dis 4

Separation of a Three-Component Mixture

Purpose

This experiment aims to isolate and determine the percent recovery of acid, basic and neutral substances through acid-base extraction using a separatory funnel and vacuum filtration.

Discussion

Acid, base, and neutral solids are soluble in diethyl ether due to its property as a non-polar solvent to dissolve non-polar solutes using the solubility principle that “like dissolves like”, so non-polar solvents dissolve non-polar solutes. The solubility of these solids in water versus diethyl ether is dependent on the intermolecular forces present in the solids and in the solvents being used. The acid benzoic acid is soluble in both water and diethyl ether due to hydrogen bonding between its carboxylic acid group (COOH) and water and Van der Waals forces acting between its non-polar ring and non-polar diethyl ether. The base 4-ethylaminobenzoate that containing both acid and base group can form a Zwitterion, wherein the amine group (NH2) deprotonates the carboxylic acid, thus making 4-ethylaminobenzoate more soluble in water than benzoic acid despite both having non-polar rings that make them both soluble in diethyl ether (Nichols, 2017). The neutral 9-fluorenone cannot dissolve in water since it cannot form hydrogen bonds or have ionic interaction. Its structure, however, is non-polar and so it is soluble in diethyl ether that is also non-polar.

( benzoic acid ) O

diethyl ether 9-flourenone ethyl-4-aminobenzoate

With the addition of 3M HCl to an ether solution, the two phases separate because water and ether are immiscible due to the difference in their polarity. 3M HCl through its H+ ion is considered to have the same density as water which is 1 g/mL whereas diethyl ether has a density of 0.71 g/mL, making water denser than diethyl ether. The denser liquid settles at the bottom which is why HCl is at the bottom layer and diethyl ether is at the top layer.

Apart from neutral 9-fluorenone that does not react with HCl, the following are the chemical reactions that occur when 3M HCl was added to the ether solution. The lone pair of electrons present in the nitrogen of the amine group (-NH2) of 4-ethylaminobenzoate protonates to form the hydrochloride salt NH3Cl.

Ethyl-4-aminobenzoate

( 1 )

Similar to the addition of 3M HCl, the addition of 3M NaOH to the ether solution forms the two phases separate because water and ether are immiscible due to the difference in their polarity. 3M NaOH through its H+ ion is considered to have the same density as water which is 1 g/mL whereas diethyl ether has a density of 0.71 g/mL, making water denser than diethyl ether. The denser liquid settles at the bottom which is why NaOH is at the bottom layer and diethyl ether is at the top layer.

Apart from neutral 9-fluorenone that does not react with NaOH, the following are the chemical reactions that occur when 3M NaOH was added to the ether solution after the initial separation. The acidic carboxylic group (-COOH) of benzoic acid reacts with NaOH to form sodium benzoate and water. Sodium benzoate (C6H5COONa) dissolved in the upper layer whereas water (H2O) dissolved in the lower layer.

+

Benzoic acid Sodium benzoate

Water

Conclusion

To conclude this experiment, the separation of a three-component mixture was made possible through acid-base extraction. Using the differences in solubility of acids and bases as a guide, each of the three substances was successfully extracted into its original acid, base and neutral component using the solvents diethyl ether, hydrochloric acid (HCl) and sodium hydroxide (NaOH). Throughout this experimentation, one of the biggest factors affecting recovery of the filtrate was knowing how to use the separatory funnel for the acid-base extraction and Buchner funnel for the vacuum filtration to recover the solid filtrate. In acid-base extraction, care had to be given to ensure that the correct molar concentration of HCl and NaOH were used, and that proper venting was observed. In vacuum filtration, maximum effort was done to ensure no spills happened, which could otherwise lessen the percent recovery yield of the original component of the solution.

References

Nichols, Lisa. “4.8: Acid-Base Extraction.” Chemistry LibreTexts, 21 Oct. 2017, chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Technique s_(Nichols)/04%3A_Extraction/4.08%3A_Acid-Base_Extraction.

“Solvent Extraction Experiment Using Liquid-Liquid Extraction.” Us.ukessays.com, us.ukessays.com/essays/chemistry/solvent-extraction-experiment-using-liquid-liquid- extraction.php. Accessed 21 Sept. 2023.

Melting Point Experiment

Purpose

This experiment aims to determine the melting points of known pure, unknown and mixed substances using the melting point capillary tube and OptiMelt Automated Melting Point System.

Discussion

Melting point is defined as the point at which materials change from a solid to a liquid. In pure substances, this is the temperature where the vapor pressures of the solid and liquid are at equilibrium (Mao, Fei, et al).

The melting process is the change of a solid substance into a liquid with the application of heat. In this experiment, melting point capillary tubes containing an aliquot of resorcinol, benzoic acid, urea, 9:1 resorcinol/benzoic acid and Unknown 2 are each analyzed with the use of the OptiMelt Automated Melting Point System. The capillary tube is inserted onto the ceramic insulator on the heating block of the OptiMelt instrument. The ceramic insulator acts to guide the capillary tubes to the heating block and to protect it from breakage, allowing the powder inside the capillary tube to melt when heated to a specific temperature. The melting process can be viewed through the observation window of the Optimelt instrument. The temperature at which the powder starts melting is called the low melting point and the temperature at which the powder fully dissolves is called the high melting point. By determining the low and high melting points, the melting point ranges of pure substances and mixtures can then be established.

The purity of benzoic acid, resorcinol and urea is determined by comparing the established melting point ranges from experimentation to the reference table provided in the lab manual. A narrow melting point range indicates purity of a substance, whereas a broad melting point range indicates presence of impurities. In mixtures, the presence of an impurity will result in melting point ranges that are greater than 4C. In this experiment, melting point ranges obtained for resorcinol, benzoic acid, and urea, were 109.9 C to 110.2 C, 123.0 C to 123.8 C and 133.0 C to 133.8 C, respectively, which indicated that these compounds were pure substances due to their narrow melting point ranges. For the 9:1 resorcinol/benzoic acid mixture, the melting point ranges obtained were 99.2 C to 103.4 C, which indicated that the mixture contained an impurity due to the melting point ranges being greater than 4C. The impurity is most likely benzoic acid because of its lower concentration of 1, in comparison to the concentration of resorcinol which is 9. The melting point ranges of Unknown 2 were 123.0 C to 128.0 C (approximate) and 122.3

· C to 123.3 C (exact). After suspecting the unknown to be benzoic acid, the melting point technique was performed on a combined Unknown 2 and known benzoic acid to test it for purity. The melting point ranges obtained were 122.6C to 123.7 C.

In identifying Unknown 2, the mix melting point analysis was used, wherein an approximate melting point range was first established using 100-195 C as the start and stop temperatures,

( 1 )

respectively, on the OptiMelt instrument, followed by an exact melting point range using the range 118-133 C as the start and stop temperatures, respectively, on the OptiMelt instrument. The exact melting point range of 122.3 C to 123.3 C was then used to compare Unknown 2 to its known value for initial identification of the substance. The unknown substance was initially identified as pure benzoic acid which had a known melting point of 122 C to 124 C. Then an aliquot of pure benzoic acid powder was mixed with Unknown 2. By combining Unknown 2 with pure benzoic acid and performing the melting point experimentation on the OptiMelt instrument, purity is the property used to ascertain that the unknown substance is benzoic acid as evidenced by an expected narrow melting point range that falls within the reference melting point range of 122 C to 124 C.

Conclusion

To conclude this experiment, the melting point technique using the OptiMelt Automated Melting Point System was a very useful tool that helped confirm the purity of the known substances urea, benzoic acid, and resorcinol in Part I of the experiment because the experimental melting point ranges of the three knowns fell within the reference melting point ranges found on page six of the lab manual. In Part II of the experiment, the mixture of 9:1 resorcinol and benzoic acid was proven to contain an impurity since the melting point ranges had a range of more than 4 C. In Part III of the experiment, the unknown substance was determined to be benzoic acid by first establishing a melting point range for the unknown (approximate and exact method) and then using this range to rule in a suspected pure substance that has a similar melting point range, and then performing the mixed melting point technique on the mixed unknown and pure substance (combination method) to ascertain the purity of the unknown. After experimentation, the melting point ranges for Unknown 2 and the combined Unknown 2 and benzoic acid were found out to be similar, and thus, Unknown 2 was determined to be pure benzoic acid. In this experiment, the data obtained was consistent with all hypothesized results.

References

Mao, Fei, et al. “Melting Point Distribution Analysis of Globally Approved and Discontinued Drugs: A Research for Improving the Chance of Success of Drug Design and Discovery.” ChemistryOpen, U.S. National Library of Medicine, 21 Mar. 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC4981057/.

“OptiMelt Automated Melting Point System.” Operation and Service Manual - MPA100 OptiMelt Automated Melting Point System, Stanford Research Systems, Oct. 2004, hanmisci.com/downloads/PDFs/Manuals/MPA100m.pdf.