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Lecture 7

CHEM 233 Lecture 7: Base Extraction of Benzoic Acid from Acetanilide followed by Recrystallization and melting point Determination


Department
Chemistry
Course Code
CHEM 233
Professor
Driver Tom
Lecture
7

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Base Extraction of Benzoic Acid from Acetanilide followed by Recrystallization and
melting point Determination
Kenil Gandhi
Partners: Victor, Zahra
March 15, 2016
Methods and Background
Benzoic Acid
Acetanilide
Chemical
Name
Molecular
Formula
Molecular
Weight
(g/mol)
Liquid Solid Solubility Potential
Hazards
b.p.
ºC
Density
g/mL
m.p.
ºC
Acetanilide C8H9NO 135.17 304 1.21 111-
115
Slightly
Soluble
Slightly
toxic
Benzoic
Acid
C6H5COOH 122.12 249 1.27 121-
123
Soluble Toxic
The goal of this lab is to perform a base extraction of benzoic acid from acetanilide. This lab also
involves identifying immiscible layers in a separatory funnel, operating a separatory funnel,
purifying a solid by recrystallization, isolating crystals by vacuum filtration, obtaining a melting
point of crystalline solid and differentiating products by IR spectroscopy. This lab incorporates
separation of 1:1 mixture of Benzoic acid and acetanilide with base extraction NaOH.
Along with the recrystallization, chromatography and distillation methods for separating and
purifying organic liquids and solids, extraction is also used to separate immiscible liquids. The
separation between the immiscible liquids is known as phase distribution because the liquids are
in two different phases. This phase distribution results in partitioning or adsorption phenomenon.
Extraction is based on the partitioning which happens due to difference in solubility of liquids
concluding selective dissolution. The two immiscible phases in which the solute will be
distributed include the extracting phase and the original phase. According to the equation, , the
amount of solid distributed in each phase is quantitatively expressed through a constant K. K is
partition coefficient calculated by dividing S(x) which is the extraction phase to S(o) which is
the original phase. If the value of K is greater than 1 that means the solute will remain largely in
the extracting solvent. At equilibrium, the concentration of the solute in both the phases is equal
and constant. If value of K is less than 1, then multiple extractions of the solute should be
performed. This means multiple smaller extractions are better than one large extraction with the
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same volume. The fraction of the solute remaining in the original solvent decreases as the
number of extractions increases. This is explained by the equation, , where n is the number of
extractions, V(o) is the volume of the original solvent, V(f) is the volume of extracting solvent
per extraction, and K is the partition coefficient. Thus, to perform successful extractions, the
extracting solvents must not react irreversibly with the solute, an ideal extracting solvent must be
immiscible with the original solvent (i.e. CH2Cl2 and H2O), it must be able to selectively remove
the desired component, and it must be easily separated from the solute.
For partitioning between nonpolar and polar solvent, the solutes with different polarities would
have different coefficient K. For example, if a mixture containing nonpolar neutral compound
and ionic polar compound is distributed between a nonpolar and polar solvent, then the solutes
will be distributed based on their relative affinities to each solvent. This theory is based on the
principle of “like dissolves like”, the neutral compound will partition into the nonpolar phase and
the ionic compound will partition into the polar phase. That is why neutral compounds will not
dissolve in water rather ionic compounds will dissolve in water due to its high polarity. In In this
lab, benzoic acid will be extracted from acetanilide in the presence of an aqueous base like
NaOH as shown in Figure 1.
Figure1. Reaction mechanism of Benzoic acid and NaOH to extract acid from acetanilide
Reactions between acids and bases are used to extract solutes. So, if an acid is mixed with a basic
solvent such as NaOH, then the acid will form its conjugate base and can be separated by
extracting the acidic compound into a basic aqueous phase. Thus, reaction can also be vice-versa
when a base is placed into an acidic solvent, then the base will form its conjugate acid as shown
in Figure 2. When the conjugate base is formed is very polar because originally it comes from an
acidic environment makes layers of liquid phase which are easier to be separated. The Keq which
is an equilibrium constant can be calculated by Keq = [products] /[ reactants]. Carboxylic acids
and phenols are organic compounds that contain functional groups that are polar, hydrophilic but
due to their R or aromatic groups, these compounds are insoluble in water and soluble in organic
solvents like CH2Cl2. The reaction of an acid with a base will lie to the right if Keq is greater than
one.
The extraction methods are used to separate two solvents based on the differences in solubility.
The solutions are poured into a separatory funnel and are inverted to enhance the separation of
the solutes. The separatory funnel is also timely vented to release any pressure in the funnel built
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in by the reaction. The bottom layer is the organic layer and the top layer is the aqueous layer
containing the conjugate base of benzoic acid. In liquid-liquid base extraction of benzoic acid
from acetanilide, neutral compound of benzoic acid is converted to its conjugate base through
extraction with an aqueous base like NaOH. This conjugate base of benzoic acid is soluble in
water and can be separated from acetanilide which remains dissolved in the organic layer.
Figure 2. Reaction mechanism of Base and acid to extract base from acid
After extraction, drying agents are used to remove water from a solution. The drying agent must
not react with the organic liquid and it must easily be separated from the liquid. In this
experiment NaSO4 was used as a drying agent because it has neutral acid-base properties.
The isolated compound from the different compound may contain impurities which can be
removed through the process of recrystallization. In solution recrystallization, the solid is
dissolved in an appropriate solvent at an elevated temperature and crystals are allowed to re-form
on cooling, whereas the impurities remain dissolved in the solution. Mostly all solids are more
soluble in a hot than a cold solvent. But in order to perform recrystallization the solvent must not
react with the desired product, the desired product is easily soluble in the solvent at elevated
temperatures, but only slightly soluble or insoluble at all or at room temperatures, impurities
must be highly soluble at all temperatures, and the solvent must be easily removed from the
crystalline product by filtration or evaporation. Filtration is a technique used to separate solids
from liquids. The solids are crystals whereas the filtrate is discarded in this lab. It is performed
by attaching the Erlenmeyer flask with the vacuum pump letting all the air in the flask out. While
the liquid is poured through the ceramic funnel from the filter paper attached to the flask. The
solution is purified while the solids stay on the filter paper.
Equations:
Fraction of solute remaining in original solvent
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