A problem relating to density and layers

[Skyblitz]

Hi, I recently had a lab where I had to separate a mixture of benzoic acid and benzil. So, I dissolved the mixture in ch2cl2 and then added NaOH in order to convert the benzoic acid into its sodium salt. At anyrate, this then produced the aqueous and organic layers.

However, I noticed that the organic layer (the yellow one) was on the bottom and the aqueous was on the top which makes sense since ch2cl2 is more dense than water, but there was a thin yellow layer on top of the aqueous layer. How did that yellow layer get up there if it is indeed more dense than water?

My yield of benzil was a little low so I'm assuming that part of it was included in my benzoic acid.

 

[chem_tr]

Hello, I remember that benzil converts into an acid derivative via the mechanism "http://themerckindex.cambridgesoft.com/TheMerckIndex/NameReactions/ONR32.htm" in alkali media, so you'll need to find another way to obtain benzil; have you tried Schiff condensation with small amines like pentylamine? Then the Schiff product precipitates, and after you filtrate, benzoic acid may be available in the filtrate, easily crystallized from water.

When we come to the yellow line on top of aqueous layer, I think that it is an emulsion; you may try two things to avoid it:

a) Hold the separatory funnel from the bottom, i.e., from the stopcock, and stir the funnel in round motions. This will help the "confused" upper layer to be in its right place.

b) If it does not help, prepare a saturated NaCl solution and add to the medium until the layer disappears; after you add some NaCl, don't forget to shake the funnel. It is due to distribution; the compound easily soluble in organic layer has some solubility in aqueous layer. If you add a compound very soluble in water, then the organic compound cannot be dissolved in water anymore.
Regards, chem_tr

 

[ravenprp]

One possible explanation for this phenomenon is that there may have been some emulsification occurring during the separation process. Emulsification is the mixing of two immiscible liquids, such as water and ch2cl2, resulting in the formation of a stable suspension. This could have happened if the mixing was too vigorous or if there were impurities present in the mixture.

Another factor to consider is the solubility of benzoic acid in the organic layer. While ch2cl2 is more dense than water, it is also a nonpolar solvent, meaning it may have a higher affinity for the nonpolar benzoic acid compared to the polar water. This could result in some of the benzoic acid being carried up into the organic layer, creating the thin yellow layer on top of the aqueous layer.

To improve the yield of benzil in future experiments, it may be helpful to ensure a more thorough mixing of the organic and aqueous layers to prevent emulsification. Additionally, using a more polar solvent for the extraction, such as ethyl acetate, may result in a better separation of the two compounds.