CHEM 233: Organic
Laboratory I Prelab Lecture
University of Illinois
at Chicago
UIC
O
H2O
protonation
+
R NH2
O
+
R NH3
H OH2
O
H2O
Na
OH
OH
H
N
amine
(conjugate base)
ammonium cation
(conjugate acid)
O
crude solid:
• not pure/clean
• depressed mp
• possibly discolored
CH2Cl2
Lab 6: Base Extraction of Benzoic Acid from Acetanilide
followed by Recrystallization and mp Determination.
H
N
CH3
O
pure solid:
• white crystals
• sharp mp
• narrow mp range
O
O
OH
(pKa = 4.20)
Keq
+
OH
O
+
H2O
(pKa = 15.7)
Todays Goals
O
O
H
H
N
CH3
O
benzoic acid
(m.p. = 121-123 ºC)
acetanilide
(m.p. = 111-115 ºC)
1. Separate ~1.0 g of a 1:1 w/w mixture of benzoic acid and acetanilide by baseextraction.
2. Purify each isolated solid by recrystallization from boiling water.
3. Measure melting points and compare to reported values.
4. Determine % yield obtained for each solid based on mass of starting mixture.
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 2
Prelab Lecture: Lab 6
General Scheme for:
Base Extraction of 1:1 Benzoic Acid/Acetanilide
1. Neutralize/
Protonate
with 3 M HCl
H2O O
O
Na
2. Vacuum filter
precipitated
benzoic acid
O
OH
crude solid:
• not pure/clean
• depressed mp
• possibly discolored
1. recrystallize
from boiling
water
2. vacuum filter
O
OH
pure solid:
• white crystals
• sharp mp
• narrow mp range
H2O
O
O
O
OH
Add 3 M
NaOH (aq)
Na
OH
separate
layers
H
N
H
N
O
O
CH2Cl2
CH2Cl2 is denser
than water =
bottom layer
CH2Cl2
evaporate
CH2Cl2
CH2Cl2
H
N
H
N
CH3
O
crude solid:
• not pure/clean
• depressed mp
• possibly discolored
O
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
1. recrystallize
from boiling
water
2. vacuum filter
H
N
CH3
O
pure solid:
• white crystals
• sharp mp
• narrow mp range
Slide 3
Prelab Lecture: Lab 6
Three Readily Ionizable Functional Groups That
are Separable by Acid or Base Extractions
O
R
O
deprotonation
O
+
H
OH
R
carboxylic acid
(conjugate acid)
O
R
H
O
+
H2O
+
H2O
+
H2O
carboxylate anion
(conjugate base)
+
O
deprotonation
OH
R
phenol
(conjugate acid)
phenoxide anion
(conjugate base)
protonation
+
R NH2
amine
(conjugate base)
H OH2
R NH3
ammonium cation
(conjugate acid)
*Hydroxide and hydronium were chosen as general examples; other specific acids and bases may be used.
The counterion on the left of the equilibria will depend upon the specific acid or base on the left.
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 4
Prelab Lecture: Lab 6
Solubility--”Like Dissolves Like”
O
O
R NH3
R
R
Water Polarity
O
water soluble
• ionic and very polar
• small organic ion
O
H
O
R NH2
R
O
H
R
O
H
water insoluble; organic solvent soluble
• nonionic, but polar functional groups
• i.e. organic solvent = CH2Cl2
University of
Illinois at Chicago
UIC
H
• VSEPR = water is bent; bond
angles of 104.5º
• strong molecular dipole
• inorganic and very polar (Ɛ=80)
• well suited to dissolve ionic
compounds, including small
organic anions and cations
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 5
Prelab Lecture: Lab 6
Choosing an Effective Extracting Acid or Base
For an Effective Acid or Base Extraction, Keq must be > 1.0
pKeq = pKa (acid left) - pKa (acid right)
Keq = 10
• remember: p = -log10
• For derivation of this
equation, see text page 154.
-[pKa (acid left) - pKa (acid right)]
Base Extraction
HA
B–
organic
acid (left)
extracting
base
Keq
Acid Extraction
Keq
BH
A–
B
HA
extracting
conjugate
acid (right)
organic
conjugate
base
organic
base
extracting
acid (left)
Example:
A–
HB
extracting
conjugate
base
organic
conjugate
acid (right)
Example:
O
O
OH
Keq
+
OH
O
N H
H
+
(pKa = 4.20)
H2O
(pKa = 15.7)
+
Keq
HCl
H3CO
H
N H
H
+
Cl
H3CO
(pKa = -8.0)
(pKa = 5.20)
pKeq = 4.20 - 15.7
Keq = 10 -(-11.5) = 3.16 x 1011
pKeq = -8.0 - 5.20
Keq = 10 -(-13.2) = 1.58 x 1013
Since Keq is large (>>1), aqueous hydroxide is a
good extracting solvent for benzoic acid.
Since Keq is large (>>1), aqueous HCl is a good
extracting solvent for 4-methoxyaniline.
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 6
Prelab Lecture: Lab 6
Several Smaller Extractions are More Effective
than One Large Extraction with Same Volume
(
FA =
Vo
KVx + Vo
FA = fraction of solute remaining in original solvent
K = partition coefficient = ([Ax]/[Ao]) →
larger K = more efficient extracting solvent
Vo = volume of original solvent
Vx = volume of extracting solvent (per extraction)
n = number of extractions
)
n
Example one: 3 extractions x 5 mL each
(15 mL total) from 20 mL of Vo. K = 2.
(
20
FA =
2*5 + 20
FA = 0.30
)
3
**30% of solute remains in the original solvent.**
University of
Illinois at Chicago
UIC
Example two: 1 extractions x 15 mL
each (15 mL total) from 20 mL of Vo. K = 2.
(
)
20
FA =
2*15 + 20
FA = 0.40
1
**40% of solute remains in the original solvent.**
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 7
Prelab Lecture: Lab 6
General Requirements for Extracting Solvent
1. Does not react irreversibly with the solute.
2. Immiscible with original solvent.
(i.e. H2O and CH2Cl2)
3. Selectively removes desired component.
(i.e. large K for component to be extracted and small K for the rest.)
4. Easly separated from the solute.
(i.e. cyrstallizatio/precipitation then filtration,distillation)
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
Slide 8
Prelab Lecture: Lab 6
Recrystallization
Dissolve product and
impurities in a minimum
amount of solvent at high
temperature.
Cool solution. Product
crystallizes out of
solution. Impurities
remain dissolved.
Desired Product
(yellow/large circles)
+
Impurities
(small/blue circles)
Requirements for Recrystallization Solvent
1. Not reactive with desired product(s).
Vacuum filter solution
through a Buchner funnel.
Mother liquor (filtrate)
contains impurities.
2. Desired product is completely soluble at elevated
temperature, but only slightly soluble–or not soluble at
all–at room temperature.
3. Undesired components (impurities) are highly soluble
at all temperatures (small temperature coefficient).
4. Easily removed from crystalline product by filtration
and evaporation.
University of
Illinois at Chicago
UIC
© 2009, Dr. Chad L. Landrie
CHEM 233: Organic Chemistry Laboratory 1
filtrate
Slide 9
Prelab Lecture: Lab 6