E28 EXTRACTION OF ORGANIC COMPOUNDS
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 (see Appendix 1).
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.
APPENDIX 1 - GENERAL ACID/BASE THEORY
Acids
A typical weak acid, HA, reacts with water according to the following equation:
HA + H2O  H3O+ + AThe equilibrium constant for this reaction is called the acid dissociation constant and is given by:
The stronger acids have the higher values of Ka. Indeed, for strong acids the reaction proceeds essentially
100% to the right and thus Ka is very large. The vast majority of organic acids, however, have Ka values
smaller than 10-3 M. In order to avoid dealing with these small numbers, it is common practice to tabulate
acid strengths as pKa values, where:
pKa = -log10Ka.
The weakest acids have the smallest Ka values. Hence, the weakest acids have the largest pKa values and the
strongest acids have the smallest (or negative) pKa values.
Sodium hydroxide is a strong base and dissociates completely in water to produce OH - ions. These OH- ions
react with a weak acid essentially to completion, as follows:
HA + OH- H2O + ABases
A typical weak base, B, reacts with water according to the following equation:
B + H2O  HB+ + OHThe equilibrium constant for this reaction, called the base dissociation constant, is given by:
The stronger bases have the higher values of Kb. The vast majority of organic bases have Kb values smaller
than 10-3 M, and it is common practice to tabulate base strengths as pKb values, or even more commonly to
merely tabulate the pKa values of the conjugate acids, from which the Kb can be deduced.
pKb = -log10Kb.
-
For a conjugate acid base pair, HA and A :
Ka(HA) x Kb(A-) = 10-14 M2
and
pKa(HA) + pKb(A-) = 14.
The strongest bases have the smallest pKb values and hence their conjugate acids have the largest pKa
values.
Hydrochloric acid is a strong acid and dissociates completely in water to produce H+ and Cl- ions. The H+
ions react, essentially to completion, with a base as follows:
B + H+  HB+
APPENDIX 2