Measuring the solubilities of benzotriazole-5-carboxylic acid; 8-bromo-octanoic acid; 4-dimethylaminocinnamaldehyde and 2-(4-hydrozyphenyl)-azobenzoic acid in ethanol, methanol and THF.

Method

24 eppendorfs were taken and split into two groups of 12. The first set were labelled with the planned contents and the second set labelled and weighed.
Eppendorfs from set one had 0.75 mL of either ethanol, methanol or THF added accordingly. Benzotriazole-5-carboxylic acid was added to eppendorfs to give saturated solutions of benzotriazole-5-carboxylic acid in ethanol, methanol and THF. The solutions were a browny-yellow colour. 8-Bromo-octanoic acid was added to eppendorfs to try and give saturated solutions. The acid was so soluble in all three solvents that the volume increased far enough to require transferring to larger glass vials. Colourless saturated solutions of 8-Bromo-octanoic acid in ethanol, methanol and THF were prepared.
4-Dimethylaminocinnamaldehyde was added to eppendorfs to give three bright yellow saturated solutions of 4-dimethylaminocinnamaldehyde in ethanol, methanol and THF. 2-(4-hydroxyphenyl-azo) benzoic acid was added to eppendorfs to give three orange saturated solutions of 2-(4-hydroxyphenyl-azo) benzoic acid in ethanol, methanol and THF. The Benzotriaxole-5-carboxylic acid, 4-dimethylaminocinnamaldehyde and 2-(4-hydroxyphenylazo) benzoic acid was not very soluble and only required addition of a small amount to produce a saturated solution. How are you mixing the solutions and for how long? JCB In this experiment I mixed the solutions by inversion of the eppendorfs and used the vortex on the larger vials. I did not use a standard time as each solution took a different length of time to become saturated.

The eppendorfs were centrifuged at 13 200 rpm for 1 minute. The glass vials could not be as there isn't a suitable centrifuge. 0.5 mL of each solution was added to it's respectively labelled group 2 eppendorf. The eppendorfs were dried under vacuum and heat using the speedevac centrifuge for 2 hours. At this point the eppendorfs were weighed and returned to the speedvac overnight under heat and vacuum Do you have an idea of the temperature and pressure? JCB. I'm afraid not. The heater function is just an on/off switch and the vacuum pump isn't attached to any pressure guages.
The eppendorfs were reweighed this morning and other changes to appearance noted. All the samples were dry except for the 8-bromo-octanoic acid, which had turned from a gel to a liquid overnight. These were returned to the speedvac for further drying, but not under heat (see discussion). After a further 2 hours on the speedvac, the remaining samples were gels once again. They were weighed and returned to the speedvac for further drying. The samples were dried on the speedvac for a further three hours but very little change in mass has occurred and the samples were still gels. it was noted that despite the heater not being on, the inside of the speedvac was hot. The remaining samples have been left open to the air for the weekend. I have now updated the spreadsheet with the results of leaving open to air over the weekend. The samples have lost no further mass, but are now yellow crystalline solids. The numbers reflect the solubility assuming the product is the acid. It seems likely that the product is not the acid so the numbers are probably wrong.

The experimental spreadsheet for this experiment can be found at http://spreadsheets.google.com/ccc?key=pUtF1N4vVD6dQBZs3znFBVQ&hl=en

Discussion

Benzotriaxole-5-carboxylic acid [you are reporting that this is almost insoluble in methanol, which is not expected from similar compounds - are you sure that you used the acid and not a salt? JCB], 4-dimethylaminocinnamaldehyde and 2-(4-hydroxyphenylazo) benzoic acid were all originally orange powders when added to the solvents to make the saturated solutions. From this, 4-dimethylaminocinnamaldehyde has returned a crystalline solid. This technique has purified this compound during the measurement of the solubilities.
8-Bromo-octanoic acid was very very soluble and the saturated solution returned a gel from ethanol and methanol after initial drying, but remained liquid from THF. (Is there a chemical reaction that could occur with some of these solvents to explain you results? JCB)
I have considered the possibility of chemical reaction. Alcohols and acids can form esters, but usually in the presence of an acid catalyst. I'm not sure if the carboxylic acid group is a strong enough acid to act as the acid catalyst as well to aid the reaction, since the usual acid catalyst for an esterification between the acid and an alcohol is H2SO4.

However, one of the products of esterification is water, and by putting the saturated solution under heat and vacuum, it may have driven off the water and altered the equilibrium of the reaction to favour an ester product. The fact that the gel was not observed with the THF, which is not an alcohol may corroberate this theory. Likewise, the gel was also observed with citric acid in methanol and ethanol but not THF.

Overnight all three samples lost mass, but the gels had dissolved to become viscous liquids again. However, it was noticed that on removing from the speedvac and standing for a few minutes whilst the eppendorfs were weighed, the liquids had started to crystallise. Therefore, the samples were returned to the speedvac to dry further, but not under heat as the crystals must have a low melting temperature. Yes they do look up and report the melting point on ChemSpider JCB. 8-bromo-octanoic acid crystals have a melting temperature of 39-41°C.
The 8-bromo-octanoic acid samples have continued to remain wet despite being on the speedvac for a number of hours. It is possible that the acid is very hygroscopic and is not willing to give up the solvent. It does not take much impurity to make a compound with a m.p. close to room temp liquid at room temp. JCB However, I am trying to find out by leaving them open to the air for the weekend (in the fume cupboard with the rack loosely covered in tissue). It might possibly evaporate naturally over the weekend in the fume cupboard due to the air flow.

This experiment has been more successful than the previous experiment as none of the samples have gained mass overnight. The numbers and solubilities given in the spreadsheet so far tie in well with the observations during the experiment of how much solid went into solution.