Expt 2. Acid/Base
Extraction, Recrystallization, and C-13 NMR.
Part 1 - Acid/Base
Extraction
Reading
Assignment - Mohrig Chapter 10, 11, 12
Background
In this experiment you
will learn how to use a separatory funnel for the
purpose of carrying out liquid/liquid extraction,
a useful separation method commonly referred to as just
"extraction". The process of extraction involves
intimately mixing a solution with an immiscible
extraction solvent so as to allow compounds in the
solution to partition between the two liquid layers that
form after mixing is discontinued. Compounds that are
more soluble in the extraction solvent than in the
original solvent will end up predominantly in that
layer, whereas compounds that are less soluble will stay
in the original solvent. Water is almost always one of
the solvents with the other solvent being an organic
compound such as diethylether (CH3CH2OCH2CH3)
or dichloromethane (CH2Cl2), so
the two layers can be referred to simply as the aqueous
layer and the organic layer. Physical separation
of the layers then accomplishes the separation of the
compounds in the solution based on their solubility
characteristics.
Acid/base extraction
is a process that allows the separation of organic
acids, organic bases, and organic neutral compounds (not
an acid or base) from each other based on the solubility
differences of the organic acid (or base) and its
conjugate base (or conjugate acid). Organic acids such
as carboxylic acids (RCOOH), phenols (PhOH), and thiols
(RSH), all have an acidic proton that can be
deprotonated by aqueous base (usually NaOH) to form a
salt form of the acid, which is much more soluble in
aqueous solution than in organic solvents, as
illustrated by equation 1 for a carboxylic acid. Hence,
the carboxylic acid can be extracted from an organic
solvent by aqueous NaOH.
RCOOH
+ NaOH(aq) → RCOO-Na+(aq)
+ HOH (1)
The original
carboxylic acid can be retrieved from the aqueous layer
by simply neutralizing the base with HCl(aq) and
reforming the carboxylic acid (eq 2). The relatively
insoluble carboxylic acid often precipitates at this
point and can be collected by vacuum filtration.
RCOO-Na+(aq)
+ HCl(aq) →
RCOOH(s) + NaCl(aq) (2)
Conversely, organic
bases (e.g., amines, RNH2, R2NH,
or R3N) are protonated by aqueous acid
(usually HCl) to form salts that are much more soluble
in the aqueous layer (eq 3). Hence, amines can be
extracted from an organic solvent by aqueous HCl.
RNH2 + HCl(aq) → RNH3+Cl-(aq)
(3)
The original amine is
retrieved by treating the aqueous layer with aqueous
base (NaOH) so as to deprotonate the salt (eq 4), which
often precipitates and can be collected by vacuum
filtration.
RNH3+Cl-(aq)
+ NaOH(aq) → RNH2(s)
+ HOH + NaCl(aq) (4)
Organic compounds that
are neither acids or bases do not react with either NaOH
or HCl and, therefore remain more soluble in the organic
solvent and are not extracted.
Overview
You will use an
acid/base extraction to separate a mixture of trans-cinamic
acid, ethyl 4-aminobenzoate, and N,N-diethyl-3-toluamide
(DEET).
Pre-Lab
The mixture given
includes an organic acid, an organic base, and a
(non-acid non-base) neutral compound. Identify which is which and
write equations for the reactions that will occur when
the mixture is extracted with HCl and then later with
NaOH. Also,
make sure to include a literature pKa value for the acid
and pKb for the base in your table of reactants and
products. Your planned procedures should explicitly
state which compounds are obtained in the respective
steps 11, 13, and 15.
Experimental
Procedures
- Dissolve 1.5 g of
the provided mixture in 20 mL dichloromethane (DCM)
and transfer the contents to a separatory funnel.
- For steps 3-7 pay special attention to
the information in chap 11.2 of Mohrig and to the
instructions provided by the instructor on the
proper use of the sep funnel. Also see https://www.youtube.com/watch?v=2A98YEKzsMI
for a nice video tutorial. It is strongly
recommended that you label all containers used to
contain the various layers separated and solids
obtained.
- Add 15 mL 1M HCl.
Cap the funnel and shake gently at first with frequent
venting. As it becomes clear that excessive
pressure is not building up, end by shaking vigorously
for 30 s or more and then allow the layers to
separate. Before you go onto step 4 make sure you
know which layer is organic and which is aqueous.
- Into separate
Erlenmeyer flasks, drain off the bottom layer through
the stopcock and then pour out the top layer through
the top of the sep funnel.
- Return the organic
layer to the sep funnel and and extract it with
another 15 mL of 1M HCl using the same procedures as
before. Combine the aqueous layers from the first two
extractions and return the organic layer to the sep
funnel.
- Extract once more
with 1M HCl, combining the obtained aqueous layer with
that obtained previously.
- Return the organic
layer to the separatory funnel and extract it with 15
mL of 1M NaOH. Separate the layers as before and
return the organic to the funnel.
- Extract the organic
layer twice more each time with 15 mL of 1M NaOH combining the
obtained aqueous layers with that obtained from the
first NaOH extraction.
- Dry the organic
layer over anhydrous Na2SO4.
Remove the drying agent by simply decanting the liquid
into a dry round bottom flask. See Chap 12 in Mohrig.
- Remove the DCM
solvent on the rotary evaporator (Mohrig, Fig 12.7),
the instructor or TA will assist with this.
- Use a Pasteur pipet
to transfer as much of the obtained oily liquid as
possible into a vial. Weigh and save for week 2.
- Carefully add 1M NaOH to the aqueous layer
from the HCl extractions so as to neutralize the pH.
You can use pH paper to make sure the solution is no
longer acidic and/or you can use the formation of
precipitate as a gauge. The organic base is only
slightly soluble in water so it precipitates as it is
reformed from the salt by deprotonation. Thus, maximum
precipitate formation indicates complete
neutralization.
- Collect the formed
solid by vacuum filtration on a Buchner funnel (Mohrig
Chap 10.4). Weigh it and allow it to air dry until
week 2.
- Use 1M HCl to
neutralize the aqueous layer from the NaOH
extractions. Do the addition carefully and slowly.
- Also collect this
solid by vacuum filtration, weigh it and and allow to
air dry until next week.
Assigned
Questions
- The original
mixture was prepared by using equal masses of
the three compounds. Use this information to
calculate the percent recovery of each compound.
(The proper word here is "recovery" not "yield"
because no net chemical reaction occurred. Instead
we merely are recovering the unchanged components of
the mixture.)
- Use the pKa's
of the acids on both sides of your equations for the
acid/base extraction reactions (see prelab
assignment) to calculate the equilibrium constants
for these reactions. Hint 1: the pKb of a base and
it's conjugate acid are related by the equation, pKa
+ pKb = 14. Hint 2: You will need to look up values
for the pKa of water and of HCl. Hint 3: You can use
this equation to calculate pKeq from pKa's: pKeq =
pKa(reactant) - pKa(product).
- Biochemists
like to use the equation pKa = pH + log([HA]/[A-])
to qualitatively predict whether an acid (HA) is
mostly deprotonated (A-) or mostly
in its protonated form (HA). The idea is that if pH
< pKa (by at least one unit) then the log term is
greater than 1.0 meaning that [HA]/[A-]
is greater than 10. Thus, under these conditions the
acid is at least 90% in its protonated form.
Conversely, if pH > pKa then [HA]/[A-]
is less than 10 and the acid is
at least 90% deprotonated. In very general terms, if
the solution is more acidic than the acid is (i.e.,
pH < pKa) then the acid is forced to stay
protonated, but if the solution is less acidic than
the acid (pH > pKa) then the acid will be fully
deprotonated. Question: Calculate the pH of 1M
NaOH(aq) and use it to determine if the organic acid
was fully deprotonated in steps 7 and 8 of the
extraction procedure.
- What do your
answers to questions 2 and 3 tell us about how much
of the carboxylic acid and amine should
theoretically have reacted when reacted with
NaOH(aq) and HCl(aq) respectively?
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