Hello,
I am synthesizing a material based on a cyclosiloxane compound, where I am attaching functional groups onto the silicon atoms. My worry is that the siloxane ring could open during the reaction, as they are notoriously weak rings, which is something I do not want to happen. The silicon atoms have 5 carbon arms, and I am attaching a compound at the end of each arm. Therefore, it is impossible to tell whether or not the ring has opened by conventional methods (H & C NMR, FTIR, MS...etc.). The only method I can think of is using Si NMR, would that prove whether or not the siloxane ring has opened?
What about proton NMR, if the ring opens it should produce a silanol (hydrogen attached to oxygen)
I think there are a few options for you to try. I'm assuming your compound is symmetric, but if it wasn't and depending on the size, you could use HMBC to confirm covalent bonds (or lack thereof). I'm guessing this isn't an option though.
1H NMR you might be able to see a hydroxyl peak and you might even see new peaks appear (very close to 0) instead of a single peak, as the electronic environment where the ring opened will be slightly different. These shifts are very small so I can't guarantee you'd see clearly resolved peaks. Also note that these shifts may appear below 0 ppm.
IR would also show a broad and diagnostic OH peak if the ring opened, which would be very quick and easy to assess. This would be the first thing I would try.
In theory, you would see a different mass on MS but I don't think these would fly well (likely be destroyed by ionization).
Finally, I would say TLC could potentially be used as you'll see a new spot with a much lower Rf. Ceric Ammonium Molybdate (CAM) should stain very strongly for hydroxyl groups; you could also try stains that give different colors (e.g., vanillin and p-anisaldehyde), as I expect the silanol group would give rise to a very different color. This wouldn't give you a conclusive answer, but it might give you enough of a clue to feel confident.
If exhausting this arsenal of characterization techniques gives you only equivocal results of whether or not your ring opened, then I would suggest going to 29Si NMR. If your institute has easy access to these more exotic NMR nuclei then this should be a good way to get a definitive answer.
- Badges
- Science topic
- Similar topics
- Biological Science
- Bioecology
- Systems Ecology
- Landscape Ecology
- Connectivity