Expt 1. Separation of the Compounds in a MixturePart 1 - Acid/Base ExtractionReading
Assignment - Mohrig Chapter 9.4, 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 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. Your
planned procedures should explicitly state which
compounds are obtained in the respective steps 4, 5, and
6. Experimental
Procedures Pay special attention to the information in
chap 10.3 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.
Assigned
Questions
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