Introduction/Background:

            Acid-Base
Extraction is one of the most useful separation techniques in organic
chemistry. This separation is composed of a solvent-solvent extraction.

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“Extraction results from the unequal distribution of solute between two
immiscible solvents.” (Solomon, 2010) The purpose of this type of extraction
falls in the usage of the acid-base properties in organic compounds and the
isolation of the same when they are present in a mixture. A separation of a
mixture of an acid, a base, and a neutral compound is formed; these mixtures
then separate into their individual components. The theory behind this type of
extraction lies in the addition of either an acid or a base to a mixture so
that it separates its components; it also acts in the recrystallization of the
same in order to be purified and isolated (Nguyen, 2013).

            The
aim of this experiment is to separate a mixture of benzoic acid (Figure 1),
4-chloroaniline (Figure 2) and naphthalene (Figure 3), and figure out the
purity of the mixture using their melting point range. The acid-base reaction
used in this experiment consists of adding either an acidic or basic solution
to an acidic mixture depending on what the desired result is. If one wants to
turn the solution more basic, an acid is added; if one wants a more acidic
solution, then a basic solution is added into the mixture.

           

 

 

 

 

 

 

 

 

 

 

Experimental
Section:

1)   
Weight
out 1:1 ratio of 3g of starting solutes

2)   
Dissolve
in diethyl ether (30-mL) and put it in 125-mL separatory funnel using 20-mL
fresh ether

3)   
Add
30-mL of 5% HCl, close the separatory funnel and shake to mix

4)   
Vent
the separatory funnel and close and shake again

5)   
Repeat
until fully mixed

6)   
Allow
2 phases to separate into layers

7)   
Draw
out the lower layer into Flask #1

8)   
Add
3M NaOH to generate amine in Flask #1

9)   
Dry
using a vacuum then weight the residue

10) Add 50-mL of 5% NaOH to
separatory funnel

11) Shake until fully mixed (as
done before)

12) Draw off the lower layer into
a Flask and label it Flask #2

13) Add 6M HCl to Flask #2 while
the flask sits in an ice bath

14) Remove, dry, and weight as
done before

15) Add 20-mL of NaCl solution to
separatory funnel and drain bottom layer

16) Label the flask: Flask #3

17) Dry and weight as done before

18) Take each product’s melting
point using a melting point apparatus to determine the purity of each solution

19) Record all of the residues’
weight and their melting points

 

 

Table of
Chemicals:

Physical
Properties

Benzoic Acid

4-chloroaniline

Naphthalene

Molar mass

122.12 g/mol

127.57 g/mol

128.17 g/mol

Melting point

122 °C

68.0-71.0 °C

80.0 °C

Vapor
pressure

10.0 mmHg
(132 °C)

0.15 mmHg
(25 °C)

0.03 mmHg
(25 °C)

Boiling point

249.2 °C

232.0 °C

218 °C

Appearance

Colorless crystalline solid

 
Pale yellow solid

White solid crystals/flakes, strong odor of coal
tar

Density

1.27 g/mL

1.43 g/mL

1.14 g/mL

Chemical
Properties

Benzoic Acid

4-chloroaniline

Naphthalene

Solubility in
water

2.90 g/L

2.60 g/L

30.0 mg/L

Flammability

Flammable

Flammable

Highly flammable

 
 
 
Reactivity

 
Incompatible with strong bases,
strong oxidizing agents, alkalies.

Incompatible with strong oxidizing
agents, acids, acid chlorides, acid anhydrides, chloroformates, nitrous acid.

 
Can react exothermically with bases.

Enthalpy of
formation

?386 kJ/mol

?33.3 kJ/mol

?5.16×103 kJ/mol

Acidity

pKa: 4.2

pKb: 10

NA

Heat of combustion

3229.0 kJ/mol

?7669.0 kJ/mol

?5288.6 kJ/mol

Table #1:
Physical and Chemical Properties of the compounds used in the experiment.

Results:

Mixtures

Mass

Percent Recovery

Melting Points:

Benzoic Acid

1.015g

101.5%

94°C

4-Chloroaniline

0.947g

94.7%

78.3°C

Naphthalene

0.499g

49.9%

85°C

Table #2: Mass,
percent recovery, and melting point of all the compounds used.

Percent
Recovery Calculations

Benzoic Acid:

4-Chloroaniline:

Naphthalene:

Discussion:

         The results for the experiment are shown
in Table #2. The table shows the mass calculated for each individual compound,
along with their measured melting points and calculated the percent recoveries.

The percent recovery was calculated using the mass recovered versus the amount
that was actually supposed to result in the experiment. Basically the amount
recovered is divided by the amount expected in the procedure then the value is
multiplied by 100 so that we have a percent value. The percent recovery value
aids in the determination of the purity of an isolated compound.

         According to the results obtained, the
highest percent recovered was that of benzoic acid, 101.5%. It was the highest percent
out of the three substances. This result shows that there was 1.5% of impurity when
extracting the benzoic acid. This may be because the solution contained more
than what was supposed to be in the mixture. The other percent recoveries
ranged below the 100%. 4-chloroaniline was calculated to be 94.7% and
naphthalene resulted in ~50%. The percent recoveries are below 100%
because, since the beginning of the experiment, the mixture may not be 1:1:1
ratio due to impurities. Also, another possible reason could have been human
error. There could have been a mistake in the transferring of the solution to
the butcher funnel. There could have been left over solution in the flask used
to collect the solution. Another human error could possibly be an error in measurement
of the substances. All these factors could alter the values obtained for the
percent recoveries for all the compounds.

         Measuring the melting points of the
compounds is another way to determine their purity. To determine each isolated
compounds’ melting points, a melting point apparatus was used.  Using the apparatus one could see that all of
the melting point values were lower than the literal values provided. There are
various ways as to why the values differed: a rapid increase in the temperature
inside the apparatus, or the compounds extracted could have had impurities when
collected. An impurity is known to alter the results obtained in an experiment.

Conclusion

        The main purpose of this experiment was
to separate a
mixture of benzoic acid, 4-chloroaniline and naphthalene, and figure out the
purity of the mixture using their melting point range. In
this experiment the acid-base extraction of an organic acid, base and neutral
was performed successfully. Even though there may have been human errors in the
calculations, the overall procedure work relatively well. In an experiment of
this sort, the chemical compounds of the mixtures can help understand the
different reactions that could occur when the same solutions are mixed with
other substances. These chemical properties can also be used to make
extractions of certain isolated compounds. They help determine what substance
can be used to separate the mixtures. Thus, in this experiment one could see
clearly the interaction of an acid with a base and how the desired product
dissolves in one of the solvents, which are immiscible (Solomon, 2010). The
percent recovery and melting point calculated in this experiment were used to
verify the purity of the extracted compounds. Acid-base reactions can be, not
only used for the separation of organic compounds, but also in the removal of
impurities from different compounds. This laboratory accomplished what set out
to do because the extractions were performed successfully and clearly using all
of the concepts related to acid-base extractions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References:

L. (2013, December
18). Acid-Base Extraction. Retrieved January 29, 2018, from https://chem.libretexts.org/Demonstrations_and_Experiments/Basic_Lab_Techniques/Acid-Base_Extraction

Nguyen, V. Prezi. http://prezi.com/–uku_uckcxy/acid-base-extraction/
 (accessed January 2018).

Solomons,
Thomas W. Graham, and Craig B. Fryhle.Organic Chemistry. 7th ed. Hoboken,
N.J.:Wiley, 2010. Print. 

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