Extraction with acid and alkaline

Objectives:

1. To perform an acid-alkaline extraction

2. To recover benzoic acid and p-dichlorobenzene from its mixture using acid-alkaline extraction method

3. To determine the percent recovery of benzoic acid and p-dichlorobenzene

4. To determine the melting point of recovered benzoic acid and p-dichlorobenzene

Introduction:

Solvent extraction is a method to separate compounds based on their relative solubility in two different immiscible solutions. Solvent extraction is also known as liquid-liquid extraction or partitioning. Solvent extraction is a method that usually used to recover a compound from solid or liquid. The sample is contacted with a solvent that will dissolve solutes of interest. Solvent extraction is a major method and most efficient method of separation of interested product from a complex feedstock. The solvent can be a vapour, supercritical fluid, or liquid whereas the sample can be a gas, liquid or solid.

Basically, the frequent cleanest separation of organic compounds can be effected by use of acid or alkaline solutions which convert the compound to be extracted to a water soluble, either insoluble salt. For example, a five or ten percent solution of sodium hydroxide converts the sodium carbonate acid. Phenolic compounds undergo similar formation with sodium hydroxide solution. Hence, a sodium hydroxide can be used to extract a carboxylic acid or phenolic compound from its solution in an organic solvent, or conversely, an organic solvent can be used to remove organic impurities from a carboxylic acid or phenol dissolved in aqueous sodium hydroxide.

An aqueous solution of sodium hydroxide bicarbonate likewise convert carboxylic acids to their sodium salts, but are not efficiently alkaline to form salts of phenolic compounds. This provides an elegant method for the separation of a carboxylic acid and a phenolic compound. First the acid may be removed from the solution in an organic solvent by means of extraction with sodium bicarbonate solution, and then the phenol with sodium hydroxide solution. The inorganic acids are regularly removed from the organic solvent by extraction with sodium hydroxide, sodium carbonate, or sodium carbonate solutions. Dilute hydrochloric acid is often used in the extraction of basic substances from mixtures or in the removal of basic impurities. The dilute acid converts the base such as ammonia or an organic amine into water soluble chloride salt. Conversely, organic impurities may be removed from amines by extraction from a dilute acid solution with organic solvents.

Sodium salts of carboxylic acids and phenolic compounds are readily reconverted to the parent compounds by the treatment with sulphuric or phosphoric acid. The chlorides of amines revert to the original amine upon addition of sodium hydroxide solution.

Apparatus: Separatory funnel ( 250 mL), Buchner funnel, litmus paper, beaker

Materials: Benzoic acid, p-dichlorobenzene, ether, 10% NaOH, conc. HCl, anhydrous CaCl₂

Procedure:

1. 1 g of benzoic acid and 1 g of p-dichlorobenzene are prepared in 40 ml of ether in a beaker.

2. The ether solution is poured into a small separatory funnel followed by 20 ml of 10% NaOH solution.

3. The funnel is stoppered and is shaked well by inverting occationally and releasing any surplus presure throug the tap and it is allowed to stand.

4. The stopper is removed and the lower aqueous layer into a conical flask.

5. The residue is shaked in the with another portion of 10ml 10% NaOH and the lower layer is run off into the same conical flask. The solution in conical flask is added with conc. HCl until the solution become acidic.

6. The ether solution is washed with 30ml of water and run off the lower layer to be discarded.

7. The ether solution is added with three anhydrous CaCl₂ and the mxture is shaked occasionally until no turbidity remain.

8. The ether soution is decanted into clean dry small conical and two boiling chips are added. Then conical flask is heated until approximately 10ml of ether remains.

9. The ether solution is left it at fume hood to evaporate. Meanwhile, the precipitated benzoic acid is filtered by using Buchner funnel and flask.

10. The benzoic acid is washed with 5ml of cold distiled water and ensure the crystals are completely dry by pressing in a folded paper.

11. The weight and melting point of recovered benzoic acid and p-dichlorobenzene is determined.

Results:

Weight of benzoic acid = 1.0000g

Weight of p-dichlorobenzene = 1.0002g

Weight of beaker = 106.5879g

Weight of two filter paper = 1.6153g

Weight of two filter paper + weight of benzoic acid recovered = 2.5437g

Weight of beaker + weight of p-dichlorobenzene recovered = 106.9271g

Weight of benzoic acid recovered = 0.9284g

Weight of p-dichlorobenzene = 0.3392g

Melting point of benzoic acid = 123 ̊C

Melting point of p-dichlorobenzene = 56 ̊C

Calculations:

Percent recovery of benzoic acid =

= 92.82%

Relative melting point of benzoic acid =

= 100.82%

Percent recovery of p-dichlorobenzene =

= 33.92%

Relative melting point of p-dichlorobenzene =

= 105.66%

Discussion:

One of the important aspects when choosing a solvent system for extraction is to pick out two immiscible solvents. Some common liquid/liquid extraction solvent pairs are water-dichloromethane, water-ether and water-hexane. In this experiment, the benzoic acid and p-dichlorobenzene is used to dissolve in ether (diethyl ether), (C₂H₅)₂O. Most extraction is includes water because it is highly polar and immiscible with most organic solvents. In addition, the compound are attempting to be extracted must be soluble in organic solvent but not soluble in water. The volatility of the solvent is very important. This is because solvent have low boiling point like ether (very volatile) which can used to drying the isolated material very fast since the boiling point of ether is very low.

Both the benzoic acid and p-dichlorobenzene are able to dissolve in the ether solution according to the theory ‘like dissolve in like’ which organic compounds are soluble in organic solvent. Both solvent does not react with the ether solution, but they just dissolve in it. After that, sodium hydroxide, NaOH is added into the ether to react with benzoic acid to form sodium salt of benzoic acid and water. The chemical reaction is shown in the diagram 1.

Diagram 1

Two layers of solution are formed which upper layer is organic solvent while the lower layer is aqueous layer. The sodium salt of benzoic acid (sodium benzoate) is dissolve in the aqueous layer which runs off into a conical flask. The second time of addition of NaOH into the solvent is used to make sure that all the benzoic acid is reacted completely with sodium hydroxide. This reaction caused the similar two layers are formed and the aqueous layer is transferred into the same conical flask by using the same method. The reaction between benzoic acid and sodium hydroxide produces a lot of vapor after few times of shaking. Thus, the surplus pressure produced from the reaction is being released through the tap of the separatory funnel for several times to reduce the pressure inside it.

Water is added into the separatory funnel to wash the solvent. Then, the granular calcium chloride is added into the organic solvent after the aqueous layer run off. The purpose of adding of calcium chloride is used to remove residual water in the organic solvent. The calcium chloride in the granular form will be preferable. The calcium chloride is known as drying agent in the organic solvent which are not dissolves in the solvent but drying the solvent. The calcium chloride clump together with the water droplets as it solidified them. In another words, the calcium chloride reacts with water to form hydrates which is their preferred form when water is available. The chemical reaction between granular calcium chloride and water is shown as below:

CaCl₂(s) + H₂O(l) à CaCl₂. H₂O(s)

When the bigger size of hydrate is formed in the solvent, the heavier hydrate is sinks to the bottom of the funnel and it is easier to be removed. An excess drying agent should be used to ensure that all the water in solvent is removed. If the water remains in the materials collected, it could interfere with the analysis.

The upper layer containing p-dichlorobenzene in ether is run off into a conical flask. Then, the conical flask is added with two or three boiling chips and is being heated on a hot plate to evaporate the ether. The boiling chips are small, insoluble, and porous stones made of calcium carbonate or silicon carbide. There are a lot of pores inside the boiling chips which provide cavities both to trap air and to provide spaces to allow bubbles of solvent can be form. When boiling chips are heated, it will release tiny bubbles which can prevent boiling over. Boiling over of solvent will cause lost of solution which may lead to inaccurate result to be obtained. The boiling chips are always used when heating a solvent. The boiling chips are never added to a hot solution because it will cause immediately boiling over of solution. If the ether in the conical flask is left 20ml, the solution is left aside in the fume hood. The ether is not continues to be heated because the crystal of p-dichlorobenzene will melt and hence the crystal cannot be recovered. The white p-dichlorobenzene is formed in the crystal form after all the ether evaporates. The weight of recovered p-dichlorobenzene is about 0.3392g with melting point of 56 . In addition, the percent recovery and relative accuracy of melting point for p-dichlorobenzene are 33.92% and 105.66%. The percent recovery of p-dichlorobenzene is very low which only has 33.92% may be due to the lost of product in the experiment. Furthermore, the relative accuracy of melting point is more than 100% because the product is not a pure p-dichlorobenzene due to some impurities exist in the product.

Next, the sodium benzoate in aqueous layer collected in the conical flask is added with hydrochloric acid, HCl. The white precipitate is formed from the reaction. Litmus paper is turns from blue to red colour when excess HCl is added. The neutralization is take place when sodium benzoate and HCl is reacted with each other to produce precipitate of benzoic acid as the main product (sodium chloride salt and water are the side product). The diagram 2 in below shown that the neutralization process between sodium benzoate and HCl:

Diagram 2

The white precipitate is washed with cold water during filtration to minimize the solubility of benzoic acid in the water. After the filtration of the white precipitate by using filter paper, the benzoic acid crystal is dry up in the oven and finally the benzoic acid crystal is obtained. The weight of benzoic acid recovered and its melting point are 0.9284g and 123 . The percent recovery of benzoic acid is 92.92% while the relative accuracy of melting point of benzoic acid is 100.82. Similarly, the product was lost during conducting the experiment. Most probably some the products are dissolved in the cold distilled water during filtration. Then, the relative accuracy of melting point for benzoic acid is more than 100% due to the existence of impurities in the products.