Separation using acid-base properties; acylation
The separation of pure components from a complex mixture is a problem that is central to practical organic chemistry, and one which cannot usually be solved by using variations in a single property such as boiling point, or solubility in a single solvent. However, by taking advantage of the presence of acidic and basic groups, it is sometimes possible to achieve clean separations of mixtures using a separatory funnel, an organic solvent such as ether, and a sequence of extractions with strong acids and bases. This separation technique uses the property that charged species are soluble in water (a polar solvent) and neutral or uncharged species are soluble in organic (non-polar) solvents.
To understand this procedure fully, it is helpful to revise some acid/base theory.
Experimental Notes
The aim of this practical is to separate a mixture of three compounds, an aromatic amine, an aromatic carboxylic acid, and a neutral compound. The separation, which will be done by extraction, depends upon the following: carboxylic acids react with strong bases to form water soluble carboxylate ions; amines react with strong acids to form water soluble ammonium ions; these reactions are reversible.
Carboxylic acids
The aromatic carboxylic acid used, benzoic acid, is a weak acid (pKa ~ 4) and so can be extracted from ether by 2 M NaOH. The carboxylate anion is water soluble and ether insoluble. After separation of the aqueous and ether layers, the carboxylic acid may be recovered by acidification of the aqueous layer.
Aromatic amines
The aromatic amine used, ethyl 4-aminobenzoate is a weak base (pKb ~ 9). The amine functional group can be protonated by dilute hydrochloric acid (2 M) to form a charged species which is then extracted from ether into water. After separation of the aqueous and ether layer, the amine may be recovered by basification of the aqueous layer. However, amines are often oily compounds that are difficult to crystallise and for ease of isolation in this case, it is converted to the more stable amide. This is done by addition of a solution of sodium acetate and acetic anhydride.
Neutral compound
The neutral compound is naphthalene, a low-melting solid (m.p. range 80-82 °C) which is soluble in ether, but insoluble in water. One of its uses is as mothballs.
The extraction procedure is summarised in the flow diagram in Appendix 2.
Extraction - the technique
Sketched to the right is a typical setup of the extraction apparatus, consisting of a separatory funnel, conical flask, retort stand and ring holder. Check the tap of your separatory funnel to ensure that it does not leak and the tap moves smoothly before using it. Pour the mixture to be separated into the separatory funnel using a funnel. Add the extracting solvent and stopper the funnel. Two layers should now be in evidence. Be sure you know which layer is which. Ordinarily, one layer is aqueous (polar) and one is organic (non-polar), with the organic layer being less dense and thus the top layer.
|
|
Remove the funnel from the stand, and hold it horizontally with a gentle but firm grip at the tap and the top, as shown below. Use your hand to keep the stopper in. Gently invert the funnel, then holding it with the tap slightly higher as in the diagram, and with the tap and stem pointing AWAY from any nearby people, open the tap to release any pent up gases. Close the tap, then gently shake the funnel and its contents. Try to avoid creating bubbles. Once again release any pent up gas. Repeat this process several times or until no gas is observed being released. Return the funnel to the stand and remove the stopper. The two liquid layers should have been thoroughly mixed by this process and the funnel now needs to be left for several minutes while the layers separate out and settle down. |
|
Once the layers have clearly separated, run the lower layer out of the funnel as shown below. Make sure the stopper has been removed prior to this or the funnel will "suck" air up through the stem, causing the layers to remix. Stop running the lower layer off when the boundary between the layer nears the tap, and let the funnel stand for 30 seconds while the layers re-settle. This allows any drops of the denser liquid that have clung to the glass to settle to the bottom with the rest of the lower layer. Once this has been done, run off the remaining lower layer. You should be able to control the tap so that the boundary is stopped at the tap itself.
|
|
