Measurement of Partition Coefficients
Experiment 2
Things to look up before lab
1.
2.
3.
4.
review acid/base chemistry
separatory funnels
definition and use of a rotovap – what does it do and how does it work?
physical properties of dichloromethane, diethyl ether and water
See also
http://college.wfu.edu/chemistry/course-materials/special-equipment#rotovap
http://college.wfu.edu/chemistry/course-materials/basic-lab-skills#extraction
Introduction
Most people realize that oil and water don’t mix. This is both a nice general metaphor and an
important empirical phenomenon in the organic chemistry lab. In the lab, “oil” is an organic solvent of
some sort while “water” is either pure water or an aqueous solution of some inorganic salt, acid or base.
When the “oil” and “water” are mixed a biphasic mixture is obtained. The solution of lower density floats
on top of the solution of higher density. Usually, this will result in the organic solution to float on the
aqueous solution. However, there are important exceptions; generally, anytime a chlorinated solvent is
used, it is denser than the aqueous solution. In any case, you should always use some thought as to which
layer is which and be sure the experiment is complete before you discard any solution.
The biphasic nature of the mixture allows a separation/purification procedure known as extraction.
A solute/analyte will partition between the two phases in some proportion that is a function of its
hydrophobicity. For example, when hexane, a hydrophobic hydrocarbon, is mixed with water, the less
dense hexane floats on top of the more dense water. If cholesterol, a hydrophobic alcohol, and sodium
chloride, a hydrophilic inorganic salt, are included in the mixture, the cholesterol will be almost entirely
in the hexane layer and the sodium chloride entirely in the aqueous layer. Cholesterol is soluble in hexane
and insoluble in water; the opposite is true for sodium chloride. Thus, when this mixture (hexane, sodium
chloride, water, cholesterol) is shaken and allowed to settle, one obtains a biphasic mixture of a
cholesterol containing hexane layer (on top) and an aqueous solution of sodium chloride. When this
mixture is placed in a separatory funnel, the aqueous layer can be let out through the bottom. While the
aqueous layer drains out the bottom, one can carefully watch and stop the drainage when the layer
boundary reaches the bottom. Thus, the hexane layer can be separated from the aqueous layer and, with it,
the cholesterol from the sodium chloride.
This is a general technique to separate compounds based on their solubility and hydrophobicity.
The aqueous solution can be adjusted to take advantage of acid/base chemistry in the solute/analyte and
various organic solvents can be utilzed as well. In this laboratory, you and your classmates will test a
variety of compounds for the preference for organic vs. aqueous layer.
Every compound has a characteristic partition coefficient (K) for every pair of immiscible solvents.
K is given by the following equation:
K = C2 / C1
Where C2 and C1 are the concentration of the analyte in solvent 2 and solvent 1, respectively. K can be
measured by dissolving a known amount of solute in a biphasic mixture and measuring how much
accumulates in either layer alone.
Procedure
In a separatory funnel, place 10 mL of dichloromethane and 10 mL of the aqueous solvent
indicated by your professor or TA. Add 50 mg of the analyte indicated by your professor or TA. Stopper
and shake the separatory funnel until the analyte is dissolved. Allow the mixture to settle. Collect the
organic layer and dry it over sodium sulfate. Decant the organic layer into a tared 25 mL round-bottomed
flask. Remove the organic solvent using the rotovap. Weigh the flask and calculate what percentage of
the analyte was in the organic layer. Calculate K for your analyte/solvent pair using the organic solvent
as solvent 2.
Report your results to the TA. The results from your classmates (who are doing different
experiments than you) will be made available for your analysis.
Questions (be sure to explain your answers clearly, but grades will not be awarded for lots of words.
Answer as clearly and concisely as possible).
1. What type of compounds are preferentially soluble in an alkaline aqueous solution?
2. What type of compounds are preferentially soluble in an acidic aqueous solution?
3. How might this observation be used in separating a mixture of octanoic acid, octyl amine and methyl
octanoate (i.e. get all three compounds pure)?
4. Would you expect a single extraction to be sufficient to collect all of the caffeine from a caffeinated
beverage? If not, how would you improve the extraction?
5. Which data is least reliable in this experiment. That is, which reported partition coefficient is least
reliable? Why?
6. Most reliable? Why?
7. What is the advantage of using a rotovap as opposed to simply boiling the solvent away on a hot plate?
organic
Solvent Mixture
aqueous
Compound
Benzoic acid
Glucose
Glucose
pentaacetate
Caffeine
DCM = dichloromethane
Et2O = diethyl ether
DCM
water
Et2O
water
DCM
Saturated
NaHCO3 (aq.)
DCM
0.1 M HCl