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!!