Heated 0. 92 g of 2,4-dinitrobromobenzen and 0. 07 mole of aniline with ethanol until the hot solvent formed a saturated solution. The color of the mixture changed to bright brown. Cooled the mixture for one hour after the entire solid had dissolved. Ran thin layer chromatography (TLC) described by Ault1 with two reagents and the product mixture using three different solvents: hexane, acetone and toluene. Solvent front on the TLC plate was reached more quickly in the acetone. Compared the RF value (Figure 1) from the TLC plate by using UV light and iodine vapor.
The RF value increased as the eluting solvent polarity increased. Color of the TLC plate changed to yellow in iodine. After one hour, ran TLC with the product mixture using toluene as the solvent. Re-crystallized the product and collected the red needle shaped products by suction filtration as described by Ault1. 0. 223 g, 24 % recovery, Mp 145-150? C (161 lit, Ault1) Discussion: Thin layer chromatography (TLC) is used to separate substances by taking advantage of the fact that different substances are partitioned differently between two phases (Ault, 1998).
Silica gel was used in the TLC as an adsorbent. Polarity refers to the dipole-dipole intermolecular relationship between the slightly positively charged end of a molecule to the negatively charged end of another molecule. Thus, molecules with high polarity have greater reactivity than non-polar molecules. Acetone had the greatest polarity among the three solvents, so the RF value was higher and the solvent front on the TLC plate formed more quickly in the acetone.
The hexane had the lowest RF value because it is a non-polar molecule. The highest RF value was the 2,4-dinitorbromobenzene due to its strong chemical affinity. The lowest RF value was the aniline because of its weak chemical affinity. Thus, 2,4-dinitrobromobenzene preferred mobile phase and the aniline preferred stationary phase on the TLC plate. Purification of a solid by re-crystallization from a solvent depends on the fact that different substances are soluble to differing extents in various solvents (Ault, 1998).
Most unwanted materials were much more soluble than the product, so ethanol was heated just enough to form a saturated solution. Recovery percentage was very low. Most of the error resulted from adding too much ethanol during re-crystallization which caused a low percentage of recovery. Most of the lost product dissolved in ethanol, so it filtered through the filtrate with unwanted substances.
Reference
Addison Ault, Techniques and Experiments for Organic Chemistry, 6th edition, University Science Books, 1998, p 48, 109-110.