Chemically active extraction Fall 2020 PDF

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Chemically active extraction Background reading -

Mohrig Chapter 10 (Extraction) Klein Chapter 3 (Acids and bases)

Introduction Extraction is a technique that can be used to isolate and possibly purify a substance. Extraction involves shaking a solution of a substance with an immiscible solvent in which the desired substance is more soluble than it is in the original solvent. Upon standing, the solvents form two layers that can be separated. Extraction may have to be repeated several times to achieve complete transfer of the substance to the second solvent. In organic chemistry, most often one of the solvents is organic and the other is water. Inorganic compounds can generally be separated from organic compounds because the former are very soluble in the water and the latter are very soluble in the organic solvent. In some cases, a single extraction may be sufficient to separate the organic compound. However, many organic compounds (particularly those containing oxygen or nitrogen, such as alcohols, aldehydes, ketones, acids, esters, and amines, which can form hydrogen bonds) are partially soluble in water. They distribute themselves between the water and the organic solvent in proportion to their relative solubilities in the two solvents. Thus, the extraction can be considered a competition between two immiscible liquids for the solute, with the solute partitioning itself between these two liquids. Depending on the degree of partitioning, multiple extractions may be necessary to completely transfer the organic compound from the water to the organic solvent. Acid/base extractions (chemically active extractions) are used to separate acidic, basic, and neutral compounds. The extractions rely on simple acid/base reactions to selectively ionize the organic acids and bases to make them water soluble. This allows each acid or base to be separated from the other compounds. After the compounds have been separated, they can be converted back to the original molecular form, again using acid/base chemistry. Acid/base equilibria and relative strengths of acids and bases: Recall from general chemistry and lecture that an acid and a base react to form their conjugate acid and conjugate base. Furthermore, the equilibrium favors the formation of the weaker acid and weaker base. Recall also that the strength of an acid is measured by its reaction with water and reported as its acid dissociation constant, K a, or as its pKa. A strong acid has a large K a, which corresponds to a negative pKa. A weak acid has a small Ka, which corresponds to a positive pKa.

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An acid can only react with a base that is stronger than its conjugate base. Therefore, according to the pKa values (see table on next page), an acid will react with any base listed below its conjugate base. For example, a carboxylic acid (RCOOH) will react with sodium hydroxide as will a phenol (ArOH). A carboxylic acid will also react with sodium bicarbonate, but a phenol will not because its conjugate base is a stronger base than bicarbonate ion. These observations suggest a method to separate carboxylic acids from phenols based on their acid/base properties.

Experimental Overview: In this experiment you will separate benzoic acid from a mixture containing the carboxylic acid, cellulose (a natural polymer of glucose) and methyl orange (a common acid/ base indicator). You will use a chemically active extraction to convert the water insoluble benzoic acid into its water soluble benzoate salt by treating the carboxylic acid with base. Finally the benzoic acid will be precipitated by adding strong acid to the carboxylate salt solution. The mixture contains:

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On the next page, you will find a flow chart illustrating the methodology you will use in today’s experiment.

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Other preparation Balanced chemical equations – you should indicate the acid-base reactions that are occurring in the chemical active extraction using the actual structures of the compounds involved (not abbreviations like RCOOH or RCOO-) Safety Avoid inhaling ethyl acetate Because pressure builds up in the separatory funnel during extractions, be sure to release the pressure as described in the PowerPoint presentation; be careful where you point the funnel to avoid spraying yourself or a neighbor with the extraction solutions. Pre-lab calculations: How much 3 M NaOH is necessary to prepare 30 mL of 1 M NaOH? Procedure: 1)

Weigh approximately 2 g (± 0.50 g) of the mixture using the balances and place it into a 125 mL Erlenmeyer flask.

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Add 25 mL of ethyl acetate to the flask.

3)

Add boiling stones and gently heat the mixture on the hot plate until the ethyl acetate begins to boil.

4)

Filter the mixture through fluted filter paper using hot gravity filtration (see Technique 9.2 for details on how to carry out a gravity filtration). Dispose of the filter paper in a trash can.

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Make sure that your stopcock is closed on the separatory funnel. Carefully pour the filtrate into a separatory funnel. Dilute desk reagent sodium hydroxide (NaOH) from 3 M to 1 M. Extract the filtrate twice with two 15 mL portions of 1 M NaOH following procedure for extraction in Techniques in Organic Chemistry. Collect and keep the aqueous layers from both extractions.

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Cool the combined aqueous extracts on ice. Acidify with small additions of 6 M HCl until the solution is acidic to litmus paper.

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Collect the benzoic acid by vacuum filtration using a Buchner funnel and side arm flask (see technique 9.4 for instructions on how to carry out a vacuum filtration). Transfer the benzoic acid into a pre-weighed (tared) labelled vial. Dispose of the filter paper in a trash can. Place the vial (uncapped) into your drawer. Dispose of the ethyl acetate filtrate into the non-halogenated waste.

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Allow at least one day for the sample to dry. Return to the lab and determine a mass of the vial and benzoic acid, a melting point, and IR spectrum. After you have finished the 5

analysis, cap the vial and leave it in your drawer until next lab period. This is the crude (partially isolated/purified) benzoic acid. 9)

Before you leave the lab today, obtain a melting point range of the initial mixture and a IR spectrum of the mixture.

Data Record in your lab notebook the following information:           

Observations – record appearance (color, crystal structure, clear or opaque, etc.) of solids and solutions throughout the experiment Mass of mixture used Mass of the empty dry vial Mass of vial and dried benzoic acid Mass of crude benzoic acid isolated Name of the melting point apparatus used and the specific apparatus used Melting point range of the mixture and crude benzoic acid Name of the IR spectrometer used File names for the IR spectrum IR spectrum of crude benzoic acid with relevant IR peaks labelled IR spectrum of the mixture – you do not need to label the IR peaks as it is a mixture

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Name:_____________________ Chemically active extraction 1) Compare the melting point ranges of the mixture and crude benzoic acid with the literature melting point of benzoic acid. What does the melting point range of the crude benzoic acid indicate about the purity and effectiveness of the extraction procedure?

2) Compare the IR spectra of the mixture and crude benzoic acid with a literature IR spectrum (look up the spectrum on SciFinder – the best one is Biorad). What does the IR indicate about the purity and effectiveness of the extraction procedure?

3) Where was the methyl orange during the extraction? Was it in the aqueous or organic layers? Relate your observations to the structure of methyl orange.

4) Amines can also be isolated using chemically active extractions. Describe how you would separate the following molecule (which is a solid a room temperature) from methyl orange.

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