Separation of Acid, Base and Neutral Compounds from a Solid Mixture by Liquid-Liquid Extraction

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Experiment #1: Separation of Acid, Base, and Neutral Compounds from a Solid Mixture by Liquid-Liquid Extraction

Introduction

Liquid-liquid extraction is a technique used to separate chemical substances in order to purify or identify the various components of a mixture. Flavors, spices, perfumes, and medicines are just some of the everyday things that are extracted from plants and other natural sources [1]. The basic principle used to carry out this separation is the mixing of two liquids that are immiscible with each other. This creates layers of liquid, which can be separated and then isolated to help to identify compounds. Another important concept to remember in this experiment is that ionic salts are polar and therefore water soluble, and neutral molecules are non-polar and will usually not dissolve in water.

The student is given an unknown containing either an acid and a neutral compound or a base and a neutral compound, but since the components are unknown, the student conducts the experiment as if all 3 compounds are present. In this particular experiment, the compounds are benzocaine, salicylic acid, and naphthalene. An appropriate organic solvent is used to create the ether phase. Diethyl ether is used because it has low solubility in water but will dissolve the components of the mixture (creating liquid layers), is not acidic or basic, is less dense than water, and can be easily removed by evaporation. To begin the separation, the student needs to produce a salt that is water-soluble, so that the ether phase can separate from an aqueous phase. To accomplish this, dilute HCl is added to the solution. The HCl protonates the base (benzocaine) in the solution to create an organic polar salt, and it therefore dissolves in water. This aqueous phase is drained from the ether phase and then deprotonated again with NaOH to reproduce the original neutral benzocaine.

The same concept is used to isolate the salicylic acid. Sodium hydroxide is added to deprotonate the salicylic acid to leave the conjugate base. This salt of the acid dissolves into the aqueous phase and is then drained from the funnel where it is then protonated again with HCl, leaving the naphthalene alone in the ether phase.

The student finishes with solid naphthalene (by rotary evaporation) and either solid salicylic acid or benzocaine. Solids are filtered off and weighed to find the percent recovery. Finally, melting points of each product are taken to confirm the identities and purities of the solids.

Experimental Procedure

As outlined in the CHEM 265L manual [2]. No deviations were made.

Results and Observations

Upon addition of HCl to diethyl ether and unknown, gas was produced and separatory funnel needed to be vented about 5 times. Funnel was shaken for 2 or 3 seconds and stopcock opened to release pressure each time. A clear line was observed between the ether and aqueous phases. The ether phase was a little cloudy, while the aqueous phase was clear. After draining the aqueous phase containing the salt of the base, some residue was observed floating on the top of the liquid in the beaker. This may have been contamination, or some diethyl ether that was accidentally released with the aqueous phase.

The addition of NaOH was similar to HCl where lots of gas was produced and the funnel needed to be vented 5-6 times before no gas was released from the solution. This time, the aqueous phase started out very cloudy and the ether phase was clear, but after letting the solutions sit for a minute or so and venting a few more times, the aqueous phase looked a lot clearer and could be extracted.

When adding the drying agent to the naphthalene solution, 1.05g was measured out and added. The sodium sulfate immediately clumped into the middle and stuck to the glass, while the solution was a little cloudy. An additional 0.8g sodium sulfate was added. After swirling the beaker, some sodium sulfate was floating around freely. The...
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