Expt 2. Acid/Base
Extraction, Mass Spectrometry, 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 diethyl ether (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 1:1:1 mixture of trans-cinnamic
acid, ethyl 4-aminobenzoate (benzocaine), 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.
Experimental
Procedures
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 separatory 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.
For the numbered steps that follow
please recognize that there is no need to go strictly
in sequence 1 --> 6. For example a more efficient
sequence of steps might be 1 --> 2 --> 5 -->
3 --> 6 --> 4. A flowchart would work well in
your prelab plan as a way of organizing the tasks that
need to be done.
- Dissolve 1.5 g of
the provided mixture in 20 mL dichloromethane (DCM).
- Acid extraction -
Transfer the solution to a separatory funnel, add 15 mL 1M HCl, stopper
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. Into
separate Erlenmeyer flasks, drain off the bottom layer
through the stopcock and then pour out the top layer
through the top of the separatory funnel. Label the
flasks as to which is organic and which is aqueous and
then return
the organic layer to the separatory
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 separatory funnel. Extract once more with 1M
HCl, combining the obtained aqueous layer with those
obtained previously. Label the combined aqueous layers
as "HCl extracts".
- Base extraction -
Return the organic layer to the separatory funnel and
extract it with 15 mL of 1M NaOH. Separate the layers
as in part 2 and return the organic to the funnel
twice more extracting each time with an
additional 15 mL of 1M NaOH. Label the combined aqueous
layers as "NaOH extracts".
- Isolation of
neutral substance - Add anhydrous sodium sulfate to
the flask containing the organic layer and allow
to stand for 10-15 minutes with occasional swirling.
Decant the liquid into a dry round bottom flask. (This
process is referred generally referred to as "drying
over Na2SO4" - 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.
- Isolation of
organic base - Carefully add
1M NaOH to the flask containing the HCl
extracts 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.
- Isolation of
organic acid - 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).
- What do your
answers to question 2 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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