Using acid base chemistry in the separation of substances can be a quite useful technique in organic chemistry. A basic extraction can be carried out by applying a hot solvent such as water to a dry substance like coffee. This will allow for the extraction of caffeine and flavor from the coffee beans. Acid base chemistry can be useful in separating neutral, basic, and acidic organic substances by controlling the pH of the aqueous layer. In acid base extraction a solvent such as ether is mixed with an aqueous solution of a different pH. This will aid in ionizing the intended compound and separate it from the mixture. Once the molecule is ionized it will precipitate out of the ether solvent and dissolve into the aqueous layer. The aqueous layer can then be removed and the pure substance can be obtained through crystallization.

1. Separation of Acidic Substances
Author: Dr. Robert D. Craig, Ph.D.

2. Separation of Acidic Substances
Author: Dr. Robert D. Craig, Ph.D.
• Application:
• Acids and Bases are one the most
fundamental principles of chemistry. Acidity
and basicity are involved in determining
chemical reactivity, separation, solubility, and
transport of molecules across membranes.

3. Background
• Aqueous (water based) solvents are very
polar. Organic solvents are much less polar
than aqueous solvents.

4. Such that

5. Background
• The underlying principle behind acid
extractions begins with the fact that many
neutral organic compounds are not soluble in
water but are soluble in organic solvents. If a
neutral organic compound is converted to an
ion by reaction with a base, then the polarity
of that compound is significantly increased.

6. Background
• . The ion typically has a high solubility in water
and a reduced solubility in the organic solvent.
This often causes the ion to migrate to the
aqueous phase from the organic phase.

7. Get to the point Rob . .
• Benzoic acid –not soluble in water
• But the “sodium salt is”

8. Get to the point . . ..
• In order to separate these compounds, we will
convert the water insoluble acid into a water
soluble salt:

9. Subsequently . . .
• Can use a strong acid to take the salt back to
organic layer soluble acid form

10. Background
• One possible experiment begins with two
neutral organic compounds (benzoic acid and
2-naphthol) dissolved in an organic solvent,
MTBE (methyl t-butyl ether). ####This is
changed (diethyl ether)

11. Diethyl ether instead
A base is added which will selectively react with
one of the materials and convert it into its
conjugate base (an ion). These ions have a high
solubility in water and a low solubility in
ether, so they will migrate (partition) to the
aqueous phase.

12. Like dissolves like
• The aqueous phase is removed along with the
conjugate base of the first compound. This
layer is later acidified, protonating the
conjugate base. The solubility of the neutral
material in water is low, and the compound
precipitates and is recovered by filtration.

13. Not MTBE
• This process is repeated with the addition of a
stronger base to the MTBE which will react
with the second compound and that material
will deprotonate, form an ion then partition to
the aqueous phase.
• Again the aqueous layer is removed, acidified
and the precipitated neutral material is
recovered.

14. Oil floats on water . . .

15. Acetanilde and Acetanilde and p-Tolic
Acid

16. Acetanilde and p-Tolic Acid

17. Some terminology
• 1. Some terminology related to Acid-base
extractions:
• Acid: In this experiment we are discussing
Bronsted-Lowry acids, hence an acid is a
material that may lose a proton and form a
conjugate base.
HA -> H+ + A-
• The stronger the acid, the weaker the
conjugate base.

18. Some Terminology
• Base: In this experiment we are discussing
Bronsted-Lowry bases, hence a base is a
material that may gain a proton and form a
conjugate acid.
B- + H+ -> HB
• The stronger the base, the weaker the
conjugate acid.

19. Some Terminolgy
• Partitioning: The distribution of a substance or
ions between two immiscible liquids.
• Extractioning: Dissolution and removal of one
constituent of a mixture in a solvent.
• Precipitation: If the concentration of a
compound in a solvent is greater than the
solubility of that compound, the compound
will no longer remain dissolved and will form a
new solid phase

20. Some terminology
• Equilibruium Constant (K): The numberical
value of the concentration of the products
divided by the concentration of the reactants.
If the value of K is smaller than one, the
equilibrium lies in favor of the starting
material. The reaction does not proceed
greatly in the forward direction.
• If the value of K

21. Want to discuss
• For octanol and water-partition and
distribution

22. Two phase system, hydrophobic (top) and hydrophilic (bottom)
for measuring the partition coefficient of compounds

23. Some terminology
• pKa: The negative log of the acid equilibrium
constant. pKa= - log Ka where the acid
equilibrium constant Ka is equal to:
The smaller the pKa the stronger the acid

24. Smaller -pKa the stronger the acid.
• 2. Useful pKa information. (Do not memorize
pKa values)

25. Smaller-pKa the stronger the acid. .
ACID (HA) pKa CONJUGATE BASE (A-)
Hydrochloric Acid (HCl) pKa= -7.0 Chloride ion (Cl-)
Benzoic acid (Ph-CO2H) pKa = 4.17 Benzoate ion (Ph-CO2-)
Carbonic acid (H2CO3) pKa = 6.35 Bicarbonate ion(HCO3-)
2-Naphthol pKa = 9.5 2-Naphthonate ion
Water (H2O) pKa = 15.7 Hydroxide ion (HO-)

26. smaller-pKa the stronger the acid. .
• First, identify the acid and the base on both sides
of the arrow.
• Compare the pKa of the acids. Determine if the
stronger acid is on the right or the left of the
arrow.
• If the pKa of the acid on the left, HA1 is a smaller
number (that is, the acid is stronger) than the pKa
of the acid on the right, HA2 then the reaction will
proceed in the forward direction

27. .which way??????
• If the pKa of the acid on the left, HA1 is larger
(that is, the acid is weaker) than the pKa of the
acid on the right, HA2, then the reaction will
not proceed in the forward direction (but it will
proceed in the reverse direction).

28. .which way??????
•HA1 + -A2 →
• -A1 + HA2

29. All possible reactions
• 4. Below are ALL the possible reactions for the lab.
@@@@the handout@@@@@
• Examine each reaction, assign acid
and base to each compound, determine if the reaction
will proceed in the forward
direction or not.
Place an X across each arrow which will not proceed in
the forward
direction. Only write the reactions that WILL proceed in
your lab notebook.

30. Possible Reactions of Mixture with
Sodium Bicarbonate (NaHCO3
• Goto handout now!!

31. Possible Reactions of Mixture with
Sodium Hydroxide (NaOH)
• See handout-show pdf

32. Protonation of anions with
hydrochloric acid (HCl)
• Show pdf

33. @@@@@@@@@@@@@
• 5. Additional structures and information.
• MTBE is Methyl t-Butyl Ether. This compound
is often added to gasoline to increase the
oxygen content and improve the burn
efficiency. MTBE has a density of 0.7404 g/mL
and is less than that of water (density 1.00
g/mL) hence MTBE floats on top of water.

34. Diethyl ether-goto portal-Now Rob

35. SEPARATION OF BENZOIC ACID, 2-
NAPHTHOL, AND 1,4-
DIMETHOXYBENZENE
• Skip to slide 44.

37. • 3. Predicting if a reaction will proceed in the
forward direction or not. To determine if any
acid-base reaction will succeed, as shown,
• HA1 + -A2 → -A1 + HA2

38. • VID 11 FIRST
•
• VID 1
• VID 2
• VID 3
•
• LAB #3: SEPARATION OF BENZOIC ACID, 2-
NAPHTHOL, AND 1,4- DIMETHOXYBENZENE

39. • 6. Aqueous solutions are very polar. Organic
solvents are less polar than aqueous.
• Like materials like other materials like
themselves. Ions are very polar.
• Typically ions have a higher solubility in water
than organic solvents. Most neutral organic
compounds have limited solubility in water
but an increased solubility in organic solvents.

40. • If a neutral organic compound dissolved in an
organic solvent is deprotonated and turned
into an ion, the solubility in the organic
solvent will be decreased and the ion will
want to move or partition into an available
aqueous solvent.

41. IF a base is chosen which will selectively
• deprotante one of a mixture of dissolved
organic compounds and turn only into an ion,
• then mixtures of compounds can be
separated. This concept is why this separation
• experiment is possible.

42. Slide 44
• 7. The two compounds are separated as ions
dissolved in water. It is desired to reform
and isolate benzoic acid and 2-naphthol as
pure solids. In order to do this, a very strong
acid is added to the aqueous solutions, this
will reprotonate the ions and turn
them back into neutral organic molecules.
Most neutral organic compounds have a low

43. • solubility in water. 2-Naphthol and benzoic
acid are not an exception. As neutral
compounds they have a low solubility in the
water, and hence will precipitate as a solid.
The solid is isolated by vacuum filtration. A
melting point will be taken to assess the purity
of these materials

44. • 8. Separatory funnels are specially designed
glassware for the separation of immiscible
liquids. They have a ground glass access port
at the top and a stopcock at the bottom. This
is the most expensive piece of glassware in
your drawers. A new sep funnel costs
approximately $110.00. Handle with care

45. • Prelab.
*Before lab begins, read the following:
Zubrick’s Chapters on Extracting and Washing
(Ch 15 and 16).
Zubrick’s Chapter on Drying Agents (Ch 10).
*Draw the balanced acid-base reactions
including the structures of benzoic acid, 2-
naphthol, and 1,4-dimethoxybenzene and the
products.

46. • *Draw a flow chart of the extraction
procedure.
*Make a table of physical constants and safety
information. Include only relevant data for this
experiment.

48. • http://www.oppapers.com/essays/Separation-Of-
A-Carboxylic-Acid-A/193808
•
•
Grading.
Completion 5 pts
(includes lab and a report turned in on time)
Prelab 3 pts
Experimental 2 pts
Results 5 pts
Conclusion 5 pts