Acid-Base Extraction

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ORGANIC CHEMISTRY LAB

OBJECTIVE

1) To separate a mixture of an acid, base and and a neutral compound into its individual components

2) Purify the unknown by re-crystallization

3) Analyze and identify the compound by melting point determination

INTRODUCTION

Extraction is a process of transferring a solute from one solvent to another. It is used to separate one or more components from a mixture. Extraction is like distillation and recrystallization. However, extraction, unlike recrystallization or distillation, rarely gives a pure products. Recrystallization may be needed to purify a crude product extracted from a mixture. Another use of extraction is to wash the solution of an organic solute in an organic solvent free of inorganic impurities. Extraction is accomplished by shaking a solution in a separatory funnel with a solvent that is immiscible with the one in which the desired substance is dissolved and in which the desired substance is more soluble. Two liquid layers are formed which can be separated from each other by draining the lower layer through the stopcock of the separatory funnel. Immiscible is used to describe two phases that do not dissolve each other.

For example, a reaction is carried out in aqueous solution and the desired product is an organic compound. The reaction mixture is then shaken with a small amount of an organic solvent such as ether where upon the organic solute, being more soluble in the organic solvent than in water transfer primarily to the organic layer. Then, the undesired aqueous layers is drawn off resulting organic solution is shaken with a small amount of distilled water to wash the organic solution to remove inorganic impurities. The new aqueous layer containing inorganic impurities is then discarded. The organic solution that remains is now ready for further treatment to isolate the desired product.

If two solutes are both soluble in an organic solvents but insoluble in water, they can separated by extraction if one of the solutes can converted to a water-soluble salt. For example, a mixture of benzoic acid and naphthalene (neutral hydrocarbon) can be separated as follows. Dissolve the mixture in a small amount of ether. Then, pour the mixture into a separatory funnel. Add a small amount of diluted NaHCO₃ solutions. The NaHCO₃ is a base and reacts with the benzoic acid to form a water-soluble salt. The naphthalene being a neutral which is remains in the ether. After that, the aqueous layer can be separated from the ether layer by opening the separatory funnel’s stopcock and draining the aqueous solution into a separate container.

MATERIALS

0.24 g of an unknown (Triphenylmethanol or 1,2,4,5-tetrachlorobenzene), 10.0 mL ether, 10.0 mL of 1.0mL NaOH, 6M hydrochloric acid, 5.0mL saturated NaCl and Na₂SO₄

APPARATUS

125 mL Erlenmeyer flask, 125 mL separatory funnel, weighing balance, hot water bath/ steam bath, Melting point apparatus, litmus/ pH paper and Pasteur pipet

PROCEDURE

1. Extraction:

i. 0.24 g of unknown sample and 0.12 g of benzoic acid was weight and placed in a test tube.

ii. 10 mL of ether was added to the mixture and was shaked to dissolve the mixture.

iii. The solution then transferred to the separatory funnel and 5 mL of 1M NaOH was added, capped tightly and the layers was mixed by swirling and shaking about 30 minutes. Pressure was vent by titling the funnel and opening the stop cork.

iv. It was shakes again after closed the stop cock. The mixture was leaved to stand and allow the layers to separate. The aqueous layer was drain into the Erlenmeyer flask and was labeled as first extraction NaOH.

v. 5 mL of the 1M NaOH solution was added to the ether layer in the funnel and the step (i)-(iv) was repeated and labeled as second extraction NaOH.

2. Filtration:

i. The flask was taken to the hood and by using a glass rod slowly stirring when 6M HCl was added into each Erlenmeyer flask until the solution was acidic by using blue litmus paper.

ii. Then, the flask was taken to cool in an ice bath for about 15 minutes or until thoroughly chilled.

iii. The solid benzoic acid was collected using Buchner funnel and filter flask.

iv. 2-3 mL water was used to wash and allowing the suction to continue for five minutes to get solid as dry as possible.

v. It was scraped gently into a watch glass and was put into an oven for complete dryness.

vi. The solid was weight and obtain melting point.

3. Remove NaOH

i. 5 mL of saturated solution of sodium chloride was added to the ether layer in separatory funnel to remove any trace of NaOH solution.

ii. The layer was separated like before,capped, mixed and vent.

iii. The ether layer was transfer into and Erlenmeyer flask and 1g of anhydrous sodium sulfate was added to remove the moisture inside.

iv. The stop cork was closed and swirled and was leaved to stand for 10 minutes

v. While the ether layer was drying, a small beaker or Erlenmeyer was weighted.

vi. The dried ether layer was transfer using a Pasteur filter pipet into a beaker. The beaker then brought to a steam bath in the fume hood to evaporate the ether.

vii. After evaporated, the beaker was cooled down at room temperature and reweighing to determine the yield of two unknown sample.

viii. The melting point was obtained and compared to the two chemicals given

DATA/ RESULT:

Actual weight of benzoic acid= 0.1264 g

Weight of benzoic acid recovered= 0.0941 g

Percent recovery of unknown sample= (0.0941 g/ 0.1264 g) *100 = 74.45%

Actual weight of unknown sample= 0.2927 g

Weight of unknown sample recovered= 0.2007 g

Percent recovery of unknown sample= (0.2007 g / 0.2927 g) * 100= 68.59%

Observed melting point of unknown sample:

Literature melting point of triphenylmethanol= 163^o C

Literature melting point of 1,2,4,5-tetrachlorobenzene= 140^o C

~White precipitate of benzoic acid and white precipitate of unknown sample was formed.

DISCUSSION

When executing experimentation of this lab, it is crucial to understand the potential results from the acid-base of the compounds. A successful extraction can then occur as long as those concepts are understood. We had to know which compounds dissolve into distinct solutions. The compounds in the mixture all dissolved in the ether solution but were insoluble in the aqueous layer until further conditions allowed for solubility. Water is a poor solvent of covalent molecules but rapidly dissolves ionic compounds.

The order of extraction occurred chronologically starting with benzoic acid substance. The melting point readings indicated that benzoic acid, which re-crystallized was indeed pure and the melting point matched the known melting range of benzoic acid. Both triphenylmethanol and the 1,2,4,5-tetrachlorobenzene had melting ranges that proved inconsistent with known melting ranges for both compounds. Since triphenylmethanol 1,2,4,5-tetrachlorobenzene substance were extracted after the benzoic acid, it is possible that the layers could have mixed and contaminated the extracted layer from the pipette.

A procedure known as backwashing was used in an effort to remove any possible contaminates from both the substance and any solutions. Any contaminates present will transfer to the ether layer and then can be removed via pipette. Though the experiment only called for a single backwash, multiple attempts would have resulted in a more pure solution. This could have gained closer accuracy in melting ranges and reflected a more pure substance.

The precision required to pipette a specific layer out of a reaction tube becomes more accurate with experience. Some level of contamination can be anticipated when multiple lab technicians are participating in this procedure which results in an experiment with a lower degree of control. In addition, glassware from the micro-lab kit utilized during experimentation could have contaminated samples of phenol and the neutral substance. Such contamination could have fluctuated melting point ranges from the expected temperature readings.

CONCLUSION

This report has discussed the separation a mixture of an acid, base and a neutral compound into its individual components, purify the unknown by re-crystallization and identify the compound by melting point determination. The objectives of this lab were to develop the necessary hardware and software to have the HC11 measure temperature and indicate whether that temperature fell outside of prescribed limits. That objectives were met. The Percent recovery of unknown sample is 63.3998%.

QUESTIONS

1) List four water-immiscible liquids other than ether could be used to extract organic compounds from aqueous solution.

Ø Cyclohexane

Ø Chloroform (trichloromethane)

Ø Carbon tetrachloride (tetrachloro-methane)

Ø Methylene chloride (dichloromethane)

2) Why is it wrong to leave a bottle of anhydrous sodium sulfate or calcium chloride open?

When open the bottle of anhydrous, the air contains of water which is it expose the anhydrous to absorb the water particles in the air. This will affect the anhydrous effectiveness during the experiment.

3) Draw the structure of the product when benzoic acid reacts with sodium hydroxide. Why is the product of this reaction easily extracted into H₂O, while the original benzoic acid is not easily extracted into H₂O?

=It is because the moreOH^-that remove a hydrogen H⁺ to form a salt, the polar salt is soluble in aqueous solution, where both OH^- and benzoate are bases, the stronger base takes the H⁺ to form the weaker base.

4) Why does a benzoic acid precipitate out when the aqueous layer is acidified with HCl?

=Benzoic ion is stronger bases than Cl^-, so each one takes H⁺ from the HCl,the acid forms are not water soluble, so they precipitate out from the solution.

5) Develop a procedure for isolating a neutral compound from a mixture containing a basic impurity by drawing a flowchart as on page 20.

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