Transcript Document

Separation of a Mixture by
Extraction: Exploring acid–base
chemistry in the organic laboratory
Dr. Kristen Procko
17 February 2010
Goals of Experiment 4
• Analyze equilibrium reactions involving mixtures of two acids
• Separate a known mixture of compounds by extraction
• Learn organic chemistry techniques
– Separatory funnel use
– Rotary evaporation
– Vacuum filtration
• Analyze products by melting point and determine the components
of an unknown mixture
Components of the mixture
O
O
OH
OH
N
H
acetanilide
p-toluic acid
p-tert-butylphenol
a neutral compound
pKa = 4.2
pKa = 10
How do we use pKavalues to determine the strength of a weak acid?
How can we use pKavalues to approximate the equilibrium constant for a
mixture of acids?
pKa values to estimate pKeq
pKeq = pKa(HA) pKa(HB)
Using this equation, we see that OH– will deprotonate p-toluic acid:
O
O
OH
+
Na
OH
pKa = 4.2
H2O
+
O
Na
pKa = 15.7
But it will deprotonate p-tert-butylphenol as well!
OH
O
+
pKa = 10
OHĞ
H2O
pKa = 15.7
+
Finding a base to get the job done
pKeq = pKa(HA) pK (HB)
We must find a weaker base ato deprotonate only the p-toluic acid
O
O
+
OH
HO
O
O
pKa = 4.2
Na
HO
O
OH
+
O
Na
pKa = 6.4
O
O
OH
+
HO
pKa = 10
O
HO
O
OH
pKa = 6.4
Sodium bicarbonate works!
*** Remember it will form carbon dioxide gas; use
caution when shaking in a separatory funnel.***
+
Using the separatory funnel
Remember:
C*A*P
Close the stopcock
Away (Point the funnel away from
yourself and others, especially when
venting.)
Pressure release (frequent venting)
Strategy for separating the
components
Strategy for separating the
components
BĞ
B
OH
A
O
OHĞ
H2O
O
+
N
H
add
NaOH
organic
layer
A,B
aqueous
layer
A
drain off
organic
layer
drain off
NaOH
layer (save)
BĞ
A
BĞ
repeat 3 X
A in
Et2O
Acidification of the conjugate bases
Vacuum filtration
Potential unknowns
Phenols
OH
4-tert-butylphenol
OH
2-naphthol
Neutral compounds
O
O
naphthalene
9H-fluorene
1,4-dimethoxybenzene