Objective

To verify if the concentration of 4-pyrenebutanoic acid obtained using SAMS in Exp061 (1.39M) is justified.

Procedure



Results

HNMR
084-1A
084-1B
Pictures
084-1
Spreadsheet

Discussion

A quick comparison of the solubility results for 4-pyrenebutanoic acid in THF can be found here.

  • Results from Exp061 had indicated that it was possible to make a solution of 4-pyrenebutanoic acid in THF of all concentrations up to 1.39M. However when a one molar strength solution was attempted, most of the acid remained insoluble even after sonicating for 45min. There could be a few reasons for the failure to make a 1M solution of 4-pyrenebutanoic acid in THF.
  1. Level of the water bath in the sonicator was not high enough for the ultrasound waves to penetrate the undissolved solid in the volumetric flask above the water level.
  2. Needed more sonication time.
  3. Measurement obtained in Exp061 was at a higher temperature than 23C, recorded in this experiment.
  4. It is simply not possible to make it, contradicting the result obtained in Exp061 using SAMS...reinforcing the 0.7M value obtained in Exp030 using an internal standard.
  • Apart from the above mentioned issue there was another problem when the solubility measurement was done using an internal standard. Concentration of the saturated solution based on the internal standard (1,2-DCE) used showed a drastic difference. It came out as 0.55M. This result although not close to 0.7M measured in Exp030, still falls in a similar range. The result may indicate that there are some problems with the solubility measurements calculated using SAMS for this particular compound. The problem may be resolved by performing another solubility run for this acid in THF, preferably by another researcher.
  • Yet another point noted while performing calculation for solubility of 4-pyrenebutanoic acid in THF using SAMS was the variability in the concentrations obtained when different solute protons were used, viz both methylene groups- 1.03M, acid proton - 0.86M, all aromatic protons -0.92M. As has been noted in previous experiments solubility measurements obtained using protons attached to sp2 carbons yielded results lower than the ones obtained using protons connected to sp3 carbons. The primary mechanism responsible for the relaxation of a proton in NMR relies on there neighbors as a source of magnetic dipole moment, therefore protons which are devoid of neighbors relax more slowly. As a result there is a decrease in the peak integrals of those protons in a regular HNMR spectrum, due to the partial saturation of slower relaxing spins which are unable to recover sufficiently between each pulse acquire sequence [ Ref 1]. [This is good -- can you go further and suggest ways to choose which proton to use in a new solute? In general, I believe a proton on an sp3 hybrid carbon is supposed to relax much faster than the ones on sp2 carbons. Therefore these type of protons integrate more than sp2.
  • Is there some way empirically to determine whether a particular proton relaxes too slowly and is therefore at risk of partial saturation?
  • Can you simply choose the proton that gives the highest solubility and assume that any lower results are due to the partial saturation problem? --BH]

Conclusion

Solubility of 4-pyrenebutanoic acid in THF has been found to be 0.55M which is much lower than previously determined.

Log


04-30-2009
12:30 Weighed out 288.3mg of 4-pyrenebutanoic acid in a dry and clean 1mL volumetric flask..(to make up a1M solution)...
12:40 Slowly started adding THF (dropwise) to the 1mL volumetric flask containing 1mmol of the acid, until the level of the solution reached 1mL mark on the volumetric flask.
12:50 Vortexed the tightly stoppered solution for 1min...Unable to dissolve the solid in the solvent.
13:00 Placed the solution with mostly undissolved solid in it (volumetric flask) in a sonicator. Temperature of the water in the sonicator was noted to be 20C.
13:10 Recorded the temperature of the water again in the sonicator; now it was 22C
13:30 Temp of the water in the sonicator 23C.
13:45 Even after sonication for more than 30min the solid remained undissolved.
14:00 Left the solution on the bench to settle down.Obtained a picture of the solution in the vol.flask. 084-1
14:30 To a clean and dry NMR tube, transferred 100uL of the supernatant from the volumetric flask, added 700uL CDCl3 and obtained a HNMR on a 500MHz varian - this is 84-1A
19:40 To the same NMR tube 100uL of 1,2dichloroethane (DCE) was added as an internal standard and another HNMR run was performed on the same 500MHz varian instrument- this is 84-1B

Reference

1. High-Resolution NMR Techniques in Organic Chemistry by T. Claridge -Elsevier - Publisher -page 38 (Limited preview from Google books).