SEPARATION OF ORGANIC MIXTURES AND IDENTIFICATION

A common problem encountered in chemistry involves the separation of a mixture of two or three compounds into single compound fractions followed by the purification and identification of each.  To effect the separation, the chemist must make use of the different properties of the components.

            We will use differences in solubility, density, acid-base chemistry and reactivity to separate a mixture of compounds.  We will then purify and identify each component.  The components will be unknown to the student except that one will be a liquid neutral organic compound with a high boiling point and the other a carboxylic acid. The carboxylic acid can react with a base such as sodium hydroxide to form an anion which is water soluble. The neutral will not react and so it will remain “neutral”. The possible organic neutral compounds are listed below:

           Table 3-1: Possible Neutral Unknown Compounds and Boiling Points

                        HYDROCARBONS

                                    Decalin  190°C (2021)                        o-Xylene  144°C (2213)

                                                         

                        ALCOHOLS

                                    Cyclohexanol 160°C (2213)               Benzyl alcohol  205°C (2213)

                                                                            

                        KETONES

                                    Cyclohexanone  157°C (2213)           Acetophenone  200°C (1212)

                                                                              

                        NITRILES

                                    Malononitrile  219°C (2223)                Benzonitrile  191°C (1211)

                                                                                   

Carboxylic acids are compounds which include one or more carboxyl functional groups.

                                                             

A functional group can be defined as an atom or group of atoms in a definite structural arrangement that influences the properties of an organic compound.  The carboxyl group gives the following possible unknowns their acidic character.

                                                                                 

TABLE 3-2  POSSIBLE ACID UNKNOWNS

ORGANIC CARBOXYLIC ACIDS

            Compound                                                      Melting Range (°C)     Hazard Code

            o-Toluic Acid (2-Methylbenzoic acid)             103-105                       1112

            Azelaic Acid                                                    106-107                       0112

            m-Anisic Acid (3-Methoxybenzoic acid)        105-107                       1112

            2-Phenoxypropionic  acid                               116-119                       3112

            Benzoic Acid                                                   121-123                       1213

            Sebacic Acid                                                   131-134                       1113

            Cinnamic Acid                                                132-135                       2112

            1-Naphthoic Acid                                            157-160                       2112

            Salicylic Acid                                                  158-160                       2112

            p-Toluic Acid (4-Methylbenzoic acid)             180-182                       1112

            p-Anisic Acid (4-Methoxybenzoic acid)          182-185                       1112

Once the unknown neutral and acid compounds have been separated and purified they will then be identified, the neutral compound via its infrared spectrum and the carboxylic acid via its melting range.

Extraction

The general formula for a carboxylic acid is

                                                                       

where R stands for any group of atoms attached to the functional group COOH.  As with any Bronsted acid, a carboxylic acid reacts with hydroxide ion, OH^-, to produce the conjugate base of the acid and water

           

            Carboxylic acid                                 Carboxylate anion

            insoluble in water                                soluble in water but

            but soluble in                                       insoluble in organic

            organic liquids                                     liquids

As is indicated, this reaction also changes the solubility properties of the acid molecule.  We will take advantage of these property changes in separating the acid from the mixture.

            The neutral component of the mixture may be any one of the hydrocarbons, alcohols, ketones, or nitriles listed in Table 3-1. Neutral compounds will not react with either an acid or a base.  They are also water insoluble but very soluble in organic liquids.

The Separation of the Aqueous and Organic Layers

            We have now identified the solubility properties of the two components of the mixture which will allow us to separate them.  We will use liquid-liquid extraction to take advantage of the differences in solubility of the components. The organic liquid or solvent will be tert-butyl methyl ether (TBME) and the polar aqueous layer will be 5% NaOH or water.

            Initially the mixture of the neutral organic and carboxylic acid unknowns will be dissolved in TBME forming an organic solution. Suppose that this organic solution is shaken with a dilute aqueous sodium hydroxide solution and then allowed to stand until the two layers separate.  During the shaking process, the hydroxide ion

will react with only the carboxylic acid      component of the mixture to form the water soluble carboxylate anion (see Equation 1 again).  This changes the solubility properties of the acid as already stated and results in most of it moving from the organic liquid layer to hydroxide ion-water layer.  The carboxylate ion thus is the solute and is extracted into the aqueous (water) phase. The two phases are then separated into two fractions: The aqueous sodium hydroxide solution containing the carboxylate anion as its salt and the organic layer containing the neutral organic compound. 

            The two phases will separate and form two separate layers based on differences in polarity and density.  The organic layer is much less polar and has a much lower density compared to the diluted NaOH solution.  Sometimes the difference in polarity and/or density of the two phases may not be great enough to effect a separation causing the formation of an emulsion.  The separation can sometimes be improved by adding more NaOH solution or TBME solvent.

            It is important to note that single extractions do not necessarily yield complete separations, and that multiple extractions may be needed.  In your work, you will extract the original organic solution two times with aqueous sodium hydroxide solution to remove the acid and water soluble impurities from the organic layer.  The two aqueous extracts are then combined and set aside as the aqueous sodium hydroxide fraction. The organic solution is further extracted once with distilled water to remove any water soluble impurities.  Once these extractions are complete, the organic solution should contain only the "neutral" compound and the organic acid unknown should be extracted into the NaOH solution.

Purification

            Once the two components have been separated, we must obtain each of them in a pure form so they may be easily identified.  The pure carboxylic acid in this sequence is a solid while the neutral compound is a liquid.  The water soluble carboxylate anion will be precipitated from the sodium hydroxide extract by adding 6M HCl and then recrystallized for purification.  The pure neutral compound can be obtained by distilling the lower boiling solvent (TBME) off, leaving only the pure neutral liquid.

Precipitation of the Acid

            The carboxylic acid is extracted into an aqueous hydroxide solution because the carboxylate anion RCOO^- dissolves more readily in water than in the organic solvent because the anion is SOLVATED by the polar molecules.  What would happen if hydrochloric acid, HCl, were added to this carboxylate anion fraction?  The hydroxide ion would first react:

OH^- + H₃O⁺ ® 2 H₂O

and then after the hydroxide ion has been consumed, the carboxylate anion would react to yield the original acid

Since the carboxylic acid has a very limited solubility in water most of it will precipitate; that is, the solid will form in the solution.  The solution is then filtered to separate the acid crystals from the aqueous solvent, water.  In general, a solid which forms rapidly is not very pure because the crystal lattice includes impurities and must be recrystallized for further purification.

Purification of the Neutral Compound

            The mixture of TBME and the neutral compound will next be treated with a drying agent to remove any trace water before the TBME is removed by simple distillation. The pure, neutral compound in this experiment is a liquid which has a characteristic boiling point significantly higher than that of the organic solvent, TBME which has a boiling point of 55°C and density 0.74 g/ml.  Simple distillation will then be used to remove the TBME from the neutral compound. The TBME collected will be recycled. A "rotary evaporator" will then be used to remove the final traces of TBME from the neutral organic compound. The rotary evaporator is used to conduct a form of vacuum distillation and can be used whenever the compounds which need to be separated have very different boiling points. The organic solvent evaporates easily when heated at the low pressure, leaving the neutral compound behind.

Identification

            The acid in the unknown will be identified by melting point and mixed melting points while the neutral compound will be identified by infrared analysis. Read and study the information in the appendix on these two techniques.

                                                                   EXPERIMENTAL

At first, it is remarkably easy to get lost in this sequence!  The flow chart on the next page is designed to help you get an overview of the procedure to help keep track of where you are and where you are heading. If you do get lost hug the nearest tree until you are found (just kidding).

PRELAB ASSIGNMENT: Review information on acid-base concepts, solvation, extraction, recrystallization, simple distillation, infrared spectroscopy, melting temperature determinations, drying agents in this experiment, the appendix, previous experiments, and the lecture text. Then answer all of the pre-lab questions at the end of this experiment