Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Extraction: Separation of Acidic Substances Notes
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.
Background:
Aqueous (water based) solvents are very polar. Organic solvents are much less polar
than aqueous solvents. The underlying principle behind acid extractions begins with the
fact that many neutral organic compounds have a low solubility in water but have a high
solubility in organic solvents. If a neutral organic compound undergoes an acid-base
reaction and is converted into its conjugate base, that conjugate base is an ion. Ions
have separate negative and positive charges. Ions have a high solubility in polar water
and a very low solubility in non-polar organic solvents. If water is present, this causes
the ion to migrate or partition to the aqueous phase from the organic phase. In CHEM
1020 Module 11A, you learned how the presence of polar groups determine if a
compound will be soluble in water or not.
One possible experiment begins with two neutral organic compounds (benzoic acid and
2-naphthol) dissolved in an organic solvent, MTBE (methyl t-butyl ether). An aqueous
base is added which will selectively react with one of the materials and convert it into its
conjugate base (an ion). This ion has a high solubility in water and a low solubility in
MTBE, so the ion will migrate (partition) to the aqueous phase. The aqueous phase is
removed along with the conjugate base of the first compound. This layer is later
acidified, protonating the conjugate base and transforming the ion back into a neutral
organic compound. The solubility of the neutral material in water is low, and the
compound precipitates. The solid is recovered by filtration. 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.
Safety considerations for this experiment:
Concentrated Hydrochloric Acid is a strong acid. It is corrosive and will cause
visible destruction of tissue and other materials upon contact. Avoid
contact with skin, eyes, and clothing. Clean up all spills immediately. If
accidental contact is made, immediately wash with copious amounts of
water. Handle HCl in the fume hoods.
Sodium Hydroxide Solution (1.5M) is corrosive even though it is not very
concentrated. Avoid contact with skin, eyes, and clothing. Clean up all
spills immediately. If accidental contact is made, wash with copious
amounts of water.
Separatory Funnels which are not properly shaken and vented can build up
pressure and spew contents onto unsuspecting faces, eyes and
neighbors. Wear eye goggles at all times during this class. Do not point
separatory funnel at yourself or others. Vent frequently.
Methyl t-Butyl Ether (MTBE) is an organic solvent with a moderately high vapor
pressure. Avoid breathing the fumes. MTBE is highly flammable, have no
arcing sparking devices or open flames in close proximity to the solvent.
Proper Waste Disposal is very important in this experiment. All excess sodium
bicarbonate solution, excess hydrochloric acid and the waste from the
vacuum flask is aqueous and not organic. Aqueous waste needs to be
disposed of down the drain with copious amounts of water. Do not place
excess sodium bicarbonate solution, excess hydrochloric acid or the
waste from the vacuum flask in the “Non-halogenated liquid organic
waste”. If sodium bicarbonate and acid are mixed in a bottle and sealed,
gasses will form and the container can rupture. If the container is glass, as
organic waste containers typically are, this can cause dangerous broken
glass projectiles. Be very careful about proper waste segregation.
Wear goggles and lab coat at all times during this lab. Be extra cautious when
handling concentrated hydrochloric acid. Neutralize and wipe up all spills
immediately.
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+ + AThe stronger the acid, the weaker the conjugate base.
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.
Partitioni: The distribution of a substance or ions between two immiscible liquids.
Extractionii: Dissolution and removal of one constituent of a mixture in a solvent.
Solubility: The amount of a solute which will dissolve in a given amount of
solvent. This is typically given as grams of solute per 100 gram 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.
Density: The mass per unit volume of a material. This is commonly listed as
grams per mL (g/mL) or grams per cubic centimeter (g/cc).
Equilibrium Constant (K): The numerical 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
is greater than one, the equilibrium lies in favor of the products. The
reaction proceeds in the forward direction.
pKa: The negative log of the acid equilibrium constant. pKa= - log Ka where the
acid equilibrium constant Ka is equal to: 𝐾𝑎 =
[𝐻3 𝑂 + ][𝐴− ]
[𝐻𝐴]
. The smaller the
pKa the stronger the acid.
Polarity: An unequal distribution of positive and negative charges. Ions with a
positive and negative charge are exceedingly polar. The polarity of a
molecule (microscopic) is often quantified by dipole moment. The polarity
of a solvent (macroscopic) can be described by dielectric constant.
Dipole Moment: The vector sum of all the individual bond moments within a
molecule.
Dielectric Constant: A measure of a materials ability to transmit an electric field.
2. Hydrocarbons typically have a low solubility in water. As the carbon number
increases, the solubility in water decreases. As a general rule of thumb, a molecule with
4 or less carbons will be soluble in water. A molecule with 7 or more carbons will not be
soluble in water. If a molecule has 5 or 6 carbons it may or may not be soluble in water.
As more polar groups are added to a carbon skeleton, the solubility in water increases.
Ethanol (2 carbons) is completely miscible in water. 1-Hexanol is only slightly soluble in
water (0.59 grams will dissolve in 100 g of wateriii), while Glucose (6 carbons and 5 –OH
groups) is highly soluble in water (133 g/100mL)iv.
3. Ionic materials, with separate positive and negative charges (Na+, K+, B-), are
incredibly polar. Hydrocarbons are very non-polar and exist at the other end of the
polarity spectrum. Intermediate polarity is determined by the polarity of the bonds within
the molecule and the intermolecular forces between molecules.
Less
Polar
Low
Solubility
in water
High
solubility
in
hexane
More
Polar
Polarity of compounds by class:
Hydrocarbons < ethers < esters <ketones < alcohols< carboxylic acids< water< ions
Examples:
High
Solubility
in water
Low
solubility
in hexane
Polarity of bonds
covalent < polar covalent < ionic
Polarity of intermolecular forces
London dispersion forces < dipole-dipole < hydrogen bonding < ionic
4. Two solvents will be used; water and Methyl t-Butyl ether (MTBE). Water has a
dielectric constant of 78 and MTBE has a dielectric constant of 47. (Hexane for
comparison has a dielectric constant of 1.0) v Water is much more polar than MTBE.
5. Compounds dissolve best in solvents with a similar polarity. Like dissolves like.
Compounds of high polarity have a high solubility in high polarity solvents and a low
solubility in low polarity solvents. Ions are very polar. Ions have a high solubility in water
and a low solubility in organic solvents. Sodium benzoate and sodium naphthoxide are
ions. They have a low solubility in MTBE and a high solubility in water. Most neutral
organic compounds have a low solubility in water but an increased solubility in organic
solvents. Neutral benzoic acid and neutral 2-naphthol have a low solubility in water and
a high solubility in MTBE.
6. Useful pKa information. (Do not memorize pKa values)
HA -> H+ + AACID (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 (C10H7-0H)
pKa = 9.5
2-Naphthonate ion ((C10H7-0-)
Water (H2O)
pKa = 15.7
Hydroxide ion (HO-)
7. 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
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. 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).
Below are ALL the possible reactions for the lab. Examine each reaction, determine if
each compound is acting as an acid or a base. Compare the two acids and determine
which acid is stronger. Next 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.
Possible Reactions of Mixture with Sodium Bicarbonate (NaHCO3)
Possible Reactions of Mixture with Sodium Hydroxide (NaOH)
Protonation of anions with hydrochloric acid (HCl)
8. Additional structures and information.
The organic solvent is MTBE or Methyl t-Butyl Ether. The IUPAC name, which is rarely
used for this material is 2-methyl-2-methoxypropane. This compound used to be
frequently 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.
or CH3OC(CH3)3
Sodium bicarbonate or sodium hydrogen carbonate is NaHCO3 or
.
This is baking soda and a weak base. We are using sodium bicarbonate dissolved in
water.
Sodium hydroxide Na+ -OH, is a strong base. In its pure form it is a white solid. We are
using sodium hydroxide as a 1.50 molar (M) solution in water.
Concentrated hydrochloric acid is 12.0 molar (M) HCl dissolved in water.
9. If a neutral organic compound dissolved in an organic solvent is deprotonated and
turned into an ion, the solubility of that material in the organic solvent will be decreased
and the ion will want to move or partition into an available aqueous solvent. If a base is
chosen which will selectively deprotonate one of a mixture of dissolved organic
compounds and turn only one of the compounds into an ion, then that compound can be
separated from the other materials. This concept enables this separation experiment to
be successful. The ion in water is subsequently treated with hydrochloric acid,
reforming a neutral organic material. If no low polarity solvent is present into which it
can partition, it will precipitate as a solid. The solid is isolated by vacuum filtration.
10. The concentration of benzoic acid and 2-naphthol in the stating MTBE solution is
2.50 x 10-2 g/mL (0.0250 g/mL). To determine the mass of starting benzoic acid and 2naphthol, the starting volume of solution that you used in milliliters (mL) needs to be
multiplied by this concentration (g/mL). This is the concentration of each material.
Starting mass
= starting volume (mL) x starting concentration (g/mL)
= 40.0 mL x (2.50 x 10-2 g/mL) = 1.00 g
If a different starting volume is used, a different starting mass of each will result. Note
proper use of significant figures.
11. The percent recovery of benzoic acid, 2-naphthol and the total will be determined.
Percent recovery is equal to the mass recovered divided by the starting mass times 100
percent. The mass recovered is the mass of product collected from the Buchner funnel.
The starting mass is determined from the starting volume and the starting concentration.
𝑚𝑎𝑠𝑠 𝑏𝑒𝑛𝑧𝑜𝑖𝑐 𝑎𝑐𝑖𝑑 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐵𝑒𝑛𝑧𝑜𝑖𝑐 𝐴𝑐𝑖𝑑 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑 =
𝑥 100 %
𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 𝑏𝑒𝑛𝑧𝑜𝑖𝑐 𝑎𝑐𝑖𝑑
The total is the combined mass recovered over combined starting mass.
𝑇𝑜𝑡𝑎𝑙 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
𝑚𝑎𝑠𝑠 𝑏𝑒𝑛𝑧𝑜𝑖𝑐 𝑎𝑐𝑖𝑑 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑 + 𝑚𝑎𝑠𝑠 2 𝑛𝑎𝑝ℎ𝑡ℎ𝑜𝑙 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
=
𝑥 100 %
𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 𝑏𝑒𝑛𝑧𝑜𝑖𝑐 𝑎𝑐𝑖𝑑 + 𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 2 𝑛𝑎𝑝ℎ𝑡ℎ𝑜𝑙
12. The purity of the recovered solids will be checked by melting point. Remember what
you learned previously about the melting point range compared to literature values.
The literature melting pointvi for benzoic acid is 122.1oC, and 2-napthol is 123oC.
13. 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
separatory funnel costs approximately $110.00. Handle with care.
Place separatory funnel in ring clamp. Always place a receiver under stopcock. If the
handle of the stopcock is aligned with the funnel, the stopcock is open and contents will
drain. If the handle of stopcock is perpendicular to funnel the stopcock is closed.
Funnels are stored with the stopper out to prevent the stopper from freezing. The
stopper must be removed to allow the funnel to drain properly. Your instructor will show
you the proper method of picking up, shaking, venting, and draining your separatory
funnel. Pay close attention. The funnel must be vented frequently to prevent vapor build
up and prevent the stopper or the contents from launching. One reaction produces CO 2
gas which creates pressure.
14. Waste MTBE must be disposed of in the non-halogenated organic liquid waste
container. Aqueous neutralized waste in which all of the organics have been removed,
should be disposed of down the drain. Do not place aqueous waste into organic waste
container. Never combine sodium bicarbonate and acid in a sealed container. Gasses
will develop and a very dangerous situation may result.
15. Two flow chart showing the separation which takes place is appended. Make sure
you understand each step involved.
16. Procedural changes may improve or diminish the success of the separation.
The reaction with sodium bicarbonate is not very efficient. If all of the benzoic
acid does not react with the weak base sodium bicarbonate, the residual benzoic acid
will remain in the MTBE and further react with the second base, sodium hydroxide. If
this happens it is common to have high purity but low recovery of the benzoic acid, and
a low purity but high (over 100%) recovery of the the 2-naphthol. Two sodium
bicarbonate extractions are performed to try to avoid or reduce this from happening.
More extractions with smaller volumes are more efficient than one extraction with
the combined volume (Three 10 mL extractions are more efficient than one 30 mL).
If a mixture of dissolved organic compounds is mixed with a strong base to
deprotonate both of them, they will both turn into ions, both migrate to the aqueous
phase and no separation will occur.
Appendix 1. Flow sheet of Extraction Experiment.
Appendix 2. Step-wise flow sheet of Acidic Extraction Experiment.
Separation and Extraction
Recovery of Solids
References
i
th
Hackh’s Chemical Dictionary, 4 Ed, McGraw-Hill, 1972, p 491
Ibid p 259
iii
International Programme on Chemical Safety, ICSC
http://www.inchem.org/documents/icsc/icsc/eics1084.htm (March 6, 2013)
iv
Sigma-Aldrich, D(+)-Glucose Product Information Sheet,
http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Product_Information_Sheet/2/g5400pis.Par.0001.F
ile.tmp/g5400pis.pdf, (March 6, 2013)
v
th
Carey, F.A., Sundberg, R. J., Advanced Organic Chemistry Part A: Structure and Mechanisms, 4 Ed,
Kluwer Academic, New York,2000, p240
vi
th
CRC Handbook of Chemistry and Physics, 65 Ed, 1985, p C-149, C-390
ii
Revised September 20, 2015 S.L. Weaver