have that still contradicts the presence of monoacetylated ferrocene is the low melting point we got. However, 3 out of 4 isn't bad! That successfully finishes the ferrocene lab, which is good because we have gotten tired of writing about it!

Along with finishing ferrocene, we finished our purple crystals lab on Tuesday. Allen was responsible for this while I (Emily!) was working on other things. He did the isolation like with the benzene inclusion compound, but this time it was just the pure [Ni(en)2(NCS)2] crystals without any benzene added to it. These crystals took so long to grow, and even after five weeks, there were still only a couple of crystals in the vial (nothing like the vial full of benzene crystals we got). We finally decided to purify them and take the melting point since they didn't seem to be growing any more crystals. After the isolation, we got some very pretty dark purple crystals:

These actually look nothing like the benzene inclusion crystals, which were not shiny and dark purple but more dull and light purple. They were also much smaller crystals than these. Here's a picture of the benzene crystals to jog your memory:

We massed the new crystals so we could find percent yield for the lab report, and then we took the melting point, which ended up being above 220 degrees Celsius (that is the limit on our thermometer, so we couldn't get more precise than that!).

Since those two things didn't take much time, we were able to set up our first reaction for our final project. We are doing our project on Cr-Arenes, in which Cr(CO)6 is reacted with an arene in a solvent mixture of dibutyl ether and THF. There are so many arenes to choose from, but we just picked 6 of them to try and synthesize, purify, and characterize. The first arene we wanted to try was benzene.

We set up a three neck flask exactly the same as in the synthesis of ferrocene except with a condenser on the middle neck of the flask:

The condenser is needed to air-cool the reaction so the solvent doesn't boil away. We vacuumed and argon-filled the flask three times and then left the argon running throughout the experiment. We then added in the two solvents via syringe. We added 8 ml of dibutyl ether and 1 ml of THF.

 Now we needed to bubble argon through the solvents to get rid of any oxygen that might ruin the reaction. We "adapted" a disposable syringe to fit into a hose from the rack and stuck the needle through the septum so that the argon bubbled through the solvents. It actually looked like the solvents were boiling because of the gas bubbles:

While this was bubbling, we got 1 g of Cr(CO)6 (which is a white solid) and measured out 1 ml of benzene in a syringe. We had to take the septum out before we could add these two reactants plus a stir bar, so we just replaced the septum with a greased glass stopper. We had to wire down all the attachments and the hoses to prevent any accidents. After that was done, we turned on the stirring mechanism on the hot plate, and turned the heating mantle on to 4. Once it started refluxing, the color of the solution changed quickly from clear with the white Cr(CO)6 floating around undissolved to a light yellow-green color:

Since we have been reading about these complexes, we have seen that they usually have a yellow or orangish color so this color change was a good sign! We left the lab on Tuesday night feeling pretty confident that we would just let it stir for 20 hours, and we would come back on Wednesday to a perfect product that we could purify and characterize.