Sciencemadness Discussion Board

Predicting the color of an organic compound

Tsjerk - 12-3-2019 at 08:25

I would like to make some predictions on the color of a compound. I'm at the moment synthesizing para-chlorobenzilidin-bis-acetoacetic ester. I'm pretty sure the reaction is running slowly but steady. I do see a bright yellow color though. I'm wondering whether it is a byproduct or an intermediate. I know the stuctures of the intermediates and if possible I would like to predict the color of these but I don't know where to start. Any ideas?

I'm running 2.05 eq ethyl acetoacetate with p-chlorobenzaldehyde in ethanol with diethylamine as catalyst. It would be nice if the yellow is an intermediate as that would mean the color can be used to follow the reaction. The color appeared within a couple of hours but doesn't seem to become stronger. The reaction has been running for 96 hours now, more and more possible product is precipitating out.

aceto.PNG - 2kB

Doesn't that look bright yellow to you?

[Edited on 12-3-2019 by Tsjerk]

12thealchemist - 12-3-2019 at 09:32

This might be of help:

DraconicAcid - 12-3-2019 at 11:58

According to the link provided above, adding an acyl group won't change the wavelength (and thus won't change the colour). Thus, this would be the same colour as cinnamic acid or (m)ethyl cinnamate (two out of the three I've made as white crystals).

Now, it's possible that there's a very strongly coloured impurity formed along with your product, which is making the solution look yellow. Or that I'm misunderstanding what the link says.

Sigmatropic - 12-3-2019 at 14:27

Based on the color of benzylideneacetone I would be made to believe this compound is yellow. I'll run a search tomorrow and let you know.

I don't think the color of the reaction mixture will fade though but who knows. Usually you don't get visual end points in organic chemistry :(.

Metacelsus - 13-3-2019 at 00:08

I think the p-chloro is missing from your drawn structure.

If you really want to predict color, a TD-DFT calculation will do it, but those are a bit tricky to run.

Keep in mind the color may be slightly different in ethanol solution vs. the solid state.

Tsjerk - 13-3-2019 at 07:28

Quote: Originally posted by Metacelsus  
I think the p-chloro is missing from your drawn structure.

Oops, yes. Forgot that one. I will read about TD-DFT.

walruslover69 - 17-3-2019 at 04:27

you can do a rough back of the envelope particle in a box calculation. Just eye balling it, it looks like the conjugated system might just be large enough to absorb in the violet.

Sigmatropic - 17-3-2019 at 14:00

The 4-fluoro has been reported as a yellow solid. I would be quite confident in predicting this compound and its 4-chloro derivative are also yellow solids.

[Edited on 17-3-2019 by Sigmatropic]