Qualitative Analysis Lab

Topics: Distillation, Nuclear magnetic resonance, NMR spectroscopy Pages: 9 (2949 words) Published: April 19, 2012
Qualitative Analysis Lab Report

Organic Chemistry Laboratory II

The purpose of this lab was to separate and purify two unknown compounds, one solid and one liquid, from an ethereal solution using the techniques of chemically active extraction, vacuum filtration, simple distillation, and recrystallization. Then identifying information was compiled about these unknowns by obtaining experimental melting/boiling point ranges, and analyzing IR and proton/carbon NMR spectrums It was determined that the unknown solid’s identity was m-nitrobenzoic acid and the unknown liquid’s identity was 4,4-dimethyl-2-pentanone. The calculated percent recovery for the purified crystals was 20.75% INTRODUCTION: 2

The goal of this lab was to isolate two unknowns, one solid and one liquid, from an ethereal solution using chemically active, or acid-base, extraction. These unknowns were then to be purified and identified using melting and boiling point ranges, IR spectrums, and proton/carbon NMR spectrums.

The first technique used in this lab was chemically active, or acid-base, extraction. This technique takes advantage of the fact that organic acids and bases in their neutral form are soluble in organic solvents, but the corresponding salts are water-soluble. Therefore if one converts these acids and bases to their corresponding salts, they can be separated from the rest of the solution by trapping them in an aqueous layer. A solution of an acid component, a basic component, and a neutral component that has been dissolved in an organic solvent, such as diethyl ether, can be separated using this type of extraction. To extract the acidic component aqueous from this solution aqueous NaOH must be added to convert the acid to its corresponding sodium carboxylate. The base and the neutral components will not react and will remain in the organic layer, while the carboxylate salt will be soluble in water and be contained in the aqueous layer. These two layers can then be separated using a separatory funnel, and the acid can be restored to it’s original form by the addition of a strong acid like HCl to the aqueous layer. The acid is then organic soluble again, so if an organic solvent is added the acid will be contained in the organic layer, and the solvent can be evaporated away. The opposite is done to extract the basic layer and neutral layers, and aqueous HCl is added to the original organic layer. The basic component will be converted to the ammonium chloride salt, making it water-soluble. The aqueous and organic layers can again be separated, with the basic component in its salt form in the aqueous layer and the neutral component still confined in the organic layer. To isolate the neutral component the organic solvent can just be evaporated off. To isolate the basic component aqueous NaOH must be added to the aqueous layer to convert the base back to its original form. Extraction with an organic solvent can then be performed, and the solvent can be evaporated to yield the isolated basic component. This type of extraction can also be depicted with an acid-base extraction flowchart, like the one depicted below: Figure #1: Example of an Acid-Base Extraction Flowchart. 2

Once the unknowns were isolated they needed to be purified. The solid unknown can be purified through a process called recrystallization, which is based on the crystal lattice theory. The crystal lattice theory states that if crystal formation is slow and nicely controlled, the crystal will only allow its “like” molecules to be incorporated into its crystal structure so the most stable structure is obtained. This makes it possible remove impurities by dissolving the impure crystals in an appropriate solvent and heating the mixture. Ut then must slowly cool and the crystals will reform as a more pure compound. Recrystallization also takes advantage of differing solubilities because the solubility of a dissolved material decreases as temperature decreases....
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