Organic Chemistry Laboratory: Acid-Base Extraction and Purification Techniques

The laboratory experiment aimed to demonstrate the principles of acid-base extraction and the purification of organic compounds, focusing on the separation of benzoic acid from a mixture. The procedure involved sequential liquid-liquid extractions, filtration, and drying techniques. The experiment also included calculations and analysis to determine the efficiency of the extraction and the identity of the unknown sample.

Acid-base extraction is a widely used technique in organic chemistry to separate compounds based on their chemical properties. In this experiment, benzoic acid was chosen as the target compound for separation.

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Benzoic acid, with the molecular formula C7H6O2, is a colorless crystalline solid and the simplest aromatic carboxylic acid. The key principle involved in the separation process is the solubility of organic acids in dilute aqueous sodium hydroxide (NaOH) due to the acid-base reaction.

Procedure:

Extraction: i. Weighed 0.24 grams of an unknown sample and 0.12 grams of benzoic acid. ii. Added 10ml of ether to the mixture and shook it to dissolve.

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iii. Transferred the solution to a separatory funnel and added 5ml of 1M NaOH. iv. Shook the layers, vented pressure, and allowed the layers to separate. v. Drained the aqueous layer (first extraction NaOH) and repeated the process for the second extraction NaOH.
Filtration: i. Added 6M HCl to the first extraction NaOH until acidic. ii. Cooled the solution in an ice bath. iii. Collected solid benzoic acid using Buchner funnel and filter flask. iv. Washed the solid with water and dried it in an oven.
Remove NaOH: i. Added saturated NaCl solution to the ether layer.

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ii. Separated layers, transferred ether layer to an Erlenmeyer flask, and added anhydrous sodium sulfate. iii. Allowed the ether layer to dry and evaporated the ether.

Benzoic Acid:
- Filter paper: 2 pieces x 0.341g = 0.682g
- Extraction NaOH (before drying): 1.897g
- Extraction NaOH (after drying): 0.091g
- Percent of benzoic acid recovered: 74.7%
Unknown Sample:
- Unknown recovered: 0.1917g
- Percent of unknown recovered: 78.7%
- Melting point: 166.5°C

The melting point of the unknown sample (166.5°C) was within the range of triphenylmethanol, though slightly higher than the literature melting point (163°C). Possible reasons for the discrepancy include residual water or ether affecting the melting point determination or variations in the apparatus used.

The percent recovery of benzoic acid and the unknown sample was not 100%, indicating potential errors during extraction and filtration. Incomplete extraction by NaOH or inadequate acidity during cooling might have left some benzoic acid in the solution. Vacuum filtration effectiveness was also considered, as some solution may not have passed through the filter paper.

Triphenylmethanol, identified as the unknown sample, is an aromatic compound with specific solubility properties. The drying agent, anhydrous sodium sulfate, might not have fully absorbed water due to an open bottle, impacting the efficiency of the drying process.

The experiment demonstrated the importance of precise techniques in acid-base extraction and purification processes. Despite obtaining slightly lower percent recoveries, the identification of the unknown sample as triphenylmethanol was consistent with its properties. Human errors and equipment variations contribute to experimental discrepancies, emphasizing the need for meticulous laboratory practices.

The laboratory experiment aims to provide a hands-on experience in the separation of organic compounds using acid-base extraction techniques. The focus is on purifying benzoic acid, a common aromatic carboxylic acid, from an unknown sample using sequential liquid-liquid extractions. The procedure involves the utilization of acid-base chemistry principles, specifically the reaction of organic acids with aqueous sodium hydroxide, followed by filtration, drying, and analysis.

Introduction:

Acid-base extraction is a powerful technique used in organic chemistry to separate and purify acids and bases from mixtures based on their chemical properties. The experiment focuses on benzoic acid, which is known for its insolubility in water but reacts with dilute aqueous sodium hydroxide, enabling its selective removal from a mixture.

Procedure:

Extraction:i. Weighed 0.24 grams of an unknown sample and 0.12 grams of benzoic acid.
ii. Dissolved the mixture in 10ml of ether, transferred it to a separatory funnel, and added 5ml of 1M NaOH.

iii. After mixing, the layers were allowed to separate, and the aqueous layer was collected.

iv. This process was repeated for a second extraction with NaOH.
Filtration:i. Added 6M HCl to the aqueous layer until acidic, chilled in an ice bath, and collected the solid benzoic acid.
ii. Washed and dried the benzoic acid, obtaining its weight and optional melting point.
Removal of NaOH:i. Added saturated sodium chloride solution to the ether layer to remove traces of NaOH.
ii. Transferred the ether layer to an Erlenmeyer flask, added anhydrous sodium sulfate, and allowed it to stand for drying.

iii. Evaporated the ether, weighed the beaker, and determined the yield of the unknown sample.

Result and Observation:

Component	Weight (g)
Benzoic Acid (Before)	1.897
Benzoic Acid (After)	0.091
Unknown Sample	0.1917

Benzoic Acid Recovery Calculation:
- Percent of benzoic acid recovered (before drying): (1.897g / 0.1218g) x 100 = 1555.8%
- Percent of benzoic acid recovered (after drying): (0.091g / 0.1218g) x 100 = 74.7%
Unknown Sample Recovery Calculation:
- Percent of unknown recovered: (0.1917g / 0.2435g) x 100 = 78.7%
- Melting point: (173°C + 160°C) / 2 = 166.5°C

The melting point of the unknown sample, identified as triphenylmethane, was slightly higher than the literature value, possibly due to residual water or ether. The experiment yielded 74.7% of benzoic acid and 78.7% of the unknown sample, suggesting potential errors in extraction and filtration techniques.

Despite the inherent challenges and potential errors, the experiment successfully demonstrated acid-base extraction principles. The importance of precise techniques, including drying agents and vacuum filtration, was emphasized. The identification of triphenylmethane as the unknown sample further highlighted the significance of comparing experimental results with literature values for accurate analysis.